James L. Sanzi

Sest, Inc., Middleburg Heights, Ohio

Donald A. Jaworske

Glenn Research Center, Cleveland, Ohio

Heat Pipes and Heat Rejection Component Testing 

at NASA Glenn Research Center

NASA/TM—2012-217205

February 2012

NETS–2011–3497
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James L. Sanzi

Sest, Inc., Middleburg Heights, Ohio

Donald A. Jaworske

Glenn Research Center, Cleveland, Ohio

Heat Pipes and Heat Rejection Component Testing 

at NASA Glenn Research Center

NASA/TM—2012-217205

February 2012

NETS–2011–3497

National Aeronautics and

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Glenn Research Center

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Acknowledgments

This work was performed for the National Aeronautics and Space Administration (NASA) Enabling Technology Development 

and Demonstration and the Fission Power Systems Projects. The opinions expressed are those of the authors and do not 

necessarily refl ect the views of NASA. The authors would like to thank Lee Mason, Project Lead Engineer; Don Palac,

Project Manager; Richard Shaltens, Branch Chief; Duane Beach, Branch Chief; Marc Gibson, Engineer; Ed Sechkar,

Design Engineer; and NASA facilities staff for their guidance and support.

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217205 1 

Heat Pipes and Heat Rejection Component Testing at 

NASA Glenn Research Center 

James L. Sanzi 

Sest, Inc. 

Middleburg Heights, Ohio 44130 

Donald A. Jaworske 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Titanium-water heat pipes are being evaluated for use in the heat rejection system for space fission 

power systems. The heat rejection system currently comprises heat pipes with a graphite saddle and a 

composite fin. The heat input is a pumped water loop from the cooling of the power conversion system. 

The National Aeronautics and Space Administration has been life testing titanium-water heat pipes as 

well as evaluating several heat pipe radiator designs. The testing includes thermal modeling and 

verification of model, material compatibility, frozen startup of heat pipe radiators, and simulating lowgravity environments. Future thermal testing of titanium-water heat pipes includes low-gravity testing of 

thermosyphons, radiation testing of heat pipes and fin materials, water pump performance testing, as well 

as Small Business Innovation Research funded deliverable prototype radiator panels. 

Introduction 

The NASA Enabling Technology Development and Demonstration Program’s Fission Power Systems 

(FPS) Project develops technologies to enable the option of using fission power systems for future space 

explorations missions. Heat pipes and heat pipe radiator technology are part of the heat rejection system 

for various space fission power systems power applications. Conceptually, heat from a reactor would be 

utilized to power several Stirling convertors to generate electricity. For a 40-kWe installation, radiators 

would be needed to dissipate approximately 140 kWt of waste heat.The notional design utilized large 

radiators of 185 square meters radiating area and operated at heat rejection temperatures ranging from 350 

to 500 K. The current radiator designs are envisioned to operate at a temperature near 400 K. A heat pipe 

is an efficient heat-dissipation and heat-spreading device and in its simplest form is a passive two-phase 

heat transfer device in a sealed tube. 

Nomenclature 

ALIP A Linear Induction Pump 

DOE Department of Energy 

FPS Fission Power Systems 

ISS International Space Station 

kWe kilowatts electric 

kWt kilowatts thermal 

LHP loop heat pipe 

MSFC Marshall Space Flight Center 

NASA National Aeronautics and Space Administration 
NASA/TM—2012-217205 2 

PCAD Power Conditioning and Distribution 

QCM Quartz Crystal Microbalance 

RGA Residual Gas Analyzer 

SBIR Small Business Innovation Research 

T Temperature (K) 

TDU Technology Demonstration Unit 

Hardware Testing 

A radiator concept design can be seen in Figure 1 (Siamidis, 2006). Several technology challenges 

quickly became evident. The large size meant traditional metal radiators, such as those used on the 

International Space Station (ISS), would carry a large mass penalty. Second, the rejection temperature 

was well beyond the capability of existing large flight radiators and would require new heat rejection 

fluids, or use of existing fluids at temperatures beyond their typical range. Copper-water heat pipes are 

well established in the electronic cooling industry with operating temperatures in the 300 to 400 K range. 

The need for heat pipes to operate in the 500 K range led to the development of heat pipe materials that 

could operate at elevated temperatures and pressures. Copper-nickel and titanium alloys were investigated 

as well as a variety of potential heat pipe working fluids. Titanium-water compatibility tests have shown 

promising results, while use of other working fluids would require further investigation and development. 

Thus water was selected water as the primary heat rejection working fluid, and the work with alternate 

fluids was stopped. Titanium was chosen as the envelope material for its potential long-term compatibility 

with water, high strength, and low density. NASA determined that the design and fabrication of multiple 

titanium-water heat pipes was necessary to accelerate heat pipe development and address questions about 

their true performance capability, long-term durability, and readiness of the manufacturing sector. 

