Test Facility Development for the 15,000 lb Thrust

Peregrine Hybrid Sounding Rocket

Zachary Dunn∗ , Jonny Dyer† , Kevin Lohner† and Eric Doran∗

Cedric Bayart∗ and Andy Sadhwani∗

Stanford University, Stanford, CA, 94305, USA

Greg Zilliac‡

NASA Ames Research Center, Moffett Field, CA, 94035, USA

Arif Karabeyoglu§

Space Propulsion Group, Inc, Sunnyvale, CA, USA

Brian Cantwell¶

Stanford University, Stanford, CA, 94305, USA

Recent breakthroughs in hybrid rocket technology centered around the development of ahigh regression rate, liquifying fuel are overcoming traditional hybrid rocket performanceshortcomings. Over the past 8 years research efforts at Stanford University and NASAAmes have identified paraffin wax as a high regression rate fuel and characterized itscombustion performance with multiple oxidizers. In order to demonstrate the maturity ofparaffin wax technology, an intermediate-scale sounding rocket named Peregrine is beingbuilt with the objective of reaching an apogee of 100 km. This project, a collaborationbetween NASA Ames, Stanford University and NASA Wallops will begin with a rigorousground test program prior to building the flight vehicle. This paper chronicles the designand early build-up of this test facility which is capable of testing 15,000 lb thrust rocketengines. Sources for more information regarding the Peregrine project are also included.

Nomenclature

Pc Combustion Chamber PressureGTF Ground Test FacilityHCF Hybrid Combustion FacilityOARF Outdoor Aerodynamics Research FacilityGOX Gaseous OxygenP&ID Plumbing and Instrumentation DiagramMAWP Maximum Allowable Working PressureCv Flow CoefficientFEA Finite Element AnalysisHTPB Hydroxyl Terminated PolyButadiene

I. Introduction

Combined research efforts at Stanford University and NASA Ames Research Center have identified paraf-fin wax as a hybrid rocket fuel with highly attractive performance properties.1 Although several small-scaleparaffin fueled hybrid rockets have been built through various collaborative efforts between Stanford, NASA∗Graduate Student, Department of Aeronautics and Astronautics, Stanford, CA, AIAA Member.†Graduate Student, Department of Mechanical Engineering, Stanford, CA, AIAA Member.‡Research Scientist, NASA Ames Research Center, Moffett Field, CA, AIAA Member.§Research Scientist, Space Propulsion Group, Sunnyvale, CA, AIAA Member.¶Professor, Chairman of the Department of Aeronautics and Astronautics, Stanford University, Stanford, CA, Fellow AIAA.

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43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 8 - 11 July 2007, Cincinnati, OH

AIAA 2007-5358

Copyright © 2007 by Zachary Wayne Dunn. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

Ames, and Lockheed Martin,2 a large-scale flight validation of paraffin as a hybrid rocket fuel has not yetbeen attempted. The Peregrine Project was initiated in late 2006 to demonstrate the viability of paraffin asa high-performance hybrid fuel in a flight vehicle designed to reach the edge of space (apogee of 100km).3

Because of its high vapor pressure at ambient conditions, storability, ease of handling, and non-toxicity,nitrous oxide (N2O) was selected as the oxidizer for Peregrine. In order to increase the feed pressure ofN2O to the combustion chamber and to prevent cavitation in the oxidizer plumbing from the run tank tothe combustion chamber, helium will be used to “supercharge” the N2O. This simply entails adding inerthelium to the ullage of the nitrous oxide supply tank; details will be discussed further in the “Plumbing”section of this report.

In order to fully characterize the performance of the Peregrine vehicle prior to flight, a 15,000 lb thrustclass hybrid ground test facility (GTF) is being constructed at NASA Ames. This facility will increasethe vehicle designer’s confidence in their design and it will also fulfill a NASA Wallops requirement for asuccessful ground test program prior to launch. Moreover, it has the potential, through a robust design, tofacilitate future NASA and Stanford propulsion research.

