Peggy A. Cornell, Edward J. Lewandowski, and Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Scott D. Wilson

Sest, Inc., Middleburg Heights, Ohio

Stirling Convertor Extended Operation Testing and

Data Analysis at Glenn Research Center

NASA/TM—2010-216231

July 2010
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Peggy A. Cornell, Edward J. Lewandowski, and Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Scott D. Wilson

Sest, Inc., Middleburg Heights, Ohio

Stirling Convertor Extended Operation Testing and

Data Analysis at Glenn Research Center

NASA/TM—2010-216231

July 2010

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Seventh International Energy Conversion Engineering Conference (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

Denver, Colorado, August 2–5, 2009
Acknowledgments

This work is funded through the NASA Science Mission Directorate. The authors wish to acknowledge the many people who 

support testing in the Stirling Research Laboratory including Jeffrey Schreiber, Gina Dugala, David Meer, David Krause, and 

Geoffrey Bruder of NASA Glenn Research Center for their counsel and support in preparing this paper.

Available from

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7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

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

Level of Review: This material has been technically reviewed by technical management. 

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2010-216231 1 

Stirling Convertor Extended Operation Testing and 

Data Analysis at Glenn Research Center 

Peggy A. Cornell, Edward J. Lewandowski, and Salvatore M. Oriti

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Scott D. Wilson 

Sest, Inc. 

Middleburg Heights, Ohio 44130 

Abstract 

Extended operation of Stirling convertors is essential to the development of radioisotope power 

systems and their potential use for long-duration missions. To document the reliability of the convertors, 

regular monitoring and analysis of the extended operation data is particularly valuable, allowing us to 

better understand and quantify long-life characteristics of the convertors. Furthermore, investigation and 

comparison of the extended operation data to baseline performance data provides an opportunity to 

understand system behavior should any off-nominal performance occur. Glenn Research Center (GRC) 

has tested 16 Stirling convertors under 24-hr unattended extended operation, including four that have 

operated in a thermal vacuum environment and two that are operating in the Advanced Stirling 

Radioisotope Generator Engineering Unit. Ten of the sixteen convertors are the Advanced Stirling 

Convertors (ASC) developed by Sunpower, Inc. with GRC. These are highly efficient (conversion 

efficiency of up to 38 percent for the ASC–1), low-mass convertors that have evolved through 

technologically progressive convertor builds. Six convertors at GRC are Technology Demonstration 

Convertors from Infinia Corporation. They have achieved greater than 27 percent conversion efficiency 

and have accumulated over 185,000 of the total 265,000 hr of extended operation at GRC. This paper 

presents the extended operation testing and data analysis of free-piston Stirling convertors at NASA GRC 

as well as how these tests have contributed to the Stirling convertor’s progression toward flight. 

Introduction 

Extended operation of free-piston Stirling convertors at NASA Glenn Research Center (GRC) is 

fundamental to the development of NASA’s next generation of Radioisotope Power Systems (RPS), the 

Advanced Stirling Radioisotope Generator (ASRG). The ASRG is being developed by Lockheed Martin 

(LM) under contract to the Department of Energy (DOE). The Advanced Stirling Convertors (ASCs), 

being developed by Sunpower, Inc. under the management and technical direction of GRC, will be 

utilized in the ASRG. Various ASC prototypes, as well as two sets of Infinia Corporation Technology 

Demonstration Convertors (TDCs), are currently under test at GRC’s Stirling Research Laboratory. The 

convertors at GRC can be used for either focused tests that are relatively short-term or extended operation 

tests to establish a reliability database and to examine operating trends over long periods (Ref. 1). The 

extended operation tests are of particular interest for radioisotope power applications since the missions 

where Stirling power conversion is often considered are generally long-duration missions of up to 

14 years. 

Ten of the sixteen convertors tested at GRC are from different phases of ASC technology 

development. Additionally, eight convertors of the next phase of technology development, the ASC–E2, 

will be on extended operation within the next year. To date, Sunpower, Inc. has produced five generations 

of ASC-related hardware evolving the technology progressively with each build. Estimates for the latest 

ASC–E2 build indicates power output of 84.5 WAC with heat input from the General Purpose Heat Source 
NASA/TM—2010-216231 2 

(GPHS) to the ASC of 224 Wthermal indicating efficiency of 37.7 percent with a maximum heater head 

temperature of 850 °C, a reject temperature of 90 °C, and a convertor mass of 1.32 kg (Ref. 2). 