Titanium-Water Life Test Heat Pipes 

Over 3 years of thermal testing on nine titanium-water heat pipes has been achieved in the Heat Pipe 

Thermal Testing Laboratory at NASA Glenn Research Center (GRC). The titanium-water heat pipes are 

being tested at 500 K on a continuous 24-hour/7-day basis. Three manufacturers, Advanced Cooling 

Technologies, Thermacore, Inc., and Swales Aerospace Inc. (ATK), each produced three 1.25-cmdiameter by 1.1-meter-long titanium-water heat pipes. Each vendor used different internal wick 

characteristics resulting in slightly different performance. The nine heat pipes have performed as expected 

with negligible degradation over the 3 years. The life test setup can be seen in Figure 2. 

 

Figure 1.—Heat pipes integrated in a radiator panel. Figure 2.—Titanium-water heat pipe life test setup. 
NASA/TM—2012-217205 3 

Thermal Interface Evaluation 

The heat exchange from a closed-loop heat source to the evaporator of a thermosyphon was 

investigated. The evaporator section was either immersed in the closed-loop heat source, with the closedloop heat source fluid delivering the heat to the evaporator through the evaporator wall, or the evaporator 

section was clamped adjacent to the closed-loop heat source, with the closed-loop heat source delivering 

heat to the evaporator wall via conduction through the clamshell arrangement of hardware surrounding 

the evaporator. Essentially identical thermosyphons were either immersed in the closed-loop heat source 

or clamped adjacent to the closed-loop heat source, and the wattage transported from the closed-loop heat 

source through the heat pipe and out through the condenser was carefully measured by a surrounding 

calorimeter. Thermocouples measuring temperature difference between the closed-loop heat source and 

the evaporator section of the heat pipes, along with the flow through the calorimeter as measured by a 

calibrated flow meter, were used to obtain thermal conductance through the immersed or clamped 

interface. A temperature control unit (manufactured by Sterling, Inc.) operating on tap water was utilized 

to provide a constant temperature closed-loop heat source. With the immersion hardware installed, 

stainless steel plumbing allowed fluid flow past the evaporator. With the titanium clamshell hardware 

installed, flow through the clamshell arrangement could also be achieved enabling the closed-loop heat 

source to operate at temperatures as high as 394 K. The CP–2 titanium-water thermosyphons were 

manufactured in house; each was 1.27 cm in diameter and 74 cm in length with a titanium screen wick to 

line the evaporator wall section only. The thermal interface test setup can be seen in Figure 3. A selection 

of interface materials were evaluated in the clamped configuration, selected based on high-temperature 

durability, pliability, and availability. Two GrafTech International grafoil products were evaluated: 

eGRAF® HITHERMTM 705 and eGRAF® HITHERMTM 1205. The HT–705 has a typical throughthickness thermal conductivity of 6.0 W/mK, while the HT–1205 has a through- thickness thermal 

conductivity of 10 W/mK. Both grafoil products were plain, with no coatings or adhesives applied. One 

Aavid Thermalloy product was evaluated: Aavid Sil-FreeTM thermal grease. Copper and silver leaf were 

selected owing to their pliability. The two leaf products were purchased through an art supply house, and 

were simply wrapped around the evaporator at the time of installation. T-MateTM, a phase change product 

by Laird Technologies, was selected owing to its attractive phase change temperature in the range of 323 

to 343 K combined with its maximum operating temperature of 398 K. Figure 4 shows the many different 

evaporators tested. 

 

Figure 3.—Thermal interface evaluation system. Figure 4.—Thermal interface machined evaporators. 
NASA/TM—2012-217205 4 

RDU (Lamps) Freeze Tests 

Radiator panels utilizing titanium-water heat pipes are flat panels approximately 2.54 cm thick by 

0.5 m wide and 1 meter long. High thermal conductivity carbon-polymer facesheets are adhesively 

bonded to an aluminum honeycomb core and three titanium-water heat pipes are embedded within the 

core to provide heat transport. The heat pipes are adhesively bonded to a graphite foam saddle and the 

foam is in turn bonded to the PMC facesheets for the purpose of simultaneously providing a thermal 

pathway and addressing mismatch in coefficients of thermal expansion. A traditional sandwich structure 

is envisioned where heat pipes are embedded between two high-thermal conductivity facesheets. The heat 

pipe evaporators are to be thermally connected to the heat source through one or more manifolds 

containing coolant. Initial radiator operation on the lunar surface would likely follow a cold soak where 

the water in the heat pipes is purposely frozen. To achieve heat pipe operation, it is necessary to thaw the 

heat pipes. One option is to allow the sunlight impinging on the surface at sunrise to achieve this goal. 

