Nicholas A. Schifer and Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Summary of Stirling Convertor Testing at NASA 

Glenn Research Center in Support of Stirling 

Radioisotope Power System Development

NASA/TM—2013-216543

November 2013

NETS–2013–6816
	
	
	








	



	

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Nicholas A. Schifer and Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Summary of Stirling Convertor Testing at NASA 

Glenn Research Center in Support of Stirling 

Radioisotope Power System Development

NASA/TM—2013-216543

November 2013

NETS–2013–6816


	



	


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NASA/TM—2013-216543 1 

Summary of Stirling Convertor Testing at NASA Glenn Research 

Center in Support of Stirling Radioisotope 

Power System Development 

Nicholas A. Schifer and Salvatore M. Oriti 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The NASA Glenn Research Center has been testing 100-We-class, free-piston Stirling convertors for 

potential use in Radioisotope Power Systems (RPS) for space science and exploration missions. The 

present class of free-piston Stirling convertors are capable of achieving high conversion efficiency, 

making Stirling attractive for meeting future power system needs in light of the limited U.S. plutonium 

inventory. Convertors currently on test include four Technology Demonstration Convertors (TDCs) 

manufactured by Infinia Corporation (formerly the Stirling Technology Company), and seven Advanced 

Stirling Convertors (ASCs) manufactured by Sunpower, Inc. Operation to date has accumulated more 

than 530 000 hr (60.5 years). Tests have been devised to address the long life performance of Stirling 

convertors for space applications including in-air extended operation and thermal vacuum extended 

operation—while other tests have been conducted to characterize Stirling performance in anticipated 

mission scenarios. Data collected during these tests have been used to support life and reliability 

projections, drive design changes and improve quality, and prepare for expected mission scenarios. This 

paper will provide a summary of convertors tested at NASA Glenn and discuss lessons learned through 

extended testing. 

Introduction 

NASA’s Glenn Research Center has been involved in the development of Stirling technology since 

the mid-1970s. Early research and testing included kinematic and free-piston Stirling for a range of 

applications including residential heat pumps, terrestrial power generation, nuclear space power, and 

radioisotope space power with a primary interest in future space power (Schreiber (2006a)). Parallel 

technology advancement efforts included creating fluid flow and heat exchangers, advancing component 

technologies, and developing Stirling convertors with elevated temperature capability, enhanced 

reliability, greater efficiency and reduced mass (Schreiber (2006a)). Since 1999, the focus has shifted to 

100-We-class free-piston power convertors for a radioisotope generator. 

The Department of Energy (DOE) is managing the development of a 100-We-class multimission 

radioisotope generator for NASA (Schreiber (2006a)). Lockheed Martin Space Systems (LMSS) of 

Valley Forge, PA, was selected by DOE as the system integration contractor for the project. Glenn has 

supplied Stirling convertor expertise and provided the capability for a range of tests involving convertors 

and components (Schreiber (2006a) and Wong and Cornell (2011)). In addition, Glenn has been asked to 

provide Independent Verification and Validation of the convertors, which includes verifying performance 

and reliability through extended operation. Two companies have produced Stirling convertors for 

potential use in the radioisotope generator. Infinia Corporation, formerly the Stirling Technology 

Company (STC), of Kennewick, WA, designed and built the Stirling Technology Demonstration 

Convertors (TDCs) under the Stirling Radioisotope Generator (SRG!110 program. The TDCs were used 

to demonstrate the feasibility of free-piston Stirling technology for a Radioisotope Power System (RPS) 

as the design was matured toward production-level status. However, one metric of interest for space 

power applications is the specific power of a fully integrated system. The TDCs had a specific power of 
NASA/TM—2013-216543 2 

about 15 W/kg, which resulted in a system-specific power of about 4 W/kg. Through an SBIR contract, 

Sunpower, Inc., of Athens, OH, developed the EE35 Stirling convertor, which was sized for half the heat 

from a general purpose heat source (GPHS) module and achieved nearly 90 W/kg. The Advanced Stirling 

Convertor (ASC), which was sized for heat from one GPHS, was also designed by Sunpower and 

demonstrated a similar specific power. Analysis indicated that an RPS using the ASCs could potentially 

achieve up to 8 W/kg, which would enable electric propulsion missions (Schreiber (2006b)). Focus has 

since shifted to development of the ASC under the Advanced Stirling Radioisotope Generator (ASRG) 

project. 