This paper provides an update of the Stirling convertor extended operation data to more accurately 

document their overall reliability after 265,000 hr of combined convertor operation at GRC. To better 

understand and quantify the long-life characteristics of the convertors advances the technology toward 

acceptance for flight. 

Nomenclature 

ACU Advanced Stirling Convertor Control Unit 

ASC (-E) Advanced Stirling Convertor (-Engineering Unit) 

ASRG (EU) Advanced Stirling Radioisotope Generator (Engineering Unit) 

DOE Department of Energy 

EMI electromagnetic interference 

GPHS general purpose heat source 

GRC Glenn Research Center 

grms gravity root mean square

HS hermetically sealed 

LM Lockheed Martin 

RPS Radioisotope Power System 

TDC Technology Demonstration Convertor 

UPS uninterruptable power supply 

Convertor Operation at Glenn Research Center 

Since June 2003, numerous convertors have been operated in GRC’s Stirling Research Laboratory for 

both short-term tests as well as extended operation. Monitoring and analysis of the performance of this 

range of convertors is valuable in that the knowledge and experience gained is directly applied to freepiston Stirling convertor development. Table I, updated since its circulation in Reference 3, lists the 

convertors tested at GRC, as well as convertors that will be tested later this year. 

TABLE I.—CURRENT AND UPCOMING CONVERTORS IN EXTENDED OPERATION AT GRC 

Convertor Test environment Nominal 

operating 

temperature 

(hot/cold), °C 

Date 

operation 

initiated 

GRC hours to date Status 

TDC #13 and #14 In air 650/80 June 2003 47,200 Ongoing test 

TDC #5 and #6 Thermal vacuum 630/70 Nov. 2004 10,543 and 10,382 Completed test 

TDC #15 and #16 In air 650/80 Mar. 2005 33,400 Ongoing test 

ASC–0 #1 and #2 In air and thermal vacuum 645/72 Feb. 2007 15,363 Ongoing test 

ASC–0 #3 and #4 In air and launch simulation 650/90 Aug. 2007 12,700 Ongoing test 

ASC–1 #3 and #4 In air 850/90 May 2007 1,817 EMI testing 

ASC–1HS #1 and #2 In air and launch simulation 850/90 Feb. 2008 2,823 and 4,106 Ongoing test 

ASC–E #2 and # 3 In air and environmental 625/70 Nov. 2008 4,900 Ongoing test 

ASC–E #1 and #4 In air and launch simulation 650/90 2009 53 and 0 Future test 

ASC–E2 #1 to #8 In air and launch simulation 2009 Future test 
NASA/TM—2010-216231 3 

Extended Operation Results to Date 

Advanced Stirling Radioisotope Generator Engineering Unit (ASRG EU) 

The ASRG EU was designed and fabricated by LM under contract to the DOE. It contains two 

convertors; ASC–E #2 is the outboard convertor located farthest from the mounting end in the A position 

and ASC–E #3 is located in the B position at the inboard end. The controller is mounted to the outside of 

the housing. 

After fabrication, the ASRG EU completed a variety of tests at the LM facility, including thermal 

performance in a vacuum chamber, mechanical disturbance, sine transient, random vibration, shock, and 

electromagnetic interference (EMI) (Ref. 4). Upon conclusion of these tests, the ASRG EU underwent an 

internal inspection. The electric heat sources, which were dynamically and dimensionally equivalent to a 

GPHS module, were replaced with a cartridge heater design furnished by GRC. These heaters are 

expected to have the higher reliability required for extended operation testing. The ASRG EU was 

operated for a total of 1,124 hr at LM before shipping to GRC. 

The ASRG EU was delivered to GRC on August 28, 2008. A special test facility was designed and 

fabricated to operate the vertically oriented ASRG EU under controlled conditions (Refs. 5 and 6).

