NASA Thermal Management Systems Technology Area Roadmap - TA14

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National Aeronautics and Space AdministrationDRAFT Thermal Management SystemsR oadmapTechnology Area 14Scott A. Hill, ChairChristopher KostykBrian MotilWilliam NotardonatoSteven RickmanTheodore SwansonNovember 2010DRAFT

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Table of ContentsForewordExecutive Summary1. General Overview1.1. Technical Approach1.2. Benefits1.3. Applicability/Traceability to NASA Strategic Goals, AMPM, DRMs, DRAs1.4. Top Technical Challenges2. Detailed Portfolio Discussion2.1. Cryogenic Systems2.1.1. Passive Thermal Control2.1.2. Active Thermal Control2.1.3. System Integration2.2. Thermal Control Systems (Near Room Temperature)2.2.1. Heat Acquisition2.2.2. Heat Transfer2.2.3. Heat Rejection and Energy Storage2.3. Thermal Protection Systems (TPS)2.3.1. Ascent/Entry TPS2.3.2. Plume Shielding (Convective and Radiative)2.3.3. Sensor Systems and Measurement Technologies3. Interdependency with Other Technology Areas4. Possible Benefits to Other National 2TA14-23TA14-23TA14-23TA14-25TA14-25

ForewordNASA’s integrated technology roadmap, including both technology pull and technology push strategies,considers a wide range of pathways to advance the nation’s current capabilities. The present state of this effortis documented in NASA’s DRAFT Space Technology Roadmap, an integrated set of fourteen technologyarea roadmaps, recommending the overall technology investment strategy and prioritization of NASA’s spacetechnology activities. This document presents the DRAFT Technology Area 14 input: Thermal ManagementSystems. NASA developed this DRAFT Space Technology Roadmap for use by the National Research Council(NRC) as an initial point of departure. Through an open process of community engagement, the NRC willgather input, integrate it within the Space Technology Roadmap and provide NASA with recommendationson potential future technology investments. Because it is difficult to predict the wide range of future advancespossible in these areas, NASA plans updates to its integrated technology roadmap on a regular basis.DRAFT

Executive SummaryThe Thermal Management Systems TechnologyArea (TA) cross-cuts and is an enabler for mostother system-level TAs. Technology developmentin the Thermal Management Systems TA is centered on the development of systems with reducedmass that are capable of handling high heat loadswith fine temperature control. Technologies within the Thermal Management Systems TA are organized within the three sub-areas of CryogenicSystems, Thermal Control Systems, and ThermalProtection Systems.Cryogenic systems require special care for numerous reasons. The primary reason is the largerange of temperatures to which the cryogenic system is subjected. Secondarily, the maintenanceand production of cryogenic propellants requireslarge amounts of power which can be a driver forsome systems of the spacecraft. Due to the Carnotpenalty, 1 watt of heat at 20K most likely requires150-200 W at 300K to maintain it. This dictatesthe need for very efficient systems so power requirements are not increased. Without effectiveinsulation, large flow rates of gases will be ventedfrom the tank. Fortunately, the high vacuum andlow temperatures of the space environment simplifies the thermal control of cryogens in some aspects.The performance and efficiency of cryogenicsystems will have to significantly increase in orderto enable the missions being considered over thenext twenty-years. New materials capable of ascent venting without performance loss or physicaldamage and self-healing Multi Layer Insulation(MLI) or other insulation concepts must be developed and demonstrated. Insulation systems thatare built into cryogenic tank structure and the useof low-conductive composite materials will offerreductions in the combined structure and insulation mass fraction while significantly reducingcryogen boil-off losses. In addition, techniques fortailoring regolith properties to increase the thermal performance as an insulation system will haveto be developed as a mission enabler for spacecraft operating on other planetary or near-Earthobjects. The development of cryocoolers and other active cryogenic fluid management systems forthermal control of cryogenic propellants in spaceis a high priority and mission enabler for cryogenic fuel depots and long duration missions outsideof low Earth orbit (LEO). Overall system goalsfor these systems are for reduced vibration, lowermass, and lower specific power. Also, developmentof large capacity liquefaction cycles (e.g., low tem-perature radiators for pre-cooling gas, two phaseflow radiators that serve as passive liquefiers) optimized for the given environment is important.Thermal control systems maintain all vehiclesurfaces and components within an appropriatetemperature range throughout the many missionphases despite changing heat loads and thermalenvironments. Effective thermal control systemsprovide three basic functions to the vehicle/system design: heat acquisition, heat transport, andheat rejection while being mindful of the operational environment and spacecraft system. Technology advances for heat acquisition devices arecentered on high thermal conductivity materialswith a high strength-to-mass ratio and increasingthe specific energy density of the systems (i.e. highthermal performance and low mass). Once wasteheat has been acquired, it must be transported toa heat exchanger or radiator for reuse or ultimaterejection to space. The specific technology employed for transport is dependent on the temperature and/or heat flux and thus a wide variety ofequipment and techniques can be used. The development of single loop architectures could savesignificant weight, reduce system complexity, andincrease reliability of the thermal design of crewedsystems. An additional heat transport technologyrequiring development is in the area of heat pipes.Loop Heat Pipes (LHP) and Capillary PumpedLoops (CPL) provide significant heat transportover long distances with low temperature drop.Thermal energy can also be stored for later use orrejection into a more favorable environment, thussignificantly reducing the thermal control systemmass by smoothing out the effects of peak andminimum thermal loads as well as the extreme environments. A method of coping with the periodic long-duration extremely-cold environmentsthat will occur on planets that do not have an atmosphere is to devise a method of amelioratingthe thermal environment which can significantly reduce the required mass of the thermal systemdesign.Thermal protection consists of materials andsystems designed to protect spacecraft from extreme high temperatures and heating during allmission phases. Reusable thermal protection systems (TPS) are also key technologies for hypersonic cruise vehicles. Despite the current trend tomove away from systems requiring this kind ofTPS there is a national need to not only maintainthis technology and its manufacturing, but also toadvance the state of the art (SOA) in several areas, particularly maintainability, system size, mass,DRAFTTA14-1

