Maxwell H. Briggs and Steven M. Geng

Glenn Research Center, Cleveland, Ohio

J. Boise Pearson

Marshall Space Flight Center, Huntsville, Alabama

Thomas J. Godfroy

Maximum Technology Corporation, Huntsville, Alabama

Summary of Test Results From a 1 kWe-Class

Free-Piston Stirling Power Convertor Integrated 

With a Pumped NaK Loop

NASA/TM—2010-216934

December 2010

AIAA–2010–7173
NASA STI Program . . . in Profi le

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scientifi c and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Offi cer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major signifi cant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of signifi cant 

scientifi c and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientifi c 

and technical fi ndings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scientifi c and 

technical fi ndings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientifi c and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientifi c, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientifi c and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question via the Internet to help@

sti.nasa.gov

• Fax your question to the NASA STI Help Desk 

at 443–757–5803

• Telephone the NASA STI Help Desk at

 443–757–5802

• Write to:

 NASA Center for AeroSpace Information (CASI)

 7115 Standard Drive

 Hanover, MD 21076–1320
Maxwell H. Briggs and Steven M. Geng

Glenn Research Center, Cleveland, Ohio

J. Boise Pearson

Marshall Space Flight Center, Huntsville, Alabama

Thomas J. Godfroy

Maximum Technology Corporation, Huntsville, Alabama

Summary of Test Results From a 1 kWe-Class

Free-Piston Stirling Power Convertor Integrated 

With a Pumped NaK Loop

NASA/TM—2010-216934

December 2010

AIAA–2010–7173

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

8th International Energy Conversion Engineering Conference (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

Nashville, Tennessee, July 25–28, 2010
Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://gltrs.grc.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2010-216934 1 

Summary of Test Results From a 1 kWe-Class Free-Piston Stirling 

Power Convertor Integrated With a Pumped NaK Loop 

Maxwell H. Briggs and Steven M. Geng 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

J. Boise Pearson 

National Aeronautics and Space Administration 

Marshall Space Flight Center 

Huntsville, Alabama 35812 

Thomas J. Godfroy 

Maximum Technology Corporation 

Huntsville, Alabama 35812 

Abstract 

As a step towards development of Stirling power conversion for potential use in Fission Surface 

Power (FSP) systems, a pair of commercially available 1 kW class free-piston Stirling convertors was 

modified to operate with a NaK liquid metal pumped loop for thermal energy input. This was the firstever attempt at powering a free-piston Stirling engine with a pumped liquid metal heat source and is a 

major FSP project milestone towards demonstrating technical feasibility. The tests included performance 

mapping the convertors over various hot and cold-end temperatures, piston amplitudes and NaK flow 

rates; and transient test conditions to simulate various start-up and fault scenarios. Performance maps of 

the convertors generated using the pumped NaK loop for thermal input show increases in power output 

over those measured during baseline testing using electric heating. Transient testing showed that the 

Stirling convertors can be successfully started in a variety of different scenarios and that the convertors 

can recover from a variety of fault scenarios. 

Introduction 

Free-piston Stirling power conversion has been identified as a viable option for potential Fission 

Surface Power (FSP) systems (Refs. 1 to 4). Recent studies have examined the use of Stirling convertors 

coupled to a low temperature (< 900 K), uranium-dioxide fueled, liquid-metal-cooled reactor for a 

potential lunar application in year 2020. In order to reduce development risk and address design questions 

related to the free-piston Stirling FSP system, NASA has begun long lead technology development on 

multi-kilowatt Stirling power conversion under the Fission Surface Power Technology Project. A key step 

in the development of Stirling technology relative to FSP systems is to demonstrate that a Stirling 

convertor can be successfully integrated with a pumped liquid metal loop. 

In support of this goal, two 1 kW free-piston Stirling power convertors were procured from 

Sunpower Inc., of Athens, Ohio. These convertors are designed to produce 1.1 kWe at their design 

operating conditions of 550 C hot-end temperature, a 50 C cold-end temperature, a mean working space 

pressure of 3.0 MPa, and a piston amplitude of 10 mm. The convertors delivered to the Glenn Research 

Center (GRC) were configured with electrically heated heads which were used to establish the baseline 

convertor performance. 
NASA/TM—2010-216934 2 

Figure 1.—Baseline performance map with P2A convertors configured with electrically heated 

heads. Power shown in plot is the total electric power output for both convertors. 