Figure 1 shows the ASRG EU test station. The ASRG EU was initially operated under AC bus control to 

characterize performance of the engineering unit and the test facility independent of the Advanced 

Stirling Convertor Control Unit (or ACU). Instrumentation is used to monitor various parameters 

including temperatures throughout the ASRG EU and the test facility, heat input, power output, mounting 

interface forces, and other parameters. Extended operation commenced on November 6, 2008. The ASRG 

EU has been operated at a hot-end temperature of 625 °C on both convertors, while the rejection 

temperature varied to nominally 69 °C on ASC–E #2 and 66 °C on ASC–E #3. The heat rejection is 

accomplished by cooled air flowing over the unit from the ASC B end to the ASC A end. Since the heat 

rejection from the two convertors is not independently controlled, and the cooled air reaches the ASC B 

end first, the ASC–E #3 cold-end temperature has been slightly lower than the ASC–E #2 cold-end 

temperature. 

Figure 1.—ASRG EU test station. ASRG EU in 

an enclosure on test stand inside the ASRG 

EU and ASC–E #1 and #4 special test facility. 
NASA/TM—2010-216231 4 

A comparison was made of ASRG EU alternator power as measured at LM versus power measured at 

GRC. The alternator power measurements were comparable within ±1.5 W. 

Changes in piston amplitude occurred during operation because of test rack impedance variation as 

well as operating setpoint adjustments. The piston amplitude fluctuated between 4.10 and 4.30 mm on 

ASC–E #2 and 4.00 and 4.39 mm on ASC–E #3. These operating setpoint adjustments, reflected in the 

performance graphs of Figure 2, were made in response to the piston amplitude increases caused by the 

impedance variations. Setpoint adjustments to decrease AC bus voltage, and thus piston amplitude and 

power, were made at 1,003 and 2,874 hr of operation. Milliohm-level increases in test rack impedance 

essentially resulted in a change in the operating point, affecting the piston amplitude and output power. 

These changes can be seen in Figure 2. 

A number of upgrades were made to the test rack after 2,197 hr of operation with the intent of 

reducing the variations in impedance. These included seating capacitor connections in a more robust 

fashion, soldering some connections, and improving other connections. Subsequent tests showed that 

power variation was reduced, especially on the ASC A side of the test rack. The abnormally high ASC 

A-side impedance was reduced to be comparable to ASC B-side impedance, resulting in ASC–E #3’s 

power now slightly above that of ASC–E #2. The data also showed that variation was not eliminated, 

especially on the ASC B side, so further improvements to the test rack are under way. 

The ASRG EU has operated to date with no measurable change in convertor performance. As shown 

in Figure 2, overall performance data for these convertors shows stable operation (with exception of 

manual adjustments). The ASRG EU has achieved over 4,900 hr of extended operation and will operate 

on ACU control in August 2009. 

ASC–1 HS #1 and #2 

Upon arrival to GRC in November 2007, the ASC–1 HS #1 and #2, which are hermetically sealed 

units with the exception of the helium fill tube, underwent heater head diameter measurements, 

thermocouple instrumentation, and vacuum bakeout before beginning extended operation in March 2008. 

These units can reach temperatures up to 850 °C with a MarM–247 heater head that allows for higher hotend temperatures. Within 8 hr of initiating 850 °C operation, a power anomaly occurred causing a hot-end 

temperature transient to trigger a shutdown of the convertors. To help minimize this type of shutdown 

incident, the temperature limits were adjusted to provide greater margin for tolerating short-term 

transients. Data from this initial run showed the anomalies were in the form of rapid and spontaneous 

power output spikes of approximately 1 W above nominal and occurred roughly every 2 hr. 

Convertor operation moved forward but at lower operating conditions of 750 and 650 °C hot-end 

temperatures. Data from these lower temperature runs showed that power variations were occurring, but 

were slightly under the threshold to trigger a shutdown since the temperature limits were previously 

adjusted. Operation at 750 °C resulted in anomalous spikes on the order of approximately 2 W every 

8 hr, and 650 °C operation resulted in anomalous spikes of approximately 1 W every 24 hr. No obvious 

Figure 2.—Power output and efficiency performance data from ASRG EU through 4,900 hr of operation. 

