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AIAA 92-2867 Design and Performance of a Cryogenic Heat Pipe Experiment (CRYOHP)

Jerry Beam Wright Patterson Air Force Laboratory

Patrick J. Brennan OAO Corporation

Met Bello Aerospace Corporation

AIAA 27th Thermophysics Conference July 6-8, 1992 / Nashvi l le, TN

For permisslon to copy or republish, contact the American instftute of Aeronautics and Astronautics 370 L'Enfant Promenade, S.W., Washington, D.C. 20024

DESIGN AND PERFORMANCE OF A CRYOGENIC HEAT PIPE EXPERIMENT (CRYOEP)

v BY

Jerry Beam, Wright Patterson Air Force Laboratory Patrick J. B r e w , OAO Corporatian

Me1 Bello, Aerospace Corporation

Abstract The Cryogenic Heat Pipe Experiment (CRYOHP) has been designed to demonstrate the thermal performance of two different axially grooved oxygen heat pipes in micro- gravity. The CRYOHP is manifested for fight aboard STS-53 which is scheduled for launch in November, 1992. This paper discusses the design and thermal vacuum performance of this experiment and planned fight tests.

CECM COP CRYOHP EBP GAS HH I/F

, LEP MDP MLI PDB SDIO STS UEP VDC

Acronyms

CRYOHP Electronic Control Module Coefficient of Performance Cryogenic Heat Pipe Experiment Electronics Base Plate Get Away Special Hitchhiker Interface Lower Emt Plate Maximum Design Pressure Multi-Layer Insulation Power Distribution Box Strategic Defense Initiative Organization Space Transportation System (Shuttle) Upper End Plate DC Voltage

Introduction

The low surface tensions of cryogenic fluids and their correspondingly low wicking heights and transport factors d e it difficult to obtain reliable I-g test data with conventional designs such as axially grnnved heat pipes.'.' Flight data is necessary to avoid excessive design margins which carry large penalties in cryogenic systems and to permit accurate modelling of steady-state and transient heat pipe bebavior for future missions. CRYOHP was designed to permit independent testing of two cryogenic heat pipes. Specific experiment objectives inchule:

*Measurement of transport capability and thermal conductances

Demonstration of reliable sh t -up from a supercritical condition

Correlation of 0-g and I-g data with existing analytical models

v Establishing a micro-gravity cryogenic thermal test bed

Oxygen was selected as the working fluid because it has the best transport and wicking properties and is the preferred fluii for operation in the 60 to 100°K range. Although nitrogen is less of a safety concern, the relatively s d amount of oxygen required for the heat pipes does MA present a safety hazard. Axially grooved heat pipes were selected befause their performance with ambient fluii such as ammonia is well understood and has been correlated with existing models. Also, several of the axially grooved designs which are commonplace in ambient temperature fight systems have sufficient performance with oxygen and nitrogen for most planned cryogenic applications.

The CRYOHP is desi& to test two heat pipes independently and two different axially grooved designs will be flown. The first design which is provided by Hughes Electron Dynamics Division has an extrapolated 0-g transport caphiity of approximately 20 W-m with oxygen in the range of 80 ~ 100°K. Unfortunately, its transport cannot he demonstrated with the available cooling capacity. The second heat pipe design was developed by TRW aod permits 0-g "dry-out" in the CRYOHP offers improved ground testability for 1-g correlation.

Exneriment Desien

Trade studies were conducted to determine an acceptable cryogenic thermal test bed design. Two basic requirements were specified:

1. CRYOHP will be flown on an STS mission in a Get Away Special (GAS) canister as a stand-alone or with Hitchhiker power, command and telemetry.

6.5 watts for 100 hours or more at 80°K or less. 2. The experiment must have a cooling capacity of at least

Passive designs which incorporated solid argon or liquid nitrogen dewars were evaluated and compared to active coolmg with cryogenic refrigerators. The argon dewar systems hd limited hold times prior to launch, and the liquid nitrogen systems although they provided longer hold time, required developmeat. These factors and system cos@ led to choosing tactical cryogenic refrigerators. T k potential problem with this approach is that mechanical vibration from the cooler(s) could be induced into the heat p i p and either facilitate or inhibit its transport acd sht-up. Also, because the Coefficient of Performance (COP) of cryogenic refrigerators is high (2540), power must be provided by the Hitchhiker and the canister must provide sufficient heat rejection or thermal capacitance.