Figure 1 shows the electric power output of the convertors during baseline testing with electrically 

heated heads. At the design operating conditions, the convertors produced 2030 W of electric power at a 

gross thermal efficiency of 28.1 percent, where gross thermal efficiency is the electrical power output of 

the alternator divided by the electrical power input into the cartridge heaters. The maximum total power 

output of 2240 We was measured at a 550 °C hot-end and 40 °C cold- end temperature with a piston 

amplitude of 10.5 mm. This power level was achieved at a gross thermal efficiency of 28.6 percent. The 

maximum gross thermal efficiency achieved by these convertors was 30.0 percent, which occurred at a 

hot-end temperature of 550 °C, cold-end temperature of 30 °C, and a piston amplitude of 9 mm. After 

baseline testing was completed, the convertors were retrofitted with new heater heads to allow the 

convertors to be interfaced with a pumped NaK loop. The convertors were then delivered to the Marshall 

Space Flight Center (MSFC) for integration and testing in the Fission Surface Power Primary Test Circuit 

(FSP-PTC). The integrated liquid metal heat exchanger/Stirling convertor configuration for a single 

convertor is shown in Figure 2. The dual-opposed convertor configuration employed two independent 

convertor/heat exchanger assemblies in opposite orientations so as to minimize vibration-induced loads 

on the test setup. A mechanical support structure connected the alternator pressure vessel portions of the 

two convertors and acted as the mounting structure for the entire assembly within the vacuum chamber 

portion of the FSP-PTC. A common supply line from the test facility provided the liquid metal to each 

convertor. The convertors mounted in the MSFC test facility are shown in Figure 3.

This report describes the testing done at the FSP-PTC and briefly discusses the results of those tests. 

Prior publications describe the test facility and setup more completely (Refs. 5 and 6). In addition the 

most comprehensive discussion of the test data can be found the official test report (Ref. 7). 
NASA/TM—2010-216934 3 

Figure 2.—Single free-piston Stirling power convertor with NaK heat exchanger prior to 

installation of multi-layer insulation. 

Figure 3.—Dual P2A Stirling power convertors mounted in the MSFC test facility. 

P2A Stirling Convertors 

NaK Heater Core 

NaK Heater Head 

Stirling Convertor 

NaK Inlet 
NASA/TM—2010-216934 4 

Methodology 

To begin all steady-state testing, the cooling water temperature set-point was changed to the desired 

cold-end temperature for the data points to be acquired. The convertors were then briefly motored to 

verify the piston motions were centered properly and that their mean position did not drift. The NaK 

pump was then powered-up at a low to moderate flow setting while the NaK temperatures and flow rates 

were monitored to ensure normal operation of the NaK system. Next, the NaK pump was adjusted to 

provide the maximum flow rate, and the NaK heaters were switched on. The Stirling convertor pistons 

were locked in place using the convertor control system until the convertor hot-end temperatures reached 

250 °C. At this point, the convertors were set to a piston amplitude of 6 mm (this is the minimum piston 

amplitude that engages the gas bearings). The convertors were started this way to avoid piston/displacer 

contact and minimize run-time prior to gas bearing operation. As the hot-end temperature increased, the 

piston amplitude was increased until both the hot-end temperature and piston amplitude reached their 

desired set-points for the performance map point to be acquired. 

Transient test procedures differed depending on the nature of the test (high flow start-up, low flow 

start-up, convertor stall, etc.). These procedures will be discussed individually below. 

Steady—State Testing 

During steady-state testing, the power output of the convertors was measured at most of the same 

operating conditions (temperatures and piston amplitudes) at which data was acquired during baseline 

testing using electric heater heads. During these tests, the convertors were operated at hot-end 

temperatures ranging from 400 to 550 °C, cold-end temperatures from 30 to 70 °C, and piston amplitudes 

ranging from 6 to 11 mm. The performance map testing was conducted with the NaK pump operating at 

full power. The NaK flow tended to decrease as the test proceeded as the pump temperature increased. 

