Laying the Foundation for Space Solar Power (Compass series)

Committee for the Assessment of NASA’s SpaceSolar Power Investment Strategy

Aeronautics and Space Engineering Board

Division on Engineering and Physical Sciences

National Research Council

NATIONAL ACADEMY PRESSWashington, D.C.

Laying the Foundation for

Space Solar PowerAn Assessment of NASA’s

Space Solar Power Investment Strategy

NATIONAL ACADEMY PRESS 2101 Constitution Avenue, N.W. Washington, DC 20418

NOTICE: The project that is the subject of this report was approved by the Governing Board of the NationalResearch Council, whose members are drawn from the councils of the National Academy of Sciences, theNational Academy of Engineering, and the Institute of Medicine. The members of the committee responsiblefor the report were chosen for their special competences and with regard for appropriate balance.

This study was supported by Contract No. NASW-99037, Task Order 105, between the National Academy ofSciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, orrecommendations expressed in this publication are those of the author(s) and do not necessarily reflect the viewof the organizations or agencies that provided support for the project.

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iv

COMMITTEE FOR THE ASSESSMENT OF NASA’S SPACE SOLAR POWER INVESTMENT STRATEGY

RICHARD J. SCHWARTZ, Chair, Purdue University, West Lafayette, IndianaMARY L. BOWDEN, University of Maryland, College ParkHUBERT P. DAVIS, Consultant, Canyon Lake, TexasRICHARD L. KLINE, United Satellite Launch Services, Great Falls, VirginiaMOLLY K. MACAULEY, Resources for the Future, Inc., Washington, D.C.LEE D. PETERSON, University of Colorado, BoulderKITT C. REINHARDT, Air Force Research Laboratory, Albuquerque, New MexicoR. RHOADS STEPHENSON, Jet Propulsion Laboratory (retired), La Canada, California

Liaison from the Aeronautics and Space Engineering Board

DAVA J. NEWMAN, Massachusetts Institute of Technology, Cambridge

Staff

KAREN E. HARWELL, Study Director, Aeronautics and Space Engineering BoardLEE SNAPP, NASA Administrator’s Fellowship Program—NRC Visiting Fellow,

Aeronautics and Space Engineering BoardGEORGE M. LEVIN, Director, Aeronautics and Space Engineering BoardMARVIN WEEKS, Senior Project Assistant, Aeronautics and Space Engineering Board

(May 2000 through March 2001)MARY LOU AQUILO, Senior Project Assistant, Aeronautics and Space Engineering Board

(March through June 2001)ANNA FARRAR, Administrative Associate, Aeronautics and Space Engineering Board

v

AERONAUTICS AND SPACE ENGINEERING BOARD

WILLIAM W. HOOVER, Chair, U.S. Air Force (retired), Williamsburg, VirginiaA. DWIGHT ABBOTT, Aerospace Corporation (retired), Los Angeles, CaliforniaRUZENA K. BAJSCY, NAE, IOM, University of Pennsylvania, PhiladelphiaWILLIAM F. BALLHAUS, JR., NAE, Aerospace Corporation, Los Angeles,

CaliforniaJAMES A. BLACKWELL, Lockheed Martin Corporation (retired), Marietta, GeorgiaANTHONY J. BRODERICK, aviation safety consultant, Catlett, VirginiaDONALD L. CROMER, U.S. Air Force (retired), Lompoc, CaliforniaROBERT A. DAVIS, The Boeing Company (retired), Seattle, WashingtonJOSEPH FULLER, JR., Futron Corporation, Bethesda, MarylandRICHARD GOLASZEWSKI, GRA Inc., Jenkintown, PennsylvaniaJAMES M. GUYETTE, Rolls-Royce North America, Reston, VirginiaFREDERICK H. HAUCK, AXA Space, Bethesda, MarylandJOHN L. JUNKINS, NAE, Texas A&M University, College Station, TexasJOHN K. LAUBER, Airbus Industrie of North America, Washington, D.C.GEORGE K. MUELLNER, The Boeing Company, Seal Beach, CaliforniaDAVA J. NEWMAN, Massachusetts Institute of Technology, CambridgeJAMES G. O’CONNOR, NAE, Pratt & Whitney (retired), Coventry, ConnecticutMALCOLM R. O’NEILL, Lockheed Martin Corporation, Bethesda, MarylandCYNTHIA SAMUELSON, Logistics Management Institute, McLean, VirginiaWINSTON E. SCOTT, Florida State University, TallahasseeKATHRYN C. THORNTON, University of Virginia, CharlottesvilleROBERT E. WHITEHEAD, National Aeronautics and Space Administration

(retired), Henrico, North CarolinaDIANNE S. WILEY, The Boeing Company, Los Alamitos, CaliforniaTHOMAS L. WILLIAMS, Northrop Grumman, El Segundo, California

Staff

GEORGE LEVIN, Director

vii

In 1968, Peter Glaser advanced the proposition thatsolar energy could be collected by Earth-orbiting satel-lites and then beamed by means of microwaves topower stations on Earth’s surface. The energy collectedwould be converted to electricity and introduced intocommercial power grids for use by customers. Boththe Department of Energy and the National Aeronau-tics and Space Administration (NASA) examined theconcept in the late 1970s and early 1980s; however, theprogram was canceled. In 1995, NASA decided to takea fresh look at the feasibility, technologies, costs, mar-kets, and international public attitudes regarding spacesolar power (SSP). This Fresh Look study1 found thatmuch had changed. Key technologies needed for theconstruction, deployment, and maintenance of SSP sat-ellites, such as composite materials, modular fabrica-tion, and robotics for construction and repair, had allshown significant advances. During this period, publicconcerns about environmental degradation grew. Thecommittee also noted that such environmental concernsare, if anything, even more intense today than in thedays of the Fresh Look study. As a result of this study,

Preface

the U.S. Congress became interested in SSP and in FY1999 appropriated funds for NASA to conduct the SSPExploratory Research and Technology (SERT) pro-gram. The SERT program and its follow-on, the SSPResearch and Technology (SSP R&T) program, con-stitute the effort assessed in this report.2

In March 2000, NASA’s Office of Space Flightasked the Aeronautics and Space Engineering Board ofthe National Research Council to perform an indepen-dent assessment of the space solar power program’stechnology investment strategy to determine its techni-cal soundness and its contribution to the roadmap thatNASA has developed for this program.3 The program’sinvestment strategy was to be evaluated in the context

1Feingold, Harvey, Michael Stancati, Alan Freidlander, Mark Jacobs,Doug Comstock, Carissa Christensen, Gregg Maryniak, Scott Rix, and JohnMankins. 1997. Space Solar Power: A Fresh Look at the Feasibility ofGenerating Solar Power in Space for Use on Earth. Report No. SAIC-97/1005. Chicago, Ill.: Science Applications International Corporation (SAIC).

2The SERT program was established in FY 1999 and continued throughFY 2000 by U.S. congressional appropriation. An additional appropriationwas also funded for SSP Research and Technology (SSP R&T) for FY2001. Decisions on internal NASA budget allocations for FY 2002 werepending during the preparation and review of this report. As a result ofrecent agencywide realignments, future SSP programs may be includedwithin other NASA initiatives. Throughout this report the term “SERTprogram” or “SERT effort” refers to both the 2-year Space Solar PowerExploratory Research and Technology (SERT) program during FY 1999and 2000 and the follow-on effort in FY 2001, referred to as the SSP Re-search and Technology (SSP R&T) program. The terms “SSP program”and “SSP effort” refer to any planned future program in SSP technologydevelopment and are used in recommendations to NASA.

3This assessment evaluates the SERT program and the follow-on SSPR&T efforts through December 15, 2000. Program changes after that dateare not included.

viii PREFACE

of its likely effectiveness in meeting the program’stechnical and economic objectives. The scope of thisstudy did not include assessments of the desirability ofspace-generated terrestrial electrical power or assess-ment of the ability of NASA’s space launch develop-ment efforts to provide the capability needed to deploya space solar power system.

The Committee for the Assessment of NASA’sSpace Solar Power Investment Strategy of the NationalResearch Council has completed an approximately 12-month study evaluating the technology investmentstrategy of NASA for SSP. A copy of the statement oftask for this study is included in Appendix A. In con-ducting its review, the committee was not asked to as-sess, and it did not comment on, the ultimate economicviability of producing terrestrial solar power fromspace. The committee sees the wisdom of investingsome of this nation’s resources in a number of potentialapproaches for dealing with future energy needs. Thisis particularly true when the committee considers thepotential payoffs from this investment to other NASA,government, and commercial programs. This reportprovides an assessment of NASA’s management of itsSSP investments and provides recommendations onhow its technical investment process can be improved.

The committee recognized that NASA deliberatelyexcluded “lowering the cost of access to space” (i.e.,development of new Earth-to-low-Earth-orbit launchvehicles) in its roadmap for SSP technology develop-ment. The committee understands and accepts NASA’srationale for this decision. NASA has a major programdevoted to lowering the cost of access to space. Giventhe relatively small amount of funding earmarked byCongress for space solar power technology develop-ment, little could be accomplished (and much would belost) by using these program resources to help lowerthe cost of access to space.

This study was sponsored by NASA and conductedby a committee appointed by the National ResearchCouncil (see Appendix B). The statement of task di-rected the committee to (1) evaluate NASA’s SSP ef-forts and (2) provide an assessment of its particularinvestment strategy for a potential program in SSPtechnology research and development. In order to ef-fectively prioritize and balance investments across sev-eral technology areas, rigorous modeling and systemanalysis studies are usually performed. NASA beganthis process during the SERT effort. Preliminary tech-nology and programmatic investments were presentedto the committee based on this modeling and seem re-

alistic, taking into account the level of funding madeavailable to the program. The committee believes thatthis approach is one useful technique for assigningtechnology investment priorities and determining therelative payoff from technology investments. The com-mittee discovered during its meetings, however, thatmany of the modeling inputs were suspect and thatmore refinement and better validation were necessarybefore additional decisions were made regarding tech-nology investment balance. Consequently, the commit-tee agreed that it would be inappropriate to evaluate theactual magnitude of funding in each technical area.Comments on the relative amounts for various tech-nologies are included.

As a result of low overall program funds during thepast 3 years, the program has been forced to make muchsmaller investments than desired for research in vari-ous technical areas. Due to this mismatch between theactual funding and program plan, the committee be-lieved it was critical to evaluate the organizationalfoundations, modeling methodologies, and programmanagement style on which the future SSP investmentstrategy will be based (despite levels of funding avail-able to the program). These issues led the committee toperform a two-part assessment of the program, provid-ing (1) an evaluation of the total program investmentstrategy, management, and organization and (2) anevaluation of each individual SSP-related technologyarea. The structure of the following report is based onthese two factors.

The committee was not asked to evaluate technol-ogy development in SSP-related areas in the UnitedStates or worldwide or to evaluate any other NASA ornon-NASA programs in technology development,whether related to SSP or not. As a result, no othertechnology program structure was assessed or men-tioned in the report. However, knowledge of the stateof the art in various technical areas is necessary to ef-fectively evaluate any research and technology effort.Various options for generating power from space havebeen suggested (and researched) during the past 30years, including the Lunar Solar Satellite Concept pro-posed by David Criswell, among others. The commit-tee did not consider such competing concepts for solarpower from space but concentrated solely on the NASASERT program. To this extent, the committee has fo-cused on the program at NASA and its relationship withindustry and other efforts in SSP-related technology.

This report has been reviewed in draft form by indi-viduals chosen for their diverse perspectives and tech-

PREFACE ix

nical expertise, in accordance with procedures ap-proved by the National Research Council’s (NRC’s)Report Review Committee. The purpose of this inde-pendent review is to provide candid and critical com-ments that will assist the institution in making its pub-lished report as sound as possible and to ensure that thereport meets institutional standards for objectivity, evi-dence, and responsiveness to the study charge. Thereview comments and draft manuscript remain confi-dential to protect the integrity of the deliberative pro-cess. We wish to thank the following individuals fortheir review of this report:

Minoru S. Araki, Lockheed Martin Corporation,retired,

Richard Green, International Power and Environ-mental Company,

Joel Greenberg, Princeton Synergetics, Inc.,Nasser Karam, Spectralab, Inc.,Thomas J. Kelly, Grumman Corporation, retired,Leeka I. Kheifets, Electric Power Research Institute

(EPRI),Mark S. Lake, Composite Technology Develop-

ment, Inc.,F. Robert Naka, CERA, Inc., andStephen M. Rock, Stanford University.

Although the reviewers listed above have providedmany constructive comments and suggestions, theywere not asked to endorse the conclusions or recom-mendations, nor did they see the final draft of the re-port before its release. The review of this report wasoverseen by Gerald L. Kulcinski, University of Wis-consin, appointed by the NRC’s Report Review Com-mittee, who was responsible for making certain that anindependent examination of this report was carried outin accordance with institutional procedures and that allreview comments were carefully considered. Respon-sibility for the final content of this report rests entirelywith the authoring committee and the institution.

The committee also thanks those who took the timeto participate in committee meetings and provide back-ground materials (see Appendix E). The committee isespecially indebted to Karen Harwell, study director,for her unflagging support of the committee and herhelp every step of the way. Lee Snapp, a NASAAdministrator’s Fellowship Program visiting fellow,contributed to the introduction and international sec-tions of the report, and George Levin, director, Aero-nautics and Space Engineering Board, was particularlyhelpful in interpreting and clarifying the committee’scharge.

Richard J. Schwartz, ChairCommittee for the Assessment of NASA’s

Space Solar Power Investment Strategy

Contents

xi

EXECUTIVE SUMMARY 1

1 INTRODUCTION 91-1 Electricity and Solar Power, 91-2 Background, 101-3 Study Approach, 11References, 11

2 OVERALL SERT PROGRAM EVALUATION 122-1 Evaluation of Total Program Plan and Investment Strategy, 122-2 Applications, 232-3 International Efforts, 25References, 27

3 INDIVIDUAL TECHNOLOGY INVESTMENT EVALUATIONS 293-1 Systems Integration, 293-2 Solar Power Generation, 343-3 Space Power Management and Distribution, 373-4 Wireless Power Transmission, 403-5 Ground Power Management and Distribution, 423-6 Space Assembly, Maintenance, and Servicing, 433-7 Structures, Materials, and Controls, 473-8 Thermal Materials and Management, 513-9 Space Transportation and Infrastructure, 533-10 Environmental, Health, and Safety Factors, 563-11 Platform Systems, 60References, 60

xii CONTENTS

APPENDIXES

A Statement of Task 65B Biographical Sketches of Committee Members 66C Example of NASA’s SERT Program Technology Roadmaps 69D Brief Overview of NASA’s Space Solar Power Program 73E Participants in Committee Meetings 77F Acronyms and Abbreviations 79

xiii

Tables and Figures

TABLES

ES-1 Proposed Space Solar Power Program Resources Allocation,FY 2002 to FY 2006, 2

2-1 NASA’s SERT Program—Model System Category Definitions, 132-2 Proposed Space Solar Power Program Resources Allocation,

FY 2002 to FY 2006, 17

D-1 Proposed Space Solar Power Program Resources Allocation,FY 2000 to FY 2016, 76

FIGURES

ES-1 Key recommendations to the NASA SSP program, 3

2-1 NASA’s SERT program: research and technology schedule of milestonesroadmap, 14

2-2 NASA’s SERT program: strategic research and technology goals, 16

3-1 NASA’s SERT program integration process, 30

C-1 Sample SERT progam executive summary chart on solar power generationactivity, 70

C-2 Sample SERT program goal chart on solar power generation activity, 71C-3 Sample SERT program milestones chart on solar power generation activity, 72

D-1 Generic space solar power system, 74D-2 Generic microwave and laser SSP systems, 74

1

NASA’S SPACE SOLAR POWER EXPLORATORYRESEARCH AND TECHNOLOGY (SERT)PROGRAM

The National Aeronautics and Space Admini-stration’s Space Solar Power (SSP) Exploratory Re-search and Technology (SERT) program1 was chargedto develop technologies needed to provide cost-com-petitive ground baseload electrical power2 from space-based solar energy converters. In addition, during its 2-year tenure, the SERT program was also expected toprovide a roadmap of research and technology invest-ment to enhance other space, military, and commercialapplications such as satellites operating with improvedpower supplies, free-flying technology platforms,space propulsion technology, and techniques for plan-etary surface exploration.

NASA focused the SERT effort3 by utilizing thedefinition of a “strawman” or baseline SSP system thatwould provide 10 to 100 GW to the ground electricalpower grid with a series of 1.2-GW satellites in geo-synchronous Earth orbit (GEO). For each of the majorSSP subsystems, NASA managers developed top-levelcost targets in cents per kilowatt-hour (kW-hr) that theyfelt would have to be met to deliver baseload power ata target of 5 cents/kW-hr. The result of this work wasa set of time-phased plans with associated cost esti-mates that provided the basis for a technology invest-ment strategy. Central to the SERT program was aseries of five or six experimental flight demonstrationsof progressively larger power-generation capacity,called Model System Categories. These demonstrationswill serve as focal points for the advancement of SSP-related technologies and will provide advancements intechnologies benefiting other nearer-term military,space, and commercial applications. NASA made ex-tensive use of cost and performance modeling to guideits technology investment strategy.

Executive Summary

1The SERT program was established in FY 1999 and continuedthrough FY 2000 by U.S. congressional appropriation. An addi-tional appropriation was also funded for SSP Research and Tech-nology (SSP R&T) for FY 2001. Decisions on internal NASAbudget allocations for FY 2002 were pending during review andpublication of this report. During recent agencywide realignments,future SSP programs may be included within other NASA initia-tives.

2Baseload power is defined as the power available to an area ata constant level during a 24-hour period. For example, most of thepower available to residential and business areas is consideredbaseload power.

3Throughout this report the terms “SERT program” and “SERTeffort” refer to both the 2-year Space Solar Power Exploratory Re-search and Technology (SERT) program during FY 1999 and 2000and the follow-on effort in FY 2001, the SSP Research and Tech-nology (SSP R&T) program. The terms “SSP program” and “SSPeffort” refer to any planned future program in SSP technology de-velopment.

2 LAYING THE FOUNDATION FOR SPACE SOLAR POWER

COMMITTEE ASSESSMENT

The current SSP technology program4 is directedat technical areas that have important commercial,civil, and military applications for the nation. A dedi-cated NASA team, operating with a minimal budget,has defined a potentially valuable program—one thatwill require significantly higher funding levels andprogrammatic stability to attain the aggressive perfor-mance, mass, and cost goals that are required for ter-restrial baseload power generation. Nevertheless, sig-nificant breakthroughs will be required to achieve thefinal goal of cost-competitive terrestrial baseloadpower. The ultimate success of the terrestrial powerapplication depends critically on dramatic reductionsin the cost of transportation from Earth to GEO. Fund-ing plans developed during SERT are reasonable, atleast during the 5 years prior to the first flight demon-stration in 2006 (see Table ES-1). The committee isconcerned, however, that the investment strategy maybe based on modeling efforts and individual cost, mass,and technology performance goals that may guide man-agement toward poor investment decisions. Modelingefforts should be strengthened and goals subjected toadditional peer review before further investment deci-sions are made. Furthermore, SERT goals could be ac-complished sooner and potentially at less cost throughan aggressive effort by the SERT program to capitalizeon technology advances made by organizations outsideNASA.

COMMITTEE RECOMMENDATIONS

Recommendations to the NASA SSP program canbe generally categorized by three main imperatives: (1)improving technical management processes, (2) sharp-ening the technology development focus, and (3) capi-talizing on other work. Figure ES-1 provides a snap-shot of the committee’s key recommendations. Eachrecommendation is numbered to correspond to the textsection in which it is discussed.

Improving Technical Management Processes

NASA’s SERT program’s technical managementprocesses need to be improved. Currently the program

has developed a set of integrated roadmaps containinggoals, lists of technology challenges and objectives,and a strawman schedule of program milestones thatguide technology investment. Appendix C contains asample set of roadmaps that have been developed forthe entire SERT program and each of the program’s 12individual technology areas. The roadmaps’ perfor-mance, mass, and cost goals are tied to research andtechnology initiatives in various technical areas neces-sary for SSP. Unfortunately, the committee did notfind adequate traceability between the goals at the sys-tem level and those at the subsystem level.

Integral to the milestone schedule are a series ofdownselect opportunities that precede each flight testdemonstration. However, there is no formal mecha-nism at this point in the program to guide thesedownselect decisions. The committee has also seenevidence that the current SERT program’s roadmapsdo not adequately incorporate the planned advances inlow-cost space transportation, both Earth-to-orbit andin-space options. Since advancements in space trans-portation are key to the SSP program’s ultimate suc-cess, the timing and achievement of technology ad-vances and cost and mass goals by the separate spacetransportation programs within NASA should be in-cluded directly in the SSP roadmaps. A periodic re-vamping of the roadmaps should be done based on theachievements of NASA in space transportation. SSP

4This assessment evaluates the SERT program and the follow-on SSP R&T efforts through December 15, 2000. Program changesafter that date are not included.

TABLE ES-1 Proposed Space Solar Power ProgramResources Allocation, FY 2002 to FY 2006 (millionsof dollars)

Investment FY FY FY FY FYArea 2002 2003 2004 2005 2006

Systems integration,analysis, and modeling 5 7 8 8 8

Total technologydevelopment 73 92 128 149 154

Technology flightdemonstrations 10 25 75 125 150

Total investment 88 124 211 282 312

SOURCE: Adapted in part from “Strategic Research and Technol-ogy Road Map.” Briefing by John Mankins and Joe Howell, Na-tional Aeronautics and Space Administration, to the Committee forthe Assessment of NASA’s Space Solar Power Investment Strat-egy, National Research Council, Washington, D.C., December 14,2000.

EXECUTIVE SUMMARY 3

Continue and Expand Technology DemonstrationsContinued Use of Model System Categories (Rec. 2-1-4)

Inclusion of Complementary Ground Testing (Rec. 2-1-6)

Use of International Space Station as Technology

Test Bed (Rec. 2-1-5)Testing of Robotics and Assembly Techniques on All

Flight Test Demonstrations, as Appropriate (Rec. 3-6-2)

Improve Technical Management Processes

Sharpen the Technology Development Focus

Capitalize on Other Work

Improve Decision Making Written Technology Plan (Rec. 3-1-1)Consistent Processes (Rec. 3-1-1)Rigorous Systems and Cost Modeling (Rec. 2-1-1)

Increased Use of Expert Critique/Review (Rec. 3-1-4)

Improve Program OrganizationContinued Use of Flight Test Demonstrations (Rec. 2-1-4)

Improvement of Advisory Structure (Rec. 3-1-5)

Address Environmental, Health, and Safety Issues Early in Program (Rec. 3-10-1, Rec. 3-10-2)

Invest in Key Enabling Technologies (Rec. 2-1-7)Solar Power GenerationWireless Power TransmissionSpace Power Management and Distribution

Assembly, Maintenance, and ServicingIn-Space Transportation

Expand Systems Integration and Testing Efforts Increased Investment in Modeling Capabilities (Rec. 2-1-1)

Component-to-System Integration (Rec. 3-1-3)

Prediction of In-Space Performance (Rec. 3-1-3)

Under current funding conditions focus on nearer-term applications but maintain the long-term investments in technology development. (Rec. 2-2-1)

Earth-to-orbit Transportation (Rec. 3-9-4)

Power Generation and Conversion (Rec. 2-1-8, Rec. 3-2-2 )

International Efforts (Rec. 2-3-1)

Orbit-to-orbit Transportation (Rec. 3-9-5, Rec. 3-9-6)

FIGURE ES-1 Key recommendations to the NASA SSP program. Each recommendation is numbered to correspond to the textsection in which it is discussed.


program technology investments, flight test demonstra-tions, and full-scale deployment should be rescheduledaccordingly.

Recommendation: NASA’s SSP program shouldimprove its organizational and decision-making ap-proach by drawing up a written technology devel-opment plan with specific goals, dates, and proce-dures for carrying out technology advancement,systems integration, and flight demonstration. TheSSP program should also establish a consistent pro-cess to adjudicate competing objectives within theprogram and specifically include timing andachievement of technology advances in robotics andspace transportation in the roadmaps.

NASA’s use of an architecture cost goal estimatebased on power costs in the future electricity market isappropriate and commended. As SSP developmentprogresses, however, the architecture cost goal shouldbe adjusted periodically to reflect changes in expecta-tions about future power markets, environmental costs,and other social costs that may arise during develop-ment.

The NASA SERT program began development ofrigorous modeling and system analysis studies, whichwere used as a basis for technology and programmaticinvestments. The approach could be developed, withimprovements, into one useful technique for determin-ing program priorities. The committee discovered dur-ing its meetings that many of the modeling inputs weresuspect and that more refinement and better validationwere necessary.

Recommendation: The SSP program should reviewits technology and modeling assumptions, subjectthem to peer review, and modify where indicated. Asingle SSP concept should be rigorously modeled,incorporating technology readiness levels and in-volving industry in conceptual design, as a means toimprove the credibility of the model input and out-put but not to prematurely select a single system forultimate implementation.

The SERT program’s oversight advisory structure(called the Senior Management Oversight Committee)includes representatives from various internal NASAorganizations, industry, and academia. Further lever-aging of technology expertise, management expertise,and funding could be obtained by including represen-

tatives from other organizations as well. Additionalinput would be beneficial from traditional aerospacecompanies, the Electric Power Research Institute(EPRI), utilities, and other government agencies (par-ticularly the Department of Defense [DOD]/U.S. AirForce, the Department of Energy [DOE], and the Na-tional Reconnaissance Office [NRO]). These additionswill provide periodic input on the investment strategyand program roadmap and provide further opportuni-ties to validate technological and economic input intothe performance and cost models. Individual researchand technology working groups have also been estab-lished to address planning and technology developmentin specific technology areas. It would be beneficial toexpand these activities.

Recommendation: The current SSP advisory struc-ture should be strengthened with industry (includ-ing EPRI and electric utility) representatives plusexperts from other government agencies (particu-larly DOD/Air Force, DOE, and NRO) in order tovalidate technological and economic inputs into theperformance and cost models. Also, due to the widebreadth of technologies related to SSP, the programshould establish similar advisory committees forspecific technologies in addition to the research andtechnology working groups currently utilized by theprogram.

In designing a full-scale SSP system, an environ-mental impact analysis must be performed that consid-ers human health issues, environmental impact both onEarth and in space, and possible risks to the SSP sys-tem itself. Currently the SERT program has placed onlya small priority on this area. However, the committeebelieves that environmental, health, and safety issuesshould be considered with more emphasis early in theprogram.

Recommendation: The SSP program should expandits environmental, health, and safety team in orderto review SSP design standards (beam intensity,launch guidelines, and end-of-life policies); assesspossible environmental, health, and safety hazardsof the design; identify research if these hazards arenot fully understood; and consider legal and globalissues of SSP (spectrum allocation, orbital space,etc.). One approach would be to involve an interna-tional organization such as the International Astro-

EXECUTIVE SUMMARY 5

nautical Federation Space Power Committee insuch studies.

Recommendation: Public awareness and educationoutreach should be initiated during the earliestphases of an SSP program to gain public acceptanceand enthusiasm and to ensure ongoing supportthrough program completion.

Sharpening the Technology Development Focus

Key Technologies for SSP

The SERT program must focus its technology de-velopment. Currently, the program is funding researchin a myriad of technologies that may have potential usein a full-scale SSP system. This research is a valuableendeavor in advancing SSP-related technologies and indetermining the extent of development necessary forindividual technologies to reach technology readinesslevels that can be certified for space flight. The com-mittee recommends that the current long-term focus ofthe program remain. However, due to current fundinglevels, most investments in individual technologies aremuch smaller than SERT program managers feel arenecessary for adequate research and development ofSSP technologies. Many investments are in areaswhere the utility and power industry should be the leadinvestor. Under current funding constraints, most ofthe investment should be focused on technologies thathave nearer-term applications in space or that may beapplied to other Earth applications. Specifically, thecommittee believes that the greatest benefit would beobtained by investing in several key enabling technolo-gies, which include solar power generation; wirelesspower transmission; space power management and dis-tribution (SPMAD); space assembly, maintenance, andservicing; and in-space transportation. Without sub-stantial advances in these critical areas, a viable, com-mercial full-scale SSP system that meets NASA’s costgoals may be unattainable in the time frame envisionedby the program.

Solar Power Generation Solar power generation isin the midst of an exciting period of advancement.NASA must collaborate with DOD, DOE, and com-mercial efforts to avoid undue duplication in researchand improve overall effectiveness. Successful attain-ment of the aggressive cost and mass goals that mustbe met if SSP is to provide commercially competitive

terrestrial power will require that NASA focus on high-reward, high-risk solar array research. Cost-competi-tive SSP terrestrial electric power will require majortechnology breakthroughs in solar power generation.