1

Copyright O American Institute of Aeronautics and AQronautics. Inc., 1992. All rights rcservcd.

CRYOHP Thermal Desipe

The CRYOHP flight experimed consists of the following subsystems: two heat p i p , five Cryogenic mechanical coolers, a CRYOHP Eledrnnic Control Module (CECM), a Power Distribution Box (PDB), and the thermal subsystem. AU elemota of the CRYOHP are housed within the standard 5 cubic foot HH canister lid. Figures 1, 2 and 3 show scbemptic views of the msjor components of the experkeat. The iodividual heat pipes are "S" shaped 88 shown in Figure 1 and are mounted to a common stainless steel structure that is supported from the canister lid. The five cryo-coolers are mounted vertically to the canister lid 88 shown in Figure 2. This lid commonly referred to 88 the uppr d plate PEP) is solid 6061-T6 alumirmm . It is a d i e d version which has the upper portion exteoded to provide a total thermal mass of 100 pounds. The mass is necsssmy to avoid excessive temperaturea (above EU'C) in the event that the CRYOHP is operated for several hours with i nc idd sun. The UEP is covered with silver teflon tape and the canister is painted white to maximize their heat rejection.

- T-T

Figure 3 illustrates the thermal design of the experiment. 'Ihe condeoser section of each heat pipe is bolted to an aluminum shunt which is in-turn coupled to individual cryocoolers by paraUel vibration isolators. Experiment 'j operation consists of cooling the thermal shunt and the heat p i p aad then applying heat to the evaporator end to test the beat pipe.

The vibration isolators consist of parallel braided copper straps that are epoxy bonded between two copper plates on either side of the isolator. The copper straps provide good thermal conductance with sufficient pliability to dampen vibration from the cooler to less than 2 milli-g in any direction at the heat pipe. This level represents less than 2% of the cnpiUary pumping head that exists in either heat pipe at 80% and will have a negligible effect on the transport capability.

Three cryncoolers are used to test the TRW heat pipe and two are used with the Hughas heat pipe. The three coolers provide sufficient capacity to conduct transport testing with the TRW heac pipe at vapor temperatures in

RI-...

Figure 1. CRYOHP Experiment Configuration

- M -

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\

,'

c- the range of 60 to 100°K. Tb Hughes bat pipe would have required a minimrm of six coolers which is impossible from the stlnapoiot of mpongeable power dissipetian. Prkaging constrpiors limited the total

v number of coolen to five and therefore three were allocated to the TRW bat pipe to insure complete testing. "Dry&" with the Hughes heat pipe should be achieved arouDd 115% with two coolers and this datapoint is useful in confirmiog the analytical model.

Grafoil is used throughout the experiment at each interface to provide good thermal conductance. The overall conductance betwee0 the heat pipe condenser aod the individual c r y o ~ o l e r cold heeds is 1.2 WPC for the TRW heat pipe and 1.0 WPC for the Hughes heat pipe as derived from the system's thermal vacuum tests.

A major consideration in the thermal design of this experiment was to minimize the heat leeks to each heat pipe and to the thermal sbunt and the other compo& on the "cold-side" of the experiment. The stainless structure which supports the heet pipes is isolatgi from the UEP by G-10 isolator blocks and G-10 tubular inserts as shown m Figure 3. The heat p i p are suspended from the strnchre at thrae locations by Kevlar cabldG- 10 clamp a r r a n g e d that is illustrated in Figure 4.

Figure 4. Heat Pipe Attachment and Isolation Supports (3 Places Each Heat Pipe)

The Kevlar cables are wnpped over the exterior of the multi-layer insulation 0 blanketa that cover each heat pipe. As mentiooed previously, the heat pipe's condensers are bolted to the s b which are supported from the staioless 8tnwium by G-10 bnckeds. The shunts are used to provide a hat sink for the instrumencation I& to minimize cbs bat krLs from these l d to the heat pipes. ?by .bo pmvide a d b n t i o n for the heat flows from the bat Pipa and to the coolers. Each shunt has a gold plated retlective finish and is covered with an MLI blanket. The entire "cold-side" of the experiment inctudmg both heat pipes and shunts is surrauoded with another MLI blanket. The inner blankets have twenty alternating layers of a l u m h i d mylaranddacron. The e x t e d blanket has ten dtesnating layers. G-10 bolts and washers were used throughout to minimize conductive heat leeks. The analyses and correlation of thermal vacuum test results that are ptesented in Ref. 4 indicate

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3

that the pansitic heat leak to each heat pipe has been limited to spproximately 0.5 watts. The total parasitic heat leal; which inchides an estimated 0.35 watts from the instnunentation leads was determined to be 2.5 to 3.0 watts at 80% for the TRW heat pipelcooler system.