Reliability is an essential requirement of a radioisotope generator for space applications. Potential 

mission lengths could be as long as 14 years, with an additional 3-year storage life prior to launch. While 

accelerated life testing is possible at the component level, it is not possible to perform an accelerated life 

test on an entire convertor. This is because there is no single parameter that influences the life of multiple 

components. While accelerated life testing at the component level has been performed and life 

requirements have been met, the remaining challenge is to verify long life of the convertor, in which wear 

mechanisms have been eliminated by design (Schreiber (2006b)). Since operating convertors for 17 years 

is not a reasonable option, the approach being taken is to build a database from operating convertors that 

can be used to support a statistical reliability estimate. A total of 39 free-piston Stirling convertors have 

operated at Glenn and have accumulated greater than 530 000 hr. Convertors have operated in both 

thermal vacuum and in-air environments, under launch vibration conditions, along with other tests to 

investigate specific situations, such as convertor start-stop, static acceleration, and various mission phase 

simulations. There are currently four TDCs and seven ASCs on extended operation at Glenn representing 

several developmental stages. This paper details extended operation and summarizes the status of 

convertors that remain on test. Table 1 summarizes ongoing Stirling convertor operation at NASA Glenn. 

Extended Operation at Glenn 

Extended operation is 24-hr/7-day unattended operation of Stirling convertors over a long period of 

time. Originally, extended operation of the TDCs was intended to be 5000 hr (208 days), but a decision 

was made to continue the test because of the value for reliability and performance data (Schreiber 

(2006a)). Glenn has been operating convertors in extended operation since 2002 and has developed and 

refined the procedures, data systems, and safety mechanisms required for safe and reliable unattended 

operation. 

TABLE 1.—SUMMARY OF ONGOING STIRLING CONVERTOR 

OPERATION AT NASA GLENN AS OF NOVEMBER 2012 

Convertor Supplier Nominal operating 

 temperatures, 

hot/cold °C 

Nominal 

per-convertor 

power output, 

We 

Date initiated, 

month year 

Per-convertor 

runtime, 

hr 

TDCs #13 and #14a Infinia 650/80 65 June 2003 72 100 

TDCs #15 and #16 650/80 65 March 2005 61 100 

ASC0 #3b Sunpower 650/90 75 Aug. 2007 32 600 

ASCEs #2 and #3 

(ASRG Engineering 

Unit (EU)) 

625/70 65 Nov. 2008 29 600 

ASCE #4 650/70 65 Dec. 2009 19 100 

ASCE2 #1 850/50 80 Mar. 2010 15 400 

ASCE2 #4 850/50 80 Aug. 2010 900 

ASCE2 #5 850/50 80 Aug. 2010 14 600 a

Technology Demonstration Convertor 

b

Advanced Stirling Convertor 
NASA/TM—2013-216543 3 

There are two procedures that are essential to successful extended operation: (1) Test Rack Checkout 

and (2) Unattended Shutdown Checkout. Any time a change is made to the power pathway, hardware in 

the test rack, or the software, these two procedures must be performed. The Test Rack Checkout 

procedure systematically verifies all connections (heater, power pathway, thermistors, thermocouples, 

etc.) in the test rack and verifies that all channels are read properly into the LabVIEW (Laboratory Virtual 

Instrument Engineering Workbench) data system (National Instruments Corporation). The Unattended 

Shutdown Checkout procedure verifies that the most critical shutdown conditions trigger a shutdown and 

that the appropriate automated actions are taken. The following limits are verified in the procedure: low 

rejection temperature, high rejection temperature, high hot-end temperature, low hot-end temperature, 

high-pressure vessel temperature, excessive vibration or overstroke, and building power loss lasting for 

more than 5 min. Low charge pressure and high charge pressure are also monitored on non-hermetic 

convertors. Upper and lower limits can be set in the LabVIEW software for any measured parameter and 

the action taken when the threshold is crossed can be set to give a warning or initiate a shutdown. Hardwired safeties are also in place for redundancy. Hot-end temperature limit controllers are typically set 

30 °C above the operating point and will disable the heaters in the event that the hot end exceeds the set 

limit. A fail-safe protection circuit (FPC) will disconnect the convertor from the controller load and apply 

a fixed load to stall the pistons within one-half of the cycle of operation. The FPC is typically used to 

monitor piston amplitude with the threshold set 0.5 mm above the operating point; however, it is possible 

to use the FPC with other dynamic signals such as an accelerometer or load cell, as is the case on the 

Advanced Stirling Radioisotope Generator Engineering Unit (ASRGEU). In the event of an automated 

shutdown, an immediate or a controlled shutdown will be initiated depending on the triggering event. If 

an FPC trip is detected, the heaters will be disabled, hot-end temperature PIDs set to 10 °C, coolant 

circulator set to 25 °C, and the piston amplitude set to zero. All other shutdown events initiate a 

controlled shutdown where the heaters are disabled, the hot-end PIDs set to 10 °C, the coolant circulator 

set to 25 °C, and the piston motion is maintained until the average hot-end temperature falls below a user 

set value listed in the test rack configuration file. All warnings and shutdowns trigger text messages to 

automatically notify lab personnel. During attended operation the operator can enable the stall load sized 

to stop piston motion by kill switches on the test rack. 