and system robustness. Additional technology development is needed to increase the robustnessand reduce the maintenance required for reusable TPS. In the area of hot structures, high temperature heat pipes hold the promise of providinghigh heat flux capability far in excess (5-10x) ofhigh temperature materials. Large inflatable/flexible/deployable heat shields enable the consideration of an entirely new class of missions – flexible TPS is enabling for deployable entry systems.For many exploration missions rigid ablative materials are an enabling technology and are needed for dual- heat pulse reentries and for very highvelocity entries. Advances are required to significantly lower the areal mass of TPS concepts, demonstrate extreme environment capability, high reliability, improved manufacturing consistency andlower cost, and dual-heat pulse capability. Froman analytical perspective, recent efforts have revived ablation analysis capabilities but these needto be further developed to include developmentof material response/flow field coupling codes, integration of ablation models into standard 3-dimensional thermal modeling codes, and groundtesting to generate data for code correlation andvalidation.Future missions show the need for higher heatrejection, cryogenic propulsion stages, and highenergy atmospheric reentry trajectories. Based onthese criteria, the Thermal Management SystemsTA has prioritized the following technical challenges for thermal management systems:1. Low density ablator materials and systems forexo-Low Earth Orbit (LEO) missions ( 11km/s entry velocity)2. Innovative thermal components and looparchitecture3. 20K Cryocoolers and Propellant TankIntegration4. Low Conductivity Structures/Supports5. Inflatable/Flexible/Deployable heat shields6. Two-phase Heat Transfer Loops7. Obsolescence-driven TPS materials andprocesses8. Supplemental Heat Rejection Devices(SHReDs)9. Hot structures10. Low temperature/power cryocoolers forscience applicationsSuccessful development of the various technologies captured under the Cryogenic, Thermal Control, and Thermal Protection System eleTA14-2ments would impact almost every figure of merit(e.g., mass, reliability, performance, etc.). Someadvancements in TPS technology fall under thecategory of “game changing,” while others wouldrepresent significant advancements in technology currently available. Implementation of a single-loop thermal control system is a significantsystem simplification thereby increasing the system reliability while decreasing integration effortsfor the system. Finally, 20 K cryocoolers capableof 20 W of refrigeration would offer a significantmass savings in cryogen storage through a significant reduction of cryogen boil off and would be amission enabler for long term cryogen storage forlong duration missions.In summary, the Thermal Management Systems TA cross-cuts and is an enabler for most other system-level TA’s with specific interdependencies identified with ten of the remaining fourteenTA’s. The primary benefits from investment in thetechnologies outlined for cryogenic systems, thermal control systems, and thermal protection systems are enabling missions, reducing system mass,& increasing system reliability. Finally, the strategic roadmap for the Thermal Management Systems TA is balanced between Technology Push &Mission Pull.DRAFT

