Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Extended Operation of Stirling Convertors at

NASA Glenn Research Center

NASA/TM—2012-217289

August 2012

AIAA–2011–5726
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Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Extended Operation of Stirling Convertors at

NASA Glenn Research Center

NASA/TM—2012-217289

August 2012

AIAA–2011–5726

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

9th International Energy Conversion Engineering Conference (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

San Diego, California, July 31 to August 3, 2011
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NASA/TM—2012-217289 1 

Extended Operation of Stirling Convertors at 

NASA Glenn Research Center 

Salvatore M. Oriti 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

NASA Glenn Research Center (GRC) has been supporting development of free-piston Stirling 

conversion technology for spaceflight electrical power generation since 1999. GRC has also been 

supporting the development of the Advanced Stirilng Radioisotope Generator (ASRG) since 2006. A key 

element of the ASRG project is providing life, reliability, and performance data for the Advanced Stirling 

Convertor (ASC). The Thermal Energy Conversion branch at GRC is conducting extended operation of 

several free-piston Stirling convertors. The goal of this effort is to generate long-term performance data 

(tens of thousands of hours) on multiple units to build a life and reliability database. Currently, GRC is 

operating 18 convertors. This hardware set includes Technology Demonstration Convertors (TDCs) from 

Infinia Corporation, of which one pair (TDCs #13 and #14) has accumulated over 60,000 hr (6.8 years) of 

operation. Also under test are various Sunpower, Inc. convertors that were fabricated during the ASC 

development activity, including ASC-0, ASC-E (including those in the ASRG engineering unit), and 

ASC-E2. The ASC-E2s also completed, or are in progress of completing workmanship vibration testing, 

performance mapping, and extended operation. Two ASC-E2 units will also be used for durability testing, 

during which components will be stressed to levels above nominal mission usage. Extended operation 

data analyses from these tests are covered in this paper. 

Nomenclature 

ASC Advanced Stirling Convertor 

ASRG Advanced Stirling Radioisotope Generator 

BOM beginning of mission 

DOE Department of Energy 

EOM end of mission 

EMI electromagnetic interference 

GRC Glenn Research Center 

GPHS General Purpose Heat Source 

LMCT Lockheed Martin Coherent Technologies 

LMSS Lockheed Martin Space Systems 

LR low rejection 

HR high rejection 

SRG-110 Stirling Radioisotope Generator, 110-We 

TDC Technology Demonstration Convertor 
NASA/TM—2012-217289 2 

1.0 Introduction 

The Thermal Energy Conversion branch at the NASA Glenn Research Center (GRC) has been 

supporting the development of free-piston Stirling conversion technology for space electrical power since 

1999 (Ref. 1). Free-piston Stirling conversion offers a thermal-to-electric conversion efficiency of greater 

than 25 percent at the system level, which is a factor of four greater than thermoelectric conversion. 

Thermoelectric-conversion radioisotope power systems have been used in space since 1961, most notably 

on the Apollo missions, the Voyager planetary probes, and soon the Mars Science Laboratory (Ref. 5). 

The higher conversion efficiency offered by free-piston Stirling techonogy reduces the required amount 

of radioisotope fuel for a given power output. Between 2001 and 2006, Lockheed Martin Space Systems 

(LMSS) was contracted as system integrator by the Department of Energy (DOE) to design a 110-We, 

Pu-238-fueled Stirling-conversion generator. This design, designated the Stirling Radioisotope 

Generator–110 (SRG-110), comprised two Infinia, Corporation free-piston Stirling convertors, each 

receiving heat from one General Purpose Heat Source (GPHS) module. During this time period, NASA 

GRC began continuous, extended operation of several Infinia, Corporation Technology Demonstration 

Convertors (TDCs) (Ref. 1). The TDCs were fabricated during technology development activities 

preceding the SRG-110 project, but are functionally the same as those of the SRG-110 design. In 2006, 

the project was redirected to use Sunpower, Inc. Advanced Stirling Convertor (ASC) technology in lieu of 

Infinia technology to increase the hot-end conversion temperature. This offered increases in the 

generator’s specific power and overall conversion efficiency. As such, the generator was renamed the 

Advanced Stirling Radioisotope Generator (ASRG). Recently, NASA has announced three candidate 

Discovery 12 missions, two of which would make use of the ASRG (Ref. 6). 

At the time of the redirection towards ASRG, GRC also began continuous, extended operation of 

several ASCs. Since dynamic energy conversion has yet to be used for a spaceflight application, 

demonstration of the technology’s reliability is paramount to the progression of Stirling technology to 

flight. The GRC convertor level testing serves the function of building a reliability database to support the 

capability of Stirling convertors for spaceflight applications. Continuous extended convertor operation 

provides performance data over a sufficient length of time to observe long-term trends (tens of thousands 

of hours). Initial Stirling-conversion radioisotope power system development envisioned missions to the 

outer planets, with cruise times on the order of 17 years. It is time-prohibitive to operate any single unit 

for 17 years, and there is no feasible method for accelerating life testing at the convertor level. Therefore, 

the tactic employed is to operate many units to accumulate convertor hours of operation useful for 

supporting reliability analyses. 

A total of 38 free-piston Stirling convertors have been operated at GRC in support of dynamic energy 

conversion for a radioisotope power system (Refs. 2 and 3), with a cumulative runtime exceeding 

400,000 hr. GRC is currently maintaining 24/7 operation of 18 convertors, for which the cumulative 

runtime exceeds 350,000 hr. Four of these are Infinia TDCs, while the remainder are Sunpower ASCs 

from various points along the ASC technology development path. A summary of the ongoing tests is 

shown in Table I. An example extended operation test station with two ASC-E2s is shown in Figure 1. 