Procedures governing data quality, archiving, and review have evolved and been refined through 

testing experience. Other papers have been published that describe improvements in the rack, and will not 

be discussed here. High- quality connections in the electrical power pathway that are not susceptible to 

changes in resistance are vital to ensure the test rack is not the cause of variations in the convertor 

performance. Much time has been invested in investigating and resolving such issues to ensure data 

quality. Each test rack undergoes an annual calibration cycle to ensure data accuracy. Increased data 

sample rates and regular review have become a requirement. During initial testing, a daily 5-minute 

average data point was used for tracking trends and reporting performance. After observing variations 

caused by power pathway connection issues in the test rack and later power variations caused by the 

convertor, this evolved into saving and reviewing data sampled every 2 sec. In other cases signals such as 

piston position and alternator parameters are sampled at rates of 50 kHz. High-data-rate oscilloscopes 

with data saving capabilities were used to collect these data. Currently, 5-min averages are collected 

hourly, data are sampled at a rate of one-half Hertz are saved and reviewed daily, and a high-data-rate 

oscilloscope is used when needed. In future test racks a feature will be added to the LabVIEW software to 

collect select signals at rates up to 10 kHz. 

One challenge of extended operation is maintaining a steady operating point throughout the test. The 

difficulty often stems from the facility and the equipment required to support convertor operation. 

Coolant circulators used to reject waste heat from the Stirling cycle require regular maintenance (coolant 

concentration adjustments, fluid replenishment, and filter cleaning) and sometimes need to be replaced. 

Heater cartridges that supply heat to the Stirling cycle have a limited life and require replacement. 

Thermocouples can fail and initiate automated convertor shutdowns. Power outages have also initiated 

shutdowns; however, a 50-kW backup generator, a facility power conditioner, and an uninterruptable 

power supply for each test station have addressed this issue. Electrical interference with the FPC circuitry 
NASA/TM—2013-216543 4 

and facility temperature variations have also caused disturbances. For test articles that are not 

hermetically sealed, regular charge pressure top-offs are required and often results in a “saw-tooth” 

pattern in the data. More frequent circulator top-offs, concentration adjustments, and charge pressure topoffs can be performed to minimize the impact on the data. 

Extended Operation of Technology Demonstration Convertors 

The TDCs were built as part of the SRG110 program by the Infinia Corporation. A total of 16 TDCs 

were built in the developmental stages, and there are currently four under extended operation testing at 

Glenn (Schreiber (2006a)). The TDC was designed to produce a nominal 65 W of electric power when 

provided <250 W of thermal heat input from a GPHS module. The nominal operating conditions of the 

TDC are a hot-end temperature of 650 °C, a cold-end temperature of 80 °C, a pressure vessel temperature 

of 45 °C, a piston amplitude of 6 mm, and an operating pressure of 365 psig (Roth and Schreiber (2004)). 

Inconel 718 (Special Metals Corporation) was chosen for the heater head material for its high-temperature 

capability and available long-term creep data that extended out through tertiary creep (Oriti (2011)). 

Noncontacting operation of the piston and displacer was achieved with a spring flexure-based bearing 

system to enable long life. A section view of the TDC and a photo of a typical TDC test station are shown 

in Figure 1 below. 

During extended operation testing at Glenn, the TDCs are configured in dual-opposed pairs, with the 

heater heads outboard, such that the convertors are dynamically balanced. The mounting orientation is 

horizontal and the test article fixture is isolated from the table with rubber mounts. Heat is input through 

electric heater cartridges, and heat is rejected at the cold end through an ethylene glycol (EG) coolant 

loop. A secondary EG cooling loop is used on the pressure vessel to control pressure vessel temperature. 