Wireless Power Transmission Investments in wire-less power transmission will also need to be focused onmore specific areas in the near-term time frame. Cur-rently, the program is funding work on several differ-ent options, both microwave and laser. As long as bud-get levels remain modest, NASA should select one ofthe three proposed microwave options, along with thelaser option, for further funding. Because of the poten-tial benefits to nearer-term space applications, invest-ment in the laser option should be aimed at bringingthis technology to the same level of maturity as themicrowave option. Ground demonstrations of point-to-point wireless power transmission should be con-ducted. NASA should also study the desirability ofground-to-space and space-to-space demonstrations.

Space Power Management and Distribution SPMADis a major contributor to the mass and cost of SSP sys-tem designs. Significant investment should be made toreduce the mass and cost of the components to be ap-plied in space while increasing their efficiency andmaximum operating temperature. Investments shouldalso be made with companies that are experienced inproducing power management and distribution(PMAD) and wireless power transmission componentsand that will one day have the capability to providehigh-volume manufacturing at low cost with high per-formance and high reliability.

Space Assembly, Maintenance, and Servicing As cur-rently envisioned by the SERT program, autonomousrobots will accomplish space assembly, maintenance,and servicing. This will require significant advances inthe state of the art of robotics. NASA’s SSP programshould perform additional systems studies directed atdetermining the optimal mix of humans and machinesand to allow for substantial human involvement on theground and possibly in space. Focused investments inadvancing robotics are expected to have benefits wellbeyond SSP.

In-Space Transportation Space transportation is keyto the deployment of any SSP system. In NASA’s ini-tial studies, approximately one-half of the system costwas allocated to ground-to-low-Earth-orbit (LEO)


transportation. Earth-to-orbit transportation costs andreliability will be crucial to the deployment of any fu-ture commercial SSP system. However, ground-to-LEO transportation is covered by the separate NASASpace Launch Initiative (SLI) program and is outsidethe scope of this assessment. LEO-to-GEO transporta-tion has little funding in other parts of NASA, so it hasbeen included as part of the SERT program. Chemical,electric, and hybrid propulsion systems are under con-sideration. In-space transportation is a critical technol-ogy that should receive significant investment.

Recommendation: The NASA SSP program shouldinvest most heavily in the following key enablingtechnologies, mainly through high-payoff, high-riskapproaches: (1) solar power generation (in collabo-ration with DOD/USAF and DOE to avoid duplica-tion); (2) wireless power transmission; (3) spacepower management and distribution; (4) space as-sembly, maintenance, and servicing; and (5) in-space transportation. The SSP program should notinvest research and development funds in groundPMAD technologies, ground-based energy storage,or platform system technologies. Utilities, industry,and other government programs already have sig-nificant investments in those areas.

Recommendation: Under current funding con-straints, the SSP program should devote a largeportion of its efforts to technologies that havenearer-term applications (e.g., low-mass solar ar-rays) while continuing to develop technology andconcepts for long-term terrestrial baseload powerapplications.

Any long-term, large program such as SSP muststrive to maintain a balance between near- and far-termobjectives and goals. Significant differences in technol-ogy development would occur if either short- or long-term goals are considered most important. The commit-tee has seen this struggle within the SERT program.Long-term progress must be made in many technologyareas before space solar power can become economicallyviable as a full-scale terrestrial baseload power source.However, due to budget realities and the need to provenear-term success, a program must also make contribu-tions to advancing nearer-term technologies that are ap-plicable to many different programs. In several technol-ogy areas, the committee sees merit in suggesting that theSSP program, as currently funded, invest in next-genera-

tion, revolutionary, high-payoff, high-risk concepts. Eachof the 11 individually numbered technical sections inChapter 3 discusses appropriate long-term recommenda-tions for the program.

Systems Integration

Systems integration is commonly applied duringthe development phase of a product. However, due tothe large number of SSP subsystems and their stronginteractions with one another, it should be of vital im-portance during early SSP technology development.NASA allocated a portion of its SERT funding to de-veloping an overall SSP concept and cost model thatincludes system cost, mass, and performance targets.Although not yet complete or independently validated,this model has been used as a tool to predict deliveredbaseload power costs, assuming that various technol-ogy goals have been achieved. The committee endorsesthis methodology as one useful technique for assigningtechnology investment priorities and urges that its de-velopment continue as an indicator of the relative pay-off from technology investments. The model is stillcoarse at this time, and the scope and detail should bebroadened so that cost and mass targets can be accu-rately allocated down to the component level. It ap-pears to the committee that many of these goals forlaunch costs and for system mass and cost must be sig-nificantly lower than those currently being used by theNASA team if the system is to produce competitiveterrestrial power. Sensitivity studies should be an inte-gral part of any large-scale modeling effort in order toquantify the impacts of departures from the nominalinput metrics, many of which are simply assumptionsfor the SERT program at this time. Nominal inputmetrics should be developed in consultation with ac-knowledged experts in SSP-related technology fieldsto assure quality and accuracy of data.

Recommendation: The SSP team should broadenthe scope and detail of the system and subsystemmodeling (including cost modeling) to provide amore useful estimate of technology payoff. Themodels should incorporate detailed concept defini-tions and include increased input from industry andacademia in the specification of model metrics. Thecosts of transportation, assembly, checkout, andmaintenance must also be included in all cost com-parisons to properly evaluate alternative technol-ogy investment options.

EXECUTIVE SUMMARY 7

Recommendation: The SSP program should reviewits technology and modeling assumptions, subjectthem to peer review, and modify where indicated. Asingle SSP concept should be rigorously modeled,incorporating technology readiness levels and in-volving industry in conceptual design, as a means toimprove the credibility of the model input and out-put but not to prematurely select a single system forultimate implementation.

Verification of SSP technology and the integrationand testing of hardware and software are necessarybefore deployment of any SSP system. A combinationof new modeling techniques and new design methods,which adaptively accommodate errors in predicted per-formance and function, may be necessary. The com-mittee saw little evidence of the depth of modeling nec-essary for such complex space platforms but expectsthat the effort will increase as candidate designs arechosen.

Recommendation: The SSP program should in-crease investments in developing spacecraft integra-tion and testing so that the performance of SSP sat-ellites can be verified with a minimum of ground orin-space testing. This may include the developmentof specialized integration, test, and verificationmethodologies for SSP spacecraft.

Technology Demonstration

A set of technology flight demonstrations (TFDs)is key to NASA’s technology demonstration plan forSSP. Use of these TFDs is commended by the commit-tee as an excellent means of testing available technolo-gies before full-scale integration and deployment. Ex-tensive use of ground demonstration milestones wasnot observed by the committee in the SERT roadmap.Use of ground demonstrations would provide a lower-cost mechanism to test new technologies before flight.Use of currently available in-space testing mechanismswould also be beneficial to any future SSP program.The current infrastructure on the International SpaceStation (ISS) could provide an excellent platform fortechnology demonstration activities. However, becausethe LEO at which ISS is located may be significantlydifferent from the GEO environment in many ways,demonstration plans should include methodologies thataccount for the differences between these orbits. Addi-tionally, testing of new robotics and assembly tech-

niques should be incorporated into all flight test dem-onstrations to further test advanced technologies.

Recommendation: The SSP program should con-tinue the use of technology flight demonstrations toprovide a clear mechanism for measuring technol-ogy advancement and to provide interim opportu-nities for focused program and technology goals onthe path to a full-scale system.

Recommendation: The SSP program should defineadditional ground demonstration milestones to beconducted prior to the far more expensive flighttests in order to test advanced technologies and sys-tem integration issues before planned downselectsof flight-demonstration technologies occur.

Recommendation: NASA should seriously considerutilizing the International Space Station as a technol-ogy test bed for SSP during the first set of flight dem-onstration milestones. Such tests would leverage ISStechnology and infrastructure, be independent of newadvances in space transportation, and provide an op-portunity to test autonomous robotic systems.

Recommendation: The SSP program should per-form near-term flight demonstrations of robotic as-sembly techniques, as well as robotic maintenanceand servicing operations. Robotics testing should beincorporated into all SSP flight demonstrations, ifpossible and as applicable.

Capitalizing on Other Work

NASA’s SSP program must capitalize on otherwork. Even if the SSP funding level increases dramati-cally, the technical challenges faced by NASA’s SSPprogram will require effective utilization of all re-sources currently being expended on SSP-related tech-nologies in a variety of government agencies (DODand DOE), commercial entities, and academia, both inthe United States and abroad.

This is especially true in reducing the cost of Earth-to-orbit transportation. NASA’s SLI is currently work-ing on cost reduction of transportation to LEO. TheSSP program must convey program information to theSLI on its transportation cost goals, optimal payload,mass, packaging, launch rate, and reliability require-ments and request that a credible plan be defined bySLI to help achieve these goals. In the case of the pro-


posed use of electric propulsion from LEO to GEO,NASA will need to collaborate with and capitalize onthe expertise of commercial firms working on electricpropulsion and other in-space transportation options.

Recommendation: The SSP program should begindiscussions between its management and that of theNASA Space Launch Initiative, so that future mile-stones and roadmaps for both programs can rein-force one another effectively. This discussion shouldinclude specific information on SSP space transpor-tation needs, including cost goals, timelines fordeployment, optimal payload mass, packaging re-quirements, launch rates, and reliability require-ments.

Recommendation: The SSP program should en-courage expansion of the current in-space transpor-tation program within NASA and interact with itstechnical planning to ensure that SSP needs anddesired schedules are considered.

Recommendation: The SSP program should in-crease coordination of industry, academic, andother NASA and non-NASA government invest-ments in advanced in-space transportation con-cepts, particularly in the areas of electric, solar-elec-tric, magnetohydrodynamic, ion, and solar-thermalpropulsion.

The components necessary for the ground PMADsubsystem are similar to those used for terrestrial pho-tovoltaic systems. Substantial research and develop-ment work is currently supported by the National Cen-ter for Photovoltaics, as well as several commercialentities that provide PMAD components for terrestrialphotovoltaic applications. In the case of the solar powergeneration components (i.e., photovoltaics), programsare currently under way in the Air Force to develophigh-efficiency, high-specific-power solar cells. Thework of the DOE’s National Renewable Energy Labo-ratory in thin-film solar cells will also be important tothe program.

Recommendation: NASA should expand its currentcooperation with other solar power generation re-search and technology efforts by developing closerworking relationships with the U.S. Air Force pho-tovoltaics program, the National Center for Photo-voltaics, industry, and the U.S. government’s SpaceTechnology Alliance.

Although it may be beyond the means of any onecountry to fund the research, development, and imple-mentation of SSP, these tasks could be more achiev-able with international cooperation, which would al-low NASA to profit from the work of expertsworldwide as well as to contribute its own expertise.

Recommendation: NASA should develop and imple-ment appropriate mechanisms for cooperating in-ternationally with the research, development, test,and demonstration of SSP technologies, compo-nents, and systems.

Many technologies for SSP (and other space mis-sions) are not currently on the critical path for any near-term NASA mission. Hence, little funding is availablethat can be leveraged by SSP to develop these tech-nologies. Without this leverage, it is unlikely that theSSP program can be the sole funding source for suchtechnologies. Examples of such technologies are free-flying robotic servicers, specific space structures, reus-able in-space transportation, and certain improvementsin thermal materials and management and in powermanagement and distribution. While it is beyond thepurview of this study to specifically recommend fund-ing increases for programs other than the SSP programassessed in this report, the committee believes that suchtechnologies are important to the ultimate success ofSSP.

SUMMARY

The committee has examined the SERT program’stechnical investment strategy and finds that while thetechnical and economic challenges of providing spacesolar power for commercially competitive terrestrialelectric power will require breakthrough advances in anumber of technologies, the SERT program has pro-vided a credible plan for making progress toward thisgoal. The committee makes a number of suggestions toimprove the plan, which encompass three main themes:(1) improving technical management processes,(2) sharpening the technology development focus, and(3) capitalizing on other work. Even if the ultimategoal—to supply cost-competitive terrestrial electricpower—is not attained, the technology investmentsproposed will have many collateral benefits for nearer-term, less-cost-sensitive space applications and fornonspace use of technology advances.

9

1

Introduction

1-1 ELECTRICITY AND SOLAR POWER

Throughout history, human progress has been fu-eled by energy. In early ages, wood to cook food, pro-vide heat, and later to smelt metals provided the pri-mary source of energy. By the seventeenth century,coal to heat homes and factories, make iron and steel,and produce steam for the engines of industrial produc-tion was a primary energy source. The twentieth cen-tury saw the advent of oil, natural gas, nuclear energy,and various renewable forms of energy, as well as thecontinuing use of coal to fuel humanity’s energy needs.The availability of inexpensive energy that can be con-verted to usable forms has provided the people of theindustrialized nations of the world a standard of livingthat would have been envied by kings of only a fewcenturies ago. A nation’s economic development andstandard of living go hand in hand with readily avail-able, useful forms of energy.

Electricity is one such useful form that can be madefrom readily available energy sources and is usedworldwide. Global demand for electricity has risen tre-mendously in recent years. In 1990, the world usedapproximately 11 trillion kW-hr of electricity peryear—a figure that is projected to be 22 trillion kW-hrby 2020 (EIA, 2000). However, as this global marketgrows, other issues have come to the public conscious-ness. Concerns have arisen about the deterioration ofEarth’s biosphere and potential long-term changes in

climate that may result from pollutants such as carbondioxide exhausted into the air as a result of fossil fuelcombustion.

Using sunlight to generate electricity has been dis-cussed for many years as an alternative source and per-haps a way to relieve some of these concerns. In 1968,in a paper published in Science, Peter Glaser proposedthat solar energy could be collected by earth-orbitingsatellites and then beamed to power stations on Earth’ssurface (Glaser, 1968). The energy collected would beconverted to electricity and introduced into the com-mercial power grid for use by terrestrial customers.Both the Department of Energy (DOE) and the NationalAeronautics and Space Administration (NASA) exam-ined the concept in the late 1970s and early 1980s.

NASA found that generating electric power for ter-restrial consumer use was not the only potential appli-cation for space solar power. Other uses have beenpostulated, including power transmission to other spacevehicles, power generation for lunar and Martian ex-ploration, power for commercial space developmentsuch as communications satellites, and as a source ofadditional power to enhance the capabilities of suchon-orbit facilities as the International Space Station(Grey, 2000). Making some or all of these uses of spacesolar power a reality requires developing, fielding, andmaking effective use of a number of complex technolo-gies within a constrained budget. The next section pro-vides a brief history leading up to NASA’s current


Space Solar Power (SSP) Exploratory Research andTechnology (SERT) program.

1-2 BACKGROUND

From 1979 to 1981, the Committee on Solar PowerSystems, Environmental Studies Board, of the NationalResearch Council (NRC) evaluated DOE’s andNASA’s work on SSP from the 1970s (NRC, 1981).The committee was tasked to perform a critical ap-praisal of this work, including identifying gaps in theDOE/NASA program and examining the results thatthe DOE/NASA study obtained. The study’s conclu-sions were not favorable for development of a satellitesolar power system. The 1981 NRC report concludedthat cost was a major prohibitive factor and the neces-sary technologies were not of the proper maturity. Es-timates of the energy outlook at the time did not indi-cate that SSP would be a cost-competitive source ofelectrical energy for the next 20 years. The size andcomplexity of financing and managing the infrastruc-ture that would be necessary would strain the abilitiesof the United States. International legal, political, andsocial acceptability caused by such issues as fear ofpossible health hazards could make SSP difficult orimpossible for the United States to achieve. That ear-lier report concluded that no funds should be commit-ted specifically to development of a satellite solarpower system during the next decade. Realizing, how-ever, that circumstances could change that would makemore advanced satellite solar power systems an optionin the more distant future, the 1981 NRC report alsorecommended vigorous investigation of technologiesrelevant to satellite solar power systems that were syn-ergistic with the goals of other programs.

In August 1981, the U.S. Office of TechnologyAssessment (OTA) also published a report on the DOE/NASA efforts that was unfavorable to continued workin SSP (OTA, 1981). According to the OTA report,too little was known about the technical, environmen-tal, or economic aspects to make a sound decision onwhether to continue further development and deploy-ment of SSP. Under the circumstances prevailing atthe time, OTA concluded that further research wouldbe necessary before any decisions could be made.When the unfavorable assessments, high initial costs,and need for more research, development, and testingwere combined with the drop in oil prices that began in1984, the urgency that drove development of an SSPsystem largely evaporated, and work essentially

stopped. There was little official interest until the mid-1990s.

In 1995, NASA took a fresh look at the feasibility,technologies, costs, markets, and international publicattitudes regarding SSP. The Fresh Look study, pub-lished in 1997, found that much had changed (Feingoldet al., 1997). Several promising concepts were identi-fied as alternatives to the original 1979 reference con-cept. The study showed that great cost savings, forexample, could be achieved over the 1979 referenceconcept by making use of modular, self-deploying unitsand on-orbit robotic assembly as opposed to the origi-nal concept, involving human-occupied, in-space con-struction bases. Modularization would also permit theuse of smaller launch vehicles in place of a two-stage-to-orbit, reusable, heavy-lift launch vehicle that wouldrequire unique ground launch infrastructure. The studynoted the critical importance of low-cost transportationto orbit and noted further that, although costs were stilltoo high, the technology to lower launch cost to orbitwas separately under development in other NASA pro-grams (although it is uncertain if or when those pro-grams will result in a new generation of launch ve-hicles or what improvements might be provided interms of performance or cost). The study asserted thattechnologies and concepts involved in SSP could be-come more feasible if both government and commer-cial non-SSP applications were considered. Finally, thestudy noted that the market for SSP, though global innature, might be uncertain for some time to come de-pending on how various nations’ policies treated SSPin comparison with other means of generating electric-ity (Feingold et al., 1997).

As a result of the Fresh Look study, both the U.S.Congress and the Office of Management and Budgetbecame interested in SSP once more. NASA conducteda follow-on concept definition study in 1998. The re-sult was funding of $22 million set aside for NASA toconduct the SERT program. In March 2000, the NASAOffice of Space Flight (Code M) approached the NRCwith a request to evaluate its technology investmentstrategy in space solar power with a view to determin-ing whether or not the strategy that the agency hadadopted would meet the program’s technical and eco-nomic objectives.

Although the current NRC committee neither ad-vocates nor discourages SSP, it recognizes that signifi-cant changes have occurred since 1979 that might makeit worthwhile for the United States to invest in eitherSSP or its component technologies. Improvements

INTRODUCTION 11

have been seen in the efficiency of crystalline photo-voltaic and thin-film solar cells. Lighter-weight sub-strates and blankets have been developed and flown. A65-kW solar array has been installed successfully onthe International Space Station, and wireless powertransmission has been the subject of several terrestrialtests. Japanese and Canadian experiments, some ofwhich are discussed later in this report, have shownthat small aircraft can be kept aloft by power transmit-ted via microwaves. The area of robotics, essential toSSP on-orbit assembly, has shown substantial improve-ments in manipulators, machine vision systems, hand-eye coordination, task planning, and reasoning. Ad-vanced composites are in wider use, and digital controlsystems are now state of the art.

Although scientific and engineering advances mayhelp make SSP more feasible, the committee noted thatpublic concerns about environmental degradation are,if anything, even more intense than in the days of theFresh Look study.

1-3 STUDY APPROACH

The NRC formed a committee of eight experts withexperience in space systems design, engineering, andlaunch; solar power generation, management, and dis-tribution; on-orbit assembly; robotics; space structures;and economics to independently assess the technicalinvestment strategy of NASA’s space solar power pro-gram. The full statement of task, found in Appendix A,asked the committee to address areas of the space solarpower investment strategy associated with develop-mental and operational issues, technical feasibility ofvarious aspects of the program, and opportunities forsynergy. The committee restricted its efforts to cri-tiquing NASA’s technical investment strategy and nei-ther advocated nor discouraged the concept of spacesolar power. Assessments of (and comparisons with)other space solar power concepts, such as the LunarSolar Satellite concept proposed by David Criswell,were not performed by the committee. The committeealso did not attempt to predict the role that space solarpower might play in the future among the many alter-natives for generating electricity. The purpose of thisassessment was to evaluate the technology investmentstrategy of the SERT program and provide guidance asto how the program can be most effective in meetingits long-term goals, not to influence those goals. This

assessment evaluates the SERT program and the fol-low-on SSP R&T efforts through December 15, 2000.Program changes after that date are not included.

The committee approached the study by adoptingthe SERT program’s definition of the term “investmentstrategy,” which includes six areas: (1) program divi-sion and organization, (2) use of developmental cycles,(3) opportunities for independent review, (4) balanceof internal and external investments, (5) use of systemsanalysis and modeling to define goals, and (6) periodicreview of technology roadmaps. This definition thenserved as an outline for the approach that the commit-tee used during its assessment.

This report focuses on two levels of assessment:(1) an overall evaluation of the technical investmentstrategy and program organization and (2) evaluationof individual technology subprograms. Chapter 2 ex-amines the overall investment strategy, the investmentstrategy methodology, program management issues,and opportunities for synergy with other programs.Recommendations and discussion are categorized inthree major areas. Chapter 3 provides individual evalu-ations of 11 of NASA’s 12 technical investment areas(economics is included in the overall assessment inChapter 2). Recommendations called out in the Execu-tive Summary and listed in Figure ES-1 were consid-ered key by the committee. Other recommendations inChapter 3 were considered important to managers ofindividual technology areas.

REFERENCESEIA (Energy Information Administration). 2000. International Energy

Outlook 2000. Washington, D.C.: U.S. Department of Energy, p. 114.Feingold, Harvey, Michael Stancati, Alan Freidlander, Mark Jacobs, Doug

Comstock, Carissa Christensen, Gregg Maryniak, Scott Rix, and JohnMankins. 1997. Space Solar Power: A Fresh Look at the Feasibility ofGenerating Solar Power in Space for Use on Earth. Report No. SAIC-97/1005. Chicago, Ill.: Science Applications International Corporation(SAIC).

Glaser, Peter. 1968. “Power From the Sun: Its Future.” Science, Vol. 162,No. 3856, November 22, pp. 857-866.

Grey, Jerry. 2000. “The Technical Feasibility of Space Solar Power.” State-ment to Subcommittee on Space and Aeronautics, Committee on Sci-ence, U.S. House of Representatives. September 7.

National Research Council (NRC), Environmental Resources Board. 1981.Electric Power from Orbit: A Critique of a Satellite Power System.Washington, D.C.: National Academy Press.

OTA (Office of Technology Assessment). 1981. Solar Power Satellites.NTIS No. PB82-108846. Washington, D.C.: U.S. Government PrintingOffice, p. 3.

12

2

Overall SERT Program Evaluation

2-1 EVALUATION OF TOTAL PROGRAM PLANAND INVESTMENT STRATEGY

The Space Solar Power (SSP) Exploratory Re-search and Technology (SERT) program was evalu-ated in the context of the “plan’s likely effectiveness tomeet the program’s technical and economic objec-tives,” as stated in the committee’s statement of task(see Appendix A). This top-level assessment leads toidentification of the most important technology invest-ment options, opportunities for increased synergy withother efforts, assessment of adequacy of available re-sources, and possible recommendations for changes inthe investment strategy to achieve desired objectives.Discussion and recommendations are grouped intothree basic areas: (1) improving technical managementprocesses, (2) sharpening the technology developmentfocus, and (3) capitalizing on other work.


Program Organization

The SERT program was charged to develop tech-nologies needed to provide cost-competitive groundbaseload electrical power from space-based solar en-ergy converters. In addition, during its 2-year tenure,the SERT program was also expected to provide aroadmap of research and technology investment to en-

hance other space, military, and commercial applica-tions such as satellites operating with improved powersupplies, free-flying technology platforms, space pro-pulsion technology, and techniques for planetary sur-face exploration.

With such a broad scope it is not surprising that theNational Aeronautics and Space Administration(NASA) centers, the Jet Propulsion Laboratory, andindustry participants have defined a myriad of tech-nologies that could be developed for the future applica-tions. It should also not be surprising that if NASA’syear-to-year expenditure remains at around $10 mil-lion or less, the program will be inadequate to meet theidentified needs. Funding has been in yearly incremen-tal add-ons by the U.S. Congress and has not been partof the formal NASA operating plan. It is impossible tomake efficient progress in technology developmentwhen funding and management support are uncertain.However, the current SERT managers have defined apotentially valuable program despite these obstacles.

Central to the SERT program is a series of experi-mental demonstrations called model system categories(MSCs) that serve as focal points for the technologydefinition. Table 2-1 outlines these MSCs (Mankinsand Howell, 2000a). Top-level schedule and resourcesfor accomplishing the technology development workas defined by NASA are shown in Figure 2-1 (Mankinsand Howell, 2000b). The committee endorses this ap-proach to defining flight test demonstration milestones

OVERALL SERT PROGRAM EVALUATION 13

to validate technology advancement. However, thecommittee also realizes, as does NASA, that the sched-ule of milestones and roadmap should be reconfiguredas research and development for components of theprogram are realized (or not realized) and new resultsare obtained. NASA demonstrated during the courseof the study that the roadmaps were revamped severaltimes during the first 2 years of the program in responseto both internal agency assessments and external peerreview. Continued annual and biannual assessment ofthe roadmaps, schedules, and goals are an inherent partof the program. The committee recommends thatfuture roadmaps, however, contain more transparent in-formation tying together cost, performance, and sched-ule. The roadmaps should also more visibly demon-strate their reliance on advances in space transportationand robotics that are entirely or largely funded by otherprograms.

NASA’s SERT program presented a concept forreviewing its time-phased plans, which include theincorporation of NASA’s strategic plans and goals, in-formation gleaned from independent program and tech-nology assessments, new innovative technology appli-cations, government and commercial application

opportunities, and research efforts in other organiza-tions. This iterative process review would be cycled atleast annually because strategic research and technol-ogy investments must be selected each fiscal year aspart of the NASA budget development process (assum-ing the work becomes part of the overall NASA pro-gram).

The committee has also seen evidence that the cur-rent SERT program’s roadmaps do not adequately in-corporate the planned advances of low-cost space trans-portation development, both Earth-to-low-Earth-orbit(LEO) and in-space options. Because any advance-ments in space transportation are key to the SSPprogram’s ultimate success, the timing and achieve-ment of technology advances and cost and mass goalsby the separate space transportation programs withinNASA should be included directly in the SSP road-maps. A periodic revamping of the roadmaps shouldbe made based on the achievements of NASA in spacetransportation. SSP program technology investments,flight test demonstrations, and full-scale deploymentshould be rescheduled accordingly. Adequate contin-gency plans also need to be developed to be able toreact positively to the failure of any flight or groundtest demonstration planned by the program.

TABLE 2-1 NASA’s SERT Program—Model System Category Definitions

NASA Model Power Flight Test Demonstration Options ProjectedSystem Category Capability (to be chosen competitively) Time Frame

MSC 1 ~100 kW Free flyer 2006-2007LEO-to-Earth power beaming research platformSolar power plug in spaceCryogenic propellant depot“Mega-commsat” demonstrator

MSC 1.5 ~1 MW GEO-to-Earth solar power satellite (SPS) demonstrator 2011-2012Lunar exploration SPS platformEarth neighborhood transportation system

MSC 3 ~10 MW Free flyer 2016-2017GEO-based SPS demonstration platformsfor wireless power transmission, solar powergeneration, power management and distribution,and solar electric propulsion

Interplanetary transportation system

MSC 4 ~1 GW Commercial space full-scale solar power satellite 2021+

SOURCE: Adapted in part from Mankins and Howell, 2000a.

14

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Performance Goals

NASA has made a determined effort in the SERTprogram to focus the effort by beginning the definitionof a “strawman” or baseline SSP system to provide 10-100 GW to the ground electrical power grid with a se-ries of 1.2-GW satellites in geosynchronous Earth orbit(GEO). Since no one knows the time scale to build,launch, and assemble on orbit such a system, the com-mittee did not comment on this particular scenario’spotential for commercial appeal (or any of the potentialscenarios for MSC 4). Chosen scenarios will be a di-rect result of the program’s investment strategy, theprogress of technology development, and a competi-tive selection process. The committee did not feel itwas appropriate to evaluate each individual scenariofor a full-scale system. As a result, various scenariosare not presented in the report. Time to market andsize of investment necessary for such a system will beissues that need to be addressed as the programprogresses; however, the SERT program previouslyfunded an independent economic analysis to evaluatesuch issues. Assessment of this analysis was outsidethe scope of this study.

Top-level cost targets in cents per kilowatt-hourwere developed for each of the major SSP systems thatNASA managers believed were necessary to finallydeliver baseload power at less than a selected target of5 cents/kW-hr.1 Major system and subsystem functionswere each allocated a “contributory” cost goal by pro-gram managers. The sum of the contributory goalsshould, in theory, be equal to the overall cost target of5 cents/kW-hr. These targets are shown for variousdesign options in Figure 2-2 (Mankins and Howell,2000b). For brevity, the specific design concepts andoptions (Mankins and Howell, 2000a; Carrington andFeingold, 2000) listed in Figure 2-2 are not presentedin the report. The NASA program plans to continuemonitoring this target as markets for electricity changeand to adjust this target and its distribution among tech-nologies accordingly. As such, the manner in whichcurrent cost goals are set is justified.