The typical drop in mass flow at full pump power during steady-state testing was 200 g/s (900g/s at start 

of test to 700 g/s at end of test). During mass flow performance mapping, the NaK mass flow started at 

maximum flow and was decreased incrementally until the maximum temperature difference across the 

head was reached (40 °C). For each point the system was allowed to dwell until steady-state was reached 

then data was averaged for 5 min in order to produce the performance map data points. 

Transient Testing 

Transient testing was conducted to study fault tolerance scenarios and possible start-up scenarios. 

Three fault tolerance scenarios were examined: 1) NaK pump loss, 2) heat source loss (i.e., loss of 

reactor), and 3) shutdown/restart of Stirling convertors. These scenarios were examined to determine the 

effects on the system, and to determine possible methods of recovery. To begin each of the fault 

scenarios, the system was brought to the design operating condition, according the steady-state procedure, 

a steady-state data point was recorded, and then a fault was initiated. 

Start-up testing was focused on varying the mass flow rate as well as the temperature at which piston 

motion was initiated. These tests started by locking the Stirling convertor pistons in place using the 

controller and setting the NaK flow rate to the desired setpoint. The NaK temperature was then increased 

at a rate of 10 °C per minute, until the desired piston initiation temperature was reached. At that point the 

Stirling convertor piston amplitude was controlled to 6 mm. From the NaK temperature continued to 

increase at 10 °C per minute and the piston amplitude was increased as well, until the design point 

operating conditions were reached. 
NASA/TM—2010-216934 5 

Results 

Steady—State Performance Mapping 

Figure 4 shows the combined power output of both convertors plotted as a function of piston 

amplitude. The power output of the convertors using NaK for thermal input is very similar to the baseline 

data acquired for the convertors equipped with the electric heater heads. The power output at the design 

condition was 2026 W for the convertor pair. The maximum power output was 2375 W at a hot-end 

temperature of 550 °C, cold-end temperature of 50 °C and piston amplitude of 11 mm. It should be noted 

that convertor efficiency was calculated at all of performance map test points; however, due to large 

uncertainties in both NaK mass flow as well as temperature difference across the NaK head, the heat input 

to the convertors and therefore the convertor efficiency measurement has substantial uncertainty. 

Therefore, the following efficiency values do not definitively quantify convertor efficiency. The 

calculated convertor efficiency was 32.2 percent at the design point, and ranged from 29.5 to 33.4 percent 

over the entire range of operating conditions tested. 

Figures 5 and 6 compare the power output of the convertors configured with the NaK heater heads 

with that of the convertors configured with the electric heater heads (baseline data). The convertors 

configured with the NaK heater heads either equaled or outperformed the convertors configured with 

electric heater heads at most operating points. The comparisons shown in Figures 5 and 6 are 

representative of comparisons made at other temperature ratios as well. These comparisons verify that 

Stirling convertors can be interfaced with a pumped NaK loop successfully, without negatively affecting 

convertor performance. 

Figure 4.—Combined power output of dual P2A Stirling power convertors configured with NaK 

heater heads versus piston amplitude. 
NASA/TM—2010-216934 6 

Figure 5.—Combined power output of dual P2A Stirling power convertors configured with NaK 

heater heads versus piston amplitude compared with baseline data. 

Figure 6.—Combined power output of dual P2A Stirling power convertors configured with NaK 

heater heads versus piston amplitude compared with baseline data. 
NASA/TM—2010-216934 7 

Figure 7.—Sensitivity of P2A Stirling power output for various NaK mass flow rates. 

Steady—State NaK Mass Flow Mapping 

During the second phase of steady-state testing, the sensitivity of the Stirling convertor electrical 

output power to changes in NaK mass flow was measured. Figure 7 shows the total power output at 

design hot-end (550 °C) and cold-end (50 °C) temperatures at piston amplitudes of 8, 9, and 10 mm. Over 

the range of flow rates tested, convertor electrical output was insensitive to reductions in NaK mass flow. 

