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NASA Technical Memorandum

AIAA-91-3528

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An Overview of the Lewis ResearchCenter CSTI Thermal

Management Program

Albert J. Juhasz

Lewis Research Center

Cleveland, Ohio

Prepared for the

Conference on Advanced Space Exploration Initiative Technologies

cosponsored by AIAA, NASA, and OAI

Cleveland, Ohio, September 4-6, 1991

NASA(NASA-IM-I05310) AN OVERVIEW OF

THE LEWIS RESEARCH CENTER CSII

THERMAL MANAGEMENT PROGRAM (NASA)

13 D

N92-31916

Unclas

G3/Z0 0111061

AN OVERVIEW OF THE LeRC CSTI THERMAL MANAGEMENT PROGRAM

Albert J. Juhasz

National Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135

ABSTRACT

This report presents an integrated multi-element projecteffort, currently being carried out at NASA LeRC, forthe development of space heat rejection subsystems,with special emphasis on light weight radiators, insupport of SEI power system technology, and inparticular the SP-IO0 program. Principal projectelements include both contracted and in house efforts.Included in the first category are two contracts, withRockwell International (RI) and Space PowerIncorporated (SPI), aimed at the development ofadvanced radiator concepts (ARC), and demonstrationof a flexible fabric heat pipe radiator concept beingconducted by DOE/PNL under an interagencyagreement. In house work is designed to guide andsupport the overall program by system integrationstudies, heat pipe testing and analytical code develop-ment, radiator surface morphology alteration foremissivity enhancement, and composite materialsresearch focussed on the development of light weighthigh conductivity fins.

INTRODUCTION

Ongoing "CSTI Thermal Management" related work atLeRC is an Integral part of the NASA CSTI HighCapacity Power Program, and specifically theTriAgency (DOD/DOE/NASA) SP-IO0 nuclear reactorspace power program [t].

The goal of the LeRC thermal management effort is todevelop space radiator and heat rejection systemconcepts, optimized for a spectrum of space powerconversion systems for planetary surface (lunar base)and nuclear propulsion applications for deep spacelong duration missions needed for the SpaceExploration Initiative [2]. The power, or energy,conversion system concepts range from static systemssuch as thermoelectric or thermionic, to dynamicconversion systems based on heat engines such as theStiding engine or the closed cycle gasturbine also

known as the Closed Brayton Cycle (CBC)o Althoughthe principal heat sources for these systems arenuclear [3], the technology being developed for theheat rejection subsystem is also applicable to low earthorbit (LEO) dynamic power systems with solar energyinput, using a concentrator and heat receiver, studiedas alternatives to photovoltaic power systems [4].. Theperformance goals for the advanced radiator conceptsbeing developed are lower radiator mass (specificmass of near 5 kg/m2), greater survivability in amicrometeoroid or space debris environment (up to 10years) at a sub-system reliability of 0.99 or higher.These performance goals may be realized by radiatorsegmentation and parallel redundancy, using a largenumber of heat pipes. Achievement of these goals maylead to a reduction of the SP-IO0 radiator specificmass by a factor of two or more over the presentbaseline design.

The project elements, shown in figure 1, includedevelopment of advanced radiator concepts underLeRC managed contracts and a NASA/DOEinteragency agreement, as well as in house workdirected at radiator design for optimum power systemmatching and integration. Also, tn house and universitysupported heat pipe research and development isbeing carried out, comprlsing both analytical computercode development for predicting heat pipeperformance, both under steady state and transientoperating conditions, along with experimental testingfor the purpose of validating analytical predictions.Contributing to the in-house advanced developmentprogram is continued research on radiator surfacetreatment techniques (surface morphology alteration),aimed at enhancing surface emissivity and resistanceto atomic oxygen attack.

The development of new radiator materials showinghigh strength-to weight ratio, and high thermalconductivity, such as graphite-carbon and graphite-copper composites for light weight radiator finapplications is another major program objective.

This paper is declared a work of the U.S. Government and

is not subject to copyright protection in the United States1

The project plan to accomplish the above programobjectives is shown in figure 2. Note that due tofunding constraints, the development of far terminnovative radiator concepts, such as liquid dropletradiator (LDR) or moving belt radiator (MBR), is notbeing actively pursued at the present time. Instead,technologies capable of development before the end ofthe decade are being concentrated on for both surfacepower and nuclear electric propulsion (NEP)applications.