Figure 1: Thermal Management Systems Technology Area Strategic Roadmap (TASR)DRAFTTA14–3/4

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1. General Overview1.1. Technical ApproachThe Thermal Management Systems TechnologyArea (TA) cross-cuts and is an enabler for mostother TAs. Thermal management runs the gamutfrom milliwatt cryogenic fluid management systems to megawatt thermal control systems for nuclear propulsion architectures; from achieving zeroboil off (ZBO) for large scale in-space cryogenicfluid storage systems to protection of vehicles toaerothermodynamic heating during reentry at velocities of 11 km/s and higher; and from integration of vehicle structure and the thermal management system to insulation systems that also serveas micrometeoroid and orbital debris (MMOD)protection. Technology development in the thermal control area is centered on the development ofsystems with lower mass that are capable of handling high heat loads with fine temperature control. Technologies within the Thermal Management Systems TA are organized within the threesub-areas of Cryogenic Systems, Thermal ControlSystems, and Thermal Protection Systems.As long as chemical propellants are the most efficient primary propulsion systems used in space,there will be the need for cryogenic propellants.The performance of LOX/LH2 engines surpasses other competing technologies. However, cryogenic systems require special care for numerousreasons. First is the large temperature range thesystem must endure. This has a wide effect on materials properties. Control or heat rejection fromthis temperature range require large amounts ofpower to produce the propellants, which can bea driver for some systems of the spacecraft. However, with proper design thermal management ofcryogenic propellants may be easier in space thanon Earth. Many of the cryogenic technologies detailed in this TA are driven by the mission pullof in-space cryogenic servicing needs of chemicalpropulsion stages in the current Human Exploration Framework Team (HEFT) Design Reference Mission (DRM) and the potential Flagshipcryogenic storage and transfer mission. However,significant technology push opportunities exist asmaterials advances allow for development of efficient low TRL heat and energy transport processes at very low temperatures.Material properties tend to change as they areoperating in the cryogenic regime. One of themost obvious is variations in coefficient of thermal expansion between materials, which can affect rotating equipment such as pumps and com-pressors. Many materials used in heat exchangershave large decreases in conductivity and specificheat as they approach 30K. Then there is brittleness issues limiting the classes of metals designerscan work with. In addition to the material complexity are thermodynamic considerations. Due tothe Carnot penalty, 1 watt of heat at 20K mostlikely requires 150-200 W at 300K to maintainit. This dictates the need for very efficient systemsso power requirements are not increased. Finally,thermal control is important since a large quantity of super-cold fluid depends on it. Without effective insulation, large flow rates of gases will bevented from the tank. This has a major impact onmission architecture. Fortunately, the vacuum andultra-cold sink temperature of the space environment help to simplify thermal control of cryogensas compared to on Earth.In its most basic form, thermal control is themaintenance of all vehicle surfaces and components within an appropriate temperature rangethroughout the many mission phases despitechanging heat loads and thermal environments.For satellites this requires that the thermal controlsystem must maintain all of the equipment within its operating and/or storage temperature range.Similar to the system for satellites, the thermalcontrol system for human-rated vehicles must alsomaintain all of the equipment within the appropriate temperature ranges. In addition to component-level temperature maintenance, the crewedspacecraft’s thermal control system must also safely maintain the internal cabin temperature withinthe proper temperature range to ensure both crewsurvivability and comfort. This section focuseson the technologies required to maintain thermalcontrol of the vehicle within the "mid" level temperature range.An effective thermal control system must provide three basic functions to the vehicle/systemdesign: heat acquisition, heat transport, and heatrejection while being mindful of the operationalenvironment and spacecraft system. The followingsections discuss the critical technologies requiredto advance these three functions with the understanding that some of the proposed technologiesoverlap two or more functions and each functionis dependent to some degree upon the other two.Thermal protection consists of materials andsystems designed to protect spacecraft from extreme high temperatures and heating during allmission phases. Reusable thermal protection systems (TPS) are also key technologies for hypersonic cruise vehicles. Extreme high temperaturesDRAFTTA14-5