0

10

20

30

40

50

60

70

80

90

100

0 1,000 2,000 3,000 4,000 5,000

Power Output, (W)

Time, (hrs)

ASC A

ASC B

0

5

10

15

20

25

30

35

40

0 1,000 2,000 3,000 4,000 5,000

Efficiency, (%)

Time, (hrs)

ASC A

ASC B
NASA/TM—2010-216231 5 

Figure 3.—ASC-1 HS #1 and #2 power anomalies before and after test rack modifications.

reasons were found for the anomalies, so an investigation was initiated to find the root cause. The 

convertors were operated at various operating temperatures in both dual-opposed and single configuration 

as well as horizontal and vertical orientation. Different alternator output connections and varying AC bus 

controller resistances were employed, and although the characteristics of the anomalies varied, the 

behavior remained. Additionally, the power anomalies appeared in both convertors, but generally 

occurred independent of each other. 

The investigation was then directed to the test rack, where a visual inspection of all components and 

wiring was performed. Furthermore, the AC power supply in the AC bus controller was used as a power 

source, and a resistive load was used in place of the convertors. The power variations were still present 

utilizing the resistive load, indicating that the test rack was responsible for the anomalies, and not the 

convertors. Further troubleshooting focused the suspected problem to the test rack’s load panel, and changes 

were made to the load panel based on these findings. These modifications included replacing the wire nuts 

on the transformer leads with solder joints, repairing a poor solder joint that was found in the alternator 

power path, and removing a relay in the power circuit. The output relay was originally installed to allow the 

test operator to switch between the AC bus controller and a Zener-diode controller. However, only AC bus 

control is now used, making the output relay unnecessary in this test rack. Although the modifications did 

not completely eliminate the power variations, analysis of the 2-sec power performance data (as shown in 

Fig. 3) indicated a fairly significant improvement in stability of the convertor operating point. 

ASC–1 HS #1 and #2 operation continued throughout the power anomaly investigation, accumulating 

nearly 3,000 hr of operation on ASC–1 HS #1 and 4,200 hr on ASC–1 HS #2, with an extensive amount 

of operation at the relatively higher hot-end temperature of 750 °C. Currently, new high-temperaturecapable heat sources are under evaluation at GRC that will allow long-term, reliable operation at 850 °C, 

and the anomaly investigation is progressing. 

ASC–1 #3 and #4 

Extended operation of the ASC–1 #3 and #4 was ceased after nearly 1,800 hr at GRC in 

December 2007. ASC–1 #3 and #4 were not operated during 2008, but were sent back to Sunpower, Inc. 

after the displacer epoxy joint failed due to an over-temperature condition encountered during an 

insulation loss test (as described in Ref. 3). 

Originally designed only for developmental activities rather than long-life testing, these convertors 

included multiple o-ring seals at the flange joints and pressure vessel ports. During Sunpower’s, Inc. 

inspection of the convertors following extended operation at GRC, it was found that oxygen had 

permeated through the o-ring seals causing damage to some internal components. Based on this finding, a 

new pressure vessel was designed to eliminate the o-rings exposed to atmosphere at the aft end to 

potentially decrease oxygen permeation. This new design required three changes to the pressure vessel; 

the alternator power feedthrough was changed to a welded, hermetic feedthrough, the fast linear 

displacement transducer (FLDT) was changed from a mount with an o-ring to a welded style, and the 
NASA/TM—2010-216231 6 

helium fill port was changed from an o-ring seal port to a brazed tube with a welded metal gland-face seal 

fitting. The main pressure vessel and heater head o-rings remained, and as during earlier operation at 

GRC, will be enclosed in an argon environment intended to eliminate the presence of oxygen. Because 

the new welded alternator feedthrough is located on the aft end of the pressure vessel, the pressure vessel 

must be made from two sections: a main cylinder portion and a cap. The cylinder will be placed around 

the alternator, the alternator wires will be soldered to the feedthrough in the cap, and the cap will be 

welded to the cylinder. After welding, disassembly of the two main sections would be accomplished by 

cutting off the cap. 

A second pressure vessel was designed and fabricated for use on the ASC–1 #3 and #4 during EMI 

testing. These test pressure vessels were made from Carpenter High Permeability Perm–49 to mitigate the 

AC magnetic emissions from the alternator. In April 2009, ASC–1 #3 and #4 completed baseline 

performance testing at Sunpower Inc., with the Perm–49 pressure vessels before returning to GRC. The 

convertors underwent EMI testing in June 2009 to characterize the emitted AC magnetic field strength. 