The remainder of this report will be devoted to a statusreport on the major project elements, concentrating onthe contracted efforts which account for the majorportion of the baseline budget.

Advanced Radiator Concepts DeveloDment Contracts

The Advanced Radiator Concepts (ARC) contractualdevelopment effort is aimed at the development ofimproved space heat rejection systems, with specialemphasis on space radiator hardware, for severalpower system options including thermoelectric (T/E)and Free Piston Stirring (FPS). The targeted improve-ments will lead to lower specific mass, i.e. lower massper unit area, higher reliability and survivability in anatural space environment, thereby leading to longerlife for the power system as a whole.

Specific objectives are to achieve specific mass values< 5 kg/m2 attained with radiator surface emissivitiesof0.85 or higher at typical radiator operatingtemperatures, and reliability values of at least 0.99 forthe heat rejection subsystem over a ten year life. Theabove figures of merit represent a factor of twoimprovement over the currently considered heatrejection subsystem for SP-100, and even greaterimprovement factors for state of the art heat rejectionsystems used in current spacecraft applications.

Phases I, II, and III of the ARC contracts have beencompleted by both contractors, SPI and RI. Based onphase III results, both contractors were selected toproceed into component level development,fabrication, and demonstration, to be accomplishedunder phase IV over a two year period, to beconcluded in FY 93.

Both, a high temperature heat rejection option (800K to830K) applicable to T/E power conversion systems,and a low temperature option (500K to 600K)applicable to Stiding power conversion systems, will bedeveloped by SPI, while RI will concentrate only onthe high temperature option for T/E power systems.

C;ontractNAS 3-25206 with $PI

Among the advanced concepts proposed by SPI arethe "Telescoping Radiator" [5,6] for muiti-megawattthermoelectric (TE) or liquid metal Rankine (LMR)power systems (figure 3) and the "Folding PanelRadiator" [6], shown in figure 4, for the 500 K to 600 Kheat rejection temperature range. The latter conceptwas based on a pumped binary lithium/sodiumpotassium (Li/NaK) loop, motivated by a desire toavoid the need for mercury heat pipe radiatorsoriginally planned for a Free-Piston Stiding (FPS)power system which rejects heat in the abovetemperature range. A detail of a typical Li/NaK heatrejection loop, which also utilizes high conductivity finsfor the heat transport is shown in figure 5.

As indicated in figure 6 (a) the advantage of using alithium-NaK mixture, rather than NaK alone (meltingpoint 261 K), lies in its combining the high heatcapacity and low pumping power of Li (melting point452 K) with the liquid pumping capability of NaK, downto its freezing temperature of -12 C.

To illustrate operation of this binary loop during systemstart-up and shutdown, a brief explanation is in order.During startup (Li frozen) liquid NaK would be pumpedthrough the inner cores of radiator tube passages inhydraulic contact with the frozen layers of Li coatingthe inner passage surfaces. As the NaK is heatedduring power system startup, it will eventually melt thesolid Li shells by direct contact forced convection heattransfer, progressively mixing with the NaK to form theall liquid Li/NaK coolant. Conversely, on shut-down ofthe power system, the molten lithium with its higherfreezing point will selectively "cold trap" or freeze onthe inner passage surfaces as their temperatures dropbelow 452 K, while the NaK continues to be pumped inits liquid state through the inner cores of the radiatorpassages.

Initial tests conducted thus far, using a trunnionmounted test chamber to isolate gravity effects(figure 6b), have demonstrated the feasibility of theconcept. In particular the feasibility of a heat rejectionsystem based on a binary Li/NaK pumped loop wasdemonstrated during transient operating conditionsrepresentative of both the cooldown (Li freezing) andthe warmup (Li melting) phases of typical alkali metalheat rejection pumped loops. The thawing processduring the warmup condition had to be controlled veryclosely to avoid plugging of the flow loop due tomolten Li re-freezing downstream at certain operatingconditions. Current efforts focus on widening the

operatingenvelope by a variety of techniques. One ofthese involvesthe use of fine mesh screens which act

as semi-permeable membranes to NaK under certainoperating conditions.