Comwnent Hanlware

Heat F'iw

The TRW heat pipes design is presented in Ref. 3. It is 1.32 m long and has an 11.2 mm OD circular cross- section with 152 mm long aluminum saddles soldered at the evaporator and condenser ends. lhese saddles provide flat surfaces for mounting the test beaten, thermostats, and temperature sensors and for attachment of the condenser section to the shunt. A 10.3 gram charge of oxygen is required for nominal operation at 80°K. The corresponding meximum design pressure at 80T is 122.4 atm. The theoretical transport capability of this design is pzwented in Figure 5 . Its theoretical 0-g transport is 5.1 watt-meter at 80%. This performance was obtained using the closed form solution presented in Ref. 5 and takes into account 0-g back-pumping. As indicated, the thermal vacnum test results are in g o d

Figure 5. Thermal Performance versus Elevation of TRW Heat Pipe

correlation with this theoretical performance. Component test results with the heat pipe that are provided in Ref. 3 are approximately 10% higher. This discrepancy is probably due to less accurate beat pipe tilt mensurements in the systems tests and implies that the closed form solution could ha underpredicting the transport capability slightly.

The H u g h heat pipe has a 1.37 m active length and incorporates the groove geometry presented in Ref. 2. Its tubing is extruded with a 15.88 mm square cross-section and then it is machined round everywhere but at the evaporator and condenser e& which again are 152 mm long. The rnnnd cross-section is required to facilitate b e d i g with the heavy wall that is required for pressure containment. This design has 33.7 grams of oxygen and a 20 W-m transpolt capability at 80°K which is extrapolated from component test data. Transport

.

tests were not conducted with thia pipe," the systems level because 'dry&' c ~ n not be s c b ~ e d except at high tilts or high opentine tepnperalu~ where p v e drainage and pddle effects m k the heat pipe's tmpo t t . Each of the fight beat p i p and their fight

respective MDP prior to chrging. After pinch-off, the heat pipea were X-rayed and oxygen leak tested. Repmentative burst units for the TRW and Hughea pipes demonstrated a minimum de4y factor of 3.7 and 3.0 respectively relative to their MDP.

Crvocoolers

The cryocoolers use a split-Stirling cycle and are advanced tactical coolerslModel No. 7'044H that are maufactured by Hughes Eleciron Dynamics Division. These coolers were selected because they provlde good cooling capacity (3.5 watts at 80%) with a COP of approximately 25. Their performance is shown in Figure 6 v e m cold hesd temperalure. These coolers an, lugged and hnve good reliability with pn operational life in excess of loo0 hours which is more than the required life for grouod and fight tests. They are also ~ m p c t and lightweight. The control electronics for the coolers are small hybrid circuits that are packaged in the motor housing. Opration with a 24-32 VDC input that is provided by HH is standanl.

spare was proof pressure tested at 1.5 times their

C O L D G O TEMPEGGRE (K)

Figure 6. Cryocooler Refrigerative Capacity

The compressor ud motor are orbeoted vertically and copper or ahuntourn ' clpmps are used to thermally couple themtotheUEP. Tbe ' ' exppnder sectionhasa heavy coppr baat sink tht ia brszed to it to permit good heat c o h c t i o n to the UEP. l%i# wan dooe to minimize the cryocooler body temperpbues which in turn results in a higher cooling capacity and a higher efficieocy.

Electronics and Insttwnen tatioq

CECM and PDB

A functional block diagram for the CRYOHP electronics is presented in Figure 7. The CECM provides data

- acquisition and signal coditioniag of the telemetry nnd power modulation and distnlmtion to the hent pipe heaters. Survival heaters which are usal to prevent the cryocoolers and the electronics from getting too cold (less than 40°C) during any do- periods are also powered by the CECM. The CECM in 0.25 x 0.25 x 0.46 m anrl weighs approximately 12 kg. Apporimrtely 20 watts are dissipated by the CECM which in covered with kapton tape to permit radiative coupling to the d t e r wall.