A corresponding set of mass, cost, and performancetargets was then used to help define where technology

funds should be applied, and detailed roadmaps havebeen developed to accomplish these technology goals.The result of this work is a set of time-phased planswith associated cost estimates, which provide the basisfor an investment strategy. The committee notes thatthere is a lack of traceability (of cost and mass goals) tothe next lower level. The committee expects that in fu-ture program documents there will be traceability ofcost and mass targets down to the subsystem level andto the component level. Without consistent cost andmass goals with clear traceability from the top level tothe component technology level, individual technologyteams may not make the most appropriate technologyinvestments.

The major SERT system cost and performance tar-gets, as shown in Figure 2-2, are extremely aggres-sive.2 Additionally, they include reliance that NASA’sseparate Space Launch Initiative (SLI) program will besuccessful in reducing Earth-to-LEO transportationcosts to $400/kg. NASA’s second-generation SLI goalis $2,200/kg, and the third-generation goal is approxi-mately $220/kg (NASA, 1999; Davis, 2000). In aSERT Program Status report (Mankins, 2000b), NASAreported that current SERT concepts (December 13,2000) result in predicted costs for power in the range of10-20 cents/kW-hr, versus NASA’s full-scale systemgoal of 5 cents/kW-hr.

NASA has adopted an allowable cost of 5 cents/kW-hr as its target goal for competitive terrestrialpower production. The committee suggests that thisvalue be revisited as the program proceeds; however, itis viewed by the committee as a reasonable startingpoint for the investment strategy. This choice sets therevenue stream level for a 1.2-GW facility. Once therevenue stream is known, the net present value of thisrevenue stream can be computed. A simplified calcula-tion was made by the committee for the required returnon investment, assuming zero operating costs and a 40-year operating period. The calculation demonstrates theimportance of strengthening the cost analysis for theoperational system. For instance, using a 10 percentrate of return, $5 billion is available for the entire sys-tem.

1This 5 cents/kW-hr goal was based on cost estimates gleanedby NASA from an independent economic analysis (Macauley et al.,2000).

2Subsystem cost, mass, and performance targets were also sup-plied to the committee for each technical area in the program’swork breakdown structure. For brevity, only the top-level programgoals are presented in this publication. More specific informationcan be obtained from the listed reference.

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Using the NASA target goal of allocating 2.5 centsof the 5 cent/kW-hr revenue stream to launch costs, in-space transportation, and ground assembly personnelallows $2.5 billion to be spent on construction and in-stallation of the system. At an assumed launch cost of$800/kg to GEO, it would cost $14 billion to launch a1.2-GW system unless the currently assumed weight ofthe 1.2-GW facility is substantially reduced. The cal-culations should be developed further consistent withstationary power plant funding procedures and perhapsusing the models for fission nuclear power introduc-tion. In any event, more stringent cost and mass goalsmust be achieved in order to meet current NASA costgoals for competitive terrestrial electric power. TheNASA team must be more rigorous in their cost andmass allocations to the subsystems and components, aswell as launch costs. Current space transportation andtechnology subsystem goals, for example, are alreadydriving near-term choices within the SERT program,creating the possibility that technology investmentchoices may be based on goals that are not stringentenough in this area.

Consequently, the committee believes that NASAnot only should reevaluate its cost goals in various tech-nology areas but also should complete a rigorous analy-sis of its cost goals in the space transportation area.Many of the goals for launch costs and system massand cost must be significantly lower than currently be-ing used by the NASA team if the system is to producecompetitive terrestrial power. Allocations should alsobe made in absolute terms, dollars and kilograms, aswell as familiar ratios such as dollars per watt and wattsper kilogram. These absolute terms can be used di-rectly by SSP technical staff in answering the question,How much can this specific technology subsystem costto attain an overall 5 cents/kW-hr cost? The committeenotes that the 5 cents/kW-hr target may be unnecessar-ily low for nonterrestrial applications of space solarpower, such as space-to-space power beaming for in-terplanetary spacecraft or space-to-planetary surfacebeaming for rovers, for example.

Resource Allocation

NASA’s Space Solar Power Strategic Research andTechnology Roadmap proposes resource allocations asgiven in Table 2-2. The figures are broken down intosystems integration, analysis, and modeling, total tech-nology development, and flight test demonstrations (re-ferred to by NASA as technology flight demonstra-

tions). Breakouts were also provided for each of themain technology development categories providing adescription of the proposed work, schedule, and costgoals (see Appendixes C and D for more detailed in-formation). The committee restricted its attention tothe first 5 years of the program (FY 2002 to FY 2006)due to the large uncertainty in the out years.

The committee’s reactions to NASA’s proposedresource allocations are as follows:

• This is a reasonable first projection of resourcerequirements through FY 2006.

• The committee supports NASA’s approach ofdividing the program into three major elements: (1) sys-tems integration, analysis, and modeling, (2) technol-ogy development, and (3) technology flight demonstra-tion. In the future, if some ground demonstrations areadded as key milestones, it would be appropriate togroup them with the technology flight demonstrationsand rename the category “technology demonstration.”

• Although it was not given a cost breakdownfor the eight subelements within systems integration,analysis and modeling, the committee would expect thesystems and infrastructure modeling to decrease mark-edly after the first few years and be replaced by tech-nology validation and studies of mission architecture.

• Technology development is broken down bymain resource applications. NASA’s chosen distribu-tion shows emphasis in the areas of greatest impact:solar power generation (SPG); wireless power trans-

TABLE 2-2 Proposed Space Solar Power ProgramResources Allocation, FY 2002 to FY 2006 (millionsof dollars)

Investment FY FY FY FY FYArea 2002 2003 2004 2005 2006

Systems integration,analysis, and modeling 5 7 8 8 8

Total technologydevelopment 73 92 128 149 154

Technology flightdemonstrations 10 25 75 125 150

Total investment 88 124 211 282 312

SOURCE: Adapted in part from “Strategic Research and Technol-ogy Road Map.” Briefing by John Mankins and Joe Howell, Na-tional Aeronautics and Space Administration, to the Committee forthe Assessment of NASA’s Space Solar Power Investment Strat-egy, National Research Council, Washington, D.C., December 14,2000.


mission (WPT); space power management and distri-bution (SPMAD); and space assembly, maintenance,and servicing. However, the relative size of the re-sources should vary much more between the vital tech-nologies and those that serve to make advances on allfronts. Additionally, it appears that this first estimatedoes not leverage the work of other agencies in suchcritical areas like SPG and SPMAD or, if it does, thatleverage is very small.

• Technology flight demonstration resourcesmay be at an appropriate level for the first flight testdemonstration (MSC 1). However, a requirements-driven conceptual study, including cost development,is needed before strong endorsement can be offered.Specifically, the requirements to be validated must beclearly defined, and alternative concepts for meetingthem should be developed, including associated costsand schedules. Potential resource support from otheragencies and international sources should be defined,resulting in a full conceptual plan that can be criticallyevaluated.

• The time schedule for each technology flightdemonstration should be tied to demonstrating desiredtechnology levels, rather than an arbitrary date.


System and Cost Modeling

A commendable start has been made on systemsand cost modeling for various SSP concepts and tech-nology choices (Carrington and Feingold, 2000;Feingold, 2000; Mullins, 2000). However, the com-mittee believes there may still be a great disparity be-tween the resources that can reasonably be expectedand the desired rate of technology developmentprogress. This will require careful evaluation of tech-nology payoffs and tough program management deci-sions.

The SERT program’s general approach of defininga baseline concept and coupling it with detailed systemand subsystem performance plus cost modeling toguide technology investment will be an excellent toolfor making some of these crucial decisions providedthe modeling is strengthened, as described in Section3-1 of this report. Additionally, the technology goalsand roadmaps concept should provide a credible goalfor individual technology areas. However, for thismethodology to be realistic, goals must flow from the

technology and cost requirements in both a top-downand bottom-up manner, involving input and decisionmaking from both technology management and tech-nologists.

Conceptually, the use of an architecture cost goalestimate based on power costs in the future electricitymarket is appropriate and commended. Specificationof this goal as a probability distribution representing arange of uncertainty would better represent the uncer-tainty inherent in any projections of future market po-tential. Over time, as SSP development progresses, thearchitecture cost goal should be adjusted to reflectchanges in expectations about future power markets,environmental costs, and other social costs that mayarise.

The SSP cost and system analysis models shouldincorporate one detailed concept definition, making itpossible to evaluate the payoff of specific technologyefforts within the broad functional systems areas ofsolar power generation and wireless power transmis-sion, among others. This concept definition should alsoinclude assembly, checkout, and maintenance tech-niques. The model needs to clearly show where SSPtechnology investments branch off to the benefit ofother missions, including both in-space and terrestrialapplications.

Input should be gathered from hardware builders,industry, academia, and other government agencies toimprove the modeling effort. The input should includeinformation on technical performance, current state-of-the-art technology, forecasts for technology advance-ment, and system cost modeling. The expansion of themodeling should also include other applications (inaddition to terrestrial power supply) that will benefitfrom the SSP technology investments.

Future comparisons in the models should includetransportation costs and delivered performance on or-bit, which will enhance decision making between alter-native technologies. This seems to have been intendedin some of the past SERT program trade studies; how-ever, the committee believes that the principle is notsufficiently ingrained in the modeling effort.

Recommendation 2-1-1: The SSP team shouldbroaden the scope and detail of the system and sub-system modeling (including cost modeling) to pro-vide a more useful estimate of technology payoff.The models should incorporate detailed conceptdefinitions and include increased input from indus-


try and academia in the specification of modelmetrics. The costs of transportation, assembly,checkout, and maintenance must also be includedin all cost comparisons to properly evaluate alter-native technology investment options.

In the economics modeling of new technologies,taking explicit account of risk and uncertainty is key toan effective understanding of the sensitivity of modelsto assumptions and the quality of the data used to in-form the models. The SERT program is currently us-ing conventional measures of technology readiness (theTechnology Readiness Level index [Mankins, 1995])and a rough measure of technological uncertainty (theResearch and Development Degree of Difficulty[Mankins, 1998]) in its modeling effort. These mea-sures address dimensions of the engineering stages ofdevelopment of new technologies but are silent as tothese technologies’ ultimate usefulness and cost-effec-tiveness, which are important factors in setting priori-ties for roadmaps. The program may be better servedby integrating measures in the cost and system model-ing that consider the cost-effectiveness of new tech-nologies, the relationships among different technolo-gies, and their effect on overall program structure.

The SERT team began to take this step in resultspresented to this committee in December 2000(Mankins, 2000b). Further development would pro-vide a more effective, resilient, and defensible roadmapfor SSP, that is, not only the integrated technologyanalysis methodology but also explicit incorporationof risk, uncertainty, cost, and benefit data. However,subsequent decision making based solely on these mod-els is not encouraged by the committee until the mod-els have been adequately validated and tested againstbaseline SSP concepts (see Section 3-1). In addition tothis validation, model assumptions and their possibleimpacts on cost, use of certain technologies, and massshould also be evaluated before further decisions aremade.

It is also useful that the roadmaps clearly distin-guish “risk” from “uncertainty”—words often usedambiguously. Risk is generally understood as describ-ing a known probability of an undesirable outcome—failure—while uncertainty refers to lack of knowledgeabout potential outcomes (Knight, 1921). The assess-ment of risk thus depends critically on the definition offailure, which in turn may depend on public and insti-tutional (e.g., government agencies, Congress) expec-tations.

Recommendation 2-1-2: The SSP team should con-tinue integration of technology readiness measure-ment and cost uncertainty modeling, both in devel-oping a consistent framework for the approach andin parameterizing the framework with the bestavailable information. Over time, the empiricalevaluation of these dimensions should become bet-ter understood as technology demonstrations begin.

Overall Program Focus in a Cost-ConstrainedEnvironment

Under the current NASA funding environment(i.e., yearly congressional earmarks), the program hasbeen provided with funds that are adequate only fortechnology roadmap development and preliminaryplanning. With these constraints, it is difficult to fundevery technological area with promise for SSP. Thecommittee suggests that if full program funding is notmade available, additional focusing of the SSP effortbe made. With the projected level of funding, manag-ers should select a single principal baseline concept andone technology per subsystem. For example, a magne-tron might be selected as part of the WPT baseline con-cept. The baseline should be altered and more ad-vanced technologies substituted when justified bytechnical progress and funding.

Recommendation 2-1-3: The committee recom-mends additional focusing of the SSP program. Forexample, if continued underfunding of the effortcontinues, it would be in the program’s best interestto choose a single baseline concept and one technol-ogy per subsystem for technology advancement.There should be periodic reviews as technologies areadvanced, and alternatives may be reintroducedinto the design when justified and affordable.

Technology Demonstration

The committee endorses the approach of definingdemonstration milestones of achievement (NASA’sMSC categorization) to provide program focus, as wellas a clear mechanism for measuring technology ad-vancement progress. Progression from the 100-kWMSC 1 demonstration to more technically challengingand larger demonstration missions (MSC 1.5, 2, 3, etc.)is reasonable. The committee appreciates the fact thatthe MSC 1 demonstration is not dependent on simulta-neous invention of a new low-cost transportation sys-


tem. The committee also encourages the continued bal-ance of technology flight demonstrations that test near-term and far-term technologies. Advancements in SSP-related technologies will be beneficial to the entirespace program. Flight demonstrations currentlyplanned in the program are designed to test the interac-tions and feasibility of advanced technologies for pos-sible use with SSP systems. These demonstrations willtest technologies at technology readiness levels that arebelow the level industry will accept. Without flight testexperiments, industry will be hesitant to accept manyof the new technologies into their programs.

Recommendation 2-1-4: The SSP program shouldcontinue the use of technology flight demonstrationsto provide a clear mechanism for measuring tech-nology advancement and to provide interim oppor-tunities for focused program and technology goalson the path to a full-scale system.

The committee also sees value in testing technolo-gies for SSP on already available space platforms. Inparticular, it believes that NASA should consider waysin which the International Space Station (ISS) can helpadvance the technology development effort in SSP.Furthermore, industry and academia should openlycompete on proposals for technology demonstration sothat their expertise is brought to bear on the technicalissues. In addition, international cooperation will mostlikely be necessary for any large-scale terrestrial SSPapplication, and international markets for SSP shouldbe considered, for economic reasons.

Flight demonstration of technologies and testingof systems should be completed early in the SSP pro-gram. The program’s use of flight demonstration mile-stones is an excellent way of achieving this. Althoughthe committee recognizes the vast difference betweenconditions and operational procedures in LEO andGEO, early milestone tests may have to be performedat LEO. To save cost and to maximize the engineeringdata returned, these experiments would have a smallerscope than the MSC system-level demonstrations.They might, for example, involve measurement of aparticular mechanical effect of zero gravity on asubscale structural component rather than on a com-plete, functioning solar array.

Serious consideration should be given to develop-ing the first flight demonstration (MSC 1) as an ISStechnology research mission and developing other ex-

perimental programs to be validated on the ISS.3 TheNASA SERT program’s 100-kW MSC 1 free flyercould be assembled from the ISS as a technology dem-onstration test bed. For instance, various solar arrayconcepts could be used on MSC 1 and then subjectedto test after release from the ISS. Space-to-space trans-mission could also be demonstrated using a co-orbitingtarget module, which also could serve as a platform forperforming experiments at lower microgravity levelsthan will be achievable on the ISS. After test comple-tion, MSC 1 could be returned to the ISS for inspectionand subsequent reoutfitting with other subsystems fora second round of free-flyer tests. Experiments testinglow-power-level transmission to Earth should also beincluded for microwave and laser wireless power trans-mission systems—both of which are being consideredin NASA’s SSP program. Transmission efficiencywould be low for these initial tests, especially for mi-crowave systems, but the experimental performancecould be corrected analytically to correspond to full-scale systems. An added advantage to ISS-based as-sembly of the MSC 1 is that various assembly, mainte-nance, and service techniques could be tested anddeveloped for future application in GEO.

Upon its completion, the MSC 1 demonstrationcould provide an enhancement for the ISS technologydemonstration program. This enhancement could beeither use of a free flyer in co-orbit with ISS or an up-grade to the ISS solar array with new technology.When the ISS program is ready to procure a replace-ment or expansion of the ISS solar arrays, the programshould consider the SSP array technology and, if pos-sible, procure an array that is on the roadmap for anSSP system. Additionally, the ISS might be used as anorbiting platform to validate techniques for structuralassembly, subsystem life, repair, and other experimentsunique to an SSP program.

Opportunities for collaboration with the Depart-ment of Defense (DOD) and international technologistsshould be considered in the MSC 1 flight demonstra-tion program for technology leveraging and sharedfunding. The Air Force Research Laboratory’s

3One previous NRC study recommended that NASA use theInternational Space Station as a test bed for engineering researchand technology development. Areas suggested that overlap withSSP-related technologies included electric power, robotics, struc-tures, and thermal control (NRC, 1996).


PowerSail program is a 30- to 50-kW thin-film photo-voltaic free-flyer demonstration intended to fly in the2004 or 2005 time frame. This may be an opportunityfor collaboration with NASA’s MSC 1 demonstration.

Recommendation 2-1-5: NASA should seriouslyconsider utilizing the International Space Station asa technology test bed for SSP during the first set offlight demonstration milestones. Such tests wouldleverage ISS technology and infrastructure, be in-dependent of new advances in space transportation,and provide an opportunity to test autonomous ro-botic systems.

The starting point for considering all of these col-laborative options is the development of a detailed setof requirements to be met by MSC 1 in support of SSPand other applications. Development of the technol-ogy demonstration concept and evaluation of the ad-vantages and disadvantages presented by using the ISSshould then follow.

There is a need for a comprehensive ongoing pro-gram to advance critical technologies from the labora-tory to operational readiness. This program must in-clude focused flight demonstrations for many of theindividual technologies, in addition to the system-levelMSC flight demonstrations. The committee recom-mends definition of additional ground demonstrationmilestones to be conducted prior to the far more expen-sive flight tests. In addition, the committee feels thateach of the demonstration projects should be evaluatedagainst the goals for the project and the timing, basedon the technology available at the time.

Recommendation 2-1-6: The SSP program shoulddefine additional ground demonstration milestonesto be conducted prior to the far more expensiveflight tests in order to test advanced technologiesand system integration issues before planneddownselects of flight-demonstration technologiesoccur.

Technology Building Blocks

Specific treatment of the SERT technology build-ing blocks can be found in Chapter 3; however, severalgeneral observations can be made from NASA’s mod-eling data that influence the investment strategy:

• By any yardstick, current expenditures of $10

million to $40 million per year cannot come close toproviding the technology development progress that isnecessary for application of terrestrial SSP in the next20 years.

• Due to uncertainties in future funding for SSP,various near-term choices must be made by SSP pro-gram managers. Even with a large increase in funding,NASA’s SSP program would be best served by focus-ing its efforts more narrowly than at present. Most ofthe technology investments should be devoted to tech-nologies that have multiple applications (in addition toterrestrial power generation). In many of the key en-abling SSP technologies, significant advances must bemade in technology performance, mass, and cost be-fore a commercial SSP system is viable. Most far-terminvestments should be in research areas that are highrisk but could provide high payoff to the SSP program.

• The SSP program should give considerableweight to nearer-term space, military, and commercialapplications of this technology, or portions of it (e.g.,low-mass solar arrays or WPT). Only a few technolo-gies unique to terrestrial power generation should befunded by NASA. Specifically, the system studies in-dicate that greatest benefit is obtained by investingmost heavily in several key technologies, describedbelow (Carrington and Feingold, 2000; Feingold, 2000;Mullins, 2000):

—Solar power generation technology is cur-rently in the midst of an exciting period of advance-ment with solar array improvements of benefit to allsolar-powered applications, including terrestrial powerand space vehicles. NASA should collaborate withDOD, DOE, and commercial efforts to avoid duplica-tion and improve the overall effectiveness of invest-ments in SPG technology.

—Wireless power transmission has possibledual application potential for free-flying platforms inspace or airborne, which could attract military or com-mercial participants. However, investments in this areaneed to be focused. Currently, the SERT program isfunding efforts in several major WPT technologies.

—Space power management and distributionis a major contributor to SSP system mass and cost.Investments should be made to reduce the mass andcost of the components while increasing efficiency andimproving operation conditions. New SPMAD tech-niques developed under the SSP program will haveapplication to the ISS and many other NASA, DOD,and commercial systems.

—Space assembly, maintenance, and servicing


are challenges common to all large space systems andplanetary exploration, particularly when launching acomplete unit is beyond the capacity of the space trans-portation vehicle. SSP systems should be designedfrom the outset to accommodate on-orbit robotic as-sembly and maintenance. The degree of structural de-ployment versus assembly should be rigorously stud-ied, and deployment concepts should be developed thatare compatible with planned robotic capabilities. Sys-tems studies are necessary to determine what level ofrobotic capability is optimal (from tele-operated tofully autonomous) and how humans are best used inthe assembly, maintenance, and servicing operations(on the ground or in orbit).

—In-space transportation is an importantdriver in establishing on-orbit SSP costs. Trade stud-ies of various concepts should be made along with sat-ellite design and assembly concepts to establish thelowest-cost methods for placing the completed SSPdesign in GEO.

• Utilities, industry, and other government programsshould make the most investment in ground powermanagement and distribution (PMAD) technologies,ground-based energy storage, and platform systemtechnologies. These areas are either utility specific orare funded adequately through other efforts. Further-more, key research and development should placeheavy emphasis on reduction of mass and cost and im-provements in efficiency, the ultimate drivers for com-mercial application of any SSP system.

Recommendation 2-1-7: The NASA SSP programshould invest most heavily in the following key en-abling technologies, mainly through high-payoff,high-risk approaches: (1) solar power generation (incollaboration with DOD/USAF and DOE to avoidduplication); (2) wireless power transmission;(3) space power management and distribution;(4) space assembly, maintenance, and servicing; and(5) in-space transportation. The SSP programshould not invest research and development fundsin ground PMAD technologies, ground-based en-ergy storage, or platform system technologies. Utili-ties, industry, and other government programs al-ready have significant investments in those areas.

Capitalizing On Other Work

It was clear from material presented to the commit-tee that the SSP program, including SSP for terrestrial

use and other technology applications, is highly syner-gistic with the related work of other U.S. agencies, andcommercial and international interests. NASA mustdevelop strong interfaces with other organizations toensure that funds are spent most effectively. Specifi-cally, the U.S. Air Force has a vigorous space photo-voltaics technology program that could support NASAand potentially benefit from many of the SSP demon-stration programs. There may also be near-term com-mercial and military applications for WPT to powerlong-duration airships and aircraft.

Similarly, DOE, with its Office of Energy Researchand the National Renewable Energy Laboratory, shouldbe involved, for both near-term benefits and long-termprogram planning. DOE currently spends approxi-mately $75 million/year on solar power technologiesand also supports research in related environmentalhealth and safety areas. Internationally, Europe andJapan are rapidly increasing funding for terrestrial pho-tovoltaics research.

The U.S. government sponsors a Space Technol-ogy Alliance to increase coordination and collabora-tion on space-related technology development. Mem-ber agencies include the Air Force ResearchLaboratory, the Ballistic Missile Defense Organization,the Defense Advanced Research Projects Agency, theNational Reconnaissance Office, the Naval ResearchLaboratory, DOE, and NASA, among others. Currentinitiatives of its Space Power Subcommittee includeinfusion of technologies such as photovoltaics, modu-lar low-cost PMAD, and efficient compact thermalmanagement into all government space programs.NASA is currently involved in the alliance; however,continued and increased involvement is suggested bythe committee to promote increased technology lever-aging.

International coordination should be fostered onenvironmental, safety, and spectrum allocation issues,as well as on other standard space technology develop-ment topics. The committee encourages mutually ben-eficial cooperation consistent with the InternationalTraffic in Arms Regulations, perhaps with a researchcooperative agreement, to maximize the effectivenessof the total investment.

Recommendation 2-1-8: NASA should expand itscurrent cooperation with other solar power genera-tion research and technology efforts by developingcloser working relationships with the U.S. Air Forcephotovoltaics program, the National Center for


Photovoltaics, industry, and the U.S. government’sSpace Technology Alliance.

2-2 APPLICATIONS

NASA’s SSP technologies have extensive applica-tions both in space and terrestrially. The program’stechnology development can also be leveraged by otherinternal NASA programs and industry. Technologydevelopment crucial for an SSP system includes solararray advancements; robotic maintenance and servic-ing development; power management and distribution;and enhanced systems integration activity for largetechnical programs. These technologies have applica-tions to many other engineering and science efforts inboth government and industry. The following sectionsoutline NASA’s current efforts in these areas and pro-vide recommendations for future activities in relationto potential applications of SSP and areas for potentialtechnology transfer.

Applications to Enable Space Science

The objective of the NASA SERT science effort isto “enable science efforts to identify, focus, and quan-tify the scientific benefit of new concepts created byproviding a source of beamed power in space and thescience missions made possible by the SSP developedtechnologies” (Marzwell, 2000). The basic premise ofthe effort is that high power and large, lightweightstructures can enable in-space science. The higherpower available from an SSP system can be used forincreased penetrating power for imaging and soundinginstruments; increased power for drilling and the vola-tilization of subsurface materials with beamed energy;and increased mobility and power for remote drillers,rovers, and moles in areas where current power sys-tems cannot provide adequate resources.

Many SSP technologies have already been identi-fied by NASA to enable science (Marzwell, 2000).Advanced space-based structures, including large ap-ertures, large photovoltaic arrays, and space-rigidizedaerobrake structures, are expected to be a secondaryproduct of SSP research and development. Laser-elec-tric and solar-thermal propulsion and beamed energypower could be directly applicable to Earth-orbit andMars-orbit missions as well as to future lunar activi-ties. SSP research and development can also improveactive sensing technology utilized to map hidden sur-faces of planets and asteroids, discover new planetary

bodies, analyze atmospheric properties, perform sur-face imaging, and track resources such as ice and wa-ter. Power beaming has application in space (Earthorbit, Mars orbit), as power from an orbit to a planetarysurface, or in transportation (including laser sails andlaser-thermal and laser-electric propulsion). SSP canalso be utilized as an inexpensive, abundant powersource for conventional orbital science. Several spe-cific applications are being pursued by NASA in theseareas.

HEDS Applications

The strategic plan of NASA’s Human Explorationand Development of Space (HEDS) effort establishes arange of visionary goals and objectives, including mul-tiple targets for human exploration outside LEO, goalsfor scientific discovery through research in space, andresearch to enable humans to live and work perma-nently in space. The SERT program has identified po-tential space applications of SSP technologies and con-cepts in relation to the HEDS effort (Mankins, 2000a).

Preliminary assessments of many applications havebeen performed by NASA. These preliminary assess-ments are an excellent initiation in providing motiva-tion for the development of a much smaller SSP systemin addition to the development of a future terrestrialbaseload power system. Assessments were divided intocategories based on the requirements of SSP technol-ogy. Nearer-term applications such as the use of evo-lutionary power systems for the ISS and solar powersystems for GEO communications satellites are con-sidered Generation I applications. Generation II appli-cations include robotic planetary outpost power sys-tems, wireless planetary power grids, and public spacetravel and tourism. Industrial space stations in LEO,power plugs in space, in-space propellant depots, andintegrated human and robotic exploration applicationsare considered Generation III and IV applications.These applications would utilize SSP technologies inthe mid- to far-term time frame (2005-2025). Manyother potential applications such as space businessparks, space utilities, interplanetary electromagneticpropulsion, and solar system resource development arevery far-term applications. The committee believesthat, while these far-term applications are important,the SSP program may be better served by focusing onnearer-term applications and technology under currentfunding conditions.


Recommendation 2-2-1: Under current fundingconstraints, the SSP program should devote a largeportion of its efforts to technologies that havenearer-term applications (e.g., low-mass solar ar-rays) while continuing to develop technology andconcepts for long-term terrestrial baseload powerapplications.

These applications for power may provide a betterbusiness case for industry development during the com-mercialization phase because such applications will beable to absorb power costs that are higher than futureterrestrial power markets will accept. The committeebelieves, however, that pursuit of such large-scale ap-plications may be possible only with increased coop-eration between NASA and foreign governments andindustry. The committee also recognizes that this pro-posed SSP program will require the development ofcloser working relationships with industry. NASAshould lay the groundwork for commercialization.

Recommendation 2-2-2: The SSP program, as wellas any future effort in space solar power on the partof NASA, should involve a concerted effort to de-velop closer ties to industry, the U.S. government,international groups, and other internal NASA ef-forts for purposes of technology development, peerreview, and possible shared resources.

There is a need to involve more outside people inthe Senior Management Oversight Committee (SMOC)or the appropriate technical interchange meetings andsystem working groups meetings (not just organiza-tions external to NASA that are funded by the program,but others as well). Continued and expanded peer re-view should be an element of any future SSP effort.This may simply be accomplished by expanding therole of SMOC, including more industry and academicresearchers in various areas germane to SSP technol-ogy development. These actions on the part of NASAmay alleviate the issues raised in other sections of thisreport on the validity and reality of technical assump-tions and forecasts (see Sections 2-1 and 3-1).