The contractor also completed a two dimensionalcomputer analysis of the coo_down (freeze) and thewarmup (thaw) process including color graphicsoutput. Although the fiat flow channel cross sectionalgeometry assumed in the analysis deviated from thecylindrical flow channel used in the experimental loop,thiscomputer code nevertheless permitted visualizationof basic phenomena taking place withinthe binary loopduring the warmup and cooldown periods. A videotape was produced illustrating several cases with andwithout plugging of the flow channel due to lithiumfreezing over the entire channel cross section.

HiahConductivity Fin DeveloDment

Progress was also made by SPI in identifying potentialsubcontractors, namely Applied Sciences Inc. (ASI)and Science Applications International Co. ($AIC), withdemonstrated capabilities in the development andfabrication of high thermal conductivity compositematerials for space radiator fin applications. Inparticular, AS1 has produced a composite by CVDdensification of closely packed (up to 60 vol. %) vaporgrown ceramic fibers (VGCF) which was shown tohave very high thermal conductivity, near 560 W/m K,at a density of 1.65 g/cc. Use of composite materialswhich exhibit specific thermal conductivity values atthis level for heat pipe fin applications has the potentialof reducing radiator specific mass by over 60 percentfor radiators that are radiative heat transfer surfacelimited, i.e. no heat transfer limitations exist at thesecondary cooling loop/heat pipe evaporator surfaceinterface.

Technology requirements for joining the highconductivity fins to the heat pipes by advanced brazingor welding techniques have also been identified.

(_ontra_:tNA$ _-25209 with RI

A sketch of the "Petal-Cone" radiator concept beingdeveloped at RI [7] is shown in figure 7. Since each ofthe "petals", or radiator panels, are composed of alarge number (384) of variable length C-C heat pipesmounted transverse to the panel axis, a majorobjective of this effort is the development of theseintegrally woven graphite carbon tubes with an internalmetallic barrier that is compatible with the intendedpotassium working fluid.

Carbon-carbon (C-C) heat pipe tube sections withintegrally woven fins were fabricated under phase III.Highlights of the fabrication process are illustrated infigure 8. Because of its low cost, commercialavailability, and ease of weaving a T-300 fiber wasselected for this demonstration of C-C heat pipepreform fabrication. This PAN fiber was judged torepresent a tradeoff between high elastic modulus, andconsequently ease of handling and weaving, atmedium thermal conductivity, as contrasted to somevery high conductivity fibers which, however, may bebrittle and difficult to weave.

Several fiber architectures were Investigated beforesettling on the angle interlock, integrally wovenconcept. In this design, the axial fiber bundles, referredto as warp weavers, are woven in an angle interlockpattern, repeatedly traversing from the ID to the ODsurface of the tube. An unfilled "Novolack"/resoleprepreg resin was selected for prepregging the wovenpreforms followed by a low pressure impregnation andcarbonization process for densification of thecomposite.

Considerable progress was made in the developmentof internal metallic coatings to ensure compatibility ofthe heat pipe surface with the potassium working fluid.A coating consisting of a 2 to 3 micron rheniumsublayer with a 70 - 80 micron niobium overlayeremerged as the final recommended coating design.Due to funding and time limitations, however, this finalcoating design and the recommended method toachieve it by a novel chemical vapor deposition (CVD)process utilizing a moving heat source could not befully implemented during phase II1. Other coatingapproaches which were tried achieved incrementalimprovements over each previous coating attempt, buta constant thickness coating over the full length of thetube without any flaws or imperfections could not beachieved.

Due to the problems encountered with achieving aflawless metallic coating on the internal C-C tubesurface, with the inception of phase IV a technicaldirection was issued to the contractor, which requiresthat the safe containment of the heat pipe working fluidbe accomplished by means of a metallic liner, ratherthan the metallic coating that had been underdevelopment during phase Ill.

Concurrently with this task a high temperature braze orother joining process needs to be developed, in orderto insure good mechanical and thermal contactbetween the thin metallic liner and the C-C internal

tubesurface.Bondingof theentireliner surface to thetube needs to be achieved to prevent partial separationand collapse of the liner at conditions where theexternal atmospheric pressure exceeds the internalpressure of the working fluid.

thermal contact with the heat rejection systemtransport duct.