~

HH IIF PDB - -

*Z8V I I

CRYOHP

I P O l E R CRYO-

COOLERS DUTIIBUTION PULSE C Y 0

EXPERIMENT LOADS

Figure 7. CRYOHP Electronics Block Diagram

The power distribution box (PDB) receives power from three HH pelts and provides a 10 Amp power line to each of the five cryocoolers. Discrete commnnds from the HH activate relays in the PDB to brm each cooler ON or OFF independently. Ths PDB is 0.13 x 0.13 x 0.2 m and weighs 3.6 kg. Its power dissipation is negligible. Both units we suppotted from an alumioum stmchire thnt is bolted to the electronics baseplate (c.f. Figure I) .

Instlumentation

Instrumentation for the CRYOHP is WllllIlYtlzed ' in Table 1. Platinum resistance themmeters (PRTs) are used to measure temperalure on the cryogenic side of the experiment. Each heat pipdcmler system has a totd of 13 PRTs. Twenty four the&m p v i d e h e k e e p i n g temperalure measurements principlly for the stmcture. the cooler expanders and compraseors, nnd the electronics. Hitchhiker provides u1 additional nine thermistors that are used to measure tempsrPtures on the canister and on the electronics asdpkts and CECM mounting brackets. Thew Laar (emperalures provide n measure of the health of the CRYOHP during periods when the experiment camc4 he turned ON.

Other telemetry consisLs of curreot monitors that are used to calculate the heater power and voltage monitors for the bus voltage and for temperablra d i m . A Hitchhiker pressure transducer is provided to measure the internal canister pressure.

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4

Platinum Resistaxe I Thermometers (PRTs)

v

Thermistors

Pressure Transducer

Current Moniton

Voltage Monitors

Heaters (Kapon Foil)

Themlorno

26

24

9 (HH)

I (HH)

I3

18

I1

33

13 Each Heat Pipe System

UEP. EBP. Pillars. Heat Pipe Structure. Crya- Cwlerr. Elecmonics

EBP. Canister. & CECM Mounting Brackeo

Canister Internal Pressure

CECM

I for Bur Voltage. 17 for Temperature Calibration

4 p r Heat Pipe. 3 Survival

Tri-Series Circuit for Each HeatCr

Table 1. CRYOHP Instrumentation

Vent Valves

, The canister is backfilled on the grouod with 16.7 pia of dry nitrogen to inhibit the m p s of moist air which would degrade the M U and compmmise the fight teats. During ascent a one-way 2 psid differentid valve provides preliminmy evacuation. Ooce in orbit and h e r deployment of the Primpry pnylosd, two solewid valves and then a motor drivea M y vdve M opmed to achieve a vacuum of leas than 0.1 millitorr. All valves are located on the LEP and the mlaooi and buaerfly valves are activated by HH c o d . 'Ibese vdvea rn closed prior to reentry to avoid the possibility of a flammable atmos#me entering the caoiatsr.

v

Heeters and Thermostats

Each heat pipe hss a primary md redwrU evaporator heater to conduct the transport toas. Ideotical primary aod redundant "trim" heaters M attached to the coodenser section to provide corn01 at a COUS~W~ tempemture during the transport teats. lbese heaters are c o d e d from the ground.

Pulse modulation from the CECM in used to provide incremental heater power. The TRW beaters can be operpted in 0.5 watt steps from 0 to 7.5 watts whereas the Hugbas heat pipe heatas have o m watt i n c m from0 to 15 watts. AU beaters uelrnpton foil types that M taped to the surfacea wing doublssided Lapton tape.

Tri-series Elmwood themstat circuits M used in series with each heater to provide a two fault tolerunce against a heater runaway. The thermostatll have n o m i d set points of 65°C or less.

A potential large heat leak to each heat pipe and the other cryogenic components is the conduction plong the PRT and heater leads. A manganin inSart WM mldered into each of these leads nesu the shuut to mlnimire this coductive heat leak.