TDCs are controlled by a zener diode style controller. Hot-end temperature is measured by five 

thermocouples and set with the hottest at 650 °C. The cold-end temperature is maintained at 

approximately 90 °C (Roth and Schreiber (2004)). The pressure vessel temperature is measured by three 

surface mounted thermocouples. The TDCs are the only convertors on extended operation that still utilize 

a gas charging system. A gas charging system allows the ability to perform vacuum bakeout, initial 

pressurization, pressure adjustments, and gas sampling. Since the TDCs are not hermetic, pressure 

top-offs are required to maintain the charge pressure. Every 1000 hr of operation a gas sample is taken 

to monitor the working fluid to look for impurities and signs of potential outgassing of the organics. 

TDCs #13 through #16 are currently on extended operation. 

Figure 1.—Section view of Technology Demonstration Convertor (TDC) (left) and photo of typical TDC test station

(right). 
NASA/TM—2013-216543 5 

Extended Operation of TDCs #13 and #14 

TDCs #13 and #14 are the longest running pair of convertors at Glenn, with over 72 100 hr 

(8.2 years) each. A plot of the convertor performance is shown in Figure 2. These convertors have been 

previously discussed in references Oriti (2011), Lewandowski and Schreiber (2008), Schreiber (2006a), 

and Roth and Schreiber (2004). These convertors were the first developed under the Infinia Quality 

Assurance program and were, therefore, considered flight prototypes (Roth and Schreiber (2004)). These 

convertors were delivered to Glenn in February 2002, first operated in April 2002, and began extended 

operation in June 2002. Early stages of extended operation were disrupted by electrical connection issues 

in the resistance heaters, which were resolved by June 2003. The convertors were not hermetically sealed 

when manufactured with o-ring seals at flanged joints. While a vacuum bakeout process was used to 

ensure a high purity helium fill, the o-rings were found to allow helium to permeate out and air to 

permeate in (Schreiber (2006a)). Oxidation of the regenerator is believed to have caused performance 

degradation during the first 18 000 hr of operation. Beginning near 18,000 hr, the convertors were 

operated at a reduced hot-end temperature to prevent further oxidation until hermetic welding of the 

pressure joints. In January 2007, at approximately 19 000 hr, the pressure joints were welded, but the 

helium fill tube remained, enabling gas sampling and mean charge pressure adjustments. At 20 000 hr, 

full-temperature operation was resumed and the observed performance became steady. Subsequent 

variations in performance did occur, but have been attributed to set point adjustments and facility-related 

items. The presence of the non-hermetic fill tube introduced the need to make periodic pressure 

adjustments approximately every 2 weeks, which appear as saw-tooth variations in power output. 

Figure 2.—Technology Demonstration Convertors (TDCs) #13 and #14 performance through 72 100 hr of operation. 

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 0

10

20

30

40

50

60

70

80

90

100

Hours of Operation

Power Output (We

)

TDC #13 & #14 Performance Data

11359:Change Circulator Settings15424:Low Temperature Checkout20024:Full Temp. Operation22039:New Heater32888:Adjust Piston Amplitude41879:Rack Calibration45390:Adjust Piston Amplitude51633:Rack Calibration54124:New heater, Rack Upgrades56471:Rack Calibration59255:Lab Power Conditioner Installed65870:Rack Calibration

TDC #13

TDC #14
NASA/TM—2013-216543 6 

Cartridge heaters also introduced performance variability as they degraded and eventually failed. This 

resulted in a variation of the heat input and therefore, power output from the convertor. The cartridges 

were replaced at 22 000 and 54 000 hr. Other variations in the performance were the result of manual 

shutdowns for instrument calibration, manual corrections to piston amplitude for controller drift, facility 

temperature fluctuations, nuisance trips of the protection circuit, building power outages, and 

maintenance of the cold-end temperature control system. No evidence of outgassing or decomposition of 

the organics has been observed in the gas samples taken every 1000 hr of operation, and no unexplainable 

change in performance have been observed over the entire operation of TDCs #13 and #14. 

Extended Operation of TDCs #15 and #16 

TDCs #15 and #16 are the second longest running pair of convertors at Glenn, with over 62 200 hr 

(7.1 years) each. These convertors have been previously discussed in the following references: Oriti 

(2011), Lewandowski and Schreiber (2008), Schreiber (2006a), and Roth and Schreiber (2004). Convertor 

performance data is shown in Figure 3. These convertors were also developed under the Infinia Quality 

Assurance program and are identical to TDCs #13 and #14. Upon receipt in February 2005, these 

convertors were operated at hot-end temperature of 500 °C until hermetic sealing to avoid regenerator 

oxidation as experienced in TDCs #13 and #14 (Schreiber (2006a)). The convertor pressure vessel joints 

were welded in the same fashion as TDCs #13 and #14 after 4400 hr of operation. A noticeable 

performance difference can be observed between TDCs #15 and #16, which is attributed to a known 

difference in clearance seals. The same sources of operational variation described for TDCs #13 and #14 

Figure 3.—Technology Demonstration Convertors (TDCs) #15 and #16 performance through 62 200 hr of operation. 