Details of test station design and operation have been discussed previously in References 1 to 4. The 

Infinia convertors were the first to begin extended operation, and have the most runtime. The leader in 

runtime is the TDC #13 and #14 pair, which have each operated for over 60,000 hr (~6.8 years). GRC 

began operating ASCs in 2007, shortly after the project was redirected to use Sunpower’s convertor 

design. Three ASC designs are currently being operated: ASC-0, ASC-E, and ASC-E2. The ASC-0 

design was the first to implement hermetic sealing of the convertor’s pressure joints. Later, ASC-E units 

arrived, which incorporated design modifications specific to LMSS’s ASRG engineering unit (ASRGEU). Two ASC-Es (#2 and #3) were assembled into the ASRG-EU by LMSS, which was subsequently 
NASA/TM—2012-217289 3 

delivered to GRC for extended operation and controller testing (Ref. 4). In parallel with the first pair of 

ASC-Es, a second pair (ASC-E #1 and #4) was fabricated and delivered to GRC. These were not 

integrated into a generator, but were assembled into a test station by GRC that aesthetcially resembles the 

ASRG-EU. The ASC-E2 design is capable of higher hot-end temperature operation. Eight ASC-E2 units 

were delivered to GRC throughout 2010 and are at various stages in the standard test sequence (Ref. 2). 

TABLE I.—SUMMARY OF ON-GOING STIRLING CONVERTOR OPERATION AT NASA GRC AS OF JULY 1, 2011 

Convertors Supplier Nominal operating 

temperatures 

(Hot/Cold, °C) 

Nominal 

per-convertor 

power output 

(We) 

Date initiated Per-convertor 

runtime 

(hrs) 

TDC #13 & #14 Infinia Corp. 650/80 65 Jun 2003 60,000 

TDC #15 & #16 650/80 65 Mar 2005 49,000 

ASC-0 #3 & #4 Sunpower Inc. 650/90 75 Aug 2007 25,000 

ASC-E #2 &# 3 

(ASRG-EU) 

625/70 65 Nov 2008 19,000 

ASC-E #1 & #4 650/70 65 Dec 2009 10,000 

ASC-E2 #1 850/50 80 Mar 2010 6,200 

ASC-E2 #2 Feb 2010 2,700 

ASC-E2 #3 & #4 Aug 2010 800 

ASC-E2 #5 & #6 Aug 2010 4,800 

ASC-E2 #7 Nov 2010 2,100 

ASC-E2 #8 Jun 2011 20 

ASC‐E2 Housing

Test Rack

User controls

Data acquisition

Software protection

Hard‐wired protection

ASC‐E2 Housing

Circulator

Cold‐end temperature control

Cold‐end circulator plumbing

Figure 1.—Example ASC-E2 extended operation test station at NASA GRC. 
NASA/TM—2012-217289 4 

2.0 Extended Operation Test Results 

2.1 TDCs #13 and #14 

TDC #13 and #14 is the longest-running pair of Stirling convertors at GRC, with 60,000 hr each. 

Operation of these units has been discussed previously by References 1 and 3. These units produce 65 We at 

a hot-end temperature of 650 °C and a cold-end temperature of 80 °C. Inconel 718 was chosen for the heater 

head material due to both its high-temperature capability and the ample available tertiary creep data (Ref. 1). 

This was important because at the time, no other candidate material had available tertiary creep data 

necessary to design a long-life heater head. The convertors have operated without failure, and all observed 

degradation has been attributed to sources other than the workings of the device. Shutdowns have been 

required only to address support facility issues. The continued operation of these units has demonstrated the 

viability of dynamic power conversion for space power. The data show that dynamic conversion can attain 

the long-life necessary for spaceflight applications by eliminating mechanical wear mechanisms via noncontacting moving components. The data also show that the non-moving components can be designed to 

attain long life, such as the heater head material at high-temperature and stress. 

These convertors began extended operation in June 2003. A plot of the convertor performance data is 

shown in Figure 2. Initially, the pressure joints were sealed only with o-rings. This was shown to allow 

inward permeation of air despite the existence of high-pressure helium gas inside the convertor. The 

permeation of oxygen into the helium working gas resulted in oxidation of the regenerator material, which 

caused the decline in convertor performance shown during the first 18,000 hr of operation. Beginning at 

approximately 18,000 hr, the convertors were operated at reduced hot-end temperature in anticipation of 

welding the pressure joints. At 19,000 hr, the pressure joints were welded, effecting a hermetic seal at 

these joints, but the helium fill line remained to enable working gas sampling and mean charge pressure 

adjustment. The convertors were then operated at reduced hot-end temperature until approximately 

20,000 hr, after which operation resumed at full temperature. Since sealing the joints and eliminating 

permeation through the o-rings, the initially-observed degradation was eliminated. The performance 

became more steady after the sealing process. There were subsequent disturbances in the performance 

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TDC #14 Power Output (We

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TDC #13 Power Output (We

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TDC #13 & #14 Performance Data

11359 : Change Circulator Settings18424 : Low temperature Checkout20024 : Full Temp. Operation22039 : New Heater32888 : Adjust Piston Amplitude41879 : Rack Calibration45390 : Adjust Piston Amplitude51633 : Rack Calibration54124 : New Heater, Rack Upgrades56471 : Rack Calibration