Following EMI testing, the convertors were returned to Sunpower Inc., where the Perm–49 pressure 

vessels will be replaced with the new pressure vessels. The combination of the argon environment 

surrounding the heater head and pressure vessel o-rings, coupled with the elimination of o-rings from the 

aft end of the pressure vessel is to enable extended operation without complications caused by internal 

component oxidation. Four candidate high-temperature heat sources are currently being evaluated at GRC 

for the higher heater head temperatures required for ASC–1 #3 and #4. The heat sources maintain 950 to 

1000 °C temperatures to make use of the MarM–247 heater head for operation up to 850 °C. Extended 

operation will resume at GRC at full design conditions in September 2009. 

ASC–0 #1 and #2 

NASA GRC has been operating several ASCs in support of the ASRG project, however, the first 

ASCs to be operated at GRC were ASC–0 #1 and #2, which include Inconel 718 heater heads and 

hermetically sealed flange joints. Delivery of the convertors was in December 2006, and in-air operation 

began in February 2007, where they operated at their nominal design conditions for approximately 600 hr. 

These operating conditions included 650 °C hot-end temperature, 90 °C rejection temperature, and 

4.50-mm piston amplitude. Following in-air operation, they were reconfigured for thermal vacuum 

operation. Convertor operation in a thermal vacuum environment was initiated in March 2007 and 

concluded in January 2009, accumulating 15,363 hr of in-air and thermal vacuum operation. The majority 

of operation in thermal vacuum was at less-than-design piston amplitude because of questionable 

instrumentation or convertor behavior, but other parameters, such as hot-end temperature, were 

maintained at the nominal design value. Several periods of nonsteady performance were observed 

throughout in-air and thermal vacuum operation and are described in Reference 3. The thermal vacuum 

test of ASC–0 #1 and #2 was concluded in January 2009 to conduct an inspection of the convertors at 

Sunpower Inc. Cursory assessment of the convertors at GRC upon removal from thermal vacuum 

revealed all hardware to be intact with the minor exception of fractured ceramic washers on the electrical 

heater leads. The convertors were returned to Sunpower Inc. for inspection, where they will be 

refurbished if necessary and returned to GRC in September 2009. 

ASC–0 #3 and #4 

The ASC–0 #3 and #4 units, designed similar to the ASC–0 #1 and #2 units with Inconel 718 heater 

heads and hermetically sealed flange joints, began operation at GRC in August 2007. Their nominal 

design conditions are equivalent to the ASC–0 #1 and #2 at 650 °C hot-end temperature, 90 °C rejection 

temperature, and 4.50-mm piston amplitude. Continuous operation of these convertors was halted in 

August 2008, with over 7,400 hr of operation, to prepare them for flight-acceptance-level and launch 
NASA/TM—2010-216231 7 

Figure 4.—Power output and efficiency performance data from ASC-0 #3 and #4 through 12,700 hr of operation. 

simulation vibration testing in three orthogonal axes at GRC’s Structural Dynamics Laboratory. The 

intent of the test was to subject the convertors to the same environmental conditions as flight convertors, 

which would experience a workmanship vibration test during manufacturing, acceptance vibration test 

when the generator is fueled, and launch vibration. ASC–0 #3 was subjected to acceptance and launch 

simulation levels of 8.7 g root mean square (grms) in all three axes using the standard RPS specification 

flight acceptance profile (Ref. 7). With data from that vibration test and the response measured by LM 

when they tested the ASRG EU, the vibration profile was adjusted to simulate the dynamic conditions 

imposed on convertors installed in the ASRG EU. This resulted in a test level for ASC–0 #4 of 8.4 grms in 

the two lateral axes, and 14.3 grms in the axial direction as measured at the pressure vessel. 

Furthermore, ASC–0 #3 and #4 underwent heater head diameter measurements by the Structures and 

Materials Division at GRC. A previously developed methodology was applied to measure the convertor 

heater head using a laser micrometer scanning technique and a National Instruments data acquisition 

system to record analog diameter versus axial position. The laser scanning technique and the configured 

data acquisition system were utilized to perform heater head diameter versus axial length measurements at 

45° circumferential intervals. The purpose of the activity was to provide baseline data for comparison of 

future heater head measurement data subsequent to GRC in-house testing, providing a means to track 

creep of the heater head assembly should creep strains accumulate throughout extended operation testing. 

The computed creep strains will be used subsequently in the heater head’s life assessment. 