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 0

10

20

30

40

50

60

70

80

90

100

Hours of Operation

Power Output (We

)

TDC #15 & #16 Performance Data

4878:Cartridge heaters were replaced24241:Rack calibration27889:Rack calibration37721:Rack calibration45021:Rack calibration54483:Rack calibration

TDC #16

TDC #15
NASA/TM—2013-216543 7 

also exist for these convertors. Cartridge heater replacement occurred at 4900 hr and was attributed to 

improper cleaning after the cartridge lead wire braze operation; however, this issue was corrected for the 

replacement cartridges. A slight decrease in performance from 5000 to 15 000 hr was attributed to 

variations in the cold-end temperature control. An open-bath circulator is used to flow a water-glycol 

mixture around the heat rejection zone of the convertor. This method is susceptible to changes in 

concentration as water evaporates, which affects the viscosity and fluid flow, and thus the cold-end 

temperature. Care has been taken to make periodic adjustments to the circulator fluid to maintain constant 

conditions. Periodic adjustments are also made to maintain the charge pressure, which results in the sawtooth variation in the power plot. As with TDCs #13 and #14, no evidence of outgassing or decomposition 

of the organics has been observed in the gas samples, and no unexplainable degradation in performance 

has been observed over the entire operation of TDCs #15 and #16. 

Extended Operation of ASCs 

The ASCs were built by Sunpower, Inc., in support of the ASRG project. To date, there have been 21 

ASC convertors operated at Glenn for a variety of tests including developmental, investigative, and 

extended operation tests. The ASCs cover a range of maturation stages from technology development 

(ASC1, ASC0, and ASC1HS), engineering model (ASCE, ASCE2, and ASCE3), and flight 

(ASCF). Sunpower delivered the following under the Glenn development contract: ASC1, ASC0, 

ASC1HS, ASCE, and ASCE2 (Wong and Cornell (2011)). The ASC1s were the first in the sequence 

and demonstrated a conversion efficiency of 38 percent and the ability to achieve hot-end temperatures up 

to 850 °C by utilizing MarM-247 as the heater head material (Oriti (2011)). However, the design included 

nine o-ring seals, which was not acceptable for extended operation due to working gas permeating 

through the seals (Lewandowski and Schreiber (2008)). The ASC0s were then developed as hermetic 

units (meaning the pressure joints were welded) with the exception of the helium fill tube, which allowed 

for gas sampling and charge pressure adjustments. Because a process had not yet been developed for 

hermetically sealing a MarM-247 heater head, Inconel 718 was used and the design was given a 

preceding model number. The ASC1HS units then concluded the technology development stage with 

high-temperature MarM-247 heater heads and hermetic sealing with the exception of the fill tube. The 

ASCE convertors were designed for integration into the ASRGEU to expedite demonstration of a 

generator. These convertors utilized an Inconel 718 head and were the first to undergo complete hermetic 

sealing via pinching the fill tubes. Integration of the ASCE units into an engineering unit generator 

required development of a comprehensive product specification, including generator interfaces and 

preliminary flight requirements, improved quality assurance, documentation, and processing and testing 

of the convertors. This build greatly advanced the state of the technology. The ASCE2s were built under 

a newly developed Quality Management System. The ASCE3 design is the last build prior to 

qualification and flight hardware (Wong and Cornell (2011)). A section view of the ASC and a photo of 

an ASC single convertor test station are shown in Figure 4. 

	

 

ASC0s #3 and #4 were delivered to Glenn in July 2007 and have accumulated 32 600 and 25 860 hr 

(3.7 and 3.0 years), respectively. These convertors have been previously discussed in Oriti (2011) and 

Cornell et al. (2008). These convertors have operated in-air since delivery and their performance is shown 

in Figure 5. The convertors produce approximately 75 We at operating temperatures of 650 °C hot end and 

90 °C cold end. After 282 hr at low-temperature operation, the convertors operated at the nominal 650 °C 

hot-end temperature until approximately 3990 hr. At this time, the convertors were shut down for rack 

maintenance and calibration. The performance then became more variable as the result of a poor electrical 

connection in the test rack, which occurred during the rack maintenance. After 7400 hr, the convertors 

were exposed to 8.7 grms random vibration in all three axes to simulate launch vibration. After vibration 

testing, heater head diameter measurements were taken to characterize Inconel 718 creep. A laser 
NASA/TM—2013-216543 8 

 

Figure 4.—Section view of Advanced Stirling Convertor (ASC) (left) and photo of ASC single convertor test station 

(right). 

micrometer Z	



Z

	
	





eight circumferential locations. These same measurements will be taken in the future to quantify creep. 