Technology Transfer and Cross-Cutting Applications

Individual technologies labeled as SSP-enablingtechnologies also have potential for terrestrial use anduse in other space or aviation activities. Basic researchin support of SSP is being performed in the areas of

photon interaction and ablative physics, wireless powertransmission, photovoltaics, robotics, and advancedmaterials development. Such technology advancementactivities are important endeavors that should be con-tinued. One previous NRC study recommended thatNASA use the ISS as a test bed to develop new spacetechnologies (NRC, 1996). Many SSP-enabling tech-nologies, such as robotics, solar arrays, structures,WPT, and assembly techniques, could be tested on theISS.

Unfortunately, little support has been seen withinNASA for cross-enterprise technology programs ex-cept from the personnel directly involved in such ef-forts (NRC, 1998). Increased leverage of currentNASA programs directly related to SSP-enabling tech-nologies should be pursued. The NASA SERT pro-gram has attempted, on a preliminary basis, to coordi-nate its research and technology roadmaps with otherNASA programs. Specific examples follow:

• HEDS Technology-Commercialization Initia-tive;

• Gossamer Spacecraft program;• Small Business and Innovative Research pro-

gram;• Spacecraft power and propulsion “core tech-

nology competency” funding , i.e., the Cross-Enterprise Technology program;

• Advanced Space Transportation and SpaceLaunch Initiative programs;

• Intelligent Systems (IS) program; and• Space science spacecraft technology demon-

stration programs (e.g., New Millennium).

The SERT program has also attempted coordinationwith various efforts external to NASA, includingDOE’s National Renewable Energy Laboratory photo-voltaics research, the National Science Foundation’sefforts in innovative manufacturing and robotics, andDOD and the Naval Research Laboratory’s work in thearea of intelligent systems. Program managers agreethat the SSP effort within NASA needs to improve itstrack record of coordination with other research andtechnology programs across NASA, the U.S. govern-ment, and non-U.S.-government organizations. Fur-thermore, there is a need to incorporate flight test dem-onstrations of key technologies on space platforms suchas the International Space Station or geosynchronouscommunications satellites. Also, as reflected in Sec-tion 2-3, the SERT program has had little detailed dis-


cussion with international efforts except in the area offrequency allocation. One improvement in interna-tional relations seen in the last 2 months of the studywas the organization of an Open Forum on Space SolarPower, with participation by several countries. Moreactivities such as this one are encouraged by the com-mittee.

Recommendation 2-2-3: The SSP program shouldassess in detail how to use research by other organi-zations to expedite the development of SSP-enablingtechnologies.

During the tenure of this study, the SERT programdid indeed begin what was termed a “gap analysis” inorder to uncover areas within NASA in which increasedtechnology leveraging may occur. The assessment ofinvestments external to NASA is scheduled to be com-pleted by the time this report is published, but as ofDecember 2000 efforts had not yet been initiated. Asthe NASA SERT program management agrees, it willbe imperative for the SSP program to carry this analy-sis a step further by actually using the knowledge fromthese yet-to-be-identified programs and leading effortsto adjust the focus of other related NASA technologydevelopment programs to help achieve SSP researchand technology goals. Factors to be considered in suchan evaluation should include information solicited fromthe outside research community and program balancebetween various issues such as technology push versusprogram pull, near-term versus far-term applications,and competitive technology development versus theneed for system design choices. An example of suchintegrated technology planning and cross-enterprisetechnology can be found in NASA’s Office of SpaceScience; it is described in a National Research Councilstudy (NRC, 1998).

As part of the 2-year SERT effort, NASA con-tracted with the American Institute of Aeronautics andAstronautics (AIAA) to perform an assessment of cer-tain aspects of NASA’s SSP research and technologyeffort (AIAA, 2000). Part II of the report discussedmultiple-use technologies and applications. It is ex-pected that these findings will be incorporated into thecurrent SSP effort at NASA. The report identifiedmultiple use of SSP technologies as a major area for in-depth consideration. The prospects for these technolo-gies were evaluated by AIAA in order to answer thefollowing questions:

• How real are these technologies and applica-tions?

• Are there other as-yet-unearthed opportunitiesfor dual or alternative applications of SSP tech-nology?

• How and to what extent should SSP technol-ogy studies be integrated with these non-SSPprograms?

• What might be the payoffs of such an integra-tion in achieving the various programs goals?

Initial areas for consideration included geocentricspace applications, lunar and planetary exploration,space science projects, national security applicationsin space, terrestrial applications, and a miscellaneouscategory (AIAA, 2000). Two areas mentioned by theAIAA report but not covered in the NASA materialpresented to this committee were national security mis-sions and terrestrial applications such as airborne ve-hicles and offshore oil platforms. These areas, as wellas the other applications, should be given greater con-sideration as potential applications for an SSP system.A need still exists to explore further potential applica-tions and technology transfer opportunities, particularlyin relation to the importance of the SSP effort havingindustrial support throughout the program’s lifetime.As previously witnessed during the Apollo and shuttleprograms, technology development in computers, ma-terials, and robotics can be transferred directly to manyeveryday industrial and personal applications.

2-3 INTERNATIONAL EFFORTS

The committee examined several activities inprogress outside the United States and noted a growingworldwide interest and involvement in space solarpower. With a new global energy market emerging,led by electricity as the fastest-growing form of energyfor users worldwide, NASA has excellent opportuni-ties to contribute to and profit from international col-laboration. Advantages to worldwide cooperation stemnot only from the synergy that is possible from coop-eration with other experts but also from the fact thatSSP has space-based components and thus no techni-cally imposed geographic limits on the countries thatcould participate in SSP’s benefits (AIAA, 2000). Thecommittee was briefed on current international involve-ment in SSP and found an optimistic global picture.Japan, France, Canada, Russia, Ukraine, Georgia, Italy,Belgium, Germany, India, Netherlands, China, and


Singapore are among the countries engaged in at leastsome facets of SSP studies, research, development, andtechnology demonstration. Several multinational or-ganizations such as the United Nations and the Euro-pean Space Agency (ESA) are also sponsoring work inSSP (Erb, 2000).

The following sections show the diversity of inter-national efforts in which NASA’s participation mightbe mutually beneficial.

SPS 2000

SPS 2000 is a planned operational test bed for sev-eral important space solar power technologies. Theproject was originally proposed by Makoto Nagatomoof the Institute of Space and Astronautical Science,Sagamihara, Japan (Pignolet, 1999), working with ateam of government and academic research scientists.The SPS 2000 proposal features a 10-MW solar powersatellite in an 1,100-km equatorial orbit. The 1,100-km orbit corresponds to about a 100-min orbital pe-riod, permitting the satellite to furnish a power burst ofabout 200 s to a rectenna4 on the ground (Moore,2000). As of November 30, 2000, 19 sites in 11 equato-rial countries had been selected as candidate rectennasites. Field visits by the SPS 2000 task team have per-mitted the team to establish local contacts, engage thepopulation in the project, and examine local impacts(Mori, 2000).

The Grand-Bassin Project

The French government, with the Centre Nationald’Études Spatiales (CNES) as its primary agent, alsofavors international cooperation, believing that successwill come when a critical mass of research that makesuse of technical synergy is applied to SSP problems(Vassaux, 1999). To that end, CNES, the University ofLa Réunion, and researchers from Japan are sponsor-ing a large-scale wireless power demonstration projecton the island of La Réunion. La Réunion is a volcanicisland in the Indian Ocean southeast of Madagascar.Because the island has few indigenous conventional

energy resources, it must import them at great expense.Local decision makers actively support new ways toprovide energy to their population (Pignolet, 1999).

The village of Grand-Bassin on La Réunion Islandis located in a deep canyon 700 m distant from an entrypoint to the island’s power grid. The Grand-Bassinproject involves transmitting about 10 kW of electric-ity via microwave energy at 2.45 GHz to provide powerfor the village. This project is notable for going be-yond current experimental feasibility of WPT to anoperational system that would be subject to real-worlddemands: exposure to tropical weather conditions,varying power demand level, continuous power pro-duction, and reasonable cost. CNES estimates that thesystem could be in place as early as 2003 (Pignolet,1999).

Demonstrations of Wireless Power Transmission

Both Japan and Canada have been involved in dem-onstrations of WPT. A survey taken in Canada identi-fied some 50 organizations in that country that haveinterests that relate to SSP. Canadian interests includespace-based collection systems, space-to-ground dem-onstrations, ground point-to-point WPT demonstra-tions, and pilot plants. Canadian activity includes plan-ning for a ground demonstration site, probably inNewfoundland, that will examine many of the sametechnical issues that are planned for Grand-Bassin, al-though in completely different climate and terrain (Erb,2000). Canada also sponsored development of a proto-type microwave-powered aircraft for use as a long-du-ration, high-altitude communications platform. Firstflights of a scale model began in 1986 (Erb, 2000).

At Kobe University in Japan in 1995, researcherssuccessfully flew a small airship using power transmit-ted from the ground by microwave energy. Research-ers estimated that they were able to generate 10 kW ofradiated power from the parabolic antenna on theground and obtained about 5 kW from the rectennaonboard the airship. Researchers were able to use thepower to make the airship climb as high as 45 m abovethe ground and to hover there for 4 min 15 s (Kaya,1999).

Other International Efforts

The committee noted numerous other projects thatdealt directly with SSP or with the development of tech-nologies that will facilitate SSP. The Technical Uni-

4A rectenna (the term was coined by the late William Brown,formerly of Raytheon), sometimes referred to as a rectifying an-tenna, is composed of a mesh of dipoles and diodes for absorbingmicrowave energy from a transmitter and converting it into electricpower (DC current).


versity of Berlin has been involved in SSP studies since1985. German work has included computer simula-tions of GEO solar power satellites that include opera-tional characteristics and life-cycle costs; modeling theacquisition and operation of SSP lunar installations;and modeling the development of SSP in conjunctionwith human exploration of the Moon and Mars (AIAA,2000).

In Russia, the Central Science Research Instituteof Machine Building has designed an SSP satellite thatwould have a maximum output of 3.14 MW. This de-sign would transmit solar power by microwave to or-biting or remote spacecraft and to lunar and Martianbases. In Ukraine, researchers are proposing experi-ments for the International Space Station dealing withsemiconductor structures for solar cells, new forms ofsolar concentrators, and enhanced forms of powertransmission (AIAA, 2000).

The nations of the European Space Agency are alsocurrently active in SSP studies and research. Early in2000, ESA published a study conducted jointly by Ger-man, Italian, and French participants that sought toidentify opportunities, capabilities, and technologiesfor exploration and utilization of space in the next 30years. Among the SSP scenarios examined in this studywere an experiment for ISS that involved microwavepower transmission to a free-flying satellite, wirelesspower transmission from one terrestrial location to an-other via a power relay satellite in GEO, and solar-power-generating satellites for terrestrial power supply(Nordlund, 2001).

NASA’s Role in International SSP Activities

It was not the committee’s intent to produce anexhaustive list of efforts worldwide but to show thatthere are opportunities for international collaboration.The committee noted that NASA hosted an Interna-tional Space Power Forum at NASA Headquarters inWashington, D.C., in January 2001. At that forum,representatives from Canada, the ESA, and Japan allindicated that increased interest and participation bythe United States would be beneficial to work they weredoing in their own countries. The committee under-stood that the International Traffic in Arms Regula-tions may impede transfer of certain technologies butalso noted that a great deal can be accomplished undercurrent law and that avenues exist for approving tech-nologies for export.

It may be beyond the means of any one country tofund the research, development, and implementationof SSP, but these tasks should be more achievable byinternational cooperation. International cooperationwould allow NASA to profit from the work of expertsworldwide, as well as to contribute its own expertise.The comments at the 2001 International SSP Forumfrom Canadian, European, and Japanese representa-tives reinforce the committee’s observation thatNASA’s collaboration internationally would be wel-come and appropriate.

Recommendation 2-3-1: NASA should develop andimplement appropriate mechanisms for cooperat-ing internationally with the research, development,test, and demonstration of SSP technologies, com-ponents, and systems.

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Marzwell, Neville. 2000. “Space Science Enabled Applications.” Briefingby Neville Marzwell, Jet Propulsion Laboratory, to the Committee forthe Assessment of NASA’s Space Solar Power Investment Strategy,National Academy of Sciences, Washington, D.C., September 14.

Moore, Taylor. 2000. “Renewed Interest in Space Solar Power.” EPRIJournal 25(1):6-17.

Mori, Masahiro. 2000. Summary of Studies on Space Solar Power Systemsof the National Space Development Agency of Japan. White Paper (No-

vember 30). Tokyo, Japan: Advanced Mission Research Center, Officeof Technology Research, pp. 31, 33.

Mullins, Carie. 2000. “Integrated Architecture Assessment Model and RiskAssessment Methodology.” Briefing by Carie Mullins, Futron Corpora-tion, to the Committee for the Assessment of NASA’s Space SolarPower Investment Strategy, National Academy of Sciences, Washing-ton, D.C., October 23.

NASA (National Aeronautics and Space Administration). 1999. SpaceLaunch Initiative Program Description. NASA White Paper. Washing-ton, D.C.: National Aeronautics and Space Administration. Availableonline at <http://std.msfc.nasa.gov/sli/aboutsli.html>. Accessed August15, 2001.

Nordlund, Frederic. 2001. “ESA and Space Solar Power (SSP).” Presenta-tion by Frederic Nordlund, European Space Agency, to the InternationalSpace Power Forum, Washington, D.C., January 12.

NRC (National Research Council), Aeronautics and Space EngineeringBoard. 1996. Engineering Research and Technology Development onthe Space Station. Washington, D.C.: National Academy Press. Avail-able online at <http://books.nap.edu/catalog/9026.html>. Accessed Au-gust 16, 2001.

NRC, Space Studies Board. 1998. Assessment of Technology Developmentin NASA’s Office of Space Science. Washington, D.C.: National Acad-emy Press. Available online at <http://www.nationalacademies.org/ssb/tossmenu.htm>. Accessed August 16, 2001.

Pignolet, Guy. 1999. “The SPS-2000 ‘Attaché Case’ Demonstrator.” SpaceEnergy and Transportation 4(3, 4): 125-126.

Vassaux, Didier. 1999. “The French Policy on Space Solar Power and theOpportunities Offered by the CNES Related Activities.” Space Energyand Transportation 4(3, 4):133-134.

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3

Individual Technology Investment Evaluations

3-1 SYSTEMS INTEGRATION

Systems integration is commonly utilized duringthe development phase of a product but is a vital con-sideration in the early space solar power (SSP) tech-nology development work due to the large number ofsubsystems and their strong interaction with one an-other. Committee comments and recommendations aregrouped into three areas: (1) improving technical man-agement processes, (2) sharpening the technology de-velopment focus, and (3) capitalizing on other work.


As discussed in Section 2-1, NASA has allocatedsome of its available SERT funding to the develop-ment of an overall SSP concept and cost model thatincludes system mass and performance targets(Carrington and Feingold, 2000; Feingold, 2000;Mullins, 2000). This model, while not yet complete orindependently validated, has been used as a tool to pre-dict delivered baseload terrestrial power costs assum-ing various technology goals have been achieved. Thecommittee endorses this methodology as one usefultechnique for assigning technology investment priori-ties and urges that its development continue as an indi-cator of the relative payoff from technology invest-ments. The model is coarse at this time and must beexpanded so that cost and mass targets can be allocated

down to the lowest (component) level. Sensitivity stud-ies, which the SERT program has begun, should be anintegral part of such modeling, to quantify the impactsof departures from the nominal input values, many ofwhich are, at this time, simply assumptions. Nominalvalues should be developed in consultation with ac-knowledged experts in the field. The modeling activ-ity should also be extended to other near-term applica-tions of SSP technologies. Integration, however,requires much more than the exercise of a modelingtool. An effective team process is necessary to coordi-nate actions and to ensure that the best possible deci-sions are made for SSP.

NASA has established a working process for inte-grating the SERT effort. This program decision-mak-ing and organization process to integrate SERT tech-nology working groups and task forces has been wellcoordinated and has succeeded in advancing the SERTprogram definition during the past 2 years. Figure 3-1provides a schematic of the relationships and interac-tions among the various groups. The committee notesthe exemplary degree to which NASA’s SERT projectorganization has been open and responsive to outsidesuggestions. That openness indicates a high degree ofobjectivity and confidence and should be encouragedin all continued work. However, three factors willemerge to force the SERT-type integration process tochange in the future: (1) funding levels must increase

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INDIVIDUAL TECHNOLOGY INVESTMENT EVALUATIONS 31

in order for the currently planned program to becomeviable; (2) interaction with other government agencies,commercial entities (including the utility industry), andother organizations should increase; and (3) toughdownselect decisions for various technologies andflight test hardware must be made, assuming that theincreased funds will still be constraining.

However, when NASA leaves the planning stageand undertakes serious technology development, theSSP program would be better served by adding a tech-nology development process complete with specificgoals, dates, and procedures to be followed. This clari-fication of the organization and decision-making ap-proach is necessary to meet technology goals at criticalprogram milestones and with efficient use of funds. Theprogram should define each of the integration functionsand clearly charge the responsible organizations withexplicit responsibilities. Although advancements in re-search and technology are difficult to schedule, ad-equate funding should be provided to each organiza-tion to undertake these assigned responsibilities at ahigh level of quality and at a reasonable pace. Theapproach should be similar to major program organiza-tions at a much later stage of the development cycle,because the program involves several NASA centers,outside agencies, and the international community.NASA should develop a written implementation planfor carrying out the work.

Definition of a consistent process to adjudicatecompeting objectives is also necessary. It is almostinevitable that institutional considerations will some-times be at odds with purely end-product goals, and itis better to address this likelihood with an objectivedecision process before positions stiffen within the SSPprogram and among the NASA centers.

Recommendation 3-1-1: NASA’s SSP programshould improve its organizational and decision-making approach by drawing up a written technol-ogy development plan with specific goals, dates, andprocedures for carrying out technology advance-ment, systems integration, and flight demonstra-tion. The SSP program should also establish a con-sistent process to adjudicate competing objectiveswithin the program and specifically include timingand achievement of technology advances in roboticsand space transportation in the roadmaps.


Many SSP issues relate to manufacturing cost, notjust performance. Cost estimation expertise is largelyto be found in industry, not in the NASA laboratoriesor academia. Appreciable funding of and consultationwith relevant industrial firms is required to gain au-thoritative costs to be used as future inputs to the inte-gration model. Investment should also be made in high-payoff, high-risk manufacturing technologies to reducethe cost of all the components since many hundreds ofduplicate pieces will be necessary.

The committee notes that technology is availabletoday to build an SSP system (i.e., solar electricity iscurrently generated in space, microwave power trans-mission has been successfully demonstrated terrestri-ally, and space transmission is in demonstration by Ja-pan). However, such technology would be impracticaland uneconomical for the generation of terrestrialbaseload power due to the high cost and mass of thecomponents and construction. The system would alsohave to be constructed and utilized by humans in LEO,not the GEO planned for future large SSP systems,which would be constructed or deployed autono-mously. It would be completely unable to “close thebusiness case” for its (presumably) commercial spon-sors and their funding sources. The committee consid-ers that the present SERT cost and performance targetsfor several of the technologies are beyond present cred-ibility, particularly in solar power generation (SPG),space power management and distribution (SPMAD),and wireless power transmission (WPT). In addition,goals in structural mass and performance may be toostringent. The committee suggests further design stud-ies in this area before additional technology investmentis made. Substantial improvements in assembly, main-tenance, and service are also essential for the successof this program. If successful, these technology ad-vancements will have many other applications. How-ever, the committee realizes that the SSP program ascurrently envisioned is planned to unfold in stages, withNASA leading demonstration projects at progressivelyhigher power levels (100 kW, 1 MW, and 10 MW) pro-vided that positive decisions are made at each mile-stone to make the investments necessary to continuewith the program.


Recommendation 3-1-2: The SSP program shouldallocate some of the technology development fundsto the aerospace system level and component indus-tries that can address manufacturing, assembly, andmaintenance design and cost. This would includearchitecture, engineering, and electrical utility con-struction firms. An added benefit of broadeningthe industrial community involved in SSP develop-ment may be significant improvements to futurebaseline designs.

The committee observes that the earliest of thecommercial satellites, should the private sector decideto join the SSP program, will be a subscale power sta-tion, perhaps a repeat of the 10-MW NASA experi-ment with improved technology, followed by a private-sector decision to construct and operate a 100-MWpilot plant. Only then will deployment decisions bemade for power plants of 1 GW or larger. Each ofthese several decisions will be based on economic aswell as technological figures of merit appropriate totheir particular time—not based on the limited under-standings of today.

Verification of SSP technology and the integrationand testing of hardware and software are necessarybefore deployment of any SSP system. The SSPprogram’s investment in developing spacecraft integra-tion and testing methodologies and processes shouldbe expanded. The goal should be to make verificationof the performance of the SSP satellite possible with aminimum of ground testing. A combination of two ap-proaches may be needed to accomplish this objective:(1) new modeling methods that accurately predict on-orbit function and performance and (2) a new designmethod to adaptively accommodate errors in the pre-dicted function and performance.

The modeling approach may require the develop-ment of (1) high-resolution (possibly many millions ofdegrees of freedom) structural models (including theability to predict damping in a zero-g environment),(2) complex, integrated structural-thermal-control per-formance models that include probabilistic sensitivityanalyses, and (3) validated component-to-system veri-fication test procedures. This effort should be supportedby a complementary set of ground and flight experi-ment activities, the goal of which would be to establishthe limits of the new modeling capabilities in predict-ing the zero-g performance of the SSP mechanical sys-tems.

The adaptive approach may require the develop-

ment of self-healing structures, self-tuning adaptivecontrol systems, autonomous robots, and failure-toler-ant structural concepts. Some of these developmentsmay be very far into the future, requiring rigorous,long-term investment before they will yield practicalsuccess.

Recommendation 3-1-3: The SSP program shouldincrease investments in developing spacecraft inte-gration and testing so that the performance of SSPsatellites can be verified with a minimum of groundor in-space testing. This may include the develop-ment of specialized integration, test, and verifica-tion methodologies for SSP spacecraft.

Because there are so many competing designs forvarious technology flight demonstrations (TFDs), itwas difficult for the committee to determine the accu-racy of the SSP system and cost models used to evalu-ate trade studies for various technologies. The currentmodels should incorporate one detailed concept defini-tion, making it possible to evaluate the payoff of spe-cific technology efforts within the broad functional sys-tems areas of SPG and WPT, among others. Thisconcept definition should also include assembly,checkout, and maintenance techniques. The modelneeds to clearly show where SSP technology invest-ments branch off to the benefit of other missions, in-cluding both in-space and terrestrial applications.

There was concern on the part of the study com-mittee over the actual values used as technologymetrics in the system and cost models. After discus-sion with NASA subsystem team leaders and systemsintegration personnel, the committee found the maxi-mum and minimum values used seem to be especiallysuspect in areas such as structures and photovoltaics.Some performance metrics seem to be extrapolationsthat cannot be traced to the structural requirements spe-cific to SSP platforms. Also, during (and after) presen-tations to the NRC committee, there were self-acknowl-edging errors in the models used to place priority ontechnical investments. This led the committee to beskeptical about the accuracy of the model input and theuse of these metrics. There was further concern aboutthe lack of comparison between assembly methods(i.e., fully autonomous, tele-robotic, or human) withinthe models. Technology metric input should be gath-ered from hardware builders, industry, academia, andother government agencies to improve the modelingeffort. Input should include information on technical


performance, current state-of-the-art technology, fore-casts for technology advancement, and system costmodeling. The expansion of the modeling should alsoinclude other concepts (in addition to terrestrial powersupply) that will benefit from the SSP technology in-vestments.

Future comparisons in the models should includetransportation costs and delivered performance on or-bit, which will enhance decision making between alter-native technology candidates. This seems to have beenintended in some of the past SERT program trade stud-ies; however, the committee believes that the principleis not sufficiently ingrained in the modeling effort.

Recommendation 3-1-4: The SSP program shouldreview its technology and modeling assumptions,subject them to peer review, and modify where in-dicated. A single SSP concept should be rigorouslymodeled, incorporating technology readiness levelsand involving industry in conceptual design, as ameans to improve the credibility of the model inputand output but not to prematurely select a singlesystem for ultimate implementation.

Capitalizing on Other Work

The committee realizes that a full-scale SSP sys-tem is greatly dependent on future space transportationtechnologies not yet defined. Such technologies in-clude ground launch to low Earth orbit (LEO) followedby transportation from LEO to geosynchronous Earthorbit (GEO). NASA’s SSP program should identifyand initiate detailed studies of the SSP program’s spacetransportation needs and requirements. This NASAstudy should include the space transportation needs andrequirements of the SSP demonstrator elements in ad-dition to the space transportation needs and require-ments of related space systems that NASA may laterembrace for other uses based on SSP technologies.Industry involvement is imperative in defining themeans and estimating the programmatics of new andevolving space transportation systems to fulfill theseneeds.

NASA’s technology development program, calledthe Space Launch Initiative (SLI), is currently chargedwith addressing the Earth-to-LEO launch issue. Coor-dination, however, is apparently not yet present be-tween the SERT program and SLI. Any future SSPeffort should work closely with SLI as future mile-stones and roadmaps are developed for both programs

in order to provide realistic goals for future implemen-tation. A similar program is also necessary to developspace transportation from LEO to GEO. NASA de-fines work in this area as in-space transportation andinfrastructure, which, if undertaken, may be separatefrom NASA’s SLI initiative. These issues are dis-cussed in further detail in Section 3.9.

The SSP technology program presented to thiscommittee (Mankins, 2000) relies on parallel technol-ogy development efforts undertaken in several NASAenterprises and has strong interagency and, potentially,international implications. It will be important to theSSP program, therefore, to have a strong advisory in-frastructure to help oversee the technology develop-ment progress efficiently. Currently, the main advi-sory body for the SERT program is the SeniorManagement Oversight Committee (SMOC), consist-ing of members from NASA and industry. Variousother research and technical working groups and a sys-tems integration working group are also used through-out the program integration process to infuse expertknowledge into the program (see Figure 3-1).

Upon evaluating the membership of the SMOCand, more important, the participation level of the indi-viduals, the committee concluded that industry andgovernment agency representation may be less thanadequate to provide competent oversight and technicalinput. More external membership in these assessmentteams and working groups should be brought togetherfrom industry and government in order to validate vari-ous technological metric inputs to the performance andcost models and to review progress at appropriate pro-gram milestones. Inputs from the Electric Power Re-search Institute (EPRI) and electric utilities will be ofgreater importance as the program proceeds. Govern-ment agencies, such as the Department of Defense(DOD), the U.S. Air Force (USAF), and the Depart-ment of Energy (DOE), should be involved, not just asliaison representatives but perhaps even as members ofthe SSP research team via the Intergovernmental Per-sonnel Act or other cooperative programs. Such ex-changes would not only foster cooperative activityamong the agencies but also leverage all governmentinvestments in technologies applicable to SSP. It isalso desirable, due to the breadth of SSP-related tech-nologies, to establish advisory committees for specifictechnologies as well as the overall program. Theseadvisory committees would be in addition to the re-search and technology working groups already used bythe program.


This addition of non-NASA experts in the varioustechnical areas will assist in providing technical peerreview of the SSP program’s research. Participation ofothers in the establishment of modeling metrics willalso be extremely helpful to systems-modeling re-searchers by providing realistic and accurate technol-ogy parameters. As remarked on earlier, this is a majorconcern of the committee in areas such as solar arrays,SPMAD, WPT, structures, and robotics. Active repre-sentation of various aerospace manufacturing indus-tries and the electric utilities will be imperative, espe-cially as flight demonstration and implementation areapproached.

Recommendation 3-1-5: The current SSP advisorystructure should be strengthened with industry (in-cluding EPRI and electric utility) representativesplus experts from other government agencies (par-ticularly DOD/Air Force, DOE, and NRO) in orderto validate technological and economic inputs intothe performance and cost models. Also, due to thewide breadth of technologies related to SSP, the pro-gram should establish similar advisory committeesfor specific technologies in addition to the researchand technology working groups currently utilizedby the program.

Various modeling techniques and methodologieshave been previously developed for use in large spacesystem integration and in cost, performance, and eco-nomic modeling. Such techniques do not seem to beused within the current SERT program or at least werenot evident in the information provided to the commit-tee. The SSP program should review previous model-ing efforts and integrate successful methods, asappropriate. For example, it does not appear to thecommittee that the current SERT program has lookedfor lessons learned from modeling efforts used duringthe 1970s space solar power efforts (Hazelrigg, 1977)or considered other recent efforts in long-term, multi-phase technology development (Hazelrigg andGreenberg, 1991; Hazelrigg, 1992). These methodsare not endorsed by the committee but are mentionedonly to provide examples of other modeling on whichNASA might capitalize.