The heavier end sections are also used for attachmentof the end caps.

Concerning the Integral fin weaving process usingT-300 fibers, improvements will lead to the eliminationof the Internal cusp formed at the fin-tube interface.This will be accomplished by changing the weavearchitecture, so that the outer rather than the innerplies will be used to form the fins.

It should be noted that with the funding allocated tothis contract the Integral fin weave will bedemonstrated with T-300 fiber yarn only.

Although the T-300 fiber resin matrix composite has alow specific thermal conductivity, the integral fin weaveadvantage eliminates the need for developing fin-to-heat pipe joining techniques. Moreover, perfecting theintegral fin weave technique with yarn spun from thehighly pliable T-300 fiber will pave the way for tacklingthe weaving process with higher conductivity but lowerflexibility fibers, which may be carried out under afuture development program.

LiqhtWeiqht Advanced Ceramic Fiber {ACF) Heat PiDeRa.diator_

The objective of this joint NASA/LeRC and Air Forceprogram with Pacific Northwest Laboratory (DOE/PNL)is to demonstrate the feasibility of light weight ceramicfabric/metal liner heat pipes for a wide range ofoperating temperatures and working fluids. Specificallythe NASA LeRC objectives are to develop this conceptfor application to Stiding space radiators with operatingtemperatures below 50OK, using water as the workingfluid. The specific mass goals for these heat pipes are< 3 kg/m2 at a surface emissivity of > 0.85.

Several heat pipes were built using titanium andcopper foil material for containment of the waterworking fluid. A heat pipe with an eight milTi liner wasdemonstrated at operating temperatures up to 475 K atthe 8th Symposium on Space Nuclear Power Systems($SNPS) in Albuquerque, In early January 1991.

The liner fabrication technique is expected to have awide range of applications, beyond the scope of thisprogram.

The water heat pipes fabricated for LeRC have beensubjected to a test program to evaluate performanceand reliability at demanding operating conditions,Including operating pressures up to 25 bar. Tests wereconducted with and without wicks with the heat pipesin various gravity tilt orientations from vertical tohorizontal. In addition a number of wick designs weretested for capillary pumping capability, both in groundtest and under low G conditions produced duringKC-135 aircraft testing.

The work was reported in a paper presented at the27th National Heat Transfer Conference (NHTC) [8].

Future thrusts of this work will be to perfect the heatpipe fabrication procedure, using very thin (1 to 2 mil)foil liners, internally texturized by exposure to highpressures.

Because of the high operating pressures, plans will bedeveloped to perform hyper-velocity and ballisticvelocity impact tests, in order to ascertain if secondaryfragments from a penetrated heat pipe will result infailures of neighboring heat pipes.

Another major challenge will be to design a heat pipewith high conductivity, light weight fins, as a first steptoward the fabrication of light weight radiator panels.

Based on results from the above a preliminary designof a representative radiator Panel and radiatorsubsystem will be designed, including heat exchangerducting, pumps and flow control devices. Fabricationand testing of this design is expected to be carried outin a later phase of this work.

SUDoortina Pro!ect Elements

An Innovative "Uniskan Roller Extrusion" process hasbeen developed at PNL and used to draw 30 mil walltubing to a 2 mll foil liner in one pass. Moreover, thisprocess eliminates the need for joining the thin foilsection to a heavier tube section for the evaporatorsection which needs to be in tight mechanical and

Space limitations prevent a detailed discussion of theremaining project elements referred to In figure 1.However, a brief paragraph highlighting each of theseactivities is warranted.

As mentioned previously, the system integration

studiesperformedin house, serve to guide the overallTM work by providing the proper framework for it. Asshown in [9], for example, a liquid sheet radiator (LSR)with lighter specific mass than a heat pipe radiator, willnot necessarily benefit all power conversion systemsequally. As shown in the reference, the LSR concept isnot suited to the heat rejection temperature profile ofa CBC power system. However, it does work well witha Stiding power system which rejects heat at a nearconstant temperature. As a further example of howradiator-power system integration studies are used toascertain radiator induced power system performancedegradation, the reader is referred to figure 9. Thecurves shown here illustrate the reduction in poweroutput and efficiency for both Brayton and Stirlingpower systems resulting from a loss of radiator area,caused for example by micrometeoroid damage. It tsreassuring to note, that even with a loss of 50 % ofradiator area, a Stiding is still capable of producingover 75 % of its design power, while a CBC producesover 65 %.