Figure 2.—TDCs #13 and #14 performance data through 60,000 hr of operation. 
NASA/TM—2012-217289 5 

data that can be attributed to support facility interruptions. The presence of the fill line introduces a working 

gas leak path, and the convertor still requires periodic charge pressure adjustments approximately every two 

weeks. These periodic charge pressure adjustments manifest as the “saw-tooth” fluctuations in the plot of 

power output. The effect of small changes in charge pressure have been quantified, but were not used to 

correct the data shown in the plot below. Cartridge heaters that provide heat input also introduced 

performance variability, since their usable life is less than the desired length of extended operation. The 

cartridges were replaced at 22,000 and 54,000 hr. Other events that introduced disturbances in the data 

included manual shutdowns for equipment calibration, controller hardware drift, ambient temperature 

fluctuations, nuisance protection shutdowns, and building power outages. The zener diode-based controller 

required adjustment at around 25,000 hr to maintain constant convertor piston amplitude. On several 

occasions, the ambient temperature drifted due to air conditioning malfunctions and caused an observable 

change in the convertor operation. Similarly, the cold-end temperature control system required maintenance, 

such as fluid concentration adjustments, or filter cleaning. On many occasions that are not explicitly 

identified in the plot, automated shutdowns were initiated by the data system monitoring software. Many of 

these were due to electrical noise interfering with the piston overstroke detection circuitry, while others were 

due to thermocouple failure. Recently, a backup generator has been connected to power the laboratory to 

sustain operation in the event of a city power grid outage. 

2.2 TDCs #15 and #16 

Two more TDCs were put on extended operation in March 2005. These units are identical to 

TDCs #13 and #14, and are the second longest-running convertors in the Stirling research laboratory with 

49,000 hr each. These convertors have been discussed previously in Reference 1 and 3. A plot of 

convertor performance data through 49,000 hr is shown in Figure 3. Initially, these convertors were 

operated at a limited hot-end temperature of 500 °C to mitigate regenerator oxidation that was observed in 

TDCs #13 and #14. The convertor pressure joints were welded at 4,400 hr in the same fashion as 

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TDC #16 Power Output (We

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TDC #15 Power Output (We

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TDC #15 & #16 Performance Data

4878 : Cartridge heaters were replaced24241 : Rack Calibration27889 : Rack Calibration37721 : Rack Calibration46021 : Rack Calibration

Figure 3.—TDCs #15 and #16 performance data through 49,000 hr of operation. 
NASA/TM—2012-217289 6 

described for TDCs #13 and #14. These convertors required cartridge heater replacement at 4,900 hr, 

which was earlier than TDC #13 and #14. This was attributed to improper cleaning after the cartridge lead 

wire braze operation. The method of lead wire attachment was revised and the replacement cartridges 

have performed without failure to date, meaning this particular set of cartridge heaters has lasted for over 

44,000 hr. The only other activities of note were manual shutdowns to calibrate instrumentation. These 

are noted on the plot at the appropriate points in time. The slight decrease in performance from 5,000 to 

15,000 hr could be attributed to variations in the cold-end temperature control hardware. Currently an 

open-bath laboratory circulator is employeed to flow a water-glycol mixture around the rejection zone of 

the circulator. This configuration runs in an open-loop mode, so changes in the mixture would affect the 

flow of the fluid, and thus affect the cold-end temperature. As such, periodic adjustments to the circulator 

fluid must be made to maintain constant conditions. Similarly, the convertors required periodic 

adjustments to the charge pressure, which manifests as the “saw-tooth” variation in the plot of power 

output. The sources of disturbance to steady operation discussed in the section for TDCs #13 and #14 also 

existed for these convertors, namely ambient temperature fluctuations, nuisance shutdowns due to 

instrumentation failures, controller hardware drift, and building power outages. 

2.3 ASC-0s #3 and #4 

The development of the Sunpower ASC began in 2003 with a NASA contract to develop a highefficiency (up to 40 percent thermal-to-electric) convertor for the next generation radioisotope power 

system. During Phases I and II of the contract, Sunpower designed and fabricated the convertor model 

ASC-1, which used MarM-247 heater heads to permit operation at 850 °C. These convertors 

demonstrated a conversion efficiency of 38 percent. The success of the initial phases led to the redirection 

of the SRG-110 project, and the replanning of the Sunpower convertor development contract. Sunpower 

then designed and fabricated the ASC-E convertors, which were tailored for the ASRG EU. Sunpower 

was also directed to supply additional units for extended operation at GRC. The first of these additional 

units was convertor model ASC-0. The ASC-1 design made use of MarM-247, but was not a hermetically 

sealed convertor. At the time the ASC-0 units were requested, the knowledge to hermetically seal a 

MarM-247 heater head had yet to be developed. To expedite convertor delivery and generation of 

extended operation data, the decision was made to use Inconel 718 for the ASC-0 design, for which the 

capability to hermetically seal was already in place. Similarly, the ASC-E design also made use of Inconel 

718. The use of Inconel 718 in the ASC-0 design is the reason it was given a preceding model number, 

even though it chronologically followed the ASC-1 design. The ASC-0 pressure joints were welded, but 

the helium fill tube was left with an attached isolation valve to permit connection to a working gas 

sampling and charge pressure management system. Thus, the convertors delivered were not completely 

hermetically sealed, but had no o-ring joints at the heater head and alternator housing that would normally 

exist in a non-hermetic convertor. 