The maximum NaK mass flow in each case was limited by the NaK pump output capacity and the lower 

limit was conservatively chosen to constrain the temperature difference across the heater head and thereby 

limit thermal stresses. In addition, the power requirement of the NaK pump decreased from roughly 4 kW 

to roughly 2 kW for a reduction in mass flow from 900 to 600 g/s, suggesting that reductions in mass flow 

during start-up could result in substantial energy savings. This statement must be qualified, however, 

because the pump used in this test is operating far from its design point, and is therefore very inefficient, 

so the benefit of reducing mass flow using a more prototypic NaK pump would be less significant. 

Transient Fault Scenarios 

Figure 8 shows the system response to the loss of the NaK pump with the convertors still running. 

Within 30 sec the average hot-end temperature had dropped more than 50 °C, which greatly exceeded the 

predetermined limit of 10 °C per minute and the temperature gradients across the heater heads had 

exceeded their upper limit of 40 °C so the test was stopped by stalling the convertors. At the conclusion 

of this test there was a slug of relatively cold NaK (490 °C) in the region of the heater heads. The 

remainder of the loop, with the exception of the NaK heater remained at 550 °C. During this time, the 

NaK heater remained on causing the stationary NaK within the heater to reach 620 °C. In order to avoid 

thermally shocking the convertor heater heads, the pump was not restarted and the entire system was 

allowed to cool to ambient temperature. This scenario is regarded as a dangerous one since it causes NaK 

temperatures to change quickly and creates hot and cold slugs of fluid throughout the system which 

prevent restart. This study suggests that both the NaK heater and the convertors should be shut down 

immediately in the event of NaK pump loss. Since immediate stall of the convertors results in an 

immediate stoppage of heat transfer to the cold-end, considerations will also have to be made in order to 

prevent freezing of the water loop. 
NASA/TM—2010-216934 8 

Figure 8.—Transient operating conditions—simulate loss of the ALIP pump/intermediate NaK loop at 

full NaK flow. 
NASA/TM—2010-216934 9 

Figure 9 shows the response of the system to a loss of heater power. In this scenario, the hot-end 

temperature gradually decreases, reducing the heat input to the convertors, resulting in decreased electric 

power output. As the heat input to the convertors approaches the heat input to the NaK due to the 

inefficiency of the NaK pump, an equilibrium temperature is reached. As stated previously, the pump 

used in this test operates very inefficiently which causes this equilibrium temperature to be somewhat 

higher than it would be in a more prototypic system. This fault scenario is thought to be more benign 

because changes in heat input to the convertors occurs slowly, resulting in slow temperature changes on 

both the hot end and cold end. However, it should also be noted that the cooling rate of the NaK seen 

during this test is specific to this system. If other systems were to cool the NaK at a faster rate it could 

violate a system constraint on transient heating/cooling time. If this were the case, some type of piston 

amplitude modification would be required to reduce the amount of heat removed from the NaK by the 

convertors. 

Figure 10 shows the transient data taken when the convertor pistons were stalled. The hot-end 

temperature was maintained at a constant value by the heater controller so the heater power was reduced, 

accordingly. In this test the convertors were then restarted at a piston amplitude of 6 mm, then brought to 

full amplitude of 10 mm. The convertors recovered to full power almost immediately after the piston 

recovered to the desing amplitude. 

Transient Start-Up Scenarios 

There is currently no established start-up scenario for the proposed fission surface power system, so it 

is important to understand that these scenarios may not be representative of actual start-up scenarios. The 

start-up scenario tests are simply an examination of the trade between optimizing conditions within the 

reactor and minimizing the required start-up energy. It is important to note that the relative importance of 

each of these is not currently well-defined. Nevertheless, several start-up scenarios were run, each 

following a similar form: 1) the NaK pump was initiated to deliver flow at the desired mass flow rate, 

2) the heater was then used to elevate the NaK temperature to an initial setpoint, 3) the convertors were 

started and operated at an initial piston amplitude of 6 mm, 4) the system was then allowed to reach 

steady-state, and 5) the NaK temperature was then ramped up to the Stirling convertor design hot-end 

temperature of 550 °C while the piston amplitude was ramped to 10 mm (design amplitude). The 

parameters varied during start-up testing were the NaK mass flow and the initial temperature setpoint. 