A. Paraffin Background

The use of paraffin wax as a fuel for hybrid rocket motors has been a focus of research at Stanford Universityand NASA Ames because of paraffin’s high burning rate (i.e. high regression rate).1 Note: the termregression rate describes the rate at which fuel recesses outward from a combustion port during a firing.Paraffin wax exhibits a regression rate 3 to 20 times greater than traditional hybrid rocket fuels such asHTPB, polyethylene and acrylic. For more information see Karabeyoglu et al.1 In comparison to thetraditional hybrid fuels the performance boost associated with higher regression rate has a two-fold benefit;first, a higher thrust can be developed for a given fuel grain geometry and second, a greater geometric fuelloading (or vehicle specific volume) can be attained. This allows for smaller, more geometrically dense motorswith fewer combustion ports.

Several studies have been conducted to characterize the regression rate of paraffin in comparison to tra-ditional fuels for a variety of oxidizers.1,4,5 Through these background studies the combustion data requiredto build a large-scale sounding rocket was acquired.

B. Peregrine Project

The Peregrine project is a two year project to design, test, and fly a paraffin hybrid rocket to 100km, theinternationally recognized edge of space. Project management and oversight is provided by NASA Ames.NASA Wallops will serve as the launch facility, and much of the engineering is being done by StanfordUniversity graduate engineering students under the guidance of Ames Research Scientist Greg Zilliac. Acomplete ground test program building on incremental testing will increase confidence in the design of thePeregrine motor before a flight is attempted. Prior to the start of the Peregrine project the core group ofstudent engineers developed their hot fire testing knowledge base with extensive work on a small-scale hybridfuel regression rate testbed.5 Additional background information concerning the Peregrine project can befound in Dyer et al.3

C. Requirements for Ground Test Program

Preliminary trajectory analysis coupled with detailed mechanical design considerations led to the design ofthe Peregrine combustion chamber to be tested at NASA Ames. Owing to budget and time constraints itwas decided to design a single combustion chamber for both the flight and the ground test hardware. Thealuminum chamber is designed to hold just over 150 lbs of paraffin, operate at 700 psig nominal Pc, and willdevelop just under 14,000 lbf at sea level. Nitrous oxide will be supplied to the chamber at 850 psig and arate of 53.4 lbs/s. The outer diameter of the combustion chamber is 14 inches.

The test site design was also created with programatic requirements in mind. As discussed in Dyer,3 anincremental testing plan will be employed to build up to the 18 second flight-duration burn. Two successfulfull duration tests are required to gain launch approval from NASA Wallops.

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II. NASA Ames Site Overview

The site chosen for the Peregrine testing is known as the Hybrid Combustion Facility (HCF) which is partof the larger Outdoor Aerodynamics Research Facility (OARF) at NASA Ames, and much of Peregrine’sGTF will be built or based upon existing HCF hardware. Although the HCF has not been active for severalyears it was maintained in a state such that bringing it to an operational status would require minimal effort.Previous testing at the HCF was in support of paraffin large-scale fuel grain regression rate determination,where paraffin was burned with gaseous oxygen (GOX). This facility was designed to handle 10 inch diameterhardware with GOX delivery rates of up to 35 lbs/s. Onsite cryogenic storage and a 560 gallon, high-pressurestorage tank both remain from this previous round of testing at the HCF, and both will be utilized duringPeregrine ground testing. Additionally, much of the plumbing from the previous testing is still in place, andcan be used for the Peregrine project in many instances. A fully functional underground blockhouse willserve as the command center for testing and will also act to protect personnel during hot firings. Zilliac etal. has written a comprehensive document detailing the HCF buildup and its configuration previous to thePeregrine modifications.6

Figure 1 below depicts the layout of the HCF as it will be used for the Peregrine project. The N2O runtank, Cryo N2O Storage tank and much of the supporting hardware has not been modified.