Four ASC-0 units were delivered to GRC in 2006 and 2007. These convertors have been discussed 

previously in References 1 and 3. The first pair, (ASC-0s #1 and #2), were operated for a short time in air 

for checkout testing, and then underwent 15,000 hr of operation in a thermal vacuum environment. The 

second pair (ASC-0s #3 and #4), began operation in August 2007 and have been operating in air since 

delivery. The convertors produce approximately 75 We at operating temperatures of 650 °C hot end and 

90 °C cold end. A summary of performance data for ASC-0s #3 and #4 through 25,000 hr is shown in 

Figure 4. Initial convertor operation was at full temperature and power. At approximately 3,990 hr, a 

manual shutdown and restart was executed for scheduled test rack maintenance, consisting mostly of 

instrument calibration. Following this maintenance activity, the convertor performance became noticeably 

more variable. This is thought to be due to a poor electrical connection in the test rack that arose during 

the maintenance activity. At 7,400 hr, the convertors underwent launch simulation vibration testing, 

which exposed each to 8.7 grms in all three axes. The convertors also underwent heater head diameter 

measurements that will provide data for Inconel 718 creep model validation. This measurement was 

performed using a laser micrometer providing a resolution of 0.05 μm. The micrometer was swept along 
NASA/TM—2012-217289 7 

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ASC-0 #3 Power Output (We

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ASC-0 #3 and #4 Performance Data

282:Full Temp Operation3990:1st Rack Maintenance7439:Vibe Test8034:2nd Thermal Loss & 2nd Rack Maintenance9078:Output Relay Removed12358:Tuning Cap Modification14451:Fill Tube Pinch & Thermal Loss14903:3rd Rack Maintenance18200:4th Rack Maintenance & Power Factor Tap Switched to Pre-cap19281:Stalled Convertor for 18 min21075:1st Shutdown Due to Power Glitch21914:2nd Shutdown Due to Power Glitch22735:Alternator leads swapped for 165 hours23254:Upgraded to an E2 Rack

Figure 4.—ASC-0s #3 and #4 performance data through 25,000 hr of operation. 

the axis of the heater head, and recorded diameter as a function of axial position at eight circumferential 

locations. The heater head will be measured in the same fashion again in the future, which will reveal 

creep if any has occurred. The convertors resumed operation following launch simulation vibration 

testing, emulating the sequence that the flight convertors will undergo. From this point and through 

14,400 hr, various activities took place to pinpoint the source of faulty power pathway connection in the 

test rack, but no root cause was identified. During this period, the variable nature of the convertor power 

output continued. 

After 14,400 hr, the convertor helium fill tubes were pinched, completely hermetically sealing the 

working gas. The convertors were leak tested before and after the fill tube pinch. Both leak rates were 

below the requirement for flight units, indicating successful hermetic sealing. Similarly, convertor 

performance was compared before and after the fill tube pinch. No negative effect on convertor operation 

was observed. In fact, the convertors appeared to perform slightly better with the pinched tube. Shortly 

thereafter, at 14,900 hr, another scheduled rack maintenance activity was performed. Interestingly, the 

convertor performance was noticeably steadier following this activity. This suggests that either the startup 

and shutdown transient or the activity in the test rack was to blame. At 18,200 hr, operation was shut 

down for yet another test rack maintenance activity, after which the more-variable convertor performance 

returned. At 23,250 hr, the entire test rack was upgraded to a more recent design that incorporated 

changes to achieve a more robust power pathway. Following this test rack upgrade, the convertor 

performance has been as steady as that initially observed before the first test rack maintenance, suggesting 

once again that the aforementioned variable performance was due to the test rack. As was true for other 

test stations, steady operation was disturbed on multiple occasions due to facility issues such as power 

outages, ambient temperature changes, circulator fluid adjustments, and nuisance shutdowns due to 

instrumentation. 
NASA/TM—2012-217289 8 

2.4 ASRG EU (ASC-Es #2 and #3) 

The ASRG EU was designed and fabricated by LMSS under contract to DOE. The generator design 

integrates two Sunpower ASC-Es in the dual-opposed configuration, and uses electric heat sources in 

place of GPHS modules (Ref. 4). Aside from the electric heat sources, the EU design emulates a flight 

unit. The ASC-E was designed specifically for the ASRG EU. It uses Inconel 718 for the heater head, and 

is a completely hermetically sealed convertor (pressure joints are welded and helium fill tube is pinched). 

LMSS delivered the ASRG EU to GRC in August 2008 after completing their own set of checkout tests. 

The ASRG EU has since accumulated 19,000 hr of operation at GRC. The ASRG EU was most recently 

discussed in Reference 4. The convertors produce approximately 65 We at operating temperatures of 

625 °C hot end and 63 °C cold end. When assembled into a generator, the convertors are cooled by 

connection of the rejection end to the housing. The EU is being operated in the vertical orientation and 

cooled with chilled air, which limits the ability to control the convertor’s cold-end temperature relative to 

other test stations in the laboratory. A summary of convertor performance for the ASRG-EU convertors 

through 19,000 hr is shown in Figure 5. The ASRG EU was operated on and alternating current (AC) bus 

control (rather than a flight-like controller) for the first 5,500 hr. During this time, fluctuations in 

convertor performance were observed and attributed to variation in the test rack power path impedance. 