The initial Stirling hot-end temperature setpoint ranged from 200 to 500 °C and the NaK mass flow 

ranged from 600 to 900 g/s (total for both convertors). The results of the start-up testing are shown in 

Figures 11 to 16. The Stirling convertors were successfully brought to the design operating condition in 

each scenario, without any issues. This suggests that limitations on the selection of the start-up scenario 

will not come from the convertors, but from other system components. However, further system level 

testing of more prototypic convertors will be necessary to validate that assertion. 

 
NASA/TM—2010-216934 10 

Figure 9.—Transient operating conditions—simulate loss of reactor power/primary NaK loop at full 

NaK flow. 
NASA/TM—2010-216934 11 

Figure 10.—Transient operating conditions—simulate shutdown/restart of Stirling convertors. 
NASA/TM—2010-216934 12 

Figure 11.—Transient operating conditions—minimum temperature start-up at full NaK flow. 

 
NASA/TM—2010-216934 13 

Figure 12.—Transient operating conditions—medium temperature start-up at full NaK flow. 
NASA/TM—2010-216934 14 

Figure 13.—Transient operating conditions—full (high) temperature start-up at full NaK flow. 

   
NASA/TM—2010-216934 15 

Figure 14.—Transient operating conditions—minimum temperature start-up at 75 percent NaK flow. 

   
NASA/TM—2010-216934 16 

Figure 15.—Transient operating conditions—medium temperature start-up at 75 percent NaK flow. 

   
NASA/TM—2010-216934 17 

Figure 16.—Transient operating conditions—full (high) temperature start-up at 75 percent NaK flow. 
NASA/TM—2010-216934 18 

Conclusions 

Performance maps generated using the pumped NaK loop for thermal input consistently show the 

convertors performing as well or slightly better than they had during operation with electric heater heads. 

A potential explanation for the slightly better performance is that the convertor-body heat losses due to 

natural convection were reduced due to operation in vacuum during testing with the NaK heater heads. 

Uncertainty in the measurement of thermal input to the Stirling convertors made it difficult to calculate 

converter efficiency precisely; however, consistent improvements in power output when operating in the 

pumped NaK configuration at similar operating conditions suggest improvements in the efficiency over 

those measured during electrical baseline testing. 

Full-power operation at nominal operating conditions was further demonstrated over a range of NaK 

mass flows (530 to 850 g/s total for convertor pair). Over the range of mass flows tested, convertor 

performance was insensitive to NaK mass flow at both nominal and off-nominal piston amplitude. 

Although system level performance was not considered in this study, the mass flow sensitivity testing 

illustrates a system level trade between convertor performance, convertor reliability, and pumping power, 

and shows that during this testing the convertors were operating beyond the point of diminishing returns 

of convertor performance, with respect to NaK mass flow. 

Several possible start-up tests were conducted to generate data relevant to future TDU and FSP 

system start-ups. The Stirling convertors were started at several different hot-end temperatures ranging 

from 200 to 500 °C. This data shows that the Stirling convertors are sufficiently robust to handle a wide 

range of start-up scenarios. This flexibility will be useful to future system designers who will likely find 

restrictions on reactor start-up and required pump power to be more constraining than any limitations 

imposed by the Stirling convertors. In addition, start-up scenarios were run at different NaK mass flow 

rates. This data shows that a reduction of mass flow from 850 to 600 g/s (total for convertor pair) resulted 

in a reduction in required pump power from about 4000 W to bout 2000 W. Furthermore, reductions in 

mass flow were shown to have little effect on total time to reach nominal operation. This data suggests 

that start-up scenarios should occur at low mass flows in order to minimize the total energy required for 

pumping. However, this statement must again be qualified by noting that the NaK pump used in this test 

is operating far from its nominal operating condition and therefore exaggerates the benefits of reducing 

NaK flow rate. Further analysis of a more representative system would be necessary in order to draw 

more specific conclusions. 