A typical example of radiator surface morphologyalteration by arc texturing for emissivity enhancementpurposes is shown in figure 10. With the arc texturingapparatus shown in figure 10 (a), the adjoining barchart, figure 10 (b), shows the surface emittanceresults achieved with various arc current values for

graphite-copper samples produced under the in-housematerials program. Additional details on the emittanceenhancement process and measurement techniquesare included In [10]. A brief overview discussion ofemittance enhancement by atomic oxygen (AO)bombardment is also given in [1].

Heat pipe performance modelling, both under steadystate and transient operating conditions is beingconducted in-house and under university grantsrespectively, with UCLA, UNM and WSU. The objectiveof this work is to develop a capability to analyticallypredict transient operation of heat pipes, particularlyduring the startup and cooldown phases. An especiallyimportant feature of this work is the development of ananalysis code capable of modeling startup with theworking fluid initially in the frozen state for a variety ofworking fluids, including water and liquid metals.Working versions of the codes have been developed,and validation of predicted performance by laboratorytesting of heat pipes is under way at LeRC, LANL andWRDC. Efforts have also been initiated to compile anexperimental database by a systematic literature searchand close communication with other researchers in thefield.

CONCLUDING REMARKS

The LeRC CSTI Thermal Management Program isdesigned to combine a number of project orientedelements to accomplish the overall objective ofreducing radiator specific mass by a factor of two at asubsystem reliability of 0.99 over a ten year life.Although the main focus is on support of the SP-IO0program by advances in heat rejection technology, theconcepts and hardware developed under this programare expected to benefit space power systems ingeneral, ranging from solar dynamic systems with apower level of a few kW to multl-megawatt powersystems with nuclear heat sources for planetarysurface and nuclear (electric) propulsion applications.With the focus on the SP-IO0 program the projectschedule goals are that the major technology advancesbe ready for implementation before the end of thedecade, so that NASA's and the nation's long termgoals in space exploration and utilization may berealized.

REFERENCES

. Winter, J. M. "The NASA CSTI High CapacityPower Program" AIAA/NASA/OAI Conference onAdvanced SEI Technologies, Cleveland, Ohio, Sept4-6, 1991.

. Bennett, G. and Cull, R. C. "Enabling the SpaceExploration Initiative: NASA's ExplorationTechnology Program in Space Power" AIAA/NASA/OAI Conference on SEI Technologies,Cleveland, Ohio, Sept 4-6, 1991.

. Juhasz, A. J. and Jones, B. I. "Analysis of ClosedCycle Megawatt Class Space Power Systems withNuclear Reactor Heat Sources" Proceedings of theThird Symposium on Space Nuclear PowerSystems, Albuquerque, NM, Jan. 13-16, 1986.

. Brandhorst, H. W.; Juhasz, A. J.; and Jones, B. I.;"Alternative Power Generation Concepts in Space",NASA TM-88876 (1986).

. Begg, L. I. and Engdahl, E. H. "Advanced RadiatorConcepts - Phase II Final Report for SpacecraftRadiators Rejecting Heat at 875 K and 600 K"NASA Contractor Report 182172, May 1989.

,

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.

Koester, K. and Juhasz A. J. "The TelescopingBoom Radiator Concept for Multimegawatt SpacePower Systems" AIAA/NASA/OAI Conference onAdvanced SEI Technologies, Cleveland, Ohio,Sept 4-6, 1991.

Rovang, R. D et al. "Advanced Radiator Conceptsfor SP-t00 Space Power Systems', NASAContractor Report 182174, October 1988.

Antonlak, Z.I., Webb, B.J., Bates, J.M,Cooper, M.F., and Pauley, K. A. "Testing ofAdvanced Ceramic Fabric Wick Structures and

their Use In a Water Heat Pipe" 27th NHTC,Minneapolis, Minn., July 28-31, 1991.

° Juhasz, A. J. and Chubb, D. L "DesignConsiderations for Space Radiators Based on theLiquid Sheet (LSR) Concept", NASA TM 106158;Presented at the 26th IECEC, Boston, MA, August4-9, 1991.