Figure 1. Peregrine GTF Site Overview.

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III. Plumbing

The plumbing system for the Peregrine GTF can be broken down into three major sub-systems accordingto the working fluid that they handle: N2O, Helium, and shop air. All three systems work in concert tosupply N2O from the high capacity site storage tank to the test run tank and later from the run tank tothe combustion chamber at a nominal flow rate of 53.4 lbs/s. Figures 2 and 3 below will help to guide thereader through the subsequent discussion of the plumbing sub-systems.

Figure 2. Peregrine GTF Plumbing and Instrumentation Diagram Color Coded by Plumbing Line Sizing.

Figures 2 and 3 are Plumbing and Instrumentation Diagrams (P&ID) for the Peregrine GTF; one iscolor-coded for line Maximum Allowable Working Pressure (MAWP) and the other for line sizing. Thesediagrams cover the entire plumbing system used for ground testing and also include valve names/descriptions.Table 1 below outlines the naming convention used in the P&IDs. Note: this section details the PeregrineGTF plumbing; please see the “Instrumentation” section for more information about instrumentation on theP&ID schematics.

As shown in the P&IDs, all trapped gas volumes are protected by both ASME certified pressure relievingdevices and solenoid or manual vent valves. Additionally, all trapped gas volumes have mechanical pressuregages connected to them so that personnel do not have to rely on pressure transducer readings before crackinga fitting. The following three subsections detail the three working fluid subsystems of the Peregrine GTF.

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Figure 3. Peregrine GTF Plumbing and Instrumentation Diagram color coded by MAWP.

A. Nitrous Oxide

The primary objective of the GTF plumbing system is to supply nitrous oxide to the combustion chamberat the appropriate flow rate, temperature and pressure. At the heart of this supply system is the “N2O RunTank” shown in dark blue at the centers of Figures 2 and 3. This 560 gallon capacity tank was fabricated in1963 by Babcock & Wilcox, and was designed to withstand a MAWP of 15,000 psi within its 6 inch thicksteel walls. Its MAWP has since been down-rated, but is still more than sufficient to handle the 1500 psigMAWP required for the Peregrine project. This run tank will be filled prior to a hot-fire or cold-flow test,and will contain enough nitrous for just over three full-duration runs. Its purpose is temporary high pressurenitrous storage, and high flow rate nitrous delivery.

To protect this tank and all components attached to it, a high-flow rate, 2 inch diameter burst disk,BD-1, is close-coupled to the tank. This disk serves as the primary pressure relieving component for thenitrous run tank and has been specified according to ASME section VIII standards. Ultimately an OsecoFAS series disk was selected with a burst pressure of 1425 psig ± 5% manufacturers tolerance. At the upperend of the tolerance this disk will rupture at 1496 psig which supports the tank MAWP of 1500 psig.

The burst disk is a final defense against a pressure run-away in the main tank. To protect the line andthe burst disk during normal operation, a Circle Seal relief valve, OV-12, is set to a cracking pressure of1000 psi just upstream of BD-1.

A long term supply of liquid nitrous oxide is held at refrigerated temperatures within the “N2O Store(refrig)” tank depicted in green in Figures 2 and 3. This cryogenic dewar, previously used for liquid oxygen

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Table 1. Valve Naming Convention

Designator Description TypeOV-x Oxidizer Valve Solenoid or BallHV-x Helium Valve Solenoid or BallPV-x Pneumatic Valve SolenoidCV-x Check Valve Check

R-x Regulator RegulatorBD-x Burst Disk Burst Disk

storage, has a 3,000 gallon capacity, and a single cryogenic nitrous delivery should be enough for the entiretest series. This tank has a MAWP of 250 psig and internal pressure relieving and venting valves. Whilereferred to as “cyrogenic,” N2O is more accurately described as a refrigerated liquid or “light cryogen” as itwill be delivered and stored at roughly −35◦F .