At the cold side of the hamess. b y o d the mangatin insert, each heat pipe's hamess WM expoxied to two small aluminum heat sinks tht wen, bolted rubseqnmtly to the thermal shunt. This WM dom to minimize ~y heat leak from the instnumotation leads to the heat pipe aod therein permit a more accurate evaluation of the 'dry- out" heat load.

CRYOHP Verification Test Propram

The CRYOHP Verification Test Program is illustrated in Figure 8. Component level qualificption tests were conducted in addition to CRYOHP system teats. EMYEMC tests were cwducted with the CRYOHP integrated to the HH avionics.

CECM CRYOHPSysIem Heat Pipes Cryo-Refrieermtor & PDB

Figure 8. CRYOHP Verification Test Program 5

CRYOHP Thermal Vacuum Tea4

Thermal vpcuum ta(s were cwduaed with the CRYOHP attached to its UEP d inst.lled within atest canister. The tests were coutucted at Goddard Space Flight Center

setup shown in Figum 9. Liqud nitrogen was used to cool the shroudn that run the length of the chamber. Heaters were attached to the circumfereoce of the canister

payload bay radiation. A detailed discussion of the test results and the CRYOHP model correlation is given in Ref. 4.

in the W l i i 4 opticrl coatings Laboratory using the

and the UEP to simulate theenvironmeotpl inpm and the

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Figure 9. Schematic of CRVOHP Thermal Vacuum Test Set-Up

The CRYOHP is located in p y l o d bay 4 and will have a view to deep a p c a thrcugbout most of the 6day mission planned for the STS-53. Ths test results shown in Figures 10 d 11 for the TRW and Hughes systems are representative of this figh wndition. Reference to Figure 10 shows that the TRW beat pipe's codeuser cools to below the critical point of oxygen (154.890 in less than 2.5 hours. Priming of the evaporator and isothermal operation are accomplished within 5.2 hours when the heat pipe has cooled to about 90%. This twt was conducted at M adverse elevation of approximately 1.78 mm (0.07 inch) aod hrll Prim;ng occurs at a relatively low temperahm due to the lower tramport associated with the adverse tilt. In flight, full priming should be achieved above 100% withi0 4.5 hours.

- Reference to Figure 5 sbows tht a mUirmm transport of 3.4 W-m was demonstrated at 82% atthis tih aod 2.2 W- m at 65% These values inch& a 0.5 watt prsaitic heat leak that is assumed to be diatnbuted uniformily over the heat pipe since more than 85% of the theoretical heat leak to the pipe is due to d i i o n . T h e pvasitics correawd to a tranSwrt of 0.33 W-m. Tha actual

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electrih heat loads f& these "dry-' were 2.7 W at 82°K and 1.8 W at 6%.

350

300

E 250

$ 150

g m w P

rw

50

TIME (HOURS)

N W * Hal Pips mbdired .I 81X wlrh cold head ternprams a i 70% and wilh 4 * e r n appllsd to the (rim huter

Figure 10. TRW Heat Pipe Transient Cooldown

TIME (HOURS)

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N W : * Applied 3.56 warn to the cr.ponlor hater at I vapor tempmare of 11VK after 6 hours Hughes heat p i p srabilired PI a vapor tempnture of 97.6%

Figure 11. Hughes Heat Pipe Transient Cooldown

In tlight amaximum 5 W heater l ad should be required to achieve "dry-out'. Results horn the tbermal v ~ c u u m tests show that sufficient cooler cappcity is available to manage a 5 watt heat load with a cold head temperature of 72.5% and a correupodmg beat pipe vapor

6

, . .

temperahre of 84%. ALSO, although the heat pipe becomes Unaercharged u its tempmhm is cooled below 80°K. cooling of ita coodeoser to below the freezing point of oxygen (54°K) in .chievable, and o ion with a 0.5

