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NASA’s New Emphasis on In-Space PropulsionTechnology Research

Les JohnsonAdvanced Space Transportation Program/TD15

NASA Marshall Space Flight CenterMarshall Space Flight Center, Alabama 35812 USA

Phone: 256–544–0614E-Mail: Les.Johnson@msfc.nasa.gov

IEPC-01-001

ABSTRACT

NASA’s Advanced Space Transportation Program (ASTP) is investing in technologies to achievea factor of 10 reduction in the cost of Earth orbital transportation and a factor of 2 reductionin propulsion system mass and travel time for planetary missions within the next 15 yr. Sincemore than 70% of projected launches over the next 10 yr will require propulsion systemscapable of attaining destinations beyond low-Earth orbit (LEO), investment in in-space tech-nologies will benefit a large percentage of future missions.

The ASTP technology portfolio includes many advanced propulsion systems. From the next-generation ion propulsion system operating in the 5–10 kW range to fission-poweredmultikilowatt systems, substantial advances in spacecraft propulsion performance are antici-pated. Some of the most promising technologies for achieving these goals use the environmentof space itself for energy and propulsion and are generically called “propellantless,” becausethey do not require onboard fuel to achieve thrust. An overview of state-of-the-art space pro-pulsion technologies, such as solar and plasma sails, electrodynamic and momentum transfertethers, and aeroassist and aerocapture, will also be described. Results of recent Earth-basedtechnology demonstrations and space tests for many of these new propulsion technologies willbe discussed.

THE LIMITS OF CHEMICAL PROPULSION

A vigorous and robust space science and explorationprogram will require a new generation of propulsionsystems. Chemical propulsion, which relies on makingchemical bonds to release energy and produce rocketexhaust, has been the workhorse of space explorationsince its beginning. However, we have reached itsperformance limits and those limits are now hindering

our continued exploration of space. The efficiency withwhich a chemical rocket uses its fuel to produce thrust,specific impulse (Isp), is limited to several hundredseconds or less. In order to attain the high speeds re-quired to reach outer planetary bodies, much less ren-dezvous with them, will require propulsion systemefficiencies well over 1,000 sec. Chemical propulsionsystems cannot meet this requirement.

* Presented as Paper IEPC-01-001 at the 27th International Electric Propulsion Conference, Pasadena, CA, 15-19 October, 2001.† This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

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ELECTRIC PROPULSION

An electric propulsion system uses electrical energy toenergize the propellant to much higher exhaust veloci-ties (Ve) than those available from chemical reactions.Ion propulsion is an electric propulsion technology thatuses ionized gas as propellant. Ionized xenon gas is elec-trostatically accelerated to a speed of ≈30 km/sec andprovides the “exhaust” for the propulsion system. Ionpropulsion is being used by commercial telecommuni-cation satellites and has been demonstrated as a primaryspacecraft propulsion system by the NSTAR demonstra-tion on the Deep Space 1 mission.

Electric propulsion thrusters can be divided into threebroad categories: (1) Electrothermal thrusters use elec-tric energy to simply heat the propellant, (2) electrostaticthrusters use charge potential differences to acceleratepropellant ions, and (3) electromagnetic thrusters useelectromagnetic forces (J × B) to accelerate a propellantplasma.

NASA is pursuing technologies to increase the perfor-mance of electrostatic thrusters by going to higher powerlevels and by increasing the Isp on a system level. Figure1 illustrates the mission benefit of using electric propul-sion to increase the payload mass fraction.

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Figure 1 – Electric propulsion systems provide up to10 times the payload capacity of chemicalrockets to the same destination.

FISSION PROPULSION

A fission reactor in space can be used for propulsion intwo ways. The energy created by the fission reaction canbe used to heat a propellant to extremely high tempera-tures, thus increasing its exhaust velocity and Isp. Alter-natively, fission energy can be converted to electricityand used to power an electric propulsion system. Thefirst space fission system is likely to use the latter ap-proach for propelling a series of robotic spacecraft tothe outer planets and beyond. Figure 2 shows the rela-tive benefits of nuclear propulsion for human and ro-botic exploration missions of interest to NASA.

A nuclear electric propulsion system for a Kuiper Beltexploration mission might use a 100–200 kWe nuclearreactor, launched “cold”—where only zero power test-ing has been conducted. The reactor would be activat-ed at a positive C3 (beyond Earth escape) to power akrypton-fueled ion propulsion system. The propulsionsystem would carry science payload on an indirect tra-jectory (heliocentric spiral trajectory), building up to finalvelocity of ≈25 AU/yr after a 10-yr run time. After en-gine burnout, the science payload would be deployed.