Anomalous behavior similar to what was discussed previously with ASC–1 HS data but of lesser 

magnitude also occurred on ASC–0 #3 and #4. As with the ASC–1 HS #1 and #2, power anomalies 

would appear in both convertors, but would generally be independent of each other. Poor connections and 

a malfunctioning relay appeared to be the root cause of the anomalies. The relays were removed from the 

power circuit since they were no longer needed, and the faulty connections were soldered. As shown in 

Figure 4, overall performance data for these convertors shows steady operation (with exception of manual 

adjustments) and no measurable change in performance after achieving over 12,700 hr of extended 

operation. 

TDC #13 and #14 

TDC #13 and #14 are the longest running pair of Stirling convertors at GRC, with over 47,200 hr 

(5.4 years) of operation. Extended operation of these convertors began in June 2003. Using TDC #13 and 

#14 as a benchmark for extended operation testing and applying the knowledge gained from these 

convertors to other test articles at GRC has directly resulted in greater overall uptime for all convertors in 

the Stirling Research Laboratory. Test facility upgrades such as improved failsafe protection circuit 

designs to eliminate false trips as well as test facility maintenance schedules are credited to experience 

gained from the operation of these long-running convertors. Ongoing test facility maintenance includes 

cleaning, replacement, or calibration of the cooling plumbing and pumps, heaters, rack electronics, 

0

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ASC-0 #3

ASC-0 #4

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ASC-0 #4
NASA/TM—2010-216231 8 

Figure 5.—Power output and efficiency performance data from TDC #13 and #14 through 47,200 hr of operation. 

sensors, building heating, ventilating, and air conditioning (HVAC), working gas management system, 

and the gas analysis system. To address the possibility of facility power loss, uninterruptable power 

supplies (UPS) provide backup power to each test station in the Stirling Research Laboratory. In the event 

of a facility power outage, the UPS at each test station is connected and a signal is sent to the LabVIEW 

system. The LabVIEW system monitors the outage, and if it extends beyond 5 min, the LabVIEW system 

initiates a controlled shutdown. The UPS system can also maintain operation for up to 30 min to perform 

maintenance on the facility electrical system without needing to shut down the convertors (Ref. 3). A 

50-kW generator provides backup power in the event of an extended facility power outage. 

TDC #13 and #14 were partially disassembled and inspected after 18,400 hr of operation. The 

inspection, performed by GRC and observed by Infinia Corporation, consisted of pressure vessel removal 

and resulted in no identified changes in condition since assembly. After 19,100 hr of operation, the o-ring 

seal flanges were hermetically sealed at Edison Welding Institute, but the helium fill tubes remain 

connected to the gas management system to allow for analysis of the working fluid throughout extended 

operation. A more detailed description of the operation through 38,400 hr can be found in References 1, 

8, and 9. 

Figure 5 shows output power and net efficiency to date for TDC #13 and #14. The slight degradation 

in power and efficiency prior to 19,000 hr is attributed to regenerator oxidation caused by oxygen 

permeation through the o-ring seals. Hermetically sealing the flanges at 19,100 hr prevented further 

oxidation. After hermetic sealing, the convertors were operated at a lower power level until 20,000 hr, at 

which time they underwent vacuum bakeout to prepare for full power operation. Shortly after returning to 

full power operation, the cartridge heaters began to degrade and the power level was again decreased, 

resulting in heater replacement on both convertors at 22,000 hr of operation. The heaters have currently 

been operating at full power for over 23,000 hr, during which time they have experienced seven thermal 

cycles to room temperature. At approximately 38,000 hr of convertor operation, power output decreased 

because of a decline in average heater temperature resulting from aging cartridge heaters as well as drift 

in the Zener-diode controller. Due to this change in operating point, the temperature and controller load 

were subsequently adjusted back to the original operating point, and convertor power output returned to 

expected levels. TDC #13 and #14 have continued to operate with negligible change in performance since 

the cartridge heater change at 22,000 hr and remain on extended operation testing as an integral part of 

GRC’s development of long-life radioisotope power systems. 