The following sections address each of the majortechnical areas targeted in the organization of the SERTprogram. The discussion and recommendations in-clude information on each technical area’s objectiveand scope as part of an SSP system, the current techni-

cal state of the art, necessary goals for economic com-petitiveness, challenges to be met by the NASA team,and recommended priorities for NASA’s SSP programactivities.

3-2 SOLAR POWER GENERATION

Objectives and Scope

The solar power generation (SPG) subsystem ofSSP consists of a large constellation of very large spacesolar photovoltaic (PV) arrays that convert the Sun’ssolar energy into electricity. The PV arrays would beconnected to transmitters that would beam down asmuch as several gigawatts of power to Earth via micro-wave or laser transmission. The PV array consists of alarge number of PV panels that each mechanically sup-port hundreds of individual PV cells. The panels areelectrically and mechanically interfaced together viacabling and a rigid support structure to form the largePV array. A constellation of these larger arrays wouldform the SSP solar power generation system.

Important figures of merit at the solar array levelare specific power (watts per kilogram), cost (dollarsper watt), areal power density (watts per square meter),and stowage volume (watts per cubic meter). For SSP,all four of these parameters are important, with com-plex trade-offs. For example, it can be argued that us-ing increasingly higher-efficiency crystalline solar cellsresults in increased solar array cost in dollars per watt,but also gives increased specific power, areal powerdensity, and stowage volume. On the other hand, ifthe paramount driver is PV array cost, as is the case interrestrial PV applications, then lower-efficiency tech-nology such as noncrystalline thin-film PV can pro-vide advantages. At the PV blanket and cell level, thelower-efficiency (~10 percent versus 30 percent forcrystalline cells) thin-film PV blanket can be 50-100times cheaper than the crystalline cells. For relativelylow power levels (1-20 kW), resulting thin-film PVarrays are expected to be at least 10 times cheaper, be2-3 times lighter (2-3 times greater specific power),and have 3 times more stowage volume density thancrystalline cell arrays, planar or concentrator, that pro-duce the same power. However, an important dis-advantage of thin-film PV technology will be muchlarger array size, 2-3 times larger than crystallinearrays, owing to the reduced cell efficiency. This inturn will increase array guidance, navigation, and con-trol needs; structural mass and complexity; and cable


mass, especially for SSP. Because the SSP system willbe very large (several square kilometers), a significantamount of cabling would be needed, tens if not hun-dreds of kilometers.

To date, there has been no comprehensive PV ar-ray trade study for SSP. The baseline conversion tech-nology indicated by the SERT program consists of ar-ray modules using high-efficiency crystalline solarcells under 8 times concentration. However, no studieswere conducted to determine the support structure andelectrical cabling that would be needed. Also, no con-sideration was given to the added support structure andguidance, navigation, and control system that would beneeded to enable the Sun-pointing requirement of ap-proximately 2°. Additional support structure would beneeded to maintain flatness for concentrator arrays tomaintain pointing tolerances, compared with planarunconcentrated thin-film arrays that suffer only a co-sine loss.

Current State of the Art

Today’s state-of-practice solar arrays provide spe-cific powers of 30-60 W/kg and utilize single-crystal,14-27 percent efficient (under the space spectrum) sili-con, gallium arsenide, or GaInP2/GaAs/Ge solar cellssupported by rigid aluminum honeycomb or flexiblepolymetric substrates (Sovie, 2001; Oman, 2001). Thecost for these arrays ranges from approximately $300to $1,000/W (Marvin, 2001). Examples of state-of-practice arrays are the Boeing 601 10-kW array at ap-proximately 50 W/kg and the International Space Sta-tion 260-kW array at 30 W/kg. Today’s state-of-the-artsolar arrays utilize either 26.5 percent (and soon 27-28percent) efficient three-junction GaInP2/GaAs/Ge so-lar cells supported by rigid aluminum honeycomb sub-strates or 17 percent high-efficiency silicon solar cellssupported by a lightweight suspended mesh structure,resulting in specific powers of 70-100 W/kg and $500-$1,000/W (Sovie, 1999). Examples include the Boeing702 15-kW array at 80 W/kg using three-junction cellsand the ABLE Engineering Ultraflex Array at 100 W/kg using high-efficiency silicon cells. The solar arraydesign baselined by the SERT program for SSP isEntech’s stretched lens array (SLA) PV concentratorutilizing 28 percent efficient, three-junction solar cellsunder 8 times concentration via a silicon-basedstretched Fresnel lens (Marvin, 2001). Measured per-formance for SLA at the module level is 378 W/kg butdoes not take into account the support structure mass

required to assemble the modules into a practical array.Projected performance for a <20-kW SLA is less than200 W/kg (O’Neill and Piszczor, 2001). However,most important, this projection does not include theadditional structure mass required to maintain the 2°pointing accuracy over the several square kilometerarray area of SSP. The specific power for the only fullSLA solar array flown in space, aboard the Deep Space1 mission, was 48 W/kg (Murphy and Allen, 1997).

The projected specific power for next-generation,flexible thin-film PVs at the blanket level is approxi-mately 2,700 W/kg for 9 percent efficient, three-junc-tion amorphous silicon on 1-mil Kapton and 770 W/kgon 0.5-mil stainless steel (Guha et al., 1999). The highspecific power of thin-film PVs derives from the depo-sition of ultrathin (<10 µm) layers of semiconductorabsorber material on thin (0.5-3 mil) flexible polymeror steel substrate blankets. The semiconductor layersare typically deposited onto large polymer or steelsheets using low-cost evaporation, sputtering, orplasma-enhanced techniques. The leading thin-filmcandidates under development are amorphous silicon(a-Si) and polycrystalline copper indium galliumdiselenide (CIGS) and its alloys. State-of-the-art effi-ciencies for large-area (0.5-1.0 ft2) a-Si are approxi-mately 8-9 percent (space spectrum) and 6-8 percent(0.2 ft2) for CIGS on stainless steel (Reinhardt, 2001a).The present development goal for space-qualified thin-film PVs under ongoing Air Force research and devel-opment programs is 10-12 percent by 2002, 15 percentby 2005-2007, and 20 percent multijunction CIGS by2010-2015, yielding projected solar-array-specificpower levels of approximately 300, 450, and 600 W/kgfor an array support structure areal density of 0.1 kg/m2.

Technical Performance Goals Needed for EconomicCompetitiveness

Successful development of an economically viableSSP will require substantial leaps in development ofspace solar array, PMAD, thermal control, wirelesstransmission, and launch technologies. However, im-provements in PV solar array technologies alone willnot enable SSP to be economically competitive withterrestrial utility electricity. The theoretical maximumsolar cell conversion efficiency is between 50 and 60percent for crystalline multijunction solar cells (Kurtzet al., 1997) and between 30 and 40 percent for amor-phous and polycrystalline thin-film photovoltaics(Reinhardt, 2001a). Even for the case of 60 percent


efficient crystalline solar cells, the array specific powerwill be limited to values less than 500 W/kg. Assum-ing that the SSP solar array must produce 3 GW ofpower (to result in 1.2 GW to the ground), the mass ofa 500-W/kg solar array alone will be 6 × 106 kg, ap-proximately 241 times that of the space shuttle’s maxi-mum cargo capability—clearly a formidable challenge.Even assuming the SERT program’s launch-to-GEOgoal of $800/kg, the cost of launching the array alonewould be $4.8 billion. In the case of thin-film PV,using even 40 percent efficient arrays at 1,200 W/kgwould require approximately 100 launches of the cur-rent space shuttle (at maximum payload capacity) forthe array to reach LEO. It is clear from this simpleanalysis that improvements in PV-based power gen-eration technologies alone, even to theoretical effi-ciency limits, will not enable SSP to be economicallyviable for competitive baseload terrestrial electricpower, regardless of solar array cost. Even if the solararray were free, the overriding factor is the cost of plac-ing it in orbit.

Challenges to Be Met

As stated above, improvements in PV solar arraytechnology alone will not enable SSP to be economi-cally competitive with terrestrial utility electricity, evenif the solar array were free and theoretical efficiencyand mass performance levels were obtained. The com-mittee believes that the greatest challenge for the SSPprogram is to develop more realistic and accurate sys-tem cost and performance models, including theoreti-cal solar array, power management and distribution(PMAD), thermal control, and wireless transmissioncost and performance parameters, that will allow thelaunch cost to be realistically quantified. The issue isnot the future cost of PV solar array technology, be-cause one day the terrestrial PV industry will reducecosts to a point competitive with utility electricity, butthe cost to place the array in orbit. Considering theparamount challenge in technology development re-quired for other SSP disciplines, and the approximate$200-$300 million invested annually in space and ter-restrial PV development worldwide, the SSP programshould make minimal investment in current PV tech-nologies. It is important to note that there is actuallylittle difference between space and terrestrial PV tech-nologies. There is virtually no difference in the electri-cally active part of the PV cell that controls conversion

efficiency, called the p/n junction. The only differencelies in packaging of the cell, a technology that requiresno significant new development.

Also, a more thorough trade study must be con-ducted to rule solar dynamic heat engines for powergeneration in or out. The option of using solar dynamicheat engines in the SERT program was briefly dis-cussed; however, a comprehensive trade study hasnot been conducted. Solar dynamic options are pres-ently 3-4 times heavier than conventional PV arrays,20 W/kg versus 60-80 W/kg, respectively. Also, cur-rent solar dynamic options are roughly 10 times moreexpensive than PV arrays, $5,000/W versus $500/W,respectively. These data, along with the additional con-cern that solar dynamic requires very high solar con-centration ratios and, hence, extremely accurate point-ing of very large solar collectors, indicate that it maynot be a good choice for SSP.

Recommended Priority for Investment

Considering the paramount challenge of reducingSSP power generation system mass to several orders ofmagnitude less than today’s PV solar array systems,concepts other than conventional crystalline PV, thin-film PV, or solar dynamic concepts need to be devel-oped. Even if SSP PV-based solar array were free, thetheoretical specific power for conventional PV arraysof 1,000 W/kg would result in a cost-prohibitive launchrequirement. Revolutionary breakthroughs are requiredin solar-to-electric power generation technology offer-ing system-specific power in the range of 2,500-10,000W/kg. Considering the small SSP investment in PVsolar array technology today (<$2 million) comparedwith the large national and international PV technol-ogy investment of more than $200-$300 million annu-ally, the committee recommends that the SSP effortfocus future investments on revolutionary high-risk,high-payoff solar-to-electric approaches that could re-sult in system specific powers in excess of 2,500 W/kg.At present, SERT is not working on such a strategy.The U.S. government’s Small Business InnovationResearch program could be one efficient avenue forexploring such high-payoff, high-risk technologies.

Recommendation 3-2-1: The SSP program shouldfocus future investment in solar power generationsolely on next-generation, revolutionary, high pay-off, high-risk concepts.


Synergy with Other Programs

The current SSP baseline power generation systemis PV solar. PV is considered a pervasive spacecrafttechnology and important for all near- and far-termDOD, NASA, and commercial space programs, as wellas for terrestrial electrical power applications. All per-formance improvements in solar array technology, suchas increased efficiency and reduced mass and cost, willgreatly benefit a multitude of future government andcommercial space and terrestrial power systems. Thereis presently significant U.S. government and privateindustry investment in the development of technologyfor space and terrestrial photovoltaic cells, arrays, andsystems. The NASA and Air Force R&D annual bud-gets for space PV are approximately $2 million and $5million, respectively, with industry investing approxi-mately $1 million of independent R&D in space PV.The DOE has an annual budget of approximately $75million for PV technology development, supported byapproximately $20 million from private industry. Ja-pan and the European Union have also increased sup-port for PV research significantly during the past fewyears.

As stated in the previous section, the recommendedpriority for investment by the SSP program in powergeneration should be in revolutionary, extremely high-risk, extremely high-payoff solar-to-electric ap-proaches that could result in system-specific powers inexcess of 2,500 W/kg. Such investment would havetremendous synergy with future NASA and DOD SPGtechnology development. Both agencies have identi-fied requirements for significantly higher levels ofspace platform electrical power for future missions,where the paramount goal is to provide the spacecraftpayload increasingly greater power and mass budgetswithout increasing total spacecraft mass or cost. Anyinvestment in next-generation, revolutionary, solar-to-electric conversion technology that results in increasedsolar array-specific power will benefit next-generationNASA and DOD spacecraft. Also, the SSP programshould better capitalize on government and industry PVtechnology investment and participate in the U.S.government’s Space Technology Alliance1 studies thataddress government strategic planning and investment

in space PV (Sovie, 1999). Further coordination withthe National Center for Photovoltaics, which includesthe DOE’s National Renewable Energy Laboratory andSandia National Laboratories, should also be pursuedin order to capitalize on other PV technology invest-ments.

Recommendation 3-2-2 [also 2-1-8]: NASA shouldexpand its current cooperation with other solarpower generation research and technology effortsby developing closer working relationships with theU.S. Air Force photovoltaics program, the NationalCenter for Photovoltaics, industry, and the SpaceTechnology Alliance’s Space Power subcommittee.

3-3 SPACE POWER MANAGEMENT ANDDISTRIBUTION


The scope of the space power management and dis-tribution (SPMAD) subsystem starts with the powersource (solar panels or heat engine) and ends with themajor loads—the wireless power transmitters and, ifused, the electric propulsion system.

Most of this section is written assuming that thepower source is photovoltaics. A short subsection nearthe end provides a discussion of the differences neces-sary in PMAD design if the power source is a heat en-gine using a thermodynamic cycle such as the Brayton(gas turbine) or Stirling.

The basic starting point is the individual solar pho-tovoltaic cell (also known as a solar cell). The solarcell can be large (1 × 3 in. for crystalline solar cells andup to 12 × 12 in. for thin-film solar cells), and manymust be connected in series to establish the desired busvoltage. A collection of solar cells, their substrate, andthe supporting structure is called a “panel.” A collec-tion of panels with additional structure is called an “ar-ray.” We assume that the PMAD interface is at thepanel level of integration.

An essential element of PMAD is the cabling thatcollects the electricity and routes it to the major loads.Since the SSP system will be very large (several squarekilometers) a large amount of cabling, tens if not hun-dreds of kilometers, is necessary and will account for asignificant fraction of the mass of the system.

The major load of this subsystem is, of course, thewireless power transmitters that beam the power toEarth (or to a receiving station in space). For the mi-

1Currently the Space Technology Alliance consists of membersfrom DOE, NASA, DOD, the Ballistic Missile Defense Organiza-tion, and the Defense Advanced Research Projects Agency.


crowave systems (magnetron, klystron, or solid-statepower amplifiers), the transmitters must be physicallyclose together, and all the solar panel power must beconcentrated at one point. For the laser power trans-mission option, the transmitters will be distributed overmany smaller satellites, thus allowing the length of thepower distribution lines to be much shorter. A second-ary load might be an electric propulsion system usedfor station keeping, obstacle avoidance, and attitudecontrol.

Other major components of the SPMAD systemare used for power conversion and conditioning. Thesecomponents raise the voltage so that the power can bedistributed with lower losses or less mass, or both, pro-viding the exact voltages to the various loads. For somedesigns, the voltage may be converted from direct cur-rent (DC) to alternating current (AC) and then back toDC.


Most spacecraft have PMAD systems and have hadthem since the beginning of the space program. Thus,in principle, one could be built for the SSP system.However, it would be impractical due to excessive costand mass. Both increased performance (higher effi-ciency, higher voltages, and higher temperature opera-tion to reduce thermal control challenges) and reducedmass and cost of the individual components are neces-sary for a practical SSP PMAD system.

The design and performance of state-of-the-artPMAD subsystems varies widely, depending on thespecific application. Many satellites have extensivebattery systems, which require battery charging anddischarge circuitry and controls. Efficiencies are 60-90 percent (Reinhardt, 2001b). For DC-to-DC invert-ers, the efficiency can be greater than 90 percent(Ashley, 2001). Masses of complete subsystems withcabling and battery chargers are 33-40 kg/kW (25-30W/kg) (Reinhardt, 2001b). Cabling must be estimatedand optimized separately for SSP because of the ex-treme lengths of the cable runs. Without including ca-bling and battery circuitry, the power density might beas high as 150-300 W/kg (Ashley, 2001). Costs are$50-$100/W (Reinhardt, 2001b; Ashley, 2001).


The NASA subsystem team has adopted the fol-

lowing goals for this SPMAD subsystem: end-to-endefficiency greater than 94 percent, mass less than 2 kg/kW (power density greater than 500 W/kg), and a costof less than $300/kW (30 cents/W) (Mankins andHowell, 2000b).


System Design Optimization

The mass and cost of the SPMAD subsystem forma major portion of the whole SSP system. For example,at 500 W/kg the PMAD mass would equal that of thesolar array. If the solar array achieved a breakthroughperformance goal of greater than $2,500/W, theSPMAD system would dominate the mass of the array.

As a result, optimization of the design of the sys-tem will be important. This is a systems design issueand must have continuing attention to help guide in-vestment priorities for the various hardware compo-nents of the whole SSP system. In other words, thisoptimization is not an R&D activity but rather a sys-tems activity. Adequate resources are necessary to doit well, but it should not have to be an area of signifi-cant investment.

Power Distribution Voltage and Insulation

Power distribution voltages and insulation are ar-eas of future research. The conductors will undoubt-edly be aluminum, which is optimum. High voltages(tens of kilovolts) are desired to reduce transmissionline losses and mass. Some design concepts even usebare wires (i.e., no insulation). Use of these high volt-ages will present challenges of electrical breakdownand performance of insulators, insulation, andswitchgear.

The power collection and distribution voltages areimportant system optimization variables. R&D invest-ments in this area include (1) space effects on conduc-tors (such as arcing and damage on bare wires from theplasma and particles resulting from possible meteoroidimpact to the SSP system as a whole) and (2) types ofinsulation for insulated systems. These topics are im-portant because they may limit the highest voltage, hav-ing a first-order impact on the system mass and cost.Electrical arcing and other breakdown limits are alsokey issues. Steady-state leakage of current can occurwithout arcing. Other solar system and spacecraftevents such as solar storms, thruster firing, outgassing,


and possible cosmic ray or meteoroid impacts can causepotential damage. Recently, communications satellitesin GEO have experienced serious arcing damage(Hoeber et al., 1998). As a result, select flight experi-ments at GEO may be necessary to ascertain optimumallowable voltage limits.

Reduction of thermal heat rejection and radiationhardness of components are major challenges in PMADcomponent design. High-temperature operation andpassive thermal control are goals. Adequate investmentmust be made in these areas to find the appropriatesolutions. In-space experiments may need to be done,at GEO conditions, to demonstrate lack of breakdownand possibly lifetime issues.

Power Conversion and Conditioning

Power conversion and conditioning are importantparts of the PMAD subsystem. These aspects of PMADare major mass and cost drivers. The componentsneeded include power transistors (probably thyristors),sensors, transformers, inductors, capacitors, and switchgear (transistors and circuit breakers). These compo-nents can be assembled into inverters to convert DC toAC (and vice versa) and to raise and lower the volt-ages. They also filter and regulate the output voltagesto those required by the loads.

These components have been an area for invest-ment since electricity was discovered—initially for ter-restrial power applications and, for the last 50 years,missiles and space. Total PMAD system efficienciesremain below 90 percent. Enormous investments havebeen made over many years by both the governmentand the private sector. Steady progress has been made;however, this is an appropriate area for investment bythe SSP program in order to reach its goal of greaterthan 94 percent efficiency.

A few percentage points of improvement in PMADefficiency will translate to a larger percentage point re-duction in SSP mass and cost, reduced system heatload, and thus fewer thermal management problems.However, large breakthroughs should not be expectedbut, rather, slow, incremental progress. Efficienciesmay improve a few points, but factor-of-two improve-ments will not be seen. Thus, reductions in mass forthis subsystem, other than by optimization, will bemodest.

This, however, is an area where investment inmanufacturing techniques may provide dramatic reduc-tions in cost. Space components, typically, are made in

very small quantities and are subject to extensive test-ing. For any SSP system, there will be a need for mil-lions of each item, so process improvement and massproduction could result in substantial cost reduction.NASA typically does not invest in manufacturing tech-nology, but for SPMAD, that investment may repre-sent the largest payoff.

Each subsystem should have mass, cost, and per-formance targets. The mass is also important becausea significant portion of the overall system cost, approxi-mately one-half, is incurred in launching all the massto GEO.

Heat Engine Usage

Some of the SSP conceptual designs being consid-ered use a heat engine, such as a Brayton or Stirlingthermodynamic cycle, to convert solar heat to electric-ity. An advantage of such a system is that the sunlightcan be concentrated at one, or several, points, and thepower conversion and distribution (cabling) mass andcosts can be reduced. Disadvantages are the large andheavy heat rejection radiators required, lifetime issueswith moving parts, and the management of angularmomentum of the rotating machinery for the Braytonoption (the Stirling will undoubtedly have a reciprocat-ing alternator).

A system-level advantage of using a dynamic sys-tem is that the electrical generating system will directlyproduce AC power at a relatively high voltage. Thiswill eliminate the inverter that is needed in the DC sys-tems to convert DC to AC.


Recommendation 3-3-1: A major investment shouldbe made in space power management and distribu-tion. This should include efficiency improvements,mass reduction, and manufacturing techniques de-signed to reduce the cost of the components. Al-most all of the manufacturing investment should goto companies that are already making such compo-nents for either ground or space applications.


SPMAD is important for all space programs—whether they be NASA, DOD, or commercial efforts.Thus, investments made in this SSP program will ben-efit other users. In addition, PMAD R&D by others


will benefit the SSP program. It is important to havefrequent coordination meetings with other SPMAD ef-forts.

3-4 WIRELESS POWER TRANSMISSION


The wireless power transmission (WPT) subsystemstarts with the electrical input to the transmitting de-vices in space (beamers), and ends with the DC outputfrom the receiving elements on the ground (or in spacefor a space-to-space application).

The transmitting elements under consideration areof four types: (1) magnetrons, (2) klystrons, (3) solid-state amplifiers for microwave systems, and (4) lasersfor the laser option.

Microwave Option

The microwave option may operate at 5.8 GHz(5.2-cm wavelength), although other frequencies arebeing considered. The microwave receiving elements,called “rectennas” (rectifying antennas), convert thetransmitted microwave energy into electric powerthrough use of a mesh of diodes and dipoles. Theyneed to be arrayed at about 0.6-wavelength spacing (3.1cm) in order to have high collection efficiency(Dickinson, 2000).

This subsystem also logically includes the inter-vening media—the space environment, the ionosphere,and the troposphere (lower atmosphere). The powerbeam may affect the media and the media may affectthe beam, causing losses in power. The microwave sys-tems will be nearly all-weather and continuously avail-able except for short semiannual periods when therewill be occultation of the Sun or when the beam mustbe interrupted for safety reasons.

For the microwave option, the individual transmit-ter element power level determines the transmittingarray size and the amount of beam steering required.The elements must be contiguous to avoid grating ef-fects causing large side-lobes (with resulting losses andelectromagnetic interference concerns) at the receivingsite. For the baseline system at 1.2-GW output to theutility interface, this results in a dense concentration oftransmitting elements in a 500-m-diameter circle.

Laser Option

The laser option will probably operate in the near-infrared spectrum at 1.03 µm (Er:YAG laser) or 1.06µm (Nd:YAG laser) wavelength.

The laser option has certain advantages in scalingbut is expected to have an end-to-end power transmis-sion efficiency about half that of microwave systems(20 percent versus 40 percent) and suffers from otherdisadvantages such as weather dependence (requiringredundant receiving stations) for terrestrial applica-tions.

The laser option, however, has many advantagesfor space-to-space applications. The beam width canbe narrowed by increasing the transmitting aperture(providing much better scaling to lower powers thanthe microwave option). The receiving elements for thelaser options are solar cells. For the frequencies underconsideration, silicon solar cells will most likely bechosen.

In current SSP designs developed during the SERTeffort, the laser systems are incoherent with one an-other, and each beam has a very small divergence angle.In order to limit intensities to eye-safe levels, the SSPsystem must be broken into many (20 to hundreds)smaller satellites and flown in a “halo” orbit at GEO.

To prevent any chance of using the laser system asa weapon, the aperture of the individual laser elementswill be kept small. This will make the beam spot of anindividual laser satellite about the same size as the re-ceiving array. All of the satellites will be focused onthat same receiving array so, by design, the beam in-tensity cannot be increased above its design level.

The laser system is affected by weather—rain andmoisture will cause reductions and sometimes completeinterruption of power. NASA proposes to mitigate thisproblem by having multiple receiver fields at widelyspaced locations to reduce the probability of simulta-neous cloud cover over all the receiver fields.


WPT has had numerous microwave demonstra-tions over the past 35 years. The highest power trans-mitted was 34 kW at S-band (Dickinson, 2000). Thelaser option has been demonstrated, but at much lowerpower levels. The laser option is preferred for lowerpower levels because it scales better to smaller sizes.For that reason, it is preferred for space-to-space appli-cations.



For the microwave system, the project has definedan overall transmission efficiency allocation that willyield 40 percent efficiency end to end from PMAD DCto receiver DC. A mass goal of 5 kg/kW and a costgoal of $2,500/kW have been established (Dickinson,2000). These are not consistent with a system-level costgoal of 5 cents/kW-hr. The NASA program shouldmake consistent performance, cost, and mass alloca-tions.


For the transmitting side, challenging issues in-clude the efficiency of the transmitters, thermal controland heat rejection, the antenna array, and beam steer-ing. The klystron and the laser system will probablyrequire active cooling systems, such as pumped fluidloop radiators. It is hoped that the solid state and mag-netron can be passively cooled with careful thermaldesign. Stringent mass and cost targets must be met.

The receiving elements are rectennas for the mi-crowave systems. The receiving elements for the laseroption will be silicon solar cells that also take advan-tage of the sunlight falling on them. Mass is not anissue for the ground subsystem, but cost is an issue.

A continuing activity will be needed to reserve thenecessary frequency allocations for the early SSP dem-onstrations, as well as for the ultimate full-up system.There are also issues to be addressed relating to elec-tromagnetic interference and electromagnetic compat-ibility effects on other ground and airborne systems.

Research will be necessary to develop intrusiondetection systems to keep people and airplanes out ofthe high-intensity parts of the beam. If a plane is aboutto enter the beam, the current plan is to rapidly defocusthe transmitted beam. This will minimize electrical,thermal, and attitude disturbances on the satellite. Amajor hurdle for the WPT subsystem is public accep-tance. There are concerns about the safety and healtheffects for both the microwave and laser options. Theproject has established guidelines to limit the beampower densities to levels thought to be safe. Theground portion of the system must be worker-safe sothat maintenance can be done while the beam is on.There are also concerns about low-level exposure “out-side the fence.” The SSP program should continue touse design specifications for the WPT subsystem that

meet public health safety standards. Refer to Section3-10 for further discussion of general environmental,health, and safety issues for SSP systems.


Recommendation 3-4-1: As long as the annual bud-gets remain modest (<$10 million), NASA shouldconcentrate its resources on the best of the micro-wave options and the laser option. Both should bedemonstrated, and the laser option should bebrought to the same level of maturity as the micro-wave options. When the resources become moresubstantial, NASA should re-examine the other mi-crowave options to make sure that the one selectedis still optimal.

Recommendation 3-4-2: The SSP program shoulddevelop the laser option to the same level of matu-rity as the various microwave options. Investmentshould be made in increasing the laser conversionefficiency, improving associated heat rejection sys-tems, and investigating possible uses of direct solarpumping.

Recommendation 3-4-3: Point-to-point powertransmission should be demonstrated with the laseroption. Initially this should be conducted on theground. The need for a space-to-space demonstra-tion should be studied, and such a demonstrationshould be conducted if warranted.

An advantage of the laser option is that a meaning-ful demonstration can be done at relatively small scalesand at moderate cost. This will benefit both the space-to-space and ground-to-space applications.

Recommendation 3-4-4: Larger-scale, system-levelwireless power transmission demonstrations shouldbe planned, either ground or space-to-space dem-onstrations. Some might involve the InternationalSpace Station.

These demonstrations should be selected to havecommercial potential wherever possible. This may at-tract cost sharing and possibly create an application thatwill serve to develop a larger market and help lowercosts. The space demonstrations are likely to be quiteexpensive.



There is interest in WPT within the DOD and vari-ous commercial ventures (Dickinson, 2000). Most ap-plications have to do with powering an airship or air-plane so that a platform can be kept above a given areafor long periods—say, weeks or months. There arealso some applications that involve powering a satel-lite either from the ground or from another satellite. Ifthe right demonstrations and technology options arechosen, it is likely that other significantly fundedprojects could be established that would be synergisticwith the NASA SSP program.

There is some interest in SSP and WPT technologyoutside the United States (Erb, 2000). There are con-tinuing space WPT sounding rocket experiments by theJapanese. The French are planning to beam 10 kW ofpower across a bay (700-m range) at La Réunion island(Pignolet, 1999) See additional discussion of interna-tional interest in Section 2-3.