The first step toward using advanced fission propulsionsystems is development of a safe, affordable fission sys-tem that can enhance or enable near-term missions ofinterest. To this objective, NASA is defining a safe, af-fordable fission engine (SAFE) test series, designed todemonstrate a 300-kW flight configuration system us-ing non-nuclear testing. The SAFE–30 test series is afull-core test capable of producing 30 kW using resis-tance heating to simulate the heat of fission. The 30-kWcore consists of 48 stainless steel tubes and 12 stainlesssteel/sodium heat pipes welded together longitudinallyto formulate a core similar to that of a fission flight sys-tem. Heat is removed from the core via the 12 heat pipes,closely simulating the operation of an actual system.

PROPELLANTLESS PROPULSION

Conventional space propulsion relies on the transfer ofmomentum from propellant to spacecraft, with the mo-mentum of the system remaining unchanged. For ex-ample, a large-mass spacecraft using chemical propulsionwill experience a small velocity change through the

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Figure 2 – Nuclear electric propulsion enables a new class of space missions.

exhaust of a small mass having a large velocity. A rocket,therefore, exchanges momentum with the propellant,striving to reduce propellant consumption by increasingthe exhaust velocity of the propellant. A rocket can ex-pel hydrogen, water vapor, antimatter annihilation prod-ucts, etc.; the principle is still the same.

A “propellantless” propulsion system simply uses adifferent form of momentum exchange to produce thrust,usually through interaction with the natural space envi-ronment. Solar sails, plasma sails, aerocapture, andtethers are examples of propellantless propulsion technol-ogies being investigated.

SOLAR SAILS

A solar sail is a propulsion concept which makes use ofa flat surface of very thin reflective material supportedby a lightweight deployable structure. Solar sails accel-erate under the pressure from solar radiation (essentiallya momentum transfer from reflected solar photons), thusrequiring no propellant. Since a solar sail uses no pro-pellant, it has an effectively infinite Isp; however, thethrust-to-weight ratio is very low, typically between10–4 to 10–5 for the 9 N/km2 solar pressure at Earth’sdistance from the Sun.

In the near term, deployable sails will be fabricated frommaterials such as Dupont Mylar or Kapton coatedwith ≈500 Å of aluminum. The thinnest available Kaptonfilms are 7.6 µ in thickness and have an areal density of≈11 g/m2. Sails thinner than this, made from conven-tional materials, have the potential to rip or tear in thedeployment process. Recent breakthroughs in compos-ite materials and carbon-fiber structures may make sailsof areal density <1 g/m2 a possibility. The reduced sailmass achieved this way may allow much greater accel-eration, greater payload carrying capability, and reducedtrip time.

PLASMA SAILS

A novel new approach to spacecraft propulsion using avirtual sail composed of low-energy plasma might har-ness the energy of the solar wind to propel a spacecraftanywhere in the solar system and beyond. Such plasmasails will affect their momentum transfer with the plen-tiful solar wind streaming from the Sun. Plasma sailsuse a plasma chamber attached to a spacecraft as the pri-mary propulsion system. Solar cells and solenoid coilswould power the creation of a dense magnetized plasma,or ionized gas, that would inflate an electromagnetic fieldup to 19 km in radius around the spacecraft. In the

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future, fission power could be used. The field would in-teract with and be dragged by the solar wind. Creatingthis virtual sail will be analogous to raising a giant physi-cal sail and harnessing the solar wind, which moves atspeeds >1 M km/hr.

Tests of the plasma sail concept are ongoing at MarshallSpace Flight Center (MSFC) and the University of Wash-ington. Thrust measurements, using a Hall thruster tosimulate the solar wind, are planned in 2002–2003.

AEROCAPTURE

Aerocapture relies on the exchange of momentum witha planetary atmosphere to achieve thrust, in this case, adecelerating thrust leading to orbit capture. Aerocapturehas not yet been demonstrated, though it is very similarto the flight-proven technique of aerobraking, with thedistinction that aerocapture is employed to reduce thevelocity of a spacecraft flying by a planet so as to placethe spacecraft into orbit about the planet. This techniqueis very attractive for planetary orbiters since it permitsspacecraft to be launched from Earth at high speed, pro-viding a short trip time, and then reduce the speed byaerodynamic drag at the target planet. Withoutaerocapture, a large propulsion system would be neededon the spacecraft to perform the same reduction of ve-locity, thus reducing the amount of delivered payload,increasing the size of the launch vehicle (to carry theadditional fuel required for planetary capture), or sim-ply making the mission impossible due to the tremen-dous propulsion requirements. Figure 3 shows thepropulsion system mass savings that are possible withan aerocapture system.

The aerocapture maneuver begins with a shallow ap-proach angle to the planet, followed by a descent to rela-tively dense layers of the atmosphere. Once most of theneeded deceleration is reached, the vehicle maneuversto exit the atmosphere. To account for the inaccuraciesof the atmospheric entering conditions and for the atmo-spheric uncertainties, the vehicle needs to have guidanceand control as well as maneuvering capabilities. Giventhe communication time delay resulting from the mis-sion distances from Earth, the entire operation requiresthe vehicle to operate autonomously while in the planet’satmosphere.