From 7400 through 14 400 hr, an investigation took place to find the source of the faulty power pathway 

connection in the test rack; however, the variable nature of the power output continued and no root cause 

was identified. The final step of the hermetic sealing process was completed after 14 400 hr as the fill 

tubes were pinched. The convertors were leak tested before and after the fill tube pinch, and in both cases 

the leak rates were below the flight requirement. Convertor performance after the fill tube pinch was 

evaluated and there was no loss in performance. At 14 900 hr, rack maintenance and calibration was 

performed. Afterwards, the convertor performance was more steady, further suggesting a link between 

performance variability and the test rack. Following rack maintenance at 18 200 hr, variable convertor 

performance returned. At 23 250 hr, the test rack was upgraded to a design with a more robust power 

pathway. Since the rack upgrade, convertor performance has been more stable suggesting a component or 

connection in the test rack was responsible. Random performance variations on the order of 0.2 We were 

observed on ASC0 #4 after the rack upgrade and it is suspected to be the result of poor connections at 

the electric feedthrough pin of the convertor. Connections at the feedthrough pins were nonstandard 

sleeves that slip over the pin. At 25 848 hr, a soldering procedure with known risk was performed to 

improve these electrical connections. During the soldering process, an internal short occurred on ASC0 

#4, rendering it inoperable. As a result, this soldering procedure was not performed on ASC0 #3. ASC0 

#3 was returned to extended operation as a single convertor in the vertical orientation. Between 28 485 

and 28 557 hr, an unexplained increase in piston amplitude of ~0.02 mm was observed. Since the ASC0 

#3 feedthrough connection was not soldered, the piston amplitude variation could potentially be the result 

of a poor connection. Near 29 109 hr, a test was performed to measure the natural frequency while 

operating. Other factors influencing the steady operation include circulator fluid adjustments, ambient 

temperature changes, facility power outages, and nuisance shutdowns due to instrumentation. While 
NASA/TM—2013-216543 9 

Figure 5.—Advanced Stirling Convertors (ASC)0s #3 and #4 performance through 32 300 and 25 860 hr of 

operation, respectively. 

variations in power output and piston amplitude have been observed in the short term and attributed to a 

poor feedthrough connection, no degradation in performance has been observed over the long-term 

operation of ' 

Extended Operation of the ASRGEU (ASCEs #2 and #3) 

The ASRGEU is the first integrated generator to be tested at Glenn. The ASRG has been previously 

discussed in Oriti (2011) and Cornell et al. (2008). It was designed and fabricated by the LMSS under 

contract to DOE and integrates two ASCEs in a dual-opposed configuration. The ASCEs utilized an 

Inconel 718 head, which allow hot-end temperatures of up 650 °C. After undergoing several system-level 

tests to qualification-level thermal and dynamic environments at LMSS, the ASRGEU was delivered to 

Glenn in August 2008. The EU is operating in a vertical orientation with heat provided through electric 

heat sources. Convective cooling is provided to the generator beryllium housing via fans blowing cooled 

air. The EU has been controlled with both an alternating current (AC) bus control and an early prototype 

ASC Controller Unit (ACU) delivered by LMSS, which controls the convertors similar to what will be 

used in flight. The generator has accumulated a total of 30 000 hr of operation at Glenn, with 15 400 

of those hours under ACU control. Convertor performance for the ASRGEU convertors is shown in 

Figure 6. The convertors produce approximately 65 We at operating temperatures of 625 °C hot end and 

63 °C cold end. The generator was operated on AC bus control during the first 5200 hr, during which time 

variations in convertor performance were observed and attributed to variations in the test rack power 

pathway impedance. Test rack upgrades were made at 2100 hr and 4800 hr. The ASRGEU operated on 

the ACU from 5200 to 21 219 hr, with the exception of one brief period where it operated on AC bus 

0 3,000 6,000 9,000 12,000 15,000 18,000 21,000 24,000 27,000 30,000 33,000 0

10

20

30

40

50

60

70

80

90

Hours of Operation

Power Output (We

)