and heating may be due to not only aerothermodynamic heating effects but engine plume and exhaust heating effects as well.Development of new thermal protection materials, systems, and technologies requires extensivetesting using unique facilities such as arc jets andradiant heat chambers. Development of high fidelity analytical models and the associated techniques, anchored in test and flight data, with anunderstanding of the physics of heat transfer,stress, surface chemistry, interaction with the aerothermodynamic convective and radiative heatingenvironments and relevant pressures and enthalpies, decomposition chemistry, and overall system performance is also required. Hence, testingand analysis, as well as a thorough characterizationof material properties are assumed to be integralparts of technology development and maturation.Finally, it should be noted that TPS technologyis integral to Entry, Descent, and Landing (EDL).The entry TPS technologies identified under thisTA are consistent with key technologies identifiedduring the EDL technology road mapping processand have been fully coordinated with the EDL TAteam.1.2. BenefitsSuccessful development of the various technologies captured under the Cryogenic, ThermalControl, and TPS elements would impact almostevery figure of merit (e.g., mass, reliability, performance, etc). Some advancements in TPS technology fall under the category of “game changing”(e.g., inflatable TPS for large mass payload delivery to Mars, heat pipes for hypersonic cruise vehicle), while others would represent significant advancements in technology currently available (e.g.lighter, cheaper, smaller, more robust, environmentally-friendly insulation materials with fewer maintenance requirements and built-in energyharvesting). A TPS fitting the previous descriptionwould save precious spacecraft weight, thereby increasing performance and payload capacity. Implementation of a single-loop thermal control system is a significant system simplification therebyincreasing the system reliability while decreasingintegration efforts for the system. Finally, a 20 Kcryocooler capable of 20 W of refrigeration wouldoffer a significant mass savings in cryogen storagethrough a significant reduction of cryogen boiloff and would be a mission enabler for long termcryogen storage for long duration missions.TA14-61.3. Applicability/Traceability to NASAStrategic Goals, AMPM, DRMs, DRAsZero boil off cryogenic storage in space has beena feature of many past and current NASA architectures, including Mars, Constellation Lunar,and current HEFT DRMs. In addition, the SpaceOperations Mission Directorate (SOMD) continues to help pull the state of the art in sensor cryogenic technology.The Fundamental Aero goals as listed in theAgency Mission Planning Manifest (AMPM) include hypersonics elements that are directly supported by the TPS technologies presented here,including enabling heat pipe technology. AblativeTPS technology advancement is explicitly identified as a must for DRM 2B, and inflatable TPSdevelopment is critical for all DRMs whose ultimate end is to land a large payload on Mars. TPSHealth Monitoring Systems (HMS) and integrated thermo-electric generators (TEGs) have beenidentified as technologies that may potentially enhance any exploration mission.1.4. Top Technical ChallengesFuture missions show the need for higher heatrejection, cryogenic propulsion stages, and highenergy atmospheric reentry trajectories. Based onthese criteria, the Thermal Management SystemsTA has prioritized the following technical challenges for thermal management systems:1. Low Density Ablator Materials and Systemsfor Exo-Low Earth Orbit (LEO) Missions( 11 km/s Entry Velocity) – For manyexploration missions, such as near-Earthasteroid and Mars missions, ablative materialsare an enabling technology and are neededfor dual heat pulse reentries and for very highvelocity entries (i.e., 11 km/s).2. Innovative Thermal Components and LoopArchitecture – An enabling thermal technologyoffering significant mass and power savingsand increased reliability will result frommore efficient systems capable of operatingover a wide range of heat loads in varyingenvironments (for example, a 10:1 heat loadrange in environments ranging from 0 to 275K). A system level approach should be takenwhich includes advanced fluids, advancedradiator design, and other components.3. 20K Cryocoolers and Propellant TankIntegration – Active thermal control ofcryogens in space can eliminate boil off anddramatically decrease required propellantmass for long duration space missions.DRAFT