Test rack improvements were made on two occasions during this time, at 2,100 and 4,800 hr. LMSS then 

delivered the ASC Controller Unit (ACU), which was designed to control operation of the convertors in a 

fashion similar to that used in flight. The control algorithm for flight employs active-power-factorcorrection, and needs no tuning capacitance (Ref. 7). As such, the controller must actively syncronize the 

piston motions to achieve dynamic balance. The ASRG EU began operating with the ACU at the 5,200-hr 

mark. The ASRG EU has continued operation on the ACU since then, except for one brief period of 

operation on AC bus control for specific tests. Between 5,200 and 13,500 hr, many other tests were 

conducted. However, from the 13,500-hr mark to present, operation has been maintained as best as 

possible at the nominal conditions. This region is most useful for observing steady long-term data. As can 

be seen, the performance is steady, and matches well to the initial ACU operation from the 6,000 to 

8,000-hr marks. This suggests the system (ASRG plus ACU) responds in a repeatable fashion when 

returning to the same operating conditions. Setpoint adjustment 1st set of rack improvements Setpoint adjustment Setpoint adjustment 2nd set of rack improvements Begin ACU control Begin heat input effect test

Resume AC bus control

Begin operation on cards 1 & 3

32 hours of EDU 2 ACU control

Removed J2 connector saver

24 hours operation on cards 1 & 2

Recalibration

Begin natural convection test

End natural convection test

Resume operation on cards 1 & 2

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ASC A - ASC-E #2

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Figure 5.—ASRG-EU (ASC-Es #2 and #3) performance data through 19,000 hr of operation. 
NASA/TM—2012-217289 9 

2.5 ASC-E #1 and #4 

Sunpower fabricated four ASC-E units in support of the ASRG EU. Two of these were integrated into 

the ASRG EU (ASC-E #2 and #3). The other two convertors (ASC-E #1 and #4) were delivered to GRC 

to undergo extended operation. These convertors were assembled into support hardware that visually 

resembles the ASRG EU housing, but uses liquid coolant flow to control the convertor cold-end 

temperature, rather than chilled air convection cooling used for the ASRG EU (Ref. 3). These convertors 

also underwent heater head diameter measurements for evaluation of long-term creep of Inconel 718 on 

an operating convertor. These convertors began extended operation in December 2009 and have 

accumulated 10,000 hr as of July 1, 2011. A plot of convertor performance is shown in Figure 6. The 

convertors produce approximately 65 We at operating temperatures of 650 °C hot end and 70 °C cold end. 

At around 1,000 hr of operation, the voltage tap for the power meter that measures convertor power 

output was moved to the convertor side of the tuning capacitance. This changed the power measurement 

only because of the resistive losses that exist between these two points. The tuning capacitance ideally 

does not dissipate any power, except for that associated with its series-equivalent resistance. There was an 

unexpected 7-We drop in power output from ASC-E #1 near 4,500 hr of operation. At this time, the power 

path was inspected for loose connections. No faulty connections were identified in the power path and 

operation resumed, but ASC-E #1 never returned to its original power output level. Between 7,000 and 

8,000 hr, the convertors were operated with the alternator leads swapped at their connection to the test 

rack. This was done to determine if the power loss was due to a faulty connection in the test rack, or an 

issue on the convertor side of the alternator connection. The ASC-E #1 power output did not return to its 

initial-operation level, suggesting the test rack was not the cause of the 7-We drop. This event is under 

investigation by a team at GRC to determine the root cause. Other instances of non-flatline power output 

have been attributed to shutdowns and fluctuations in ambient temperature. 

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ASC-E # 4 Power Output (We

)

971 : Moved Power Measurement Pre-Cap4434 : Power Drop Event on ASC-E #16049 : Annual Calibration6926 : Swap Alternator Leads7924 : Return Alternator Leads

Figure 6.—ASC-Es #1 and #4 performance data through 10,000 hr. 
NASA/TM—2012-217289 10 

2.6 ASC-E2s #1 and #2 

Another task under the Phase III redirection of the Sunpower ASC development contract was the 

design and fabrication of several hermetically-sealed MarM-247 heater head convertors. MarM-247 was 

successfully demonstrated on a convertor during Phase II of the contract, on the ASC-1 units, but these 

were not hermetically sealed. Following completion of the ASC-E units, Sunpower proceeded to design 

and fabricate eight ASC-E2 units. The ASC-E2 design is the same as the ASC-E, but uses MarM-247 for 

the heater head, thus ASC-E2 is the first convertor model to have the high-temperature heater head 

material and be completely hermetically sealed. MarM-247 permits hot-end operating temperatures up to 

850 °C, which increases the conversion efficiency. The ASC-E2 was designed to be as flight-like as 

possible, and is physically representative of the units that will be integrated into the flight generator. The 

ASC-E2s have completed or are undergoing a GRC test sequence consisting of the following: 

workmanship vibration testing, performance mapping, and extended operation (Ref. 2). Workmanship 

vibration testing was performed following convertor final hermetic sealing and just prior to delivery to 

GRC. Performance mapping consisted of operating the convertor at different cold-end temperatures and 

heat inputs to simulate conditions expected during a mission. An operating condition representing the 

beginning-of-mission (BOM) low- rejection (LR) temperature was chosen for extended operation. This 

represents operation of the generator with maximum heat input from the GPHS module and exposure to a 

deep-space sink temperature. Other operating conditions simulated included end-of-mission (EOM) LR 

and BOM or EOM high rejection (HR). 

ASC-E2 #1 and #2 were the first convertors of this design to be fabricated. The convertors were not 

delivered as a pair, due to the discovery of a heater head manufacturing flaw in ASC-E2 #1 resulting in 

leakage of the helium working gas over time. ASC-E2 #2 was delivered first to GRC as a single unit in 

February 2010 (Ref. 2). ASC-E2 #2 completed the GRC test sequence, and has accumulated 2,700 hr of 

operation thus far. A plot of convertor performance is shown in Figure 8. The majority of this operation 

was at 850 °C hot end and 50 °C cold end, which corresponds to BOM LR. The initial 280 hr of operation 

included checkout tests and performance mapping, after which extended operation was initiated at BOM 

LR. A series of nuisance shutdowns occurred around 400 hr and these data do not reflect unsteady 

convertor performance. Normally, the heat input to the convertor is controlled via a constant temperature 

control loop. With this, the hot-end temperature stays constant but the thermal power input could dither as 

the control loop maintains its set point. At other times, constant heat input was explored to more 

accurately simulate heat input from a radioisotope source. Such was the case between 400 and 742 hr. 