3-5 GROUND POWER MANAGEMENT ANDDISTRIBUTION


The ground PMAD subsystem begins with the out-put of the receiving elements of the WPT subsystem—either a “rectenna” (rectifying antenna) for the micro-wave options or solar cells for the laser option—andends at the interface onto the utility power grid. In theutility and terrestrial PV communities, this collectionof subsystems is usually referred to as balance-of-sys-tem. For space-to-space power transmission, thePMAD for the receiving end will be similar to thePMAD of conventional satellites.

In contrast to the space PMAD subsystem, theground PMAD subsystem does not need a mass goal.However, it does need cost and efficiency goals.


The components needed for this subsystem are thesame categories as for the SPMAD—namely, cabling,insulation, inverters, transformers, power transistors,inductors, capacitors, switch gear, sensors, and so on.For the microwave options, the receiving array will belarger than the solar arrays in space, so the cablinglengths will be even longer. The same issues of highvoltage and AC collection will be present here as well.

The final output could be high-voltage AC or DC de-pending on the utility network to which the system isbeing connected.

For the laser option technical needs are similar.The receiving panels will consist of solar cells and forall practical purposes will behave like terrestrial PV.The array will be much smaller than for the microwaveoption, so the cabling lengths can be shorter. Theshorter cable lengths will undoubtedly result in loweroptimum DC collection voltages.

The terrestrial PV industry is currently focusing ondistributed applications of 10-100 kW. There no longerseems to be government or industry interest in largePV farms of hundreds or thousands of megawatts.


The components in the ground PMAD subsystemare familiar to the utility industry for conventional aswell as terrestrial PV applications. The utility industryhas a desire for lower cost components, and competi-tive pressures by suppliers should result in R&D in-vestments necessary to improve performance and re-duce unit costs. This investment will probably waituntil a larger market is perceived.

The National Renewable Energy Laboratory(NREL) estimates a cost of 15-25 cents/W for 1-MWand greater power systems (Koproski and McConnell,2001). NREL has funded a PV Manufacturing Tech-nology (PVMat) program in conjunction with SandiaNational Laboratories, which included funding of in-verter technology at approximately $1 million/yr forseveral years. Southern California Edison uses an esti-mating figure of 30 cents/kW-hr for the balance of thesystem. A consulting firm reports that the largest U.S.installation of PV is 1 MW at the University of Califor-nia, Davis, with an estimated cost of 10 cents/W at highvolumes (Whitaker, 2001).

The SERT program has established an efficiencygoal for the ground PMAD subsystem of 85 percentand a cost goal of less than 0.5 cents/kW-hr. It appearsthat the cost goal is not stringent enough.


The ground PMAD subsystem will be similar toterrestrial solar power systems, so the challenges aresimilar. It will also utilize components that are famil-


iar to the utility and supplier industries. The biggestchallenge is cost.

Another issue is energy storage for the SSP sys-tem. The SSP program wants to apply the gold stan-dard of baseload power to compete with other ground-based alternatives. This will require storage to providecontinuous power during annual eclipses of the satel-lites (a few hours per day for a few days) and for occa-sional beam interruptions for safety reasons (e.g., anairplane about to fly through the beam). Therefore,NASA has included ground energy storage in its workbreakdown structure.

When the first SSP system is operational, it willrepresent a tiny fraction of the nation’s or a givenutility’s electricity supply. The utility will probablyintegrate it into its system without storage. As SSPbecomes a larger fraction of the electricity supply, agiven utility system may want to include storage, butprobably not at each individual receiving station. Stor-age is a network and system issue. Using storage a longdistance away from the SSP receiving station may wellbe optimum. The solution will most likely be utilityspecific. The utility industry also has other needs forenergy storage and will undoubtedly make the invest-ment necessary to develop and build such systemswhen they are needed.


Recommendation 3-5-1: NASA should neither in-vest in R&D for ground PMAD technologies norspend resources on ground-based energy storage.

If, for comparison purposes, NASA wants to quotecost figures for baseload systems with storage, thenstorage should be included only analytically in the eco-nomic models. NASA should not conduct research anddevelopment in this field. The Terrestrial Photovolta-ics Roadmap being followed by DOE covers this areaof investment.


The DOE NREL and Sandia National Laboratoriesjointly manage the National Center for Photovoltaics.This center funds various photovoltaic deployments ofdistributed PV (typically 10- to 30-kW installations).These deployments are no longer considered “demon-strations.” Each application increases the market, andtechnical improvements occur. They also conduct the

PVMat program on manufacturing technology, andabout $80 million has been spent since 1992, includingabout 60 percent cost share by industry (Bower, 2001;Koproski and McConnell, 2001). Almost all of theirwork is directly applicable to the SSP ground PMADsubsystem.

The utility industry and the Electric Power Re-search Institute have changed their emphasis since thederegulation and restructuring of the electrical utilityindustry. They used to fund aggressive programs inrenewable energy, including PV, but this is no longerthe case.

3-6 SPACE ASSEMBLY, MAINTENANCE, ANDSERVICING


The technology discussed in this section is that re-quired to assemble, maintain, and service a full-scaleSSP system, with NASA’s stated goal being to use ro-botics for all of these operations. Considerable R&Dmust be performed to bring robotic capabilities to alevel consistent with this goal. These efforts can begrouped into the following general areas: robotic sys-tem architectures, robotic component technologies, androbotic control modes. It is not the objective of thissection to identify which specific robotic architectures,technologies, or control modes warrant investment.This selection process should be performed in conjunc-tion with systems studies that include the cost of robot-ics and in accordance with the technology downselectprocedures described in Section 2-1 of this report (tech-nology readiness level, research and development de-gree of difficulty, and the integrated technology andanalysis methodology).

The design details of any SSP system should inte-grate assembly and maintenance requirements from theoutset. Design of the robots will clearly be a functionof the hardware to be assembled; however, the con-verse is true as well: SSP hardware will need to bedesigned to accommodate the type of robotic assemblybaselined for the time of construction. For example,robotic assembly may require the installation of grapplehard-points at regular locations on the structure, andmaintenance may require that robot transportation railsbe incorporated. Clearly, it will be essential to con-sider robotic assembly and maintenance early in thedesign phase of space solar power system concepts.The cost and logistics related to assembly operations


should also be included in the program’s early systemsstudies.

The NASA SERT program’s robotics goal for thegigawatt-size solar power system (MSC 4) is to per-form the entire assembly using a fully autonomous ro-bot team (Culbert et al., 2000). This is an ambitiousgoal, even in the 20-year time frame allotted. The cur-rent (i.e., December 2000) SERT program does notconsider human assembly, maintenance, and servicingfor its full-scale system (except the use of ground per-sonnel). Human assembly is expected by the programfor early flight test demonstrations. Because no analy-ses, systems studies, or data have been shown to thiscommittee demonstrating that full autonomy is the leastexpensive or optimal assembly method, discussions inthis section address the more general question of opti-mal assembly techniques for large SSP systems. Con-ceivably, SSP assembly could be accomplished usingrobotic capabilities anywhere in a range from com-pletely tele-operated (at one end of the robotic spec-trum) to fully autonomous (at the other end). The opti-mal choice of autonomy level, as well as the choice ofwhere to locate humans controlling the robots (on theground or in orbit) should be made using results of sys-tems studies that explicitly include the cost of bothhuman and robotic involvement.

According to the SERT roadmap for space assem-bly, inspection, and maintenance, the early flight dem-onstrations (MSC 1 and MSC 1.5) do not use robotictechnology for assembly but rely instead on deploy-ment and extravehicular activity. While this approachis understandable because robotic technology needs arepostponed to a later date, it removes robotics from theinvaluable operational experience developed with regu-lar flight experiments. Robotics should be consideredas a possible component of every SSP developmentmission and used, if applicable, for each flight demon-stration, consistent with the technology available at thetime and integrated into the space operations necessaryfor that mission. To achieve the level of robotic capa-bility necessary for assembly of the full-scale system,other flight experiments, in addition to the MSC flights,will likely be necessary.


Operational space flight robotics to date have beenlimited to simple sampling systems (Surveyor, Viking)and a large crane-type positioning manipulator (theshuttle’s Remote Manipulator System [RMS]) with an

end effector specialized for a single grapple fixturedesign. The next-generation positioning manipulatorwill be the Space Station RMS, which will incorporatethe ability to maneuver end over end but will still belimited to a single grapple fixture design. This systemwill eventually support the Special Purpose DexterousManipulator, which will be capable of many special-ized servicing tasks.

Assembly of space structures by robots in a realis-tic simulation environment has been limited to theBeam Assembly Teleoperator and Ranger NeutralBuoyancy Vehicle of the University of Maryland(Spofford and Akin, 1984). The Ranger Tele-roboticShuttle Experiment (TSX) is under development for a2003 flight demonstration and represents the type ofenhanced dexterous capability that could be incorpo-rated on an early SSP flight demonstration.

Future applicable robotic systems that are currentlyin laboratory development are the Carnegie MellonUniversity Skyworker and the NASA Johnson SpaceCenter Robonaut; the latter is particularly interestingin that it features near-human manipulative capabilitieswith anthropomorphic articulated five-fingered hands.Systems such as these could be advanced to flight dem-onstrations within a decade and should be available forassembly of SSP flight experiments.

Far more advanced robotic technologies and sys-tems include fully autonomous humaniform robots (de-scendents of Robonaut), self-replicating systems,nanobots, and cellular automatons. While these aretopics of active theoretical research interest, they re-quire breakthroughs well beyond current technologyand may not offer any significant advantages for SSPover more near-term robotic systems. More applicableperhaps to SSP assembly, maintenance, and servicingare novel concepts for self-assembling, self-monitor-ing, and self-repairing structures or electrical systems.Thus, for example, smart structures with embeddedsensors or self-reconfiguring electronics that could re-route around damaged solar cells could reduce the needfor robotic inspection and repair. Integrated “smarts”of this nature already exist in modern military aircraftand will probably become indispensable for spacecraftof the future, particularly large remote solar platforms.

Control of most robotic systems used to date inspace has been through some form of tele-operation(human directly commanding manipulator motion).Low-level supervisory control (human commanding awell-defined task, then supervising task completion) iscommonly used with ground-based manipulators, and


high-level supervisory control (human commanding agoal and letting the robot figure out how to accomplishit) has been demonstrated in limited laboratory condi-tions. Full robotic autonomy requires some form ofartificial intelligence in the robot. While much progresshas been made, full autonomy has not yet been demon-strated for space robotic systems.

For all but the most autonomous systems, humansmust be present somewhere and communicating withthe robots in some manner to control them. Manyforms of remote control have been demonstrated, fromoperating a manipulator in the next room to driving arover on Mars. In general, once the operator is removedfrom the work site, the separation distance is importantwhen the control time delay becomes noticeable. Con-trol time delay will be an issue at GEO for tele-oper-ated systems.

Technical Performance Goals Needed for SSP

Comprehensive robotic performance is difficult tomeasure because there are numerous aspects of impor-tance, including mobility, manipulator capabilities,control modes, speed, strength, precision, and reachenvelope, among others. One cannot define quantita-tive threshold values that must be met in any of thesecategories to enable SSP robotic construction becausethese values are entirely dependent on the hardware tobe assembled. It is possible to discuss qualitatively,however, the types of robotic activities and levels ofperformance that may be necessary to enable SSP as-sembly and maintenance.

While a solar power satellite is orders of magni-tude beyond anything ever built in space, it is similar inscale to a number of everyday Earth constructionprojects, including large bridges, deep-water drillingplatforms, skyscrapers, or large ships. These engineer-ing projects are equivalent in mass and similar in sizeto SSP and are composed of a much larger number andvariety of components under much greater load. Thesestructures are quite effectively built without huge teamsof finely interacting robots, or self-replicating systems.Indeed, optimal design for these systems does not callfor hundreds or even dozens of interacting agents work-ing with precise coordination but instead uses smallteams of agents (humans) working semi-autonomouslyat various locations around the structure. Assemblyand maintenance of SSP should be modeled on thesetypes of systems.

Robotic technologies have already been flightproven (RMS), dexterous and capable robotic systemsare nearing flight readiness (Ranger Tele-roboticShuttle Experiment), and highly capable anthropomor-phic systems are currently in laboratory testing(Robonaut). All of these, as well as Earth-based simpletechnologies such as pick-and-place automation, havereal and practical benefits in an SSP assembly scenarioand should be fully evaluated as components in a ro-botic work site.

It is interesting to note that early SSP assemblystudies performed in the 1970s, when the SSP conceptfirst arose, envisioned robotic systems such as Ranger,Robonaut, and Skyworker as being necessary and suf-ficient for the SSP assembly process (Akin et al., 1983).Yet now, when these systems are nearing operationalstatus, current assessments of robotic technologies nec-essary for SSP assembly indicate the need for largenumbers of cooperating autonomous robots (Culbert etal., 2001), a capability still very far off today. There isno doubt that further engineering development can leadto advanced autonomous abilities, greater aptitude forcooperation among robots, and reduced demands onsupport (physical and mental) from Earth. All of theseadvancements will improve the capability to assemblelarge space systems effectively and efficiently. How-ever, it is paramount that the technology investmenttoday not be focused exclusively on the most ambi-tious goals. Certainly some funding might beneficiallybe invested in revolutionary breakthroughs in robotictechnologies, but most investment should be to nurtureand support the less esoteric robotic capabilities cur-rently nearing flight status and the technologies neces-sary for the MSC flight demonstrations.

Some robotic technologies can be identified as be-ing so basic and useful that they will most likely bedeveloped regardless of how an SSP system is ulti-mately designed, assembled, and maintained. Suchtechnologies include:

• Manipulator end effectors and dexterity to al-low for delicate handling of lightweight SSP structuressuch as inflated tubes, film-like blankets (reflectors,thin-film solar cells, thermal blankets), alignment-sen-sitive mechanisms (concentrator arrays), and other dif-ficult-to-handle components (stiffening cables or wirebundles);

• Robot-compatible connectors and interfaces,not just for the mechanical connection of components,


but also for electrical wiring, fluids routing, and ther-mal conduction;

• Improved vision systems, including image rec-ognition, work site position and pose calibration, andsurface inspection in extreme lighting conditions;

• Advanced control and human interfaces forenhancing robotic performance, including mitigationof time delays and accommodating limitations in real-time video feedback;

• Mobility systems, including positioning ma-nipulators (e.g., RMS), “walking” systems (Sky-worker), and free-flight mobility (AERCam, RangerTSX);

• Decreased maintenance and servicing require-ments (i.e., longer times between scheduled and un-scheduled maintenance operations) and more robustactuator systems;

• Decreased power requirements (longer timebetween refueling or recharging);

• Basic research in remote work site technolo-gies and use of robots in weightlessness, including hu-man-robotic cooperation; and

• Ongoing development and performance evalu-ation of innovative robotic architectures and concepts,such as serpentine manipulators, modular recon-figurable systems, and free-flying miniature inspectionrobots.


One of the biggest impediments to robotic avail-ability for SSP assembly is the lack of NASA supportinfrastructure for robotic technology development to-day. While there are identified agency programs (suchas the NASA Institute for Advanced Concepts and theOffice of Aerospace Technology) to support technol-ogy at technology readiness levels (TRLs) 1, 2, and 3,2

established flight programs try to achieve mission as-surance by minimizing the use of advanced technolo-gies to the degree possible. This makes TRLs 4, 5, and6 a no-man’s land,3 where promising technologies liefallow for years for lack of advocacy or established

programs to nurture them through to flight demonstra-tion. It is essential that a well-supported and compre-hensive program in robotics and automation technolo-gies be re-established and aimed not just at enablingadvanced research, but at taking basic research all theway from concept (TRLs 1 and 2) through flight dem-onstration (TRL 9).4

A viable SSP concept design will have to considermanufacturability: design from the outset to facilitateassembly and checkout on orbit. The design of the plat-form will be interwoven with the design of the assem-bly process. Trade studies (such as type and quantity ofrobots, specific capabilities of robots, and balance ofroles and responsibilities between humans and robots)must be performed in the context of the overall system,allowing the trade space to encompass the design of theplatform itself, as well as the components of the assem-bly system. Results of these systems studies will helpidentify what level of robotic capability is necessaryand how much human involvement is optimal, thus in-dicating the robotic technologies in which to invest.No evidence of this type of analysis was presented tothe committee, yet it is essential to establishing appro-priate research goals for robotics.

Determining the optimal mix of humans and ro-bots in a space assembly workforce is actually a com-plex study that would need to include not just the de-sign parameters of the space platform but also the costof having robots or humans, or both, in orbit with alltheir supplies and support equipment. Since the cost oftransporting and maintaining humans in orbit is high, itis undoubtedly ineffective to use humans for simpleand repetitive assembly tasks on orbit. However, in theabsence of detailed systems studies, there is insuffi-cient justification for insisting a priori that all tasks beperformed exclusively by robots, particularly intricateand complex tasks such as repair and maintenance ofrobots and production equipment. It would require agreat deal of development effort to reach this level ofautonomous robotic capability and might not (argu-ably) produce a higher likelihood of assembly successthan a system that provides the opportunity for humansto intervene directly when necessary. Experts in thefield of automation and robotics for space applications

2Technology readiness level 1 (TRL 1) refers to basic researchwhere basic principles have been observed and reported. TRL 3commonly refers to technologies that have advanced through acharacteristic proof-of-concept evaluation.

3TRLs 4-6 consist of technology development and demonstra-tion both in the laboratory and in a relevant environment.

4TRL 9 involves system test, launch, and operations duringwhich an actual system is flight proven through successful missionoperations.


stress the value of having some direct human presencefor real-time accommodation of unexpected events,robustness, and capacity for innovation, all of whichwill yield improved mission assurance (Akin andHoward, 2001).


Recommendation 3-6-1: The SSP program shouldperform systems studies to investigate the trade-offbetween robotic autonomy and human involvementfor SSP assembly, maintenance, and servicing op-erations. It is essential to determine the optimallevel of robotic autonomy for SSP operations andthe optimal number and location of humans in-volved in these operations.

Recommendation 3-6-2: The SSP program shouldperform near-term flight demonstrations of roboticassembly techniques, as well as robotic maintenanceand servicing operations. Robotics testing should beincorporated into all SSP flight demonstrations, ifpossible and as applicable.

Recommendation 3-6-3: SSP systems should be de-signed from the outset for robotic assembly andmaintenance.


Robotics applications are everywhere, and spin-offtechnologies from SSP will find uses in at least as manyground-based applications as space-based applications.Examples follow:

• Robotic maintenance and repair of the Interna-tional Space Station (ISS) and its future derivatives;

• Robotic maintenance and repair of scientificspacecraft (such as the Hubble Space Telescope or NextGeneration Space Telescope) and commercial space-craft (communications satellites);

• Robotic exploration (and possible use) of otherplanets and space objects (comets, asteroids, librationpoints, missions out of the solar system, etc.);

• Robotic capabilities for undersea operationsand exploration of other Earth environments danger-ous or toxic to humans (e.g., Antarctica, volcanoes,nuclear power plants);

• Factory automation and ground-based manu-facturing;

• Robotics for medical applications (precisionsurgery); and

• Robotics for military applications (groundcombat troops).

Because these applications are so widespread andpotentially powerful, a number of organizations andcountries are already investing in robotics develop-ment, including NASA Johnson Space Center, the JetPropulsion Laboratory, and Defense Advanced Re-search Projects Agency in the United States, as well asorganizations in other countries, including Japan. Muchof this investment may be applicable to SSP assemblyand maintenance operations. However, a definite needremains for new investment in robotic technologies thatare unique to SSP, including flight experiments withearly SSP demonstrations, systems analyses to quan-tify costs and identify critical robotic technologies, anddevelopment of man-machine interfaces that allow pro-gressively more autonomous systems.

3-7 STRUCTURES, MATERIALS, AND CONTROLS

This section reviews the investment strategy for theSSP structural concepts, materials, and controls re-search effort. Rather than providing a specific critiqueof individual program technologies, the intent of thissection is to provide a basis for prioritizing technicalinvestment within this area. Specific findings and rec-ommendations are provided, with a focus on the uniquerequirements of SSP structures.

Historical Perspective and State of the Art

Much of the basic structural technology needed forSSP was developed in the late 1970s, at a time when anexpected, inexpensive access to space via the spaceshuttle motivated concepts of large platforms in orbit.The early concepts for SSP were some of these. In the1980s, NASA investment in construction of large spacestructures was largely funded and motivated by theproblem of constructing early concepts of the Interna-tional Space Station. This research was embodied inthe 1987 Space Station Freedom baseline configura-tion.

One of the engineering points of view that devel-oped at that time was that the most efficient structuresfor large space platforms would necessarily be as-sembled in orbit. It was recognized that, from the pointof view of cost, mass, and packaged density, the best


arrangement of material to form structures in spacewould probably be ones assembled and integratedthere, rather than ones integrated on the ground anddeployed. For example, the 1984 Reference Design forSpace Station was completely deployable, but by 1987NASA had changed the concept to a more mass-effi-cient assembled structure.

However, this point of view did not persist. Whenthe assembly of the Space Station structure was aban-doned in the early 1990s, NASA investment in this arealargely stopped. The current “preintegrated” Space Sta-tion structure is, in fact, a multisegment assembly oflaunch-rated pieces. It is not a structure meant to func-tion only in space. In this respect, unlike Freedom, thecurrent ISS structure is not the kind of lightweight(some would say “gossamer”) structure that SSP mightrequire. Thus, although ISS is being assembled on or-bit, the state of the art is arguably no closer to spaceconstruction now than 10 years ago.

In the intervening years, much of the focus withinNASA has been on the development of highly efficientdeployable structures. This has included inflated andlightweight unfurled structures, with a concomitant setof new concepts and materials. The SSP structuresroadmap reflects this change in focus over the past de-cade. It would be useful, however, to reconsider theSSP structures roadmap in light of the basic goals andrequirements of space construction. This may reinforcesome of the decisions already made within the SERTprogram, but it may also lead to a selection of tech-nologies different from those currently included in theSSP structures roadmap. The following section ex-pands on this idea.

SSP Structural Design Requirements and TechnicalIssues

One basis for rational examination of the design ofSSP structures is provided by Hedgepeth (1981) andHedgepeth et al. (1978). The purpose of a structuraland control subsystem is to provide a stable geometricshape, arrangement, and orientation of the componentsof the SSP. These requirements must be maintainedunder on-orbit loads. For SSP, like all large space struc-tures, the loads include gravity gradient, pointing andslew accelerations, and thermal gradients. In addition,SSP loads include assembly, verification, and mainte-nance loads, which are loads not ordinarily considereddesign drivers in modern satellite structures.

The primary design objective for the structural sub-

system is to meet the geometric requirements under theprescribed loads while minimizing cost. A space struc-ture has several design influences, which for SSP canbe separated as follows: (1) mass, (2) package density,(3) fabrication complexity, (4) integration and test,(5) lifetime, and (6) impact on other subsystems.

Mass

For SSP, without a doubt the total mass of the struc-ture is an important component of the overall cost. Theamount of mass for the concepts presented to this com-mittee varied but was easily on the order of 1,000equivalent shuttle payloads per SSP satellite (Olds,2000; Carrington et al., 2000). Of this mass, the frac-tion allocated to the structure also varied but was onthe order of 25 percent of the total mass of the SSPsatellite.

The mass of the structural subsystem should not beestimated without due consideration of the require-ments and specified loads. Different structures in spacewill have different masses, even if their size and shapeare similar. As an extreme example, if a structure hasno shape requirements, or if it experiences no loads,then it can have zero mass. The most “gossamer” ofstructures does nothing as a structure. So the structuremust have finite mass in accordance with the require-ments set forth in its design.

This actuality makes it difficult to extrapolate massestimates for SSP from other space structures unlessthose structures satisfy the SSP requirements and loads.One should not, for example, extrapolate the mass ofan SSP boom from the mass of a solar sail boom. Ac-cording to material presented to this committee(Carrington, 2001; Canfield, 2001; Collins, 2000),some mass extrapolation was done to support two ofthe system analyses (the Abacus and the IntegratedSymmetrical Concentrator concepts). For instance,“mass estimates for the inflatable rigidizable truss ele-ments were derived from equations supplied by [amanufacturer]” (Carrington, 2001) and “mass elementsfor truss elements . . . were derived from [a Space Sta-tion prototype truss built at NASA Langley]”(Carrington, 2001). Some of the mass estimates werealso based on the studies from the 1970s.

It is not asserted here that these mass estimates arewrong, but since they are based on extrapolations fromthe past, they may be misleading the specification oftechnological goals for the SERT effort. Without a top-down flow of the structural requirements, it is difficult


to assess whether the stated technological goals aresufficient for SSP.

For large space structures such as SSP, it is recog-nized that the most important design criterion thatdrives the mass will be the stiffness required from thestructure to withstand a given load and achieve a givenrequired stability. When the loads are inertial, such asgravity gradient or attitude control accelerations, theyvary with the structural mass. For this reason, it is theratio of stiffness to mass that will drive the structuraldesign of an SSP system.

This means that vibration frequency (the ratio ofstiffness to mass), and not overall stiffness, is a pri-mary structural design criterion for space structures.Usually the inertial loads will determine the frequencyrequirement. Sometimes, however, the pointing stabil-ity and bandwidth requirements for the attitude controlsystem will determine it. Nevertheless, the first vibra-tion frequency of the structure is often a top-level re-quirement levied on the structure (Hedgepeth, 1981).

This means that two structures for a given func-tional requirement or application can be compared withrespect to how much mass they use to meet a first vi-bration mode requirement. In fact, one can look at thescaling of the first vibration mode as a basis for ex-trapolating structural mass (Lake, 2001; Lake et al.,2001). It is suggested here that the goals itemized in theSSP structures roadmap would be better supported ifcast within such an overall design requirement.

Package Density

While mass is often considered the primary costdriver for the structure, the packaging density can havean important effect as well. If the mass is low but thestructure still does not fit within the volume allocatedby the booster, more launches will be required for thesame amount of mass. It is for this reason that struc-tural development goals should include the packagingdensity (kg/m3) as well.

For example, it has been pointed out that existingspace structures, whether communication antennas orthe Hubble Space Telescope, are designed for pack-aged densities on the order of 65 kg/m3 (Lake, 2001).This is close to the typical payload density of existinglarge boosters. This reference also points out that thetrend in inflatable or unfurled concepts may not bemeeting this requirement, meaning that those structuresmay in fact be volume critical, not mass critical, unlessthis requirement is factored into their development.

Fabrication Complexity

An SSP structure with a low mass that meets thepackaging density requirement may still be too expen-sive to fabricate. That is to say, the number and com-plexity of the parts used in the structure can be an im-portant design consideration. The number of partsmight be a method for capturing this cost, measured innumbers of drawings for design time and fabricationtime.

What this means, for example, is the best overallsolution might be higher mass. In other words, it maybe possible for a higher-mass structure to more thanoffset the cost of the extra launch mass if it is suffi-ciently simpler to design and build. Whether this is anissue for SSP should be examined (it is for currentspacecraft). It is possible, however, that the mass pro-duction of SSP structures may offset this cost contribu-tion.

Integration and Test

Perhaps the most challenging requirement for anSSP system is that it may be the first application tointegrate and test the structure in orbit, perhaps the firstapplication of space construction. It is standard prac-tice today for space structures to be integrated andtested on the ground, then deployed on orbit. In fact,this activity is an essential risk mitigation activity. Onemust ensure that the structure will deploy reliably onorbit upon command. The need to mitigate this risk canadd complexity to the system in the form of redun-dancy and in the form of repeated deployment and test-ing in 1 g.

An SSP structure, however, cannot be tested on theground. The size of the structure would be difficult toaccommodate. Moreover, when space structures mustbe integrated and tested on the ground, their design canbe driven by the loads in 1 g, not the loads in 0 g. Theinertial loads at GEO can be three to six orders of mag-nitude smaller than in 1 g, which means the first vibra-tion mode required for an SSP structure will be smallerby the square root of the same factor. Integrating andtesting an SSP system on orbit is a fundamental neces-sity.

However, integrating and testing structures inspace (a.k.a. space construction) would also representa fundamental change in engineering practice. Funda-mental changes like this require not only new proce-dures, but also confidence born of success. This in turn


requires that the procedures be proven in developmen-tal applications and flight experiments. One majorchange that might be needed for SSP is the reliance onpredictive models of structural performance once inorbit. Namely, ground-based testing may need to bereplaced by analytical verification through simulations.Currently, in spite of advancements in computationalcapability, it is still not possible to predict the first vi-bration modal frequency or damping from fundamen-tal mechanics with arbitrary accuracy. Damping, inparticular, is almost always determined from tests, notfrom models. Either this needs to be remedied or theuncertainty in structural behavior needs to be accom-modated in the design.

Lifetime

The lifetime of the structure is determined by boththe degradation of material properties and the damageto the structure from orbital debris and maintenanceactivities. There are two consequences to this: (1) SSPsystems will require long-lifetime materials and(2) SSP systems will require some level of structuralrepair and maintenance. The second concern may needto be considered in the development of the structuralconcepts. The repair or replacement of a truss member,for example, might not be possible if the structure can-not maintain overall integrity during the repair.