Fault recovery tests were also conducted in order to assess the effect of component failures and to 

identify possible recovery procedures. The loss of the heat source was shown to be a somewhat benign 

failure for this system resulting in slow reductions in hot-end temperature and power output. The 

convertor stall test revealed that the system could return quickly to full power operation if control of the 

convertors is regained. However, precautions must be taken to ensure that a sudden reduction in heat flow 

to the Stirling convertor cold-end is handled appropriately. Finally, simulation of a NaK pump failure 

while maintaining convertor operation revealed that the NaK contained in the Stirling convertor heater 

heads and heater head metal temperatures will drop rapidly. During this fault scenario, temperature ramprate restrictions are likely to be violated as well as maximum temperature gradient restrictions within the 

hot-end. In addition, if the convertors are permitted to run without NaK flow, the NaK contained in the 

heater head volume could over-cool and cause a thermal shock in the convertor heater head once NaK 

pump operation is restored. The only method of recovery is to wait for the system to reach thermal 

equilibrium, which could take many hours. Therefore, the convertors should be stopped immediately in 

the event of a NaK pump failure and precautions must then be taken to accommodate the loss of heat 

input to the coolant. 
NASA/TM—2010-216934 19 

References 

1. Mason, L.S., “A Comparison of Fission Power System Options for Lunar and Mars Surface 

Applications,” NASA/TM—2006-214120, 2006. 

2. Mason, L.S., “A Practical Approach to Starting Fission Surface Power Development,” NASA/ 

TM—2006-214366, 2006. 

3. Mason, L.S., Poston, D., Qualls, L., “System Concepts for Affordable Fission Surface Power,” 

NASA/TM—2008-215166, 2008. 

4. Geng, S.M., Mason, L.S., Dyson, R.W., Penswick, L.B., “Overview of Multi-Killowatt Free-Piston 

Stirling Conversion Research at GRC,” NASA/TM—2008-215061, 2008. 

5. Garber, A., “Modifications to the Fission Surface Power Primary Test Circuit,” International 

Congress on Advances in Nuclear Power Plant, Paper 8254. 

6. Briggs, M., Geng, S., Robbie, M., “Status of Kilowatt-Class Stirling Power Conversion Using a 

Pumped NaK Loop for Thermal Input,” 7th International Energy Conversion Engineering Conference 

(IECEC), AIAA–2009–4597, Denver, CO, 2009. 

7. Geng, S.M., Briggs, M.H., Penswick, L.B., Pearson, J.B., Godfroy, T.J., “Test Results From a Pair of 

1 kWe Dual-Opposed Free-Piston Stirling Power Convertors Integrated with a Pumped NaK Loop,” 

NASA/TM—2010-216266, 2010. 
REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188 

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB 

control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

01-12-2010 

2. REPORT TYPE

Technical Memorandum 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Summary of Test Results From a 1 kWe-Class Free-Piston Stirling Power Convertor 

Integrated With a Pumped NaK Loop 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Briggs, Maxwell, H.; Geng, Steven, M.; Pearson, J., Boise; Godfroy, Thomas, J. 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 463169.04.03.01.02 

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

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

8. PERFORMING ORGANIZATION

 REPORT NUMBER

E-17387 

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

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

10. SPONSORING/MONITOR'S

 ACRONYM(S)

NASA 

11. SPONSORING/MONITORING

 REPORT NUMBER

NASA/TM-2010-216934 

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited 

Subject Category: 20 

Available electronically at http://gltrs.grc.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

As a step towards development of Stirling power conversion for potential use in Fission Surface Power (FSP) systems, a pair of 

commercially available 1 kW class free-piston Stirling convertors was modified to operate with a NaK liquid metal pumped loop for thermal 

energy input. This was the first-ever attempt at powering a free-piston Stirling engine with a pumped liquid metal heat source and is a major 

FSP project milestone towards demonstrating technical feasibility. The tests included performance mapping the convertors over various hot 

and cold-end temperatures, piston amplitudes and NaK flow rates; and transient test conditions to simulate various start-up and fault 

scenarios. Performance maps of the convertors generated using the pumped NaK loop for thermal input show increases in power output over 

those measured during baseline testing using electric heating. Transient testing showed that the Stirling convertors can be successfully 

started in a variety of different scenarios and that the convertors can recover from a variety of fault scenarios. 

15. SUBJECT TERMS

Free-piston Stirling power conversion; Fission surface power; NaK pumped loop 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF

 ABSTRACT

UU 

18. NUMBER

 OF 

 PAGES

25 

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email:help@sti.nasa.gov) 

a. REPORT

U 

b. ABSTRACT

U 

c. THIS 

PAGE

U 

19b. TELEPHONE NUMBER (include area code)

443-757-5802 

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18