TDC #15 and #16 

TDC #15 and #16 are the second longest running pair of Stirling convertors at GRC, with over 

33,400 hr (3.8 years) of operation. Extended operation of these convertors began in March 2005. The 

o-ring seal flanges were hermetically sealed on these convertors at Edison Welding Institute after 4,200 hr 

of operation, and as with the first set of TDCs, the helium fill tubes remain connected to the gas 

management system to allow for analysis of the working gas throughout extended operation. A more 

detailed description of the operation through 24,600 hr can be found in References 3 and 9. 

0

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TDC #13

TDC #14

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Efficiency, (%)

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TDC #13

TDC #14
NASA/TM—2010-216231 9 

Figure 6.—Power output and efficiency performance data from TDC #15 and #16 through 33,400 hr of operation. 

Figure 6 shows output power and net efficiency to date for TDCs #15 and #16. Prior to hermetic 

sealing, the convertors operated at a reduced hot-end temperature to help prevent internal oxidation. After 

hermetic sealing, the convertors were operated at a lower power level for about 100 hr before undergoing 

vacuum bakeout. They returned to full temperature operation at 4,400 hr. As with the first set of 

convertors following hermetic sealing, the cartridge heaters on TDC #15 and #16 also began to degrade 

after returning to full temperature operation, resulting in heater replacement on both convertors at 

4,900 hr of operation. The heaters have currently been operating at full power for over 27,000 hr, during 

which time they have experienced eleven thermal cycles to room temperature. TDC #15 and #16 have 

continued to operate with negligible change in performance since the cartridge heater change at 4,900 hr 

and remain on extended operation testing as an integral part of GRC’s development of long-life 

radioisotope power systems. 

ASC–E #1 and #4 

ASC–E #1 and #4 units, delivered to GRC in December 2008 and May 2009, respectively, are being 

prepared for extended operation testing at GRC’s Stirling Research Laboratory. The ASC–E #1 is the 

spare convertor from the fabrication of the ASRG EU, and the ASC–E #4 was built using high-value 

spare parts from the earlier ASC–E builds including the heater head and cold-side adapter flange. These 

convertors utilize Inconel 718 heater heads and therefore operate at a maximum hot-end temperature of 

650 °C and a rejection temperature of 40 to 90 °C. The units have been hermetically sealed by welding 

the flange joints, and the fill tube has been pinched off. Shortly after their arrivals at GRC, ASC–E #1 and 

#4 underwent heater head diameter measurements with the Structures and Materials Division. 

Vibration testing was successfully completed at GRC’s Structural Dynamics Laboratory on 

ASC–E #1 with the ACU in February 2009. As part of an extended life testing program combined with a 

test plan to validate software and test configuration changes, the ACU operated the ASC–E at launch 

conditions while the convertor was exposed to qualification-level vibration in the axial direction and 

flight-acceptance-level vibration in the lateral axes. The convertor, shown in Figure 7, operated at 

approximate launch conditions of 625 °C hot-end temperature, 60 °C rejection temperature, and 4.25-mm 

piston amplitude throughout the test. Comparison of operational characteristics of the units prior to and 

after vibration testing further confirmed the integrity of the hardware. The test represents the first 

operation of a Stirling convertor under control of the ACU at GRC. ASC–E #1 operated at expected 

launch conditions during all vibration testing. ASC–E #4, which was motored during vibration, 

successfully passed workmanship-level vibration testing in all three axes at GRC’s Structural Dynamics 

Laboratory in April 2009. Structural integrity was confirmed by comparison of sine sweeps conducted 

prior to and after exposure to workmanship random vibration. ASC–E #1 and #4 will be in extended 

operation at GRC in August 2009. 

0

10

20

30

40

50

60

70

80

90

100

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000

Power Output, (W)

Time, (hrs)

TDC #15

TDC #16

0

5

10

15

20

25

30

35

40

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000

Efficiency, (%)

Time, (hrs)

TDC #15

TDC #16
NASA/TM—2010-216231 10 

Figure 7.—ASC-E in mounting fixture for vibration testing at GRC. 

Conclusions 

Stirling convertors have been operated for a total of more than 265,000 hr in GRC’s Stirling Research 

Laboratory and have completed various specialized tests to help refine the hardware and test stations. 