Impact on Other Subsystems

The structure often is at the center of the systemdesign because other systems rely on its function tomeet, in part, their own requirements. The developmentof structural technology should be factored into otherSSP systems.

For example, the structural concepts should be de-veloped in coordination with technology developmentin the robotic system. In fact, assembly and mainte-nance must be realized without inducing loads that be-come design drivers, or the mass of the structure mayneed to be increased. A mass-efficient SSP structureshould not be designed for assembly loads.

Another possible synergy might be the use ofPMAD components for part of the load-bearing struc-ture (the notion of a multifunctional structure). ThePMAD system represents in some concepts as much as30 percent of the SSP mass, so use of a portion of thisas structure might have a substantial impact on lower-ing the overall mass.


NASA provided no basis for establishing whatgoals are currently achievable with existing structuraltechnology. It is not clear, for example, that an SSPsystem cannot be built using existing materials andcontrols technologies. Currently, goals in the structuralarea may be leading the NASA engineers to overde-velop structure for an SSP system. The following chal-lenges are of concern: (1) requirements flow-down tostructural stiffness and frequency requirements,(2) development of analysis-based verification tech-niques, and (3) development of modular structural con-cepts with low fabrication and assembly costs.

The need for requirements flowdown is perhapsvery important to do early in the SSP program. A par-ticular consequence of a structural requirementsflowdown might be a refocus of planned R&D efforts.For instance, the required stiffness and mass of the con-cepts currently planned may be adequate for what isrequired; then again, they may be overly stiff. It is notthe intent here to identify all possible impacts of such areconsideration, but the following example illustratesthe need for such an analysis.

The SERT roadmap calls for an overall systempointing accuracy of 3°. This can be used to developbasic requirements on the structure and the control sys-tem. It can be shown that a pointing accuracy of 3° forthe solar array requires a controller bandwidth of63 µHz (Hedgepeth et al., 1978). The concepts NASApresented to the committee have a structural naturalfrequency easily on the order of 100 times this band-width (0.007 Hz).

If the first mode of the structure is this far abovethe bandwidth of the attitude control system, it is un-likely that the attitude control will excite motion in thestructure. Furthermore, it is also unlikely that struc-tural vibrations will impact the gain margin of the con-troller, unless the damping ratio of the structure issmaller than what is typical in current space structuresby a factor of more than 100. Thus, in this one ex-ample, it might be apparent that there is no need for afuture SSP program to invest in control-structure inter-action technology. Those funds might be more prop-erly redirected to other areas. On the other hand, itmight be that the structure is considerably stiffer thannecessary and the mass required for the structure mightbe significantly reduced if this requirement were prop-erly allocated to the structure.


Recommendations

The current structures and controls investmentstrategy is apparently not based on a quantitative analy-sis of the requirements for SSP. This produces two sig-nificant uncertainties for the SSP program. First, thestructural mass estimates for the selected concepts maybe considerably in error. Second, the real needs for theSSP structure and control may not be addressed by theselected technologies. Without this analysis, it isequally possible that either the selected technologieswill fall short of the actual SSP system needs or SSPsystem needs might be met with no further investmentin structural or control technologies. Further studiesshould be made to determine the structural require-ments (strength versus flexibility, structural dynamicloads, mass, etc.) before large commitments of researchfunding are given to advanced structural technologydevelopment for the SSP program.

Also, based on the data presented to the commit-tee, the SSP attitude control requirements should bemet by a very-low-bandwidth attitude control systemthat avoids adverse interaction with the structural vi-brations. Therefore it does not appear that there are anyremaining technical challenges in structural controltechnologies and NASA might better use such fundselsewhere.

Particular attention should be given to (1) the deri-vation of structural mass scaling laws derived from SSPload and stability requirements; (2) an examination ofthe degree of interaction, if any, between the attitudecontroller and the structural vibrations; and (3) pos-sible impacts of structural concept complexity on thefabrication cost.

Recommendation 3-7-1: The SSP program shouldperform a requirements-driven analysis of the over-all structural mass, stiffness, and vibration fre-quency requirements needed for selected architec-tures. Technology goals, roadmaps, and budgetsshould be adjusted to support the results of thisanalysis.


While it may be tempting to recognize synergybetween an SSP system and civilian communication ormilitary satellite structures, their structural require-ments are radically different. In fact, one should becautious about presuming without analysis that there is

overlap in the technology needed by SSP and that be-ing developed by other programs.

As discussed above, the frequency and stiffnessrequired for an SSP system are very low, even com-pared with the requirements of current microwave geo-synchronous satellites. While an SSP system might re-quire only 3° of pointing control, Earth-pointingsatellites might require much less than an arcsecond oferror. The structural frequency requirements differ inproportion to the allowable error. This means SSP canhave a vibration frequency perhaps 1/10,000th that ofan Earth-pointing satellite.

3-8 THERMAL MATERIALS AND MANAGEMENT

Thermal Management Concepts

Thermal management is a critical spacecraft capa-bility needed to maintain all spacecraft components andparts within a specified operational temperature range.Not only are spacecraft made of many dissimilar mate-rials that expand and contract at different rates withrespect to temperature, causing mechanical fatigue andeventual failure, but all electrical components have aspecified operating temperature range needed to func-tion properly. Without thermal management, a space-based system would quickly cease to operate. How-ever, thermal management is generally one of the lasttechnologies considered in spacecraft design. Thermalmanagement encompasses everything from keepingcomponents warm with heaters to dissipating excessheat via radiators. The dissipation of waste heat gener-ally requires the greatest resources (mass, volume, andpower) and includes collecting heat from electroniccomponents (power converters, batteries, computer, rfantenna, etc.), transporting it away from these compo-nents, and rejecting it to the outside environment via athermal radiator.


While major improvements have been made overthe last decade in other areas of spacecraft bus technol-ogy (generation, storage, structures, etc.), minimalprogress has been achieved in developing next-genera-tion thermal management technologies such as heatpipes and deployable radiators. As a result, thermalmanagement is rapidly becoming a limiting factor indesigning larger, more powerful spacecraft, resultingin serious mass and volume penalties.


The state-of-the-art thermal management system isarguably the Boeing 702 spacecraft’s deployable-loopheatpipe radiator, estimated to have a specific mass ofabout 4 kg/kW (Gerhart, 2001). Interestingly, the long-term (15-20 years) specific mass goal identified by theSERT program is 4 kg/kW. This figure greatly under-estimates what is possible. A 4-kg/kW system wouldbe far too heavy for systems larger than 25 kW. A newultralightweight, flexible, deployable radiator has beendeveloped by Creare, Inc., having a projected specificmass of ~1 kg/kW and areal mass density of 0.75 kg/m2 using mechanical pumps and inflatable deploymentstructures presently under development.5 Consideringthe present minimal investment in next-generation ther-mal management technologies, the metrics of 1 kg/kWand 0.75 kg/m2 can be expected to achieve and remainthe state of the art over the next 5-10 years.

Assuming that the heat generated by the SSPPMAD system is actively dissipated by the thermalmanagement system—that is, 100 percent of the wasteheat passes through an active cooling loop—then theresulting mass and size for the thermal managementsystem would be formidable. For example, assumingthat 3 GW of electrical power must pass from the solararray through the SPMAD electronics to enable 1.2GW to reach the terrestrial utility power grid (the solararray must be oversized to account for SPMAD andWPT transmission and collection losses) and assumingthe PMAD system is 90 percent efficient, then the heatgenerated by the SPMAD system would be 300 MW.The required thermal management system mass andarea to support PMAD alone would be 3 × 105 kg and4 × 105 m2, respectively, requiring approximately 12flights of the current space shuttle at maximum pay-load capacity to simply lift the PMAD components toLEO. However, it is noted that a significant reductionin thermal control mass could be achieved if the SSPPMAD system was distributed throughout the largearea of the solar array, potentially enabling passivecooling via heat radiation directly out to space.


Successful development of an economically viableSSP system will require large advances in development

of space solar array, PMAD, thermal control, wirelesstransmission, and launch technologies. Improvementsin thermal management technologies alone will notenable SSP to be economically competitive with ter-restrial utility electricity. However, significant ther-mal management mass and cost savings could beachieved via use of heat pumps to increase radiator tem-perature. The size and mass of the radiator is deter-mined by the maximum temperature of the radiator;the greater the temperature, the smaller and lighter theradiator. The 1-kg/kW and 0.75-kg/m2 performancemetrics above are based on a rejection temperature of120°C driven by today’s maximum PMAD operatingtemperature. It is projected that heat pump technologycould be available in the 2010-2015 time frame, allow-ing an increase in radiator temperature to as high as300°C, enabling a reduction in power density to 0.4 kg/kW (Gerhart, 2001). The SERT program did not ad-dress the potential impact of future heat pump technol-ogy on thermal management performance.


As stated above, improvements in thermal man-agement technology alone will not enable SSP to beeconomically competitive with terrestrial utility elec-tricity. The committee believes that the greatest chal-lenge to support SSP is to develop more realistic andaccurate SSP system cost and performance models thatinclude theoretical thermal management, solar array,PMAD, and wireless transmission cost and perfor-mance parameters, to be able to realistically quantifySSP mass, volume, cost, and launch challenges.


There is no single or simple solution for reducingthe large size and mass of the thermal management sys-tem required to dissipate SSP PMAD system wasteheat. In general, what is required is development ofhigher operating temperature PMAD electronics andthermal management components. To support higheroperating temperatures, advanced heat pump develop-ment should be accelerated.

Recommendation 3-8-1: The SSP program shouldfocus efforts in thermal management and materialson revolutionary high-payoff, high-risk approacheshaving potential cost and mass savings not only for

5Research was funded on this radiator through a U.S. Air ForceSmall Business Innovation Research program entitled “Freeze-Tol-erant, Lightweight, Flexible Radiator,” Contract Number F29601-98-C-0115.


SSP-related activities but also for most other spacetechnology development efforts.

Recommendation 3-8-2: The goals of the SSP pro-gram in the thermal management and materialsarea should be re-evaluated, taking into consider-ation the current state of the art in thermal man-agement and projections of technology advance-ment in the next 10-20 years.


Thermal management is considered a pervasivespacecraft technology and important for all near- andfar-term DOD, NASA, and commercial space pro-grams. Thermal management is also critically enablingfor all DOD, NASA, and commercial aircraft. Anyperformance improvements in thermal managementtechnology, such as reduced specific mass (kg/kW) andareal mass density (kg/m2), will greatly benefit a mul-titude of future government and commercial space andaircraft systems.

3-9 SPACE TRANSPORTATION ANDINFRASTRUCTURE

Space transportation will be a vitally important as-pect of a commercial-scale SSP initiative. Past studieshave shown that a major portion of the total deploy-ment cost for this program will be attributable to thisone area (Davis, 1978). Two space transportation ac-tivities are now expected: (1) launch of the massiveelements of an SSP system from Earth into LEO and(2) subsequent transfer of parts or partly assembled el-ements of an SSP system into the operational GEO lo-cation currently favored by the program because of itsinherent orbital properties, lower levels of orbital de-bris, and room for assembly and maintenance activi-ties. For an operational program, large reusable spacelaunch vehicles are expected to serve the former needs,while highly efficient electric propulsion may be usedfor orbit transfer.

Launch Vehicle Needs

NASA demonstrated that in order to reduce costsfor the system as a whole, vehicles and technology use-ful for continuous build and launch capability wouldbe necessary. The scale of space launch activities nec-essary to place meaningful capacity for supplying

Earth’s needs for electrical power was illustrated to thecommittee as follows (Olds, 2000): 450 flights per yearfor 30 years of a launch vehicle capable of delivering40 tons per flight or over 18,000 tons (40 millionpounds) per year. This level of activity is 80 times thatof current space launch activity. For example, eightflights per year of today’s space shuttle will produceuseful mass in LEO of about 160 tons. If other spacelaunch vehicles deliver half this amount, the multiplierof 80 is confirmed.

If future studies and demonstrations indicate quali-tative and cost viability of space power, two importantquestions will be the rate and the timing of penetrationof the global market for this new source of electricalenergy. If SSP is shown to be sufficiently economicalto begin such a program, the demand to place thesesatellites may rapidly grow well beyond that indicatedin the NASA SERT studies. Other large-scale uses ofspace by both the global private and public sectors mayoccur. These new uses are expected by many analyststo be contemporary with an SSP system and will multi-ply the levels of launch activity associated with it(Andrews and Andrews, 2001).

No one can accurately forecast today if these hugeincreases in traffic to space will be realized. Similarly,although extrapolations and estimates may be made,no one can accurately determine, today, the costs ofspace transportation 30 and more years in the future.As a result, decisions on the viability of future spacesolar power efforts should not be based solely on thespace transportation segment of the program.

If today’s expendable launch vehicles persist foranother four decades and if present forecasts made bytheir suppliers’ of an inelastic, or fixed-size, launchmarket prove to be accurate, continued high launchcosts will render power from space uneconomical. If,however, new concepts and approaches for spacelaunch are adopted and evolved and if new space launchtraffic is attracted by lower costs and higher reliability,then costs will fall, perhaps quite dramatically.

Future space transportation work within any near-term SSP program should be focused on the needs of amajor NASA SSP demonstration project (i.e., MSC 3).Such a program will require significant space transpor-tation. If a 10-MW full-scale demonstration is selectedand if the specific mass is that of earlier NASA studies,the satellite may require 150 to 200 tons of mass placedinto orbit. If the demonstrator satellite is to be placedinto GEO, additional mass in LEO will be necessary.Extended experimental use of this demonstration unit


is likely, further increasing the space transportationneeds of this program. Thus, the MSC 3 SSP demon-stration satellite program may produce a space launchtask approaching the magnitude of placing the Interna-tional Space Station into orbit.

During the same interval, NASA and others mayembark on ventures in space as large or larger than thisdemonstration of power from space. These may in-clude establishing permanent bases on the Moon, hu-man exploration missions to the Martian moons or thesurface of Mars, exploitation of asteroid resources, orpublic space travel.

The NASA SERT program has established, for acommercially viable SSP system, a goal of $400/kg ofmass as the allowable price for each of the two trans-portation legs—Earth to orbit (ETO) and in space (or-bit to orbit). Determining whether this can be achievedis not within the scope of the present assessment; how-ever, the committee feels that this goal may not be lowenough to support a commercially viable SSP terres-trial power system. The costs of space transportationmust be reduced below $800/kg if the present cost goalsof the NASA SERT program are to be realized. Massgoals must also be tightened for other SSP subsystemssince these will determine the size of the transportationtask. Re-evaluation of these goals is important to SSPstrategy because it will influence the technologies thatshould be pursued. Current space transportation goalsare already driving near-term choices within the SERTprogram, creating the possibility that technology in-vestment choices may be made based on goals that arenot stringent enough in this area.

Recommendation 3-9-1: The SSP program shouldre-evaluate its cost goals in the space transportationarea. NASA should ensure that its space transpor-tation technology development activities considerthe revised SSP goals.

In-Space Transportation Needs

One important driver in establishing on-orbit SSPcost is in-space transportation (i.e., LEO-to-GEO trans-portation). The current SERT program has not yet pri-oritized investments in this area, although budgetschedules for the program show increased activity inthis area by 2006. Low-cost options for in-space trans-portation may require large advancements in technol-ogy—advances that may not occur within the next 5-10 years without program stimulus. As a result,

investments need to be made in this area, whether un-der the purview of the SSP program or as a separateNASA initiative. Applications in addition to SSP willbe available for effective use of such new in-spacetransportation options.

Technologies for in-space transportation such aselectric, solar-electric, magnetohydrodynamic, ion, andsolar-thermal propulsion could all be considered aspossible candidates. Low cost and high rate of usagewill be driving factors for SSP since multiple use ofvehicles will be required. Preliminary cost analyses bythe SERT program have also shown that costs relatedto ownership and use of in-space transportation, as wellas launch vehicles, will have impact on the commercialviability of terrestrial power from space. Detailed tradestudies of various in-space transportation conceptsshould be made along with satellite design and assem-bly concepts in order to establish lowest cost methodsfor placing SSP elements in GEO. A preliminary as-sessment of these needs and ideas to meet those needswas performed during the SERT program through auniversity-funded research grant. Again, as for launchvehicles, cost and mass goals must be tightened be-cause they will influence the technologies that shouldbe pursued.

Recommendation 3-9-2: Detailed trade studies forvarious in-space propulsion options should be per-formed early in the program. Issues involving cost,packaging, mass, trip time, and risk assessmentmust be incorporated into the vehicle choice.

Flight Test Demonstration Transportation Needs

Although not dependent on major advances inspace transportation within the next 5 years, plannedSSP flight test demonstrations will require detailed as-sessment of needs in the area of space transportation.The committee understands that configurations havenot yet been chosen for the future SSP technology flightdemonstrations (TFDs). However, planning must be-gin now to determine if current launch vehicles will beadequate and if sufficient mass and volume are avail-able on the shuttle or suitable expendable launch ve-hicles in the time frame anticipated.

Other than very preliminary mass estimates forvarious TFD configurations, little systems work hasbeen seen by the committee in the area of space trans-portation. Most space transportation analysis withinthe program has been concentrated on space transpor-


tation needs for the full-scale MSC 4 system. This isan important factor in determining the appeal of a com-mercial terrestrial SSP system; however, launch vehicleand in-space transportation needs for the early MSCsmust be assessed during the planning stages.

Recommendation 3-9-3: The SSP program shoulddevelop detailed space transportation requirementsfor all technology flight demonstrations (MSC 1,MSC 1.5, and MSC 3) that include data from stud-ies on packaging, cost and mass estimates, and otherimportant parameters.

Opportunities for Synergy

NASA’s efforts toward the new Space Launch Ini-tiative are intended to reduce risk for NASA’s futureneeds, with funding of $290 million to NASA in FY2001 and plans by NASA to expend up to $5 billion inthis area during the next 5 years. However, nowhere inthe descriptions of the SLI provided to the committeeis power from space mentioned (Davis, 2000).

Improved coordination is needed between NASAspace transportation efforts and the future needs of theUnited States for space launch, including any futureSSP program. Work should be conducted to explorethe ramifications of other new markets and to describethe evolutionary paths that might bring this low-costlaunch capability into operation. Information shouldbe provided to SLI on the SSP program’s needs forfuture TFDs, including cost goals, optimal payloadmass, packaging, launch rates, reliability needs, andscheduled need dates to achieve the SSP goals.

Recommendation 3-9-4: The SSP program shouldbegin discussions between its management and thatof the NASA Space Launch Initiative, so that futuremilestones and roadmaps for both programs canreinforce one another effectively. This discussionshould include specific information on SSP spacetransportation needs, including cost goals, timelinesfor deployment, optimal payload mass, packagingrequirements, launch rates, and reliability require-ments.

NASA’s Advanced Space Transportation (AST)program (NASA, 1999) was created to achieveNASA’s goal of significantly lower space transporta-tion costs. The program has initiatives in four majorareas, including the support of long-term technology

research in advanced chemical and nonchemical pro-pulsion systems for use in space. Advancements in theareas of high-power electric propulsion (Hall and ionthrusters), cryogenic engines, spacecraft miniaturiza-tion, solar-thermal powered transfer, electrodynamictether orbit transfer, and in-situ propellant utilizationare expected under the program. Propulsion researchis also being supported in the advanced concept areasof magnetic levitation, pulse detonation, beamedpower, magnetohydrodynamics (MHD), fusion, anti-matter annihilation, and breakthrough physics. Cur-rently, there is little evidence of interaction betweenthe SSP program and the AST program in developingnew in-space transportation options.

Other NASA efforts not labeled specifically withinthe AST program are also being made in electric andsolar electric propulsion, including programs at severalNASA centers and the Jet Propulsion Laboratory.Many of these efforts are in conjunction with academicinstitutions. Efforts in NASA also include advancedchemical and MHD propulsion, beamed energy andmomentum propulsion, and fission and fusion propul-sion. NASA co-sponsors, in conjunction with the Uni-versity of Alabama in Huntsville, an annual joint con-ference on advanced space propulsion concepts6 fromwhich current technology advances can be identifiedand the efforts of the academic community better uti-lized.

Recommendation 3-9-5: The SSP program shouldencourage expansion of the current in-space trans-portation program within NASA and interact withits technical planning to ensure that SSP needs anddesired schedules are considered.

Currently, many other non-NASA investments arebeing made in orbit-to-orbit transportation research,including major programs within the Air Force and in-dustry. There is little evidence of the SERT programleveraging these program investments or partneringwith internal NASA space transfer technology pro-grams. The Air Force Research Laboratory’s Propul-sion Directorate currently has research programs inelectric propulsion, solar thermal propulsion, pulsed

6Proceedings for the last three meetings are available online at<http://www.eb.uah.edu/maglev/aspw/>. Accessed on August 16,2001.


plasma propulsion, and Hall effect thrusters.7 The U.S.Navy Research Laboratory’s Naval Center for SpaceTechnology also supports research efforts in the areasof MHD, tether, and electric propulsion technology.8

Recommendation 3-9-6: The SSP program shouldincrease coordination of industry, academic, andother NASA and non-NASA government invest-ments in advanced in-space transportation con-cepts, particularly in the areas of electric, solar-elec-tric, magnetohydrodynamic, ion, and solar-thermalpropulsion.

3-10 ENVIRONMENTAL, HEALTH, AND SAFETYFACTORS

Objective and Scope

This section considers the environmental, health,and safety issues of SSP systems. Environmental fac-tors include any significant effects on the space sur-rounding the satellite in orbit, on the media throughwhich power is transmitted, and on the neighborhoodof the receiving station. These effects generally includecontamination of the environment in the form of com-munications interference, generation of debris in orbit,or possible effects on the atmosphere (i.e., power trans-mission to Earth receiving sites). Safety factors in-clude any health hazards to Earth biota associated withpower beaming, whether in the form of microwave orlaser transmission. All of the concerns associated withenvironmental, health, and safety factors must be ad-dressed early in the program, with particular emphasison public awareness and public perception. If any ofthese risks are found to be of significant public con-cern, the public may be reluctant to host an SSP groundreceiving station regardless of the economic competi-tiveness of SSP. Environmental, health, and safety risksmay be minimized through appropriate technology in-vestments and technology design guidelines. However,higher priority should be placed on these areas by theSSP program.

In designing a full-scale SSP system, the environ-mental impact analysis must also include pollution thatmay occur during emplacement of the facility (produc-tion and launch of transport vehicles), as well as envi-ronmental effects during the operational phase. How-ever, it is also essential to include, in the case of SSPfor terrestrial power generation, positive effects suchas decreased pollution in comparison with other formsof power generation (i.e., fossil fuels). The great ap-peal of terrestrial SSP, of course, is that, once in placeand operating, its contribution to greenhouse gases inthe atmosphere is zero.

Current Issues

Environmental, health, and safety issues are nowrecognized as essential concerns to be addressed asearly in a program as possible. Some 60 percent of theeffort in the early solar power satellite studies was de-voted to investigating environmental and societal is-sues (Koomanoff, 2001). During the SERT program, aworking group was assembled to look at environmen-tal, health, and safety factors. Preliminary findings ofthis group show that the following are likely to be is-sues (Anderson, 2000):

• Lack of information on the risks associatedwith exposure to low-level microwaves, particularlylong-term human exposure to low-level microwaves;

• Lack of data on effects of laser beaming op-tions on humans;

• Spectrum allocation issues (international con-siderations);

• Orbital space allocation issues (in geosynchro-nous orbit);

• Land use availability for receiving systems forterrestrial SSP;

• Transmission interference concerns (both onorbit and on the ground); and

• Possible use of SSP systems as a weapon.

The effects of power beaming are primarily a func-tion of the frequency and the energy density of thebeam. Currently, the frequencies and energy levels forSSP power transmission are estimated to be as follows:microwave, 5.8 GHz (5.2-cm wavelength) and 1 kW/m2 power level; and laser, 1.03-mm wavelength and ayet-to-be-determined kW/m2 (note: design will be eyesafe due to distributed transmitting sources).

These values represent very low energy levels, and

7More information on the Air Force Research Laboratory’sspace propulsion programs is available online at <http://www.pr.afrl.af.mil/>. Accessed on August 16, 2001.

8More information on the Naval Center for Space Technology’sprograms in space propulsion is available online at <http://ncst-www.nrl.navy.mil/>. Accessed on August 16, 2001.


in both cases, the beam will be designed to be quickly“turned off.” Microwave transmissions can bedephased, dramatically reducing energy density. Laserstrengths will be planned only at eye-safe levels. Cur-rent design specifications for any future SSP systemallow only low energy densities and design-safe stan-dards to prevent further focusing of the beam. The cen-ter of the beam’s Gaussian distribution is planned forenergy densities of about 23 mW/cm2, or about one-fifth the intensity of summer sunlight at noon(Mankins, 2000a; Moore, 2000). Additionally, any re-sidual energy outside the rectenna’s protective fencewould be far below current microwave safety stan-dards, between 0.01 and 1 mW/cm2 (Moore, 2000).

Most research in the area is focused on the effectson humans, animals, and biota of radiation from house-hold devices, digital phones and other electronic equip-ment, and electric utilities (NIEHS, 1999). Little re-search has been performed at field levels specific toSSP application. However, research has been done onbees and birds exposed to microwave radiation at twicethe dose expected for a creature flying through a typi-cal microwave power transmission beam. Results todate indicate that there is no effect, at least on theanimal’s directional flying ability (Koomanoff, 2001).Other testing has been performed on monkeys and isnow under way with humans exposed to low-level mi-crowave radiation. Results to date from this testingindicate that such exposure apparently does not renderthe subject sterile or result in cataracts or any otherdeleterious effects (Kolata, 2001). The larger problemmay be preventing animals from roosting on therectenna or damaging portions of the receiving site—issues that are already of concern to terrestrial solarpower generation farms.

Research was previously funded on the effects ofmicrowave radiation on plants as well (Michaelson andLin, 1987); however, only a small body of work hasbeen performed or published in this specific area(Skiles, 2001). Results from the previous research havebeen inconclusive. Studies have shown that micro-waves (i.e., 2.45 GHz) generally have an inhibiting ef-fect on plant growth (Picazo et al., 1999) and that re-duced growth rates were demonstrated for higherpower densities (Urech et al., 1996). Other studiesshow that effects of radio frequency waves may varywith the stage of plant development (Magone, 1996)and that no significant differences between exposedand unexposed cells were seen for plants exposed to41.6 GHz frequency (Gos et al., 1997). The SERT

program is currently funding at least one experiment todetermine the effects of microwave fields on alfalfa(Skiles, 2001). The committee believes that furtherstudies in the areas of environmental, health, and safetyare warranted.

One of the topics currently under investigation isdetermining how much small transmitters such as cel-lular phones may interfere with aircraft avionics andcommunications. Results of these studies may indicatewhether there is reason for concern about aircraft fly-ing through an SSP power beam. Many different indus-tries and government institutions are also performingresearch to determine if microwave transmitters cancause cancer. So far, results appear to show that nocorrelation exists, but public perception is driving cellphone manufacturers to change their designs even with-out clear scientific evidence. The SSP program maywell be subject to the same sort of public relations re-quirements. The committee understands that the rel-evance of such cellular phone research to space solarpower may be questionable. However, program man-agers must be aware that the long-term effects of SSPwireless power transmission on humans must be quan-tified before public acceptance is found.

As mentioned previously, a major issue is the lackof research on the environmental, health, and safetyeffects of exposure to this WPT system. Technologistsmust be able to prove very limited risk during use. In-terference effects of the chosen WPT method on satel-lites and other spacecraft in the vicinity of the SSP sys-tem should be investigated as well.

Concerns are very real about the effect of the or-bital debris environment on large space structures inLEO (NRC, 1995; NASA, 2000). The problem is notjust the present orbital debris population, but the wors-ening evolution of this debris population, increasedperhaps by SSP operations. The orbital debris pollu-tion in both LEO and GEO will remain and worsen.Research is currently under way to model the orbitaldebris environment more accurately and predict its in-crease in time. In addition, testing of hypervelocityimpacts on spacecraft materials is ongoing. Also,methods of collision avoidance are being developed toaid spacecraft in avoiding collision with mid- to large-sized debris that can be tracked.

Technical Goals Needed for SSP

Systems studies should be done to clearly show notonly that the SSP system is safe but also that the overall


environmental impact of a space solar power system isa positive one. Although once in operation the SSPsystem essentially generates power with zero pollution,the pollution produced during the construction and de-ployment of an SSP system (i.e., multiple spacelaunches) must be within acceptable levels. This studyof balance would have to include, for example, anyenvironmental pollution that occurs during emplace-ment of the facility, such as harmful exhaust from thelarge number of launches necessary to transport thecomponents of the system to orbit (currently the SERTprogram estimates between 500 to 900 Earth-to-LEOlaunches, each with a 40-metric-ton capacity, for one1.2-GW SSP facility, depending on the chosen design).