planned to determinu the tnnaport at undercharged conditions (e.g, 60 and 70%) and to startup from at least a partially frozen d i i o n . "kse are obviously valuable data points for amhi i M well M cryogenic tlnids. The transient cooldown and startup of the Hughes heat pipe is presented in Figure 11. This test was condncted with the heat pipe oriented at a positive tilts of approximately 6.3 mm (0.25 inch). Schedule constraints did mt permit testing the Hughes heat pipe at adverse tilts hut only at the positive tilt that resulted wben the CRYOHP canister WM oriented to test the TRW heat pipe. The Hughes heat pipe is fully primed at 145% within 4.2 hours after cooldown is initiated. A later test at a 4.3 mm (0.17 inch) positive tilt exhibited the same startup. It is believed that these results are fairly representative of this heat pipe's 0-g startup because of its relatively high transport capability. If' this is the case, 0-g startup should he achieved within five hours when the Hughes beat pipe has cooled to IWK or 80. Once the pipe has primed, it will be tested to determine its transport caplbility in the range of 115 to 14% where it goes from an overcharged to an undercharged codition as the operating temperature is increkced. It will also be operated at 80% and helow as its cooler capacity permits. No-load operation at 65% was achieved in g d tests.

The fight plan listed in Table 2 bas been developed on the basis of the g d test results and mission objectives. A total of five cycles of startup and transport tests with each heat pipe can be achieved in the sixday mission.

W WBtt heat load WM danoostnted at X F l i g h t tests are

v

Conclusions

The CRYOHP fight offers the first oppoltunity to obtain startup and transport data with an oxygen heat pipe. Extensive test data should be obtaioed from the sixday mission which will permit I-g data correlation of the analytical models for two differant d y grooved heat pipe designs. Performance at different temperaturea ranging from 60 to 14591 will .Is0 fmtnblish operation at underfilled and overfilled d i and provide additional datn for analytical cornlatinma tht will apply to ambient as well as cryogeaic fluids. F d y . a successful mission will &bh the CRYOHP as a proven test bed for fuhlre flight v a i t h t i i of &r cryogenic heat p i p , heat pipe diodes, t h e d switches, pbase change matexi& and cryocmlers.

The CRYOHP experiment is funded jointly by the Air Force and the Strategic Defema Initiative Organization (SDIO) through Wright Patterson Air Force Lahoratory and the NASA through Goddanl Space Flight Center. Jeny Beam and Roy McIntosh are co-investigators for the respective centers.

- 7.5

8.5 8.5 + 8 . 5 . . 14 14.16 16* 16. - 20 20. . 2 2 2 2 r - 24 2 4 + - 29 2 9 - . 32 32 f 3 2 * . 36 36* . 38 38+ - 39 39- . 4 2 42 + 42 f . 44 44r . 4 8 46+ . 4 8 4 6 1 - 50 50r - 53

58 + 58+ - 5 9 r 59+ . e 1 81 + . 63 s3r - 65 6 5 + - 68 68+ - 71 71 - . 7 5 7 5 + . 79 77+ . 79

53+ . 5 e

70 * I _ .

79r - 81 81 t . 8 3 83+ - 85 85+ - 87 8 7 1 . 9 2 92+ . 9 7 97+ - 101 101. . 102 102+ . 104 I04r - 108 106+ - 129 1 2 9 r - 142

1 4 2 + . 145 145+ l 4 5 r

5 .5 2.0

4 0 2.0 2.0 5 0 3.0

4 0 2.0 1 .o 3 0

2 0 2 0 2 0 2 0 30 3 0

3 0 2 0 2 0 2 0 3 0 3 0 4 0 2 0 2 0

2 0 2 0 2 0 2 0 5 0 5 0 4 0 1 0 2 0 2 0 23 0 13 0

3 0

Table 2. CRYOHP Flight Operations

Refereoces 1. Schlitt. K.R.. Brennan. P.J. .nd Kirkmtn 'ck. J.P..

2.Fleishmao. G.L., Chraog, T.C. .nd Ruff, R.D., 'Oxygen Heat pipe 0-g P e r f o m Bvlhution Based on 1-g Tests.' AIAA Paper No. 91-1358.

3. Antoniuk, D. and Pohner. J., 'Developmeat of an Oxygen Axial Groove Heal Pip for Micmgravity Flight Experiment.' AIAA Rpsr No. 91-1357.

4. Brennan. P., et. al.. 'Perfomum of the Cryogenic Heat Rpe Experiment.' To he published in the 2 2 d

July 1992.

Study.' Center, Contract NASS-22562. July 19n .

International Conference on En ' . I Sysfems.

5 . B & K Engineering lnc.. 'Axial Oroovo Heat Rpe Prepared for NASA Gaidud Spne Flight

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