ASC-0 #3 & #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 Rack24165:Alternator leads returned to normal25036:Heater Failure on ASC-0 #325851:ASC-0 #3 Single Vertical Operation27791:Shutdown: UPS at fault28533:Unprovoked piston amplitude change29109:Measurement of Natural Frequency Test

ASC-0 #3

ASC-0 #4
NASA/TM—2013-216543 10 

Figure 6.—Advanced Stirling Radioisotope Generator Engineering Unit (ASRGEU) performance through 30 000 hr 

of operation. 

control for specific tests. Tests were conducted at the request of LMSS between 5200 and 13 500 hr. 

From 13 500 to 21 219 hr, operation was maintained at the nominal conditions. This region best 

represents steady-state long-term operation under ACU control and matched well to the initial steady 

period of ACU operation from 6000 to 8000 hr. This suggests that the system, ASRG plus ACU, 

responded in a repeatable fashion when returning to the same operating conditions. At 21 219 hr, control 


X^{Z	

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time is attributed to a known issue in the ACU control algorithm that results in jitter, or what appears as 

random variations in the piston amplitude. From 21 	
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under AC bus control and the power output has been steady. During this period of AC bus control, 

disturbances to steady operation include a rack recalibration and ambient temperature variations. 

Extended Operation of ASCEs #1 and #4 

ASCE #1 was fabricated in support of the ASRGEU, but not integrated into a generator system. 

ASC-E #4 was later fabricated from spare parts and included updates intended for E2s. These two 

convertors were delivered to Glenn and began extended operation in December 2009 where ASCE #1 

has accumulated 10 381 hr and ASCE #4 has accumulated 19 100 hr of operation. These convertors 

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

operating convertor. They were previously discussed in Oriti (2011) and Cornell et al. (2008). A summary 

of convertor performance for the ASCEs #1 and #4 convertors is shown in Figure 7. The convertors 

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

1000 hr of operation, the voltage tap for the convertor power output measurement was moved to the 

convertor side of the tuning capacitors. The power measurement shows this shift, which is caused by the 

resistive loss that exist between the two points. Near 4500 hr, a 7-We drop in power output was observed 

on ASCE #1. The power path was inspected for loose connections, but none were found. Operation 
NASA/TM—2013-216543 11 

Figure 7.—Advanced Stirling Convertor (ASC)Es #1 and #4 performance through 10 381 and 19 100 hr of 

operation, respectively. 

resumed '^			

	
Z	|Z	

convertors operated with the alternator leads connected to opposite sides of the test rack to determine if 

the power loss was the result of a faulty connection in the test rack or an issue on the convertor side of the 

	
	

	

	
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inspection was performed to determine the root cause of the power loss. Upon disassembly, it was found 

that the piston was rubbing on an out-of-round and undersized cylinder, which liberated metallic debris. 

The debris blocked gas bearing flow passages and caused improper check valve operation. This resulted 

in a change in piston mean position and backflow through the check valve. The calculated power loss 

	

	

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Z	
		'^

has continued operation in a single, vertical configuration. Between 10 400 and 10 850 hr, 

characterization testing w	
	

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measurements of convertor natural frequency, acoustic emissions, and vibration. These measurements 

were taken to serve as a baseline for comparison to future measurements for health monitoring purposes. 

Between 10 874 and 10 901 hr, launch simulation and flight acceptance vibration tests were performed. 

Vibration levels of 15.8 grms in the axial direction, and 8.9 grms in the lateral directions were achieved. At 

16 683 hr, the circulator, which cools the cold-side adapter flange (CSAF), malfunctioned causing a 

disturbance to the operating point. Other variations in power output have been attributed to shutdowns 

and fluctuations in ambient temperature. In particular, between 16 000 and 16 800 hr, the air conditioning 

in the Stirling laboratory malfunctioned.

0 2,500 5,000 7,500 10,000 12,500 15,000 17,500 20,000 22,500 25,000 0

10

20

30

40

50

60

70

80

90

100

Hours of Operation

Power Output (We

)

ASC-E #1 & #4 Performance Data

971:Moved Power Measurement Pre-cap4434:Power Drop Event on ASC-E #16049:Rack Calibration6926:Swap Alternator Leads7924:Return Alternator Leads10300:ASC-E #1 inspection; #4 continued ext ops10874:Flight Acceptance Vibration Testing16157:Rack Calibration16683:Circulator Malfunction

ASC-E #1

ASC-E #4
NASA/TM—2013-216543 12 

Extended Operation of ASCE2 #1 

ASCE2 #1 is the longest running E2 convertor at Glenn, with a cumulative runtime of 15 400 hr. 