Downstream of the cryogenic dewar one finds a filter followed by a cryogenic pump and an ambient airheat exchanger or vaporizer. The pump serves to draw the lightly cryogenic N2O from the dewar and thento pressurize it from 250 psi to the tank storage pressure of about 850 psig. The pressurized nitrous is thenpumped through the vaporizer which is used to increase the temperature of the liquid from -35◦F to justbelow ambient. Note: in this application the vaporizer does not actually vaporize the nitrous, but ratherincreases the temperature of the liquid. From the vaporizer, the ball valve OV-1 is used to control the flowof pressurized, warmed nitrous to the run tank for storage prior to a test.

Finally, nitrous oxide must be delivered from the run tank to the combustion chamber at 53.4 lbs/s. Thisis accomplished using both 2 inch piping and tubing. Working from the run tank downstream, OV-7 tapsoff of a bleed ring between two RTJ flanges and allows liquid nitrous to flow through a 1

4 inch bypass tube.By allowing nitrous to bypass OV-2 prior to a test, the trapped volume between OV-2 and OV-4 can bepressurized. As a result, when the full-flow OV-2 ball valve is opened there is not a slug of high-pressurefluid traveling downstream and impacting the closed ball valve OV-4. This eliminates the potential forN2O decomposition and subsequent line rupture caused by adiabatic heating of any trapped gas that wouldbe compressed by the high-pressure slug of N2O released from OV-2. Downstream of OV-2 is the oxidizermass flow meter; a Flowell sub-critical venturi with a Rosemont differential pressure transducer used tomeasure pressure drop across the venturi constriction. This venturi has been designed by Flowell to developa 30 psid differential pressure at nitrous oxide maximum flow rate (53.4 lbs/s) and is connected to a 2 inchcustom flexible hose manufactured by USHose Corp. This flexible tube is used to prevent the feedline fromimpacting hot-fire load cell thrust measurements by slightly flexing during a test and allowing the load cellsto accept nearly all of the rocket engine thrust. The flexible line delivers nitrous to the main oxidizer valve,OV-4. This valve, a full-port, 2 inch FlowTek ball valve will be actuated by a custom built high-pressure,high speed actuator. Finally the nitrous oxide goes through a custom designed throttling valve, describedin greater detail by Dyer,3 before being injected into the combustion chamber. The nitrous experiencesa pressure drop of 70 psi while flowing through these components in the ground test configuration. Thispressure drop will be overcome through helium supercharging.

B. Helium

There are three uses for helium in the Peregrine GTF. First, it is used to pressurize or “supercharge” thenitrous oxide beyond its vapor pressure at all points within the oxidizer delivery system. Increasing thissupply pressure prevents cavitation in the feed-line caused by the 70 psi pressure drop described in theprevious paragraph and increases allowable chamber pressure through a greater supply pressure. Second, itis used as a combustion chamber purge gas to quench combustion following a test or in the event of a testabort. Finally it is used, at high pressure, to quickly actuate the main oxidizer valve. Two high pressuresources are used to supply the GTF with helium; a large supply is used for supercharging and purge while asmall bottle is used for main valve actuation. These supplies are labeled on the P&IDs as “He Supply” and“He Run,” respectively.

The primary helium supply, a manifolded collection of standard Praxair “T” cylinders is regulated down

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to pressures suitable for supercharge and purge through a Tescom 26-1100 high Cv, dome-loaded aluminumregulator (R-1). Dome-loading pressure is supplied by an ultra-lightweight BB series Tescom regulator, R-2.Together these regulators weigh 4.5 pounds and will be used on the flight vehicle as well as the GTF.

Downstream of the Tescom 26-1100 regulator, the helium supply tees with the lower branch supplyingthe chamber purge and the upper branch supplying the N2O run tank. Chamber purge is simply actuatedby a Circle Seal SV series solenoid valve followed by a Circle Seal metering valve to control the purgeflow rate. Finally, a Circle Seal check valve is used to prevent combustion gas blow-back into the heliumdelivery plumbing. A high-flow ball valve, HV-3, is used to control the supply of supercharging helium intothe combustion chamber. Because the supply has been regulated by R-1 there is no need to meter thesupercharge supply; once ready for a test, HV-3 is opened thereby increasing the nitrous run tank ullagepressure to the level set by R-1.