Monitoring and analysis of the performance of this range of convertors, in both air and thermal vacuum 

environments, is valuable in that the knowledge and experience gained is directly applied to the 

advancement of free-piston Stirling convertor development. As currently planned, the present build, 

designated the ASC–E2, will be on extended operation testing at GRC within the next year. The 

subsequent phase of technology development, the ASC–E3 convertor, will be designed with a focus on 

refining the manufacturing processes (Ref. 10). GRC’s contribution of convertor testing and analysis is 

critical to the development and application of the next generation of practical radioisotope power 

applications, the ASRG Qualification Unit. 

References 

1. Schreiber, J.G., “Summary of Stirling Convertor Testing at GRC,” NASA/TM—2006-214429, 2006. 

2. Wong, W.A., Wood, J.G., and Wilson, K., “Advanced Stirling Convertor (ASC)—From Technology 

Development to Future Flight Product,” NASA/TM—2008-215282, 2008.

3. Lewandowski, E.J., et al. “Extended Operation Testing of Stirling Convertors in Support of Stirling 

Radioisotope Power System Development,” NASA/TM—2009-215481, 2008. 

4. Chan, J., Hill, D., Hoye, T., and Leland, D., “Development of Advanced Stirling Radioisotope 

Generator for Planetary Surface and Deep Space Missions,” AIAA–2008–5768, 2008. 

5. Lewandowski, E.J., Schreiber, J.G., Oriti, S.O., Meer, D.W., and Dugala, G.M., “Design of a Facility 

to Test the Advanced Stirling Radioisotope Generator Engineering Unit,” Proceedings of the Seventh 

International Energy Conversion Engineering Conference (IECEC 2009), American Institute for 

Aeronautics and Astronautics, 2009. 

6. Dugala, G., “Test Rack Development of Extended Operation of Advanced Stirling Convertors at 

NASA Glenn Research Center,” Proceedings of the Seventh International Energy Conversion 

Engineering Conference (IECEC 2009), American Institute for Aeronautics and Astronautics, 2009. 

7. Meer, D.W., Hill, D., and Ursic, J., “Advanced Stirling Convertor Dynamic Test Plan and Results,” 

AIAA–2009–4552, 2009. 
NASA/TM—2010-216231 11 

8. Roth, M.E., Schreiber, J.G., and Pepper, S.V., “Extended Operation of Stirling Convertors,” 

NASA/TM—2004-213388, 2004. 

9. Schreiber, J.G. and Thieme, L.G., “Final Results for the GRC Supporting Technology Development 

Project for the 110-Watt Stirling Radioisotope Generator (SRG110),” Proceedings of the Space 

Technology and Applications International Forum, (STAIF 2002), American Physical Society, 2002. 

10. Schreiber, J.G., Thieme, L.G., Wong, W.A., “Supporting Technology at GRC to Mitigate Risk as 

Stirling Power Conversion Transitions to Flight,” AIAA–2008–5790, 2008. 
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Stirling Convertor Extended Operation Testing and Data Analysis at Glenn Research Center 

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14. ABSTRACT

Extended operation of Stirling convertors is essential to the development of radioisotope power systems and their potential use for longduration missions. To document the reliability of the convertors, regular monitoring and analysis of the extended operation data is 

particularly valuable, allowing us to better understand and quantify long-life characteristics of the convertors. Furthermore, investigation and 

comparison of the extended operation data to baseline performance data provides an opportunity to understand system behavior should any 

off-nominal performance occur. Glenn Research Center (GRC) has tested 16 Stirling convertors under 24-hr unattended extended operation, 

including four that have operated in a thermal vacuum environment and two that are operating in the Advanced Stirling Radioisotope 

Generator Engineering Unit. Ten of the sixteen convertors are the Advanced Stirling Convertors (ASC) developed by Sunpower, Inc. with 

GRC. These are highly efficient (conversion efficiency of up to 38 percent for the ASC-1), low-mass convertors that have evolved through 

technologically progressive convertor builds. Six convertors at GRC are Technology Demonstration Convertors from Infinia Corporation. 

They have achieved greater than 27 percent conversion efficiency and have accumulated over 185,000 of the total 265,000 hr of extended 

operation at GRC. This paper presents the extended operation testing and data analysis of free-piston Stirling convertors at NASA GRC as 

well as how these tests have contributed to the Stirling convertor’s progression toward flight.

15. SUBJECT TERMS

Free-piston Stirling convertors; Radioisotope power systems; Advanced Stirling radioisotope generator 

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