10. Rutledge, S., Forkapa, M., and Cooper, J "ThermalEmittance Enhancement of Graphite-CopperComposites for High Temperature Space BasedRadiators" AIAA/NASA/OAI conference on SEITechnologies, Cleveland, Ohio, Sept 4-6, 1991

CSTI HIGH CAPACITY POWER- THERMAL MANAGEMENTPROJECT ELEMENTS

@,, H@

SYSTEMS ANALYSIS. I_0

Figure 1: Thermal Management Project Elements

THERMAL MANAGEMENT

BASELINE BUDGET

Advinced r._buity _slrilkms

RadialofConcepts I,x_s_ r* ccwmu:n_

Corllraclors _SPI, R_ >

HIGH CONDUCTIVITY COMPOSITE FIN DEVELOPMENT_

I- I= I,' I- I- I- I- I - 1'71

Figure 2: Thermal Management Project Plan

NASA ADVAN_:ED RADIATOR CONCEPTS

Figure 3: Multimegawatt Telescoping Radiator Conceptfor TE and LMR Space Power System

NASA Advanced Radiator Concepts1.7 MW_,600 K Heal RejectionRadiatorfor SpaceNuclearStirlingCycle Power Systems

Figure 4: Folding Panel Radiator Concept for FPS andCBC Power Systems

Figure5: Pumped Li/NaK Binary Loop Radiator ConceptUnder Development at SP;

PUMPING CHARACTERISTICSOF Li 'NaK MIXTURES

2oooF .B - s 5 MW_Transported,Fluid T = 100"C,

1"rso .7 ? 15 Parallel Loops (>@ S cm Ola.

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100% LI % NIK In LI 100% NIK

(a) (b)

Figure 6: Binary U/NaK Radiator Development (a) DesignCharacteristics; (b) Test Facility

Shut'tktBoy Envtlr,pefor Stow_ Rmdiator

Figure 7: Cone-Petal Radiator Concept under Developmentat Rockwell International

ARC- ROCKWELL CONCEPT

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Figure 8: Finned Graphite/Carbon Heat Pipe FabricationProcess

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Figure 9: Radiator-Power System Integration StudySample Results

ARC TEXTURING FACILITYARC TEXTURED ORAPHITE--COPPEI_.

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Surface Emittance Enhancement by Arc Texturing(a) Texturing Facility(b) Emittance Enhancement of Gr/Cu Samples

IMI"rI"AWCIIAT i i Jill

N NMI _I'rAH_AT 112 •

11

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1991 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

An Overview of the Lewis Research Center CSTI Thermal

Management Program

6. AUTHOR(S)

Albert J. Juhasz

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space AdministrationLewis Research Center

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAMES(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

WU-590-13-21

8. PERFORMING ORGANIZATIONREPORT NUMBER

E-6542

10. SPONSORING/MON_ORINGAGENCY REPORT NUMBER

NASA TM-105310

A1AA-91-3528

11. SUPPLEMENTARY NOTES

Prepared for the Conference on Advanced Space Exploration Initiative Technologies cosponsored by AIAA, NASA,

and OAI, Cleveland, Ohio, September 4-6, 1991. Responsible person, Albert J. Juhasz, (216) 433-6134.

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Unclassified - Unlimited

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13. ABSTRACT (Maximum 200 words)

This report presents an integrated multi-element project effort, currently being carried out at NASA LeRC, for the

development of space heat rejection subsystems, with special emphasis on light weight radiators, in support of SEI

power system technology, and in particular the SP-100 program. Principal project elements include both contracted

and in house efforts. Included in the first category are two contracts, with Rockwell International (RI) and Space

Power Incorporated (SPI), aimed at the development of advanced radiator concepts (ARC), and demonstration of a

flexible fabric heat pipe radiator concept being conducted by DOE/PNL under an interagency agreement, in house

work is designed to guide and support the overall program by system integration studies, heat pipe testing and

analytical code development, radiator surface morphology alteration for emissivity enhancement,' and composite

materials research focussed on the development of light weight high conductivity fins.

14. SUBJECT TERMS

Spacecraft radiators; Thermal radiation; Heat radiators

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