The small “He Run” tank is a light-weight 150cc pressure vessel which allows for the actuation of themain oxidizer valve up to 4 times per filling. The valves that this tank supplies are designated as HV-5(a-d)on the P&ID figures, and are 4 compact, light-weight solenoid valves. These valves control the supply ofhelium to and from a custom-built main valve actuator. This assembly, part of the helium subsystem, hasthe sole purpose of quickly actuating the main valve so that the combustion chamber is supplied with a fullflow of oxidizer as fast as possible once the test initiation signal has been issued.

C. Shop Air

The shop air subsystem is the most straight-forward subsystem, and it operates at MAWP of 125 psig asallowed by the solenoid valves PV-2 and PV-6. All orange lines in Figure 3 are shop air pneumatic supplylines, and the valves that are actuated by the air supply can be easily traced. Relief valve PV-8 ensurespersonnel and equipment protection in the unlikely event of a shop air over-pressurization.

IV. Instrumentation and Control

Figure 4. Peregrine GTF LabView Control GUI.

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All control and sequencing for the Peregrine project is handled by a flash-based microcontroller runningreal-time software capable of 1 ms control resolution. The microcontroller is linked through a high speedRS-485 serial connection to a LabView-based control console being operated in the block house (Figure 4).

The console issues commands to, and receives feedback from, the microcontroller via an efficient androbust communication protocol. Control system fault tolerance is ensured through a 3 tiered approach. Atthe lowest level, all valves are fail safe in their powered-off condition. Additionally, a master Emergency StopButton is hard-wired into the valve power system that resides with the Test Director in the block house.At the next level the microcontroller code has been designed to be deterministic, and a watchdog timer isutilized as a secondary safety measure, resetting the controller should a race condition occur. Finally, themicrocontroller continually checks the integrity of the communications link through a query/response systemby pinging the console software every 100 ms. If a response is not received within 100 ms, the microcontrollerreverts to a safe state.

During testing, all valve sequencing and timing is handled on-board the microcontroller with the consoleserving only for data feedback and remote soft-abort initiation. Timing sequences are stored on the micro-controller’s internal FLASH memory, and a crystal based oscillator is used as a time base. Furthermore,several critical sensors are monitored continuously by the controller through its on-board Analog to Digitalconversion hardware, and if pre-designated system parameters are deemed out of range, the controller willgracefully shutdown the test by closing OV-2 and OV-4 followed by a combustion chamber helium purge.

The data acquisition system for the Peregrine GTF conditions, samples and stores the data neededto analyze the performance of the motor. Sensors have been categorized as either “critical” for hardwareintegrity or “non-critical.” Critical sensors are able to detect failure modes that cannot be detected by thetest operators from within the blockhouse. Figures 2 and 3 depict these transducers and provides theirlocation relative to the test article, while Table 2 provides an explanation of the instrumentation namingconvention.

Table 2. Instrumentation Naming Convention

Designator DescriptionP-x Pressure Transducer

PIT-x Rosemount Pressure TransducerPG-x Mechanical Pressure Gage

T-x TemperatureDP-x Differential Pressure TransducerFM-x Flow Meter

A conditioning module converts the raw sensor data into a 0-5V analog signal, which is then low-passedfiltered and converted into a 16 bit digital signal. Critical sensors are monitored by the controller in orderto provide automatic shut-off if critical measurements go beyond pre-determined ranges. Data from alltransducers are stored in an autonomous data logger and can be retrieved post-test after the test stand hasbeen secured.