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Figure 3 – Aerocapture dramatically reduces thepropulsive requirements for planetarycapture maneuvers.

ELECTRODYNAMIC TETHERS

A predominantly uninsulated (bare wire) conductingtether, terminated at one end by a plasma contactor, canbe used as an electromagnetic thruster. A propulsive forceof F = IL × B is generated on a spacecraft/tether systemwhen a current, I, from an onboard power supply is fedinto a tether of length, L, against the electromagneticforce induced in it by the geomagnetic field, B. This con-cept will work near any planet with a magnetosphere(Earth, Jupiter, etc.) by exchanging momentum with thatplanets’ rotational angular momentum. This was dem-onstrated in Earth orbit by the Tethered Satellite SystemReflight (TSS–1R) mission; the orbiter experienced a0.4-N electrodynamic drag thrust during tether opera-tion. No instrumentation was flown to actually measurethis thrust; it is derived from the physics of the electro-dynamic interaction.

An electrodynamic tether upper stage could be used asan orbital tug to move payloads within LEO after inser-tion (Figure 4). The tug would rendezvous with the pay-load and launch vehicle, dock/grapple the payload, andmaneuver it to a new orbital altitude or inclination withinLEO without the use of boost propellant. The tug couldthen lower its orbit to rendezvous with the next payloadand repeat the process. Such a system could conceivably

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Figure 4 – An orbit transfer vehicle propelled by anelectrodynamic tether would be highlyreusable and require no resupply.

perform several orbital maneuvering assignments with-out resupply, making it a low, recurring-cost space asset.The same system can be used to change the orbital incli-nation of a payload as well.

MOMENTUM-EXCHANGE ELECTRODY-NAMIC REBOOST TETHERS

An Earth-orbiting, spinning tether system can be used toboost payloads into higher orbits with a Hohmann-typetransfer. A tether system would be anchored to a rela-tively large mass in LEO, awaiting rendezvous with apayload delivered to orbit. The uplifted payload wouldmeet with the tether facility which then begins a slowspin-up using electrodynamic tethers (for propellantlessoperation) or another low thrust, high Isp thruster. At theproper moment and tether system orientation, the pay-load is released into a transfer orbit, potentially to geo-stationary transfer orbit (GTO) or lunar transfer orbit.

Following spin-up of the tether and satellite system, thepayload is released at the local vertical. The satellite isinjected into a higher orbit with perigee at the releaselocation; the orbital tether platform is injected into a lower

orbit with apogee at the release location. Momentum istransferred to the satellite from the orbiting tether booststation. The satellite then enters a GTO trajectory andaccomplishes the transfer in as little as 5 hr. The plat-form then reboosts to its operational altitude. The sys-tem thus achieves transfer times comparable to a chemicalupper stage with the efficiencies of electric propulsion.The ability of a momentum exchange tether boost sta-tion to reduce launch vehicle size and cost is shown inFigure 5. This type of system could be used to reducelaunch vehicle requirements or to increase injected pay-load mass for any interplanetary mission.

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BIBLIOGRAPHY

Bangham, M.E.; Lorenzini, E.; and Vestal, L.: “Tether Trans-portation System Study,” NASA/TP—1998–206959,NASA Marshall Space Flight Center, Huntsville, AL,1998.

Frisbee, R.: Personal Communication, NASA Jet PropulsionLaboratory, Pasadena, CA, 2000.

Gallagher, D.L.; Johnson, L., et al.: “Electrodynamic TetherPropulsion and Power Generation at Jupiter,” NASA/TP—1998–208475, NASA Marshall Space Flight Center,Huntsville, AL, 1998.

Garner, C.; and Leipold, M.: “Developments and Activities inSolar Sail Propulsion,” 36th AIAA Joint Propulsion Con-ference, JPC–00–0126, Huntsville, AL, 2000.

Gilchrist, B.E.; Johnson, L.; and Bilen, S.: “Space Electrody-namic Tether Propulsion Technology: System Consider-ations and Future Plans,” 35th AIAA Joint PropulsionConference, AIAA–99–2841, Los Angeles, CA, 1999.

Mewaldt, R.A.; and Liewer, P.C.: “An Interstellar Probe Mis-sion to the Boundaries of the Heliosphere and NearbyInterstellar Space,” AIAA SPACE 2000 Conference andExhibit, Long Beach, CA, September 2000.

Sorensen, K.: Personal Communication, NASA MarshallSpace Flight Center, Huntsville, AL, 2001.

Winglee, R.M.: “Laboratory Testing of A Mini-Magneto-spheric Plasma Propulsion (M2P2) Prototype,” SpaceTechnology and Applications International Forum, Albu-querque, NM, February 2001.

Figure 5 – A momentum-exchange electrodynamic reboost tether system could reducelaunch vehicle size and cost for a variety of planetary exploration missions.