Testing was conducted in a thermal vacuum chamber to simulate the lunar sunrise and additional 

modeling was conducted to identify steady-state and transient response. It was found that sunlight 

impinging on the radiator surface at sunrise was insufficient to solely achieve the goal of thawing the 

water in the heat pipes. However, starting from a frozen condition was accomplished successfully by 

applying power to the evaporators. Figure 5 shows a radiator panel on framework with quartz lamps and 

inside the vacuum chamber cold wall. 

Second Generation Radiator Demonstration Unit 

In support of the FPS project, the Second Generation (2nd GEN) Radiator Demonstration Unit (RDU) 

was developed by Material Innovations Inc. (MII) in Huntington Beach, California, and delivered to 

NASA GRC in Cleveland, Ohio. The 2nd GEN RDU is shown in Figures 5, 6, and 7. It consists of a 

titanium-water fluid manifold coupled to 16 titanium-water heat pipes that are sandwiched between two 

composite facesheets enhanced with white thermal control paint. The radiator assembly is supported by 

an aluminum frame that can be suspended in the vacuum facility. A thermocouple patch panel below the 

water manifold provides the interface for instrumentation cabling. The 2nd GEN RDU was designed to 

reject at least 6000 W to a 250 K thermal sink with 400 K water inlet temperature and both manifolds at 

0.25 kg/s. The radiator also was operated over a wide range of water inlet temperatures, flow rates, and 

sink temperatures. 

Figure 5.—Framework above the RDU panel with 

four quartz lamps. 

Figure 6.—2nd GEN radiator panel. 
NASA/TM—2012-217205 5 

 

Figure 7.—Infrared camera image (T Inlet 430 K). Figure 8.—Thermal testing of ammonia LHP. 

 

Figure 9.—Square heat pipe. Figure 10.—Low-g flight test. 

Loop Heat Pipe 

The loop heat pipe (LHP) with ammonia working fluid is a common heat transport device used in 

spacecraft. The LHP used here was manufactured in 1997 by Thermacore, Inc., and shipped to Goddard 

Space Flight Center (GSFC). The LHP was shipped to GRC in 2002. In the summer of 2007, the LHP 

was removed from storage and tested at GRC as part of NASA’s Exploration Technology Development 

Program. The LHP survived an approximate 10-year dormant period and was in good working order. 

There were several important accomplishments during this investigation: 1) A mini-loop heat pipe was 

tested after approximately 10 years in storage and appeared to work normally, 2) this testing demonstrates 

that this novel method for surviving the cold lunar environment is practical, 3) this testing has shown that 

an ordinary loop heat pipe was robust enough to withstand temperatures well below the design 

temperature and well below the temperature of frozen ammonia, 4) it appears that larger startup heat is 

necessary under these conditions when compared to room temperature startup conditions, and 5) it was 

shown that the reduced gravity of the Moon does not affect the performance of this LHP, when compared 

to performance in a zero-gravity environment. Figure 8 shows LHP test setup. 

Low-g Heat Pipes

There are several concepts for evaluating thermosyphon performance in a reduced-gravity 

environment. The simplest technique is to tilt the thermosyphon. The angle of tilt is selected utilizing the 

cosine function, such that the gravity vector in the axial direction of the thermosyphon is reduced by the 

desired amount. An angle of 9.5° simulates the lunar surface. Figures 9 and 10 shows a square heat pipe 

to simulate low-g operation by evaporating and condensing on one surface. Figure 10 shows 

thermosyphon array controlled operation for zero g flight test article. 
NASA/TM—2012-217205 6 

Figure 11.—SBIR contributions to the fission power systems project. 

Small Business Innovative Research 

SBIR grants are an important and continuous source of component development hardware. The FPS 

Project is continuously seeking unique and innovative component technology innovations. SBIRs have 

developed several heat pipe radiators, material evaluations such as high-temperature coatings, and life test 

of materials and reliability models of system functions. Figure 11 illustrates the many SBIR contributions 

to the FPS Project. 

Component Radiation 

An important area of space fission power systems is reliability of components. Fission Power 

Systems components in the proximity of an operating reactor must tolerate radiation exposure, or be 

shielded. Most FPS components can be constructed of high radiation-tolerant materials; however, the 

radiation tolerance some radiator materials needed to be characterized for the prescribed configuration 

and environment of their application in a space fission power system. At the Sandia National Laboratories 

radiator components will be exposed, which includes titanium-water heat pipes with a composite fin, 

foam saddle, and a candidate epoxy to hold the components together. 