V. Structural

As previously mentioned, much of the infrastructure to be used with the Peregrine GTF is already inplace at the HCF. Structural design and analysis was therefore required for neither the N2O storage tank orthe associated plumbing. The fully-functional blockhouse meant that no barriers or further protective deviceswould need to be built for personnel. The only remaining structure for the GTF that required enigneeringwas the thrust stand which reacts to the thrust of the motor when firing. A workhorse design was selectedutilizing structural steel I-beams resulting in a factor of safety of greater than 15 on yield. A CAD model ofthe stand including the three Omega load cells used to measure thrust can be seen in Figure 5.

In addition to the GTF structural engineering performed on the thrust stand, a comprehensive FiniteElement Analysis (FEA) was completed on the fight-weight, aluminum combustion chamber. CosmosWorks,an add-on to the SolidWorks CAD program was used to perform the analysis. As a design tool, the analysisproved to be extremely useful, and after multiple iterations the design was finalized. The worst-case scenario

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Figure 5. Peregrine Combustion Chamber Thrust Stand.

minimum factor of safety for the fore-end of the chamber was 2.4 on yield, and for the aft-end it was 1.7 onyield. Note: for all stress calculations, von Mises stresses were used in all factor of safety calculations.

Figure 6 below is a CAD rendering of the combustion chamber, and Figure 7 depicts FEA exaggerateddisplacements for the worst-case chamber pressure scenario. This figure also indicates factors of safety (FOSon the figure) where they occur in the combustion chamber.

Figure 6. Peregrine Combustion Chamber.

VI. Progress and Tentative Schedule

Site buildup is actively taking place at the NASA Ames HCF. Plumbing, controls and instrumentationare being assembled and installed on-site, and assembly of the thrust stand will begin shortly. Currently theplan is to begin ground testing in August or September of 2007, with the largest potential for schedule slip

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(a) Fore-End (b) Aft-End

Figure 7. Peregrine Combustion Chamber FEA Results.

arising from the machine shop fabricating the combustion chamber. Launch of the flight vehicle is tentativelyscheduled for fall 2008.

VII. Conclusion

In order to prove the design of the Peregrine sounding rocket propulsion system, a specialized test facilitycapable of testing hybrid rocket motors in the 15,000 pound thrust class is being developed at NASA Ames.Progress is being made in converting the pre-existing Ames HCF to satisfy the requirements of the Peregrineproject, and testing is expected to begin in August or September of this year. Successful ground testing willvalidate Peregrine’s readiness for launch which is scheduled to take place at NASA Wallops in the fall of2008.

Acknowledgments

The author wishes to graciously thank NASA Ames and Stanford University for financial support of thisproject.

References

1A. Karabeyoglu, et al. Scale-Up Tests of High Regression Rate Paraffin-Based Hybrid Rocket Fuels, AIAA Journal ofPropulsion and Power, Vol. 20, No. 6, November-December 2004.

2D. Van Pelt, et al. Overview of a 4-Inch Paraffin-Based Hybrid Sounding Rocket Program, AIAA-2004-3822, Joint Propul-sion Conference 2004.

3J. Dyer, K. Lohner, E. Doran, Z. Dunn, et al. Design and Development of a 100km Nitrous Oxide/Paraffin Hybrid RocketVehicle, AIAA-2007-5362, Joint Propulsion Conference, 2007.

4E. Doran, K. Lohner, J. Dyer, Z. Dunn, et al. Nitrous Oxide Hybrid Rocket Motor Fuel Regression Rate Characterization,AIAA-2007-5352, Joint Propulsion Conference 2007.

5K. Lohner, J. Dyer, E. Doran, Z. Dunn, G. Zilliac Fuel Regression Rate Characterization Using a Laboratory Scale NitrousOxide Hybrid Propulsion System, AIAA-2006-4671, Joint Propulsion Conference 2006.

6G. Zilliac, et al. Ames Hybrid Combustion Facility, NASA/TM-2003-211864, National Aeronautics and Space Adminis-tration, 2003.

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