4.5.6.7.8.9.Development of low temperature cryocoolersand cryocooler to tank integration techniquesare needed.LowConductivityStructures/Supports– Current propulsion stages use highconductivity aluminum as supports, leadingto high heat leak. Low thermal conductanceor reconfigurable supports will reduce thisheat leak and minimize power requirementsfor active cooling systems.Inflatable/Flexible/Deployable Heat Shields -Analytical studies have shown that large heatshields provide a means to increase the downmass to the Martian surface. Large inflatable/flexible/deployable heat shields enable theconsideration of an entirely new class ofmissions.Two-phase Heat Transfer Loops – Thistechnology allows the transfer of small orlarge amounts of waste heat (typically a1:100 ratio) over long distances, with verylittle temperature drop. Advanced two-phaseloops allow heat load sharing thus conservingenergy.Obsolescence-Driven TPS Materials andProcesses – This effort continues developmentof replacement cryoinsulation, primer,adhesive, and ablator TPS materials that arecurrently facing obsolescence. These fourclasses of materials are each subject to uniqueobsolescence issues that will limit theiravailability for future programs.Supplemental Heat Rejection Devices(SHReDs) – Future technology developmentefforts should focus on heat rejection hardwarerequired for transient, cyclical applications.Depending on the duration of the missionphase, this function can be accomplishedusing either Phase Change Material (PCM)heat exchangers or evaporative heat sinks. Anevaporative heat sink utilizes a consumablefluid and future development efforts shouldfocus on the efficient use of this consumablewhen an evaporator is used as a SHReD.PCM heat exchanger development, on theother hand, should focus on improving theenergy storage capacity of these devices whileminimizing the hardware mass. Particularattention should be focused on combining thefunction of PCM with radiation shielding forcrew members.Hot Structures – Advancements in hightemperaturematerials,environmentalcoatings, material characterization, structuraldesign and manufacturing processes, and lifeand damage assessment methods will enablethe design optimization of advanced re-entryand hypersonic flight vehicles.10. Low Temperature/Power Cryocoolers forScience Applications – Advanced lowtemperature cryocooler technology enablesoperation of detectors for scientific observationof the universe. Advances in size, efficiency andreduced vibration/interference are needed.2. Detailed Portfolio DiscussionThe Thermal Management Systems TA hasidentified and detailed numerous technologies inthe following subsections that are a mix of bothTechnology Push and Mission Pull. The missionsthat have been identified as Mission Pull candidates for technologies from this TA are identifiedacross the top row of Figure 1 which is identifiedas Major Milestones.For Cryogenic Systems, the Mission Pull opportunities are the Cryostat Demonstration and theCryogenic Propulsion Stage which will requireadvanced multi-layer insulation and high-capacity20K cryocoolers. Thermal Control Systems haveidentified Mission Pull opportunities for advancedphase change materials, advanced thermal controlsystem fluids, and variable heat rejection radiatorswhich are demanded by NEO pre-cursor roboticand Crew-to-LEO missions. Technology push opportunities for Thermal Control Systems includehigh temperature materials and components formegawatt systems; high flux cooling with precisetemperature control; and advanced heat exchangers and lightweight radiators. Finally, for ThermalProtection Systems, the Mission Pull suite of technologies that are identified are rigid ablative TPSwhich will be pulled by Crew-to-LEO, Mars Sample Return, Mars pre-cursor, Hypervelocity EarthReturn Demo, and Crewed NEO missions; obsolescence-driven TPS will be pulled by Crew-toLEO; and structurally integrated TPS and multifunctional TPS will both be pulled by MMSEVand Deep Space Habitat.The following subsections are devoted to the description of the current state of the practice, limitations of the practice, identification of the technologies to exceed these limitations, and an estimateof the current technology readiness level (TRL)and timeframe required to advance the technology to TRL 6 for the Thermal Management TA.The first-level Technical Area Breakdown Struc-DRAFTTA14-7