Environmental effects of the SSP power beam onthe atmosphere, on Earth biota, and on the space envi-ronment surrounding the SSP satellite will clearly haveto be minimal in all respects. It is not sufficient tosimply show scientifically that effects are likely to beminimal; it is essential to mitigate from the outset anyand all perceptions that the system may be harmful ordangerous in any way to humans, animals, or the envi-ronment. Politics and public opinion will weigh inheavily on the ultimate viability of SSP systems. Issuesof electromagnetic compatibility between various hard-ware components must also be treated during the courseof technology development. As technology readinessis improved, studies should be performed on individualtechnologies to determine the level of electromagneticcompatibility between the SSP system componentsthemselves and between the wireless power transmis-sion system and external electronic hardware such asavionics, terrestrial wireless communications systems,communications satellites, and medical devices.

It will also be essential to show that the systemcannot be used as a weapon. Fail-safe features shouldbe designed into the system to prevent pointing a full-strength beam anywhere but at the receiving site towhich it is designated. Current plans are to keep beamdensity low. For the laser option, for example, a per-son standing in the center of the receiving site shouldbe able to look up at the beam and experience no eye orskin damage. In addition, to assuage any fears that thistechnology could be reworked to be used as a weapon,the program should be international in structure, withfull disclosure of information to all participating coun-tries so that no strategic advantage can be gained byany one nation.


In terms of environmental, health, and safety fac-tors, the greatest challenges for SSP are to develop asystem with benefits that outweigh the costs and a highlevel of safety that can be proven to the public. Thesechallenges can be addressed by funding near-term andcontinuing research on the topics listed in the follow-ing two sections at frequencies, power levels, and or-bital locations specific to SSP. The lists are neitherexhaustive nor prioritized.

Environmental Challenges

The following topics are areas of environmentalresearch that must be investigated before deploymentof a full-scale SSP system (by NASA, other appropri-ate organizations, or industry):

• Quantify the net environmental effect of hav-ing terrestrial SSP available in comparison with reli-ance on conventional sources of energy, including thepolluting effects of building and launching SSP hard-ware and the possible deleterious effects of powerbeaming through the atmosphere.

• Develop construction methods for SSP systemsthat minimize the generation of additional orbital de-bris.

• Provide for efficient and final disposal meth-ods for solar power platforms once their useful life isover; otherwise, they too will contribute to the orbitaldebris problem not just in LEO but in GEO as well(Anderson, 2000).

• Determine how SSP systems will alter the ra-diation environment around them and quantify the re-sulting impact in terms of orbital space allocation andinterference effects at GEO.

• Assess power transmission side-lobe effects(energy transmitted outside the nominal beam diam-eter) and the impact on communications in the vicinityof the power beam as well as around the receiving sta-tion (ground based or in orbit).

• Perform additional research on dual use ofrectenna sites for terrestrial solar power application, sothat sufficient real estate can be found in all potentiallocations.


Health and Safety Challenges

The following topics are areas of health and safetyresearch that must be investigated before deploymentof a full-scale SSP system (by NASA, other appropri-ate organizations, or industry):

• Perform further research on the effects, espe-cially long-term ones, of low-level microwave and la-ser radiation on Earth biota (humans, other animals,and plants).

• Design the SSP system so that use as a weaponis impossible under any circumstance.

• Establish public awareness and education out-reach programs covering the benefits of SSP, thetechnology behind it, and the built-in safeguards. Suchprograms might include development of demonstrationmodels (Pignolet, 2001), a children’s book on SSP, orop-ed pieces on space solar power.

Recommendations

Recommendation 3-10-1: The SSP program shouldexpand its environmental, health, and safety teamin order to review SSP design standards (beam in-tensity, launch guidelines, and end-of-life policies);assess possible environmental, health, and safetyhazards of the design; identify research if these haz-ards are not fully understood; and consider legaland global issues of SSP (spectrum allocation, or-bital space, etc.). One approach would be to involvean international organization such as the Interna-tional Astronautical Federation Space Power Com-mittee in such studies.

Recommendation 3-10-2: Public awareness andeducation outreach should be initiated during theearliest phases of an SSP program to gain publicacceptance and enthusiasm and to ensure ongoingsupport through program completion.

Recommendation 3-10-3: The SSP program shouldcollaborate more effectively with other NASA pro-grams in space-related environmental, health, andsafety and with external industry and governmentagencies currently performing research related tosuch issues. Funding should be increased to covergaps in research, specifically in areas that overlapSSP-related technology. NASA should initiate re-search that evaluates the effects of microwave and

laser wireless power transmission at levels plannedto be utilized by a full-scale SSP system.

Recommendation 3-10-4: Studies should be initiatedto determine the effects of the electromagnetic in-compatibility of wireless power transmission sys-tems and avionics, terrestrial wireless communica-tion systems, and SSP-related electronics.


Research is currently being conducted at NASAand DOD on possible strategies to address orbital de-bris in both LEO and GEO (NSTC, 1995). These strat-egies include radar tracking and de-orbit stabilization,among others. Programs are also in place in fail-safebeam pointing technology and wireless power trans-mission. The Federal Communications Commissionwill be concerned about interference of radio frequencybeams with current and planned communications sys-tems and issues of spectrum allocation. Before deploy-ment of a full-scale system, international issues in spec-trum allocation, orbital space allocation, and orbitaldebris evolution must also be considered.

Research is being conducted by the wireless com-munications industry, the federal government, andother interested parties to settle questions aboutwhether microwave transmission can be harmful tohumans. The Electric Power Research Institute cur-rently has a substantial multidisciplinary program thatevaluates all aspects of health, risk, and field manage-ment research in respect to electricity and power.9 Thisprogram includes efforts in epidemiology, toxicology,engineering, and other sciences. The National Instituteof Environmental Health and Safety convened a work-ing group on the effects of electromagnetic waves andcurrently has in place a program of public informationand dissemination on the effects of electric and mag-netic fields (NIEHS, 1999). Although frequencies andbeam strength are different, inclusion of representativesfrom these organizations in any future environmentalworking groups would be beneficial. The SSP programshould also capitalize on expert knowledge through

9More information on the Electric Power Research Institute’sresearch in environmental, health, and safety issues of electricpower is available online at <http://www.epri.com/>. Accessed onAugust 15, 2001.


work with the Committee on Space Research estab-lished by the International Council for Science—inparticular, its work on environmental, health, and safetyissues. The group should also continue its work withthe International Astronautical Federation Space PowerCommittee.

3-11 PLATFORM SYSTEMS


This subsystem of the SSP system consists of allthe additional systems that are needed to construct acomplete, integrated spacecraft, including attitude con-trol sensors (Sun sensors, star trackers, and gyros),flight computers, the telemetry and data handling sys-tem, the command system, communications, and so on.


All of the above-mentioned subsystems currentlyexist on most spacecraft. The state of the art is ad-vanced, and further advances will be made indepen-dent of the SSP application.


The aforementioned subsystems make up a negli-gible fraction of the mass and cost of the SSP system.Therefore, economic competitiveness is not affected bythese subsystems. Autonomous robots will place thehighest demand on space computing.


There are many improvements possible in varioussatellite subsystems technologies. However, any ad-vancements will be driven by many other applications.


Recommendation 3-11-1: The SSP program shouldnot expend any resources to develop advanced tech-nologies for standard satellite subsystems (e.g., atti-tude control sensors, flight computers, telemetryand data handling systems, and communicationssystems).


There are many other programs in NASA and DODto improve the performance of these subsystems withwhich the SSP program could collaborate.

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Kolata, G. 2001. “Tuning In to the Microwave Frequency.” New York Times,January 16. Available online at <http://www.nytimes.com/learning/stu-dents/pop/010117wodwednesday.html>. Accessed on August 16.

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Appendixes

65

A

Statement of Task

The Aeronautics and Space Engineering Board(ASEB) will assess the technology investment strategyof the “Solar Power from Space” Program to determineits technical soundness and contribution to the roadmapthat NASA has developed for this program. The ASEBwill assemble a committee with expert knowledge insolar power and associated technologies to conduct anindependent technical assessment that will embody thefollowing:

1. Critique the overall technology investment strat-egy for the Solar Power from Space Program in termsof the plan’s likely effectiveness in meeting theprogram’s technical and economic objectives.

2. Identify areas of highest technology investmentnecessary to create a competitive space-based electricpower system.

3. Identify, where possible, opportunities for in-creased synergy with other research and technologyefforts, including the application of these technologiesto commercial programs or programs associated withNASA’s science and exploration enterprises.

4. Provide, where possible, an independent assess-ment of the adequacy of available resources for achiev-ing the plan’s technology milestones.

5. Recommend changes in the technology invest-ment strategy, as appropriate. In particular, identifygaps or omissions in the program’s technology invest-ment strategy that must be filled, if NASA is to field afull-scale system.

The ASEB will draw upon other elements of theNRC, as appropriate, in conducting this study. A finalreport will be issued at the end of the study.

Richard J. Schwartz (Chair) has been dean of theSchools of Engineering at Purdue University since July1995. He has been on the faculty at Purdue since 1964and has served as a consultant to a number of corpora-tions, both large and small. Dr. Schwartz served as thechairman of the Science and Technology AdvisoryCommittee for the Department of Energy’s NationalRenewable Energy Laboratory. He serves on the Ad-visory Committee for the National Center for Photo-voltaics and has served on the Board of Directors of theNational Electrical Engineering Department HeadsAssociation and on the International Committee for theEuropean Union’s Photovoltaic Solar Energy Confer-ence. He has served as general chairman of the 23rdInstitute of Electrical and Electronic Engineers (IEEE)Photovoltaic Specialists Conference and as a memberof the International Committee for the World Confer-ence on Photovoltaic Energy Conversion. In 1987, Dr.Schwartz was named a fellow of the IEEE for his re-search work on the analysis, design, and developmentof high-intensity silicon solar cells. In 1998, he re-ceived the IEEE William Cherry Award for his contri-butions to the field of photovoltaics. He received theB.S.E.E. from the University of Wisconsin-Madison in1957 and the S.M.E.E. and Sc.D. degrees from theMassachusetts Institute of Technology in 1959 and1962, respectively. While a graduate student, he wasone of eight founders of Energy Conversion, Inc., amanufacturer of thermoelectric materials, devices, and

systems. He served as vice president of engineering atEnergy Conversion, Inc., where he developed newtechniques for the growth of single-crystal quaternarythermoelectric materials and high-performance ther-moelectric heat pump modules.

Mary L. Bowden is currently visiting professor in thedepartment of Aerospace Engineering at the Univer-sity of Maryland, affiliated with the Space SystemsLaboratory. Her research interests include assembly ofstructures in extravehicular assembly, large spacestructures, and the dynamics of space structures. Shehas been employed in the area of solar array design andmaterial selection by the Able Engineering Company(AEC). While employed by AEC, Dr. Bowden workedin design and test support analysis for deployable struc-tures and other space mechanisms. She has alsoworked for the American Rocket Company and Ameri-can Composite Technology in the areas of dynamicstructural model development and smart structures. Dr.Bowden graduated with Sc.D. and M.S. degrees fromthe Massachusetts Institute of Technology (MIT) and aB.A. from Cornell University. She was named SpaceEducator of the Year in 1995 by the Western SpaceportTechnological and Educational Council and awarded aNational Aeronautics and Space Administration(NASA) Group Achievement Award for the Experi-mental Assembly of Structures in EVA (EASE) FlightExperiment. Dr. Bowden was awarded a Zonta Amelia

66

B

Biographical Sketches of Committee Members

APPENDIX B 67

Earhart Fellowship and a DuPont fellowship during hertenure at MIT.

Hubert P. Davis has been an independent consultantsince 1985 performing systems engineering and inte-gration studies. His clients have included the NASAJohnson Space Center, the Large Scale Programs Insti-tute, the University of Texas, United Technologies, theBoeing Company, Rocketdyne, NASA Langley Re-search Center, and the Lawrence Livermore NationalLaboratories. In 1980, Mr. Davis founded Eagle Engi-neering, Inc., in Houston, Texas, a consulting companycoupling the experience of Apollo Program leaderswith outstanding recent graduates. Throughout the1970s, Mr. Davis managed Future Programs for theNASA Johnson Space Center, where he developed theInertial Upper Stage and solid rocket concepts and es-tablished the early NASA studies of the space solarpower satellite concept. Throughout the 1960s, Mr.Davis had a lead engineering role in the design anddevelopment of power and propulsion systems for theApollo Lunar Landing program. Mr. Davis currentlymaintains a leadership role in the development of spacesolar power system concepts.

Richard L. Kline is president of Klintech, a technicalconsulting company. He is also president and chiefexecutive officer of United Satellite Launch Services,a project to convert Russian missiles to provide scien-tific research and commercial satellite launch services.Mr. Kline was employed by the Grumman Corporationfrom 1956 until retiring in 1991. He served as vicepresident and deputy director, Grumman Space StationProgram Support Division. Previously he served asprogram vice president for civil systems and ledGrumman’s work in space solar power station conceptdesign. He also initiated and led Grumman’s partici-pation in space commercialization. Mr. Kline was em-ployed at NASA, Washington, D.C., from 1992 until1997 in a number of positions, including directing theInteragency National Facilities Study. He was com-mended by the Vice President for his contributions toreinventing government and received the NASA Ex-ceptional Achievement Medal for his leadership. In1997, he joined ANSER as vice president, internationalactivities, and led ANSER’s work to promote mutuallybeneficial scientific and commercial international part-nerships in space, primarily with Russia. Mr. Klinehas been elected fellow of the American Institute of

Aeronautics and Astronautics (AIAA), the AmericanAstronomical Society, the British Royal AeronauticalSociety, the British Interplanetary Society, the Ameri-can Society of Mechanical Engineers, and the Societyof Automotive Engineers. He is a licensed professionalengineer in New York and Virginia. Mr. Kline is anaffiliate professor at George Mason University and is amember of its School for Computational Science Ad-visory Board. He is a co-chair of the International As-tronautical Federation’s World Space Congress 2002Technical Program Committee. Mr. Kline receivedAIAA’s von Braun Space Management Medal and waselected to the International Academy of Astronautics.

Molly K. Macauley is a senior fellow with Resourcesfor the Future (RFF), Washington, D.C. She has beenDirector of Academic Programs at RFF since 1996.Since 1983, Dr. Macauley’s research at RFF has in-cluded the areas of public finance, energy economics,regulation of toxic substances, environmental econom-ics, advanced materials economics, the value of infor-mation, and economics and policy issues of outer space.Dr. Macauley’s space research includes the valuationof nonpriced space resources, the design of incentivearrangements to improve space resource use, and theappropriate relationship between public and privateendeavors in space research, development, and com-mercial enterprise. Dr. Macauley has been a visitingprofessor at Johns Hopkins University, Department ofEconomics, and at Princeton University, WoodrowWilson School of Public Affairs. Dr. Macauley testi-fied before Congress on the Commercial Space Act of1997, the Omnibus Space Commercialization Act of1996, the Space Business Incentives Act of 1996, andspace commercialization. Dr. Macauley has served onmany national-level committees and panels, includingthe congressionally mandated Economic Study ofSpace Solar Power (chair); the National ResearchCouncil’s (NRC’s) Board on Physics and Astronomy,Helium Reserve Committee; the NRC Space StudiesBoard Steering Group on Space Applications and Com-mercialization; and the NRC Space Studies Board TaskForce on Priorities in Space Research. Dr. Macauleyhas published extensively over the past 16 years, withmore than 70 journal articles, books, and chapters ofbooks. Dr. Macauley serves on the Board of Directorsof Women in Aerospace and has served as president ofthe Thomas Jefferson Public Policy Program, Collegeof William and Mary.


Lee D. Peterson is an associate professor of aerospaceengineering sciences at the University of Colorado,Boulder. He has been an associate professor or assis-tant professor at the University of Colorado since 1991.Dr. Peterson is also director of the McDonnell-Dou-glas Aerospace Structural Dynamics and Control Labo-ratory and is a member of the multidisciplinary Centerfor Aerospace Structures. His principal area of researchis in high-precision deployable spacecraft structures foruse in optical telescopes and interferometers. His re-search group has experimentally characterized andmodeled a new class of nonlinear mechanics that limitsthe stability of such space structures at nanometer lev-els of motion. He has also made research contributionsin experimental structural dynamics, system identifica-tion, parameter identification joint modeling, and ac-tive structural control. Dr. Peterson is also activelyinvolved in the University of Colorado’s new under-graduate aerospace curriculum and served as the tech-nical director of the Integrated Teaching and LearningLaboratory from 1995 to 1997. From 1989 to 1991, Dr.Peterson was assistant professor of Aeronautics andAstronautics at Purdue University. From 1987 to 1989,he was a member of the technical staff at Sandia Na-tional Laboratories, Albuquerque, New Mexico.

Kitt C. Reinhardt is an electrical engineer conductingphotovoltaic device research and development in theSpace Vehicles Directorate of the U.S. Air ForceResearch Laboratory, Albuquerque, New Mexico. Dr.Reinhardt was the Air Force nominee and the year 2000winner of the Rotary National Award for SpaceAchievement, an early career award based on Dr.Reinhardt’s pioneering work in the development ofhigh-efficiency, multijunction solar cells as well asultralightweight flexible thin-film photovoltaics fornext-generation space systems. Dr. Reinhardt led thesuccessful development and commercialization of thefirst 25 percent-efficient space solar cell, as well as theinvention and current development of the first 30-35percent efficient space solar cell. In addition, he hasbeen instrumental in several revolutionary areas, suchas thin-film photovoltaics and advanced thermal-to-electric conversion. Most recently, Dr. Reinhardt, to-gether with Hong Hou from Sandia National Laborato-

ries, invented an entirely new approach capable ofachieving 35-40 percent solar-to-electric conversionwith a four-junction solar cell design. A patent for thedevice was granted in August 1999.

R. Rhoads (Rody) Stephenson retired from the JetPropulsion Laboratory (JPL) in 1998, where he hadbeen deputy director of the JPL technology programsince 1991 and acting director since 1995. The tech-nology program included all of JPL’s technology de-velopment efforts, including robotics and its spacepower work. In this capacity, Dr. Stephenson was in-volved in many studies of space power beaming toEarth. He also worked, in conjunction with LangleyResearch Center, on large space structures and the JPLprogram on control-structures interaction, providing atechnology base for the space interferometer project.Between 1981 and 1991, Dr. Stephenson was managerof the Electronics and Control Division at JPL. Thedivision included the power section, which had respon-sibility for all forms of space power, including solarpower, and it participated in solar cell development andtesting and in the solar power beam transmission stud-ies of that period. Most recently in his 36-year careerat JPL, the laboratory turned to Dr. Stephenson to serveas a member of the Galileo and Cassini Review Boards,to chair the Mars Pathfinder Board, and to lead the in-ternal failure Review Board for the Mars Observer mis-sion.

Dava Newman, Aeronautics and Space EngineeringBoard liaison to the Committee for the Assessment ofNASA’s Space Solar Power Technical InvestmentStrategy, is an associate professor of aeronautics andastronautics at the Massachusetts Institute of Technol-ogy and a MacVicar faculty fellow. She conductsmultidisciplinary efforts combining aerospace bioengi-neering, human-in-the-loop dynamics and control mod-eling, biomechanics, human interface technology, lifesciences, and systems analysis and design. Dr. Newmanserved as a member of the NRC Committee on Ad-vanced Technology for Human Support in Space andthe Committee on Engineering Challenges to the Long-Term Operation of the International Space Station.

69

C

Example of NASA’s SERT Program Technology Roadmaps

Figures C-1, C-2, and C-3 are examples of theroadmaps and programmatic charts for each individualtechnology area in NASA’s SERT program. The fig-ures are presented in original, unedited form.

REFERENCEMankins, John and Joe Howell. 2000. “Strategic Research and Technology

Road Map.” Briefing by John Mankins and Joe Howell, National Aero-nautics and Space Administration, to the Committee for the Assessmentof NASA’s Space Solar Power Investment Strategy, National ResearchCouncil, Washington, D.C., December 14.

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73

D

Brief Overview of NASA’s Space Solar Power Program

BASELINE SSP SYSTEMS

A space solar power system requires integration ofmany technologies in order to generate electricity fromthe Sun. Figure D-1 depicts a generic solar power sys-tem, from collection of the solar power to receipt of thesolar power on Earth and delivery to the grid. This con-cept is based on the use of microwaves. Options usinglasers would involve constellations of small individualsatellites, each with its own transmitter, as depicted inFigure D-2. The National Aeronautics and SpaceAdministration’s (NASA’s) Space Solar Power (SSP)Exploratory Research and Technology (SERT) pro-gram, as of the date of this report, has not yet chosen abaseline system. Several possible variations of flightdemonstrations and systems have been presented to thecommittee, each classified according to four model sys-tem categories (MSCs). Refer to Section 2-1 for a moredetailed description of these demonstrations and pro-gram milestones.

Despite the differences in these concepts, all spacesolar power systems have a set of common technologyareas and work in the same general manner. Solar en-ergy is collected in geosynchronous Earth orbit (GEO)by a solar power generation technology, probably con-sisting of photovoltaic (PV) arrays that capture radia-tion from the Sun and convert it (using the photovoltaicprocess) into direct electric current. These PV arraysblanket a surface that faces the Sun at all times. The

electric current is collected and transformed throughthe power management and distribution system. Trans-mitters then beam the power via wireless power trans-mission to a specific collector (either on Earth’s sur-face or in space). Receivers (on Earth’s surface or inspace) collect the incoming microwave or laser trans-mission energy and convert it into electricity. For mi-crowave systems, this collector is referred to as arectenna. For laser-based transmission, the collector isconstructed from solar arrays. For space-to-space sys-tems, the collector is application specific. The con-struction of such SSP systems, each on the order ofseveral square kilometers in size, is handled almostentirely through autonomous robotic assembly, inspec-tion, and maintenance in GEO and requires numerouslaunches of heavy payloads into space. In-space trans-portation of SSP components is also required to movepayloads from low Earth orbit to GEO. Various riskmanagement and systems design tools also need to bedeveloped during the design stages of any SSP system.

OVERVIEW OF NASA’S SPACE SOLAR POWER(SSP) EXPLORATORY RESEARCH ANDTECHNOLOGY (SERT) PROGRAM

NASA’s SERT program mainly involves researchon technologies and design methods that is necessaryfor such a huge undertaking. The program has identi-


FIGURE D-1 Generic space solar power system. SOURCE: Adapted in part from Nansen, 2000.

Microwave Beam Laser Beams

FIGURE D-2 Generic microwave and laser SSP systems. SOURCE: Adapted in part from Dickinson, 2000.

APPENDIX D 75

fied several flight demonstration milestones in order totest technologies and concepts in the near-term andmid-term in preparation for transferring the technolo-gies to industry for final full-scale development andimplementation. A more specific treatment of theseflight demonstrations and key program milestones canbe found in Section 2-1.

NASA has chosen to break its research into 12 ar-eas for funding:

1. Systems integration, analysis, and management 2. Solar power generation 3. Wireless power transmission 4. Space power management and distribution 5. Structural concepts, materials, and controls 6. Thermal management and materials 7. Space assembly, inspection, and maintenance 8. Platform systems 9. Ground power systems (GPS)10. Space transportation (Earth-to-orbit and

in-space)11. Environmental, health, and safety12. Economic analysis

Each area (with the exception of economic analysis)has been allocated a portion of the earmarked govern-ment funding provided to the SERT program for tech-nology roadmap development and prioritization andwas charged with (1) developing a set of cost and tech-nology goals, (2) compiling a list of important technol-ogy challenges, (3) developing potential applicationsof technology advancements, (4) developing a break-down of the specific work necessary for advancement,and (5) developing a schedule of technology milestonesthat parallel the milestones of the total program. Anexample of these roadmaps and goals for the solarpower generation portion of the program can be foundin Appendix C. The program has identified an invest-ment portfolio for a future SSP program with plannedresource allocation through 2016 (see Table D-1). Thisallocation will be affected by choices made by NASAand the President’s Office of Management and Budgetin space solar power. Technology flight demonstra-tions (referred to by NASA as MSCs) are scheduled inFY 2006-2007, FY 2011-2012, and FY 2016.

The SERT program has several levels of organiza-tion stemming from management at the NASA Officeof Space Flight. A schematic of this organizationalstructure, which incorporates many NASA field cen-ters as well as industry and academia, is shown in Chap-

ter 3, Figure 3-1. The program has created several lev-els of oversight through its Senior Management Over-sight Committee and various technical and systemsworking groups. The program has also obtained vari-ous external evaluations from groups such as the Na-tional Research Council; Resources for the Future, aneconomic research group; and professional technicalsocieties such as the American Institute of Aeronauticsand Astronautics. External comment has also been pro-vided through involvement in various international or-ganizations and symposiums such as the InternationalForum on Space Solar Power.

REFERENCESDickinson, Richard. 2000. “Wireless Power Transmission.” Briefing by

Richard Dickinson, Jet Propulsion Laboratory, to the Committee for theAssessment of NASA’s Space Solar Power Investment Strategy, Na-tional Academy of Sciences, Washington, D.C., September 13.

Mankins, John and Joe Howell. 2000. “Strategic Research and TechnologyRoadmap.” Briefing by John Mankins and Joe Howell, National Aero-nautics and Space Administration, to the Committee for the Assessmentof NASA’s Space Solar Power Investment Strategy, National Academyof Sciences, Washington, D.C., December 14.

Nansen, Ralph. 2000. “The Space Solar Power Solution: An Industry/Gov-ernment Partnership.” Briefing by Ralph Nansen, Solar Space Indus-tries, to the Committee for the Assessment of NASA’s Space SolarPower Investment Strategy, National Academy of Sciences, Washing-ton, D.C., October 23.

76T

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77

E

Participants in Committee Meetings

The full committee met four times between Sep-tember 2000 and March 2001. Outside participants arelisted below, grouped by organization:

Auburn UniversityHenry Brandhorst

Canadian Space AgencyBryan Erb

Carnegie Mellon UniversitySarjoun SkaffPeter Steritz

Dow Jones NewswiresBryan Lee

Futron CorporationCarie Mullins

Georgia Institute of TechnologyJohn Olds

NASA Ames Research CenterCharles NeveuHans Thomas

NASA Glenn Research CenterSheila BaileyJames DolceJames DudenhoeferLee MasonBarbara McKissockRichard Shaltens

NASA HeadquartersJohn Mankins

NASA Jet Propulsion LaboratoryRichard DickinsonBrad KennedyNeville MarzwellDavid Maynard

NASA Johnson Space CenterChristopher Culbert

NASA Langley Research CenterChris Moore

NASA Marshall Space Flight CenterJeffrey AndersonConnie CarringtonDaniel DavisJoe Howell


President’s Office of Management and BudgetBrant Sponberg

Pacific Northwest National LaboratoryJames Dooley

Science Applications International CorporationHarvey Feingold

Solar Space IndustriesRalph Nansen

Strategic Insight, Ltd.John Fini

Sunsat Energy CouncilFrederick Koomanoff

79

F

Acronyms and Abbreviations

AC alternating currentAFRL Air Force Research LaboratoryAIAA American Institute of Aeronautics and

Astronauticsa-Si amorphous siliconAST Advanced Space Transportation

(program)

B billion

CIGS copper indium gallium diselenidecm centimeterCNES Centre National d’Études Spatiales

(French Space Agency)

DC direct currentDOD Department of DefenseDOE Department of Energy

EH&S environmental health and safetyEOL end of lifeEPRI Electric Power Research InstituteESA European Space AgencyETI Earth-to-orbit transportation and

infrastructureETO Earth to orbit

ft foot

g gravity level at Earth’s surfaceGEO geosynchronous Earth orbitGHz gigahertzGPS ground power systemGW gigawatt

HEDS Human Exploration and Development ofSpace (NASA)

in. inchIS intelligent systemISS International Space StationISTI in-space transportation and infrastructureITAM integrated technology analysis

methodology

kg kilogramkm kilometerkW kilowattkW-hr kilowatt-hour

LEO low Earth orbit

M millionm2 square meterm3 cubic meterMBG muti-band gapMHD magnetohydrodynamics


µm micrometerMSC model system categorymW milli-wattMW megawatt

NASA National Aeronautics and SpaceAdministration

NRC National Research CouncilNREL National Renewable Energy LaboratoryNRO National Reconnaissance Office

OTA Office of Technology Assessment

PMAD power management and distributionPS platform systemsPV photovoltaicPVMat PV Manufacturing Technology

R&D research and developmentR&T research and technologyRMS Remote Manipulator System

SAIM space assembly, inspection, andmaintenance

SCMC structural concepts, materials, andcontrols

SERT SSP Exploratory Research andTechnology (program)

SIM systems integration and managementSLA stretched lens arraySLI Space Launch InitiativeSMOC Senior Management Oversight CommitteeSPG solar power generationSPMAD space power management and distributionSPS solar power satelliteSSP space solar power

TFD technology flight demonstrationTMM thermal materials and managementTRL technology readiness levelTSX Tele-robotic Shuttle Experiment

USAF U.S. Air Force

W wattWPT wireless power transmission

YAG yttrium aluminum garnetyr year

Laying the Foundation for Space Solar Power (Compass series)

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