Between 406 and 568 hr, the downward trend in power output was due to the use of constant heat input 

mode. Malfunctions in the thermal power input feedback signal used by the constant heat input controller 

resulted in unsteady heat input. These issues were resolved at the 568-hr mark and performance became 

noticeably steadier. Between 742 and 1,265 hr, other tests were performed at the request of LMSS, which 

deviated from the BOM LR operating condition. A similar test took place between 2,084 and 2,327 hr. 

Outside of these regions, the operating conditions were held steady at the BOM LR conditions. During 

the times that conditions were steady, the convertor performance was steady. Other minor disturbances in 

the plot were caused by ambient temperature fluctuations. ASC-E2 #2 was shut down after 2,446 hr of 

operation, after which it was paired with ASC-E2 #1 and underwent an electromagnetic interference 

(EMI) characterization test. Currently, ASC-E2 #2 is idle and will continue operation in the dual-opposed 

configuration with ASC-E2 #1 once testing is completed. 

ASC-E2 #1 was delivered to GRC in April 2010. Delivery of ASC-E2 #1 was delayed due to the 

discovery of a heater head manufacturing flaw that results in helium leakage. A plot of convertor 

performance is shown in Figure 7. ASC-E2 #1 completed performance mapping and initial extended 

operation within the first 888 hr of operation. Tests similar to those performed on ASC-E2 #2 were also 

performed, such as an operating frequency sweep that took place between the hours of 605 and 888. ASCE2 #1 then underwent EMI characterization in the dual-opposed configuration with ASC-E2 #2, which 

took place between 888 and 1,143 hr of operation. ASC-E2 #1 was then manually shut down and fitted 

with a “compact” heat source. The compact heat source was designed to reduce thermal losses through 
NASA/TM—2012-217289 11 

the insulation relative to the standard heat source used for ASC-E2 operation. For comparison, with the 

baseline ASC-E2 heat source, the losses are approximately 170 Wth at 850 °C hot-end temperature, while 

with the compact heat source the losses were reduced to 75 Wth. Performance mapping with this heat 

source took place between 1,143 and 1,476 hr, after which extended operation at BOM LR was initiated. 

Steady conditions were maintained between 1,476 and 3,849 hr. During this time, the effect of the known 

working gas leakage manifested. Notice the downward trend in power output that was corrected on two 

occasions (at 2,629 and 3,223 hr). The corrections were made by increasing the piston amplitude to return 

to the baseline BOM LR power output. Data from this region are being used to validate the prediction of 

power loss versus time due to the helium leakage. Between 3,849 and 4,696 hr, another off-nominal 

operating condition test was performed. The compact heat source required replacement at 4,696 hr, at 

which time a standard ASC-E2 heat source was installed. From 4,696 to 

6,097 hr, operation continued at the baseline BOM LR condition. The downward trend in power output 

was observed again, but was more variable due to nuisance shutdowns and temporary disturbances in the 

ambient temperature. Between 6,097 and 6,193 hr, a test was performed to experimentally measure the 

convertor natural frequency while operating. This test will be performed periodically in the future to 

further track the effect of the helium leakage. To date, ASC-E2 #1 has operated for over 6,200 hr, making 

it the leader in 850 °C convertor operation. ASC-E2 #1 will be paired with ASC-E2 #2 in the future for 

continued extended operation. 

Figure 7.—ASC-E2 #1 performance data through 6,500 hr. 
NASA/TM—2012-217289 12 

 0 500 1000 1500 2000 2500 0

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Hours of Operation

ASC-E2 #2 Power Output (We

)

ASC-E2 #2 Performance Data

280 : Checkout Completed, Initiated Ext Ops406 : Shutdown/Restarts ----------------------------------------------------------------568 : Shutdown/Restart742 : Init Hot-End Temp Var.810 : Init Alt Housing Temp Variance905 : Trans back to BOM LR1102 : Freq and Htr Pwr Var1265 : Trans. back to BOM LR2084 : Init Freq Var2327 : Trans. back to BOM LR2446 : Shutdown

Figure 8.—ASC-E2 #2 performance data through 2,500 hr. 

2.7 ASC-E2s #3 and #4 

The second pair of ASC-E2 units, #3 and #4, were delivered to GRC in April 2010. These convertors 

were delivered as a pair, and were operated exclusively in the dual-opposed vertical configuration, similar 

to the arrangement of the convertors in the ASRG EU (Ref. 2). A plot of convertor performance is shown 

in Figure 9. Checkout testing and performance mapping took place during the initial 213 hr of operation. 

From 213 to 618 hr, user adjustments were made to maintain the BOM LR condition. Both constant heat 

input and constant temperature modes were used for heat input control. Adjustments were necessary to 

hone in on the desired operating condition in response to transients. These transients have been observed 

to take on the order of one week to achieve steady operation. This can be attributed to the slow thermal 

response time of the hot-end insulation assembly. Between 618 and 802 hr, a frequency sweep was 

performed similar to that described for ASC-E2s #1 and #2. Outside of user adjustments and special tests, 

the convertor performance was steady with other minor disturbances due to ambient temperature 

fluctuations. Following the operation described here, the convertors were transported to Lockheed Martin 

Coherent Technologies (LMCT) for controller development. These convertors will be returned to GRC 

for continued extended operation in 2012. 
NASA/TM—2012-217289 13 

0

10

20

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90

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ASC-E2 #4 Alternator Power (We

)

 0 150 300 450 600 750 900 0

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Hours of Operation

ASC-E2 #3 Alternator Power (We

)

ASC-E2 #3 & #4 Performance Data

24 : Begin Performance Mapping125 : Shutdown: User Initiated213 : End Performance Mapping259 : User adjustments made283 : User adjustment to piston amplitude334 : User adjustments to heat input352 : User adjustments to heat input383 : User adjustments to piston amplitude and heat input407 : User adjustments to piston amplitude and heat input430 : Returned to const temp mode477 : Circulator Top-off618 : Began frequency variation test694 : Repeat frequency variation test802 : End frequency variation test

Figure 9.—ASC-E2s #3 and #4 performance data through 800 hr of operation. 

2.8 ASC-E2s #5 and #6 

The third pair of ASC-E2 units, #5 and #6, was delivered to GRC in July 2010. These were also 

delivered as a pair, and operated exclusively in the dual-opposed vertical configuration (Ref. 2). A plot of 

convertor performance is shown in Figure 10. The convertors completed workmanship vibration testing, 

performance mapping, and are undergoing extended operation at the baseline BOM LR condition. The 

performance data between 1,194 and 1,914 hr show an increase in power output. This has been attributed 

to fluid concentration changes in the cold-end circulator. For performance mapping, a specific mixture of 

glycol and water was required to achieve the desired temperatures. It has been observed that during 

extended operation, the water may slowly evaporate, altering the viscosity of the fluid. Adjustments to the 

circulator fluid temperature and concentration were made after the 1914-hr mark to maintain the BOM 

LR condition. As can be seen, once the fluid was stabilized, the convertor performance also stabilized. 

Between 2,202 and 2,323 hr, another off-nominal test was performed during which the alternator housing 

temperature was varied. Between 2,323 and 2,480 hr, the convertors were operated at EOM LR, which 

resulted in a lower power output as would be expected. During this time, the ASC-E2 #5 heat source 

began failing, which resulted in the fluctuation in power output near the end of that time period. At 

2,665 hr, each convertor heat source was replaced and operation resumed at EOM LR. Between 2,883 and 

4,233 hr, operation at the high rejection temperature was explored to ascertain its effect on convertor 

performance. During this time, the convertor performance was visibly less steady, but this can also be 

attributed to the aforementioned variability in the rejection temperature control. At the high rejection 

temperature, the fluid of the open-bath circulator requires more frequent adjustments to maintain steady 

operating conditions. Many adjustments were made at and around the 3,019-hr mark. Furthermore, a 

particularly large disturbance in the ambient temperature resulted in the performance disturbance seen at 

the 3,823-hr mark. After 4,233 hr, the operating condition was returned to BOM LR. Convertor 

performance has been steady since this time, and the convertors are still operating. As of July 1, the 

convertors have each accumulated 4,800 hr of operation. 
NASA/TM—2012-217289 14 

0

10

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ASC-E2 #6 Power Output (We

)

 0 1000 2000 3000 4000 5000 0

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Hours of Operation

ASC-E2 #5 Power Output (We

)

ASC-E2 #5 & #6 Performance Data

140 : Checkout Testing855 : Checkout Complete, Initiated Perf Map Restart1194 : Initiated Extended Operation at BOM LR1914 : Operating Point Adjustments2202 : Alt Housing Temp Adjustment2323 : Shut Down/Restart EOM LR2480 : #5 Heat Source Failure2665 : Shutdown/Restart EOM LR2833 : Moved to EOM HR3019 : Several Operating Point Adjustments3192 : Shutdown/Restart BOM HR3486 : Shutdown/Restart BOM HR3823 : Temporary AC loss4233 : Moved back to BOM LR

Figure 10.—ASC-E2s #5 and #6 performance data through 4,800 hr of operation. 

2.9 ASC-E2s #7 and #8 

Two ASC-E2 units (#7 and #8) were selected to undergo additional “durability” tests (Ref. 2). These 

tests are intended to experimentally demonstrate the margins that exist in the ASC-E2 design. The 

convertors will be subjected to conditions beyond that expected during normal operation. The tests 

include start/stop cycling, overstroke due to launch vibration, and centrifugal acceleration. The tests are 

not intended to cause damage that will shorten convertor life. The alternator housing will be removed and 

the convertor components will be inspected after each durability test. For this purpose, the convertors 

were not hermetically sealed, but instead have a removable alternator housing. Following these tests, the 

convertors will undergo the standard hermetic sealing, and then continue extended operation. 

3.0 Maintaining Steady Operating Conditions 

The data presented thus far show that convertor performance has been steady when the operating 

conditions were precisely maintained. When user adjustments were made, the response in convertor 

performance was evident. When disturbances were introduced involuntarily, the response was also 

evident. If ignoring periods where these events occurred, the performance data were flatline. User 

adjustments were necessary on several occasions to perform investigative tests, or to supply data to 

LMSS. However, there were several periods during which the user made no adjustments, but the 

operating conditions were disrupted by spontaneous changes in support systems and the ambient 

environment. These disruptions have been attributed to variability of temperature control methods, 

nuisance shutdowns, ambient environment fluctuations, and long-term thermal transients. In the case of 

temperature control methods, changes in the heat input hardware such as thermal connectivity between 

the heat source and convertor, or long-term aging of the hot-end insulation have caused long-term thermal 

transients that require periodic adjustment. Similarly, changes in the fluid circulation system that is 

employed to control convertor cold-end temperature also caused deviations in the operating point. Since 

the circulator is an open bath, the glycol-water mixture viscosity may change over time as water 
NASA/TM—2012-217289 15 

evaporates. It is currently a regularly-scheduled maintenance task to add distilled water to the fluid to 

maintain level and concentration. It was also discovered that ambient temperature fluctuations manifest in 

convertor performance data. While the room air has a dedicated temperature control system, it is still 

affected by the season, and the environment surrounding the test article may vary by 3 °C because of this. 