Figure 2. First-Level Thermal Management TA TABSture hierarchy is classified via the temperature re- Thermal Control, and System Integration. Thegimes that thermal management systems are re- lower-tiered TABS for Cryogenic Systems is proquired to operate: Cryogenic Systems, Thermal vided in Figure 3.Control Systems, and Thermal Protection Sys- 2.1.1. Passive Thermal Controltems. This first-level hierarchy of the TABS is provided in Figure 2.2.1.1.1. Large-Scale Multi-Layer Insulation(MLI)2.1. Cryogenic SystemsMLIsystemshave been in use for in space cryoThe first major technology area within the Ther- genic propulsionapplications for many years butmal Management TABS is Cryogenic Systems use has been limitedin size and performance. Exwhich refers to those systems that are operating amples include the AtlasDelta upper stagesbelow -150 C. Cryogenic Systems is further dis- which typically have threeandlayersof MLI and arecretized into Passive Thermal Control, ActiveFigure 3. Cryogenic Systems TABSTA14-8DRAFT

intended for a few hours use. Evaporation ratesare on the order of 2 percent per day. Future applications such as Earth departure stages requirelarger volumes and much longer orbital storagetimelines. This will require an order of magnitude increase in thermal performance with passiveevaporation rates on the order of 0.2 percent perday. Application of large number of layers and integration with vapor cooled shields on high surface area tanks must be demonstrated. Of particular interest are methods of minimizing losses atseams and penetrations and other areas of changing geometry by overlapping of layers, heat stationing blankets, or using hybrid aerogel/MLI systems. Materials capable of ascent venting withoutperformance loss or physical damage must be developed and demonstrated. Currently, high performance large scale MLI systems for space applications is at Technology Readiness Level (TRL)3 and successful development to TRL 6 will take3-5 years.2.1.1.2. Advanced MLI SystemsCurrent MLI concepts utilize a number of layersof radiation shielding separated by layers of lowthermal conductance spacers to minimize conduction losses across the radiation layers. NewMLI concepts have been proposed that eliminatethe need for low conductivity layers of paper between the radiation shields. These new insulationsuse more rigid metallic layers separated by a system of discrete molded polymer spacers to precisely control spacing and layer density. Lowermass and higher performance are predicted benefits over current systems. The possibility also existsto expand these concepts so the outer layer is capable of supporting a soft vacuum while on Earth,compressing slightly while being supported by thespacer system. This offers large performance benefits during the ground and launch phases of themission where typical MLI systems are not veryeffective. Materials development and processing and manufacturing improvements needed tobring this to TRL 6 will take approximately 3-5years.2.1.1.3. Multifunctional Insulation/MMODProtectionIntegration of multi-functional insulating materials into other spacecraft systems can reducespacecraft mass and increase simplicity. For instance, MLI has shown some ability to serve as aneffective MMOD protection. Analysis and optimization of MLI systems to increase this protection effect is needed. Self-healing materials thatcan repair damage from handling or micrometeoroids while maintaining thermal performanceshould be investigated. These self healing systemsare perhaps 15-20 years in the future. A more nearterm goal is insulation built into tank structures,such as evacuated honeycomb tank walls or aerogel filled annular tanks. Multifunctional systemsthat serve as cryotank insulation as well as hightemperature thermal protection is an ideal longterm goal. Demonstrations of these cross cuttingcapabilities will take approximately 8-10 years.2.1.1.4. Ground to Flight InsulationA high percentage of the overall heat transferredto flight tanks occurs during the ascent phase ofthe mission. There is as much thermal energytransfer during the ascent phase as there is during 6 days of steady state orbital operations using conventional MLI. MLI is very effective whilein vacuum but not as good in soft vacuum or atmospheric conditions as other insulation methods. Cryopumping of atmospheric moisture canalso damage MLI and hurt on orbit performance.Hybrid insulation schemes that are effective during ground and ascent phases while still offeringoptimal performance for long duration on orbitstorage are needed. Foam/MLI and aerogel/MLIhybrid schemes are potential options for development. Hydrophobic materials or coatings can beconsidered. Deployable ground insulation panels which work during launch countdown butare then detached to minimize mass to orbit areanother potential solution. While hybrid foam/MLI systems have been tested in ground chambers, aerogel/MLI and deployable insulations areat TRL 3 and will require 3-5 years of development to achieve TRL 6.2.1.1.5. Low Conductivity SupportsConduction heat leak across mechanical supports such as struts, skirts, and feedlines can begreater than the convection/radiation heat leakacross the tank surface. Innovative methods ofminimizing/eliminatin

area roadmaps, recommending the overall technology investment strategy and prioritization of NASA's space technology activities. This document presents the DRAFT Technology Area 14 input: Thermal Management Systems. NASA developed this DRAFT Space Technology Roadmap for use by the National Research Council (NRC) as an initial point of departure.

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