This convertor was previously discussed in Oriti (2011) and Oriti and Wilson (2011). ASCE2 #1 was 

delivered to Glenn in April 2010 with a known heater head flaw that results in helium leakage. 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 the beginning-of-mission low-rejection (|*LR) condition as defined per 

the E2 product specification. ASCE2 #1 completed performance mapping, initial extended operation, 

and frequency sweep tests during the first 888 hr. ASCE2 #1 was then paired and operated in the dualopposed configuration with ASCE2 #2 for electromagnetic interference characterization from 888 to 

1143 hr. ASCE2 #1 was then fitted with a “compact” heat source and returned to operation in the single 

vertical orientation. The compact heat source was installed to investigate environmental losses and better 

quantify net heat input to the convertor. Its design reduced thermal losses through the insulation relative 

to the standard heat source used for ASCE2 operation. The baseline ASCE2 heat source losses were 

approximately 170 Wth at 850 °C hot-end temperature, while the compact heat source losses were 75 Wth. 

Performance mapping with the compact heat source took place between 1143 and 1476 hr. Extended 

operation at the BOMLR temperature began immediately after performance mapping and steady 

conditions were maintained between 1476 and 3849 hr. During this time, a downward trend was observed 

in the power output as a result of the helium leakage. On two occasions (at 2692 and 3223 hr) corrections 

were made by increasing the piston amplitude to return to the baseline |*`X power output. Between 

3849 and 4696 hr, another off-nominal operating condition test was performed. At 4696 hr the compact 

Figure 8.—Advanced Stirling Convertor (ASC)E2 #1 performance through 15 400 hr of operation. 

0 2,500 5,000 7,500 10,000 12,500 15,000 0

 10

 20

 30

 40

 50

 60

 70

 80

 90

100

Hours of Operation

Power Output (We)

ASC-E2 #1 Performance Data

296 : Initiate Extended Operation BOM-LR606 : Initiate Frequency Sweep Test889 : Shutdown1141 : Restart w/compact heater, Init Perf. Map1477 : Initiate Extended Operation BOM-LR2629 : Operating Point Adjustment3223 : Operating Point Adjustment3848 : Intiate Ac Bus Variation Test4697 : End AC Bus Variation Test5183 : Restart: BOM-LR5571 : Temporary AC Loss6191 : Natural Frequency Test6502 : Begin Rod Heating/Cooling test6852 : Heater Failing7437 : Reduced Piston Amp for Test8014 : End test, Rack Cal, Helium Leak Measurements8444 : Piston Amplitude Adjustment to match BOM-LR10295 : Shutdown: Helium Leak Measurements12983 : Shutdown: Rack Cal & Helium Leak Measurements13152 : Natural Frequency Test15340 : Piston Amplitude Adjustment
NASA/TM—2013-216543 13 

heat 
	Z	Z	'^
		


	


	

|*`X


ZZ		
Z	

Z

more variable due to nuisance shutdowns and temporary disturbances in the ambient temperature. 

Between 6097 and 6193 hr, a test was performed to experimentally measure convertor natural frequency 

Z
		
	
	

'^	
	


Z	


versus time due to helium leakage, and periodic tests to experimentally measure the convertor natural 

frequency are being performed as a metric to track the effect of helium leakage. Other natural frequency 

tests were performed near 8015 and 13 152 hr. A plot showing measured performance and Sage predicted 

performance versus time is shown in Figure 9. The initial assumptions used in the Sage model under 

predicted the performance of the convertor by 1 to 4 W depending on the leak rate used; however, the 

measured data appears to follow the trend predicted by Sage for a leak rate of 9×106 std cc/sec. Between 

6500 and 8015 hr, another off nominal operating condition test was performed. From 8114 to 10 295 hr, 


	

|*`X



Z}

|*

LR condition at 8444 hr. At 8576 hr, an ambient temperature disturbance caused the power output to 

change. The downward trend continued as helium was lost and several adjustments to piston amplitude 

were made to maintain a nominal value of 4.20 mm. During this period the piston amplitude was adjusted 

only to maintain amplitude and not to match the original power output for this operating condition. At 

10 295 hr, the convertor was shut down and helium leak measurements were made as part of the 

performance prediction effort. Operation then continued at the |*`X

	




12 983 hr at which time the convertor was shut down for an annual