Similarly, when the room air conditioning system malfunctions, an even larger rise in the ambient 

environment results. Care has been taken when analyzing convertor performance data to attribute 

performance changes to such an event when necessary. While the desire is to maintain non-stop 

operation, nuisance shutdowns have been found to be unavoidable over long periods of time. The 

majority of nuisance shutdowns have been attributed to instrumentation failure, such as a broken 

thermocouple lead, which the data system interprets as an over-temperature alarm, or interference in the 

overstroke protection circuit signal. Other nuisance shutdowns have been experienced due to electric grid 

supply failures. Despite the presence of an uninterruptible power supply (UPS), even short-term noise on 

the city power grid has been found to trip controlled shutdowns, as in some cases the UPS interprets this 

as a loss of grid power. Recently, a power conditioner, automated transfer switch, and backup natural gas 

generator have become operational, which makes the test stations immune to both complete grid outages 

and temporary “dirty” grid power. 

4.0 Conclusion 

For the purpose of generating life and reliability data in support of Stirling energy conversion for 

spaceflight, NASA GRC has been operating several Stirling convertors in extended mode. Convertor 

performance data are recorded continuously, and analyzed periodically to ascertain long-term trends. To 

date, GRC has operated 38 convertors with a cumulative runtime exceeding 400,000 hr. Currently, 18 

convertors are either operating continuously, or are slated to continue continuous operation. These 18 

convertors have accumulated over 350,000 hr. Included are four Infinia TDCs, and 14 Sunpower ASCs 

from various stages of technology development. The data support the viability of Stirling conversion in 

space, as long life can be achieved by using non-contacting oscillating component technology. The 

performance data show that long-term convertor operation is steady when operating conditions are 

maintained, and disruptions from the support facility are discarded. Disruptions of steady operating 

conditions have been attributed to variability in temperature control methods, nuisance shutdowns, 

ambient environment fluctuations, and long-term thermal transients. 

References 

1. Schreiber, J.G., “Supporting Technology at GRC to Mitigate Risk as Stirling Power Conversion 

Transitions to Flight,” Proceedings of 6th Energy Conversion Engineering Conference (IECEC 

2008), Cleveland, OH, July 2008. 

2. Oriti, S.M., “Advanced Stirling Convertor (ASC-E2) Performance Testing at NASA Glenn Research 

Center,” Proceedings of 1st Nuclear Emerging Technologies for Space (NETS 2011), Albuquerque, 

NM, February 2011. 

3. Wilson, S.D., “Stirling Convertor Extended Operation Testing Overview,” Proceedings of 8th 

International Energy Conversion Engineering Conference (IECEC 2010), Nashville, TN, July 2010. 

4. Lewandowski, E.J., “Testing to Characterize the Advanced Stirling Radioisotope Generator 

Engineering Unit,” Proceedings of 8th International Energy Conversion Engineering Conference 

(IECEC 2010), Nashville, TN, July 2010. 

5. El-Genk, M.S., “A Critical Review of Space Nuclear Power and Propulsion 1984-1993,” p. 18, 

American Institue of Physics Press. ISBN 1-56396-317-5. 

6. NASA Discovery Program News. http://discovery.nasa.gov/news/index.cfml?ID=1034. 

7. Leland, D., “Development of a Power Electronics Controller for the Advanced Stirling Radioisotope 

Generator,” Proceedings of 5th International Energy Conversion Engineering Conference (IECEC 

2007), St. Louis, MO, June 2007. 
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14. ABSTRACT

NASA Glenn Research Center (GRC) has been supporting development of free-piston Stirling conversion technology for spaceflight 

electrical power generation since 1999. GRC has also been supporting the development of the Advanced Stirilng Radioisotope Generator 

(ASRG) since 2006. A key element of the ASRG project is providing life, reliability, and performance data for the Advanced Stirling 

Convertor (ASC). The Thermal Energy Conversion branch at GRC is conducting extended operation of several free-piston Stirling 

convertors. The goal of this effort is to generate long-term performance data (tens of thousands of hours) on multiple units to build a life and 

reliability database. Currently, GRC is operating 18 convertors. This hardware set includes Technology Demonstration Convertors (TDCs) 

from Infinia Corporation, of which one pair (TDCs #13 and #14) has accumulated over 60,000 hr (6.8 years) of operation. Also under test 

are various Sunpower, Inc. convertors that were fabricated during the ASC development activity, including ASC-0, ASC-E (including those 

in the ASRG engineering unit), and ASC-E2. The ASC-E2s also completed, or are in progress of completing workmanship vibration testing, 

performance mapping, and extended operation. Two ASC-E2 units will also be used for durability testing, during which components will be 

stressed to levels above nominal mission usage. Extended operation data analyses from these tests are covered in this paper.

15. SUBJECT TERMS

Advanced Stirilng Radioisotope Generator (ASRG); Stirling power conversion 

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