Allcomp heat pipe radiator, Creare pyrolytic fin radiator, Creare heat pipe radiator, Thermacore, Inconel 718 life test 

ACT heat pipe development, ACT Stirling VCHP, ACT intermediate heat pipe fluids, Alion high temperature coating 
NASA/TM—2012-217205 7 

Sodium-Potassium (NaK) Eutectic Detection 

The pressure of NaK during a leak event is difficult to measure. The Technology Demonstration Unit 

(TDU) piping will have multilayer insulation so knowing the exact temperature and pressure of the NaK 

during an unexpected leak event would be unknown. Several different methods and instruments were 

reviewed to determine which hardware would be best suited for the task. At completion of the evaluation 

it was determined that a Residual Gas Analyzer (RGA) and a Quartz Crystal Microbalance (QCM) were 

the best candidates for near-term evaluation. The test results have given conclusive data that detection of 

NaK in a space simulated environment can be achieved using both the QCM and RGA methods together. 

Using sodium as the signature atomic mass, the RGA will be the primary detector with a partial pressure 

range down to 1.0×10–10 torr. Quantifying the amount of NaK being released will be done using the QCM 

by monitoring the deposition thickness and rate. Together, these instruments should provide enough data 

to successfully detect a NaK leak and provide opportunity to follow shutdown procedures and minimize 

any potential damage to the facility and test hardware. 

Future Work/Conclusion 

The Fission Power Systems Project is in the process of procuring components for a TDU, which will 

simulate an operating fission power reactor at NASA GRC. All the components (except a fission reactor) 

have been tested individually and are being upgraded and integrated into an operating system. The reactor 

simulator loop has been fabricated at NASA Marshall Space Flight Center (MSFC) and satisfactorily 

tested. A Linear Induction Pump (ALIP) has been fabricated by the Department of Energy (DOE) and 

installed in the reactor simulator loop and successfully tested. A full-scale radiator panel was fabricated 

my Material Innovations, Inc., and successfully tested at GRC. Stirling convertors have been successfully 

tested using pumped liquid metal (NaK) as the heat source at MSFC. A Power Conditioning and 

Distribution (PCAD) System is jointly being developed by NASA and DOE. 

References 

Siamidis, J., and Mason, L., “A Comparison of Coolant Options for Brayton Power Conversion Heat 

Rejection Systems,” Space Technology and Applications International Forum—2006, edited by 

M.S. El-Genk, American Institute of Physics, Albuquerque, NM, 2006, pp. 682–693. 

Sanzi, J. L., “Thermal Performance of High Temperature Titanium-Water Heat Pipes by Multiple Heat 

Pipe Manufacturers,” Space Technology and Applications International Forum—2007, edited by 

M.S. El-Genk, American Institute of Physics, Albuquerque, NM, 2007, pp. 681–691. 

Jaworske, D.A., Beach, D.E., and Sanzi, J.L., “Heat Rejection Systems Utilizing Composites and Heat 

Pipes: Design and Performance Testing,” 5th International Energy Conversion Engineering 

Conference, St. Louis, MO, 2007, AIAA–2007–4822. 

Golliher, E., Jentung Ku, Licari, A., Sanzi, J., “Investigation of Loop Heat Pipe Survival and Restart 

After Extreme Cold Environment Exposure,” Proceedings of the ASME 2009 Summer Heat Transfer 

Conference, HT2009, July 19–23, 2009, San Francisco, CA, USA. 

Gibson, M., Sanzi, J., Ljubanovic, D., “Experimental Studies of NaK in a Simulated Space Environment” 

Draft Proceedings of Nuclear and Emerging Technologies for Space 2011 Albuquerque, NM. 

Mason, Lee, “Fission Surface Power System Initial Concept Definition” NASA/TM—2010-216772, 

August 2010, NASA GRC Cleveland, OH.
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14. ABSTRACT

Titanium-water heat pipes are being evaluated for use in the heat rejection system for space fission power systems. The heat rejection 

system currently comprises heat pipes with a graphite saddle and a composite fin. The heat input is a pumped water loop from the cooling of 

the power conversion system. The National Aeronautics and Space Administration has been life testing titanium-water heat pipes as well as 

evaluating several heat pipe radiator designs. The testing includes thermal modeling and verification of model, material compatibility, 

frozen startup of heat pipe radiators, and simulating low-gravity environments. Future thermal testing of titanium-water heat pipes includes 

low-gravity testing of thermosyphons, radiation testing of heat pipes and fin materials, water pump performance testing, as well as Small 

Business Innovation Research funded deliverable prototype radiator panels.

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Heat pipes; Heat rejection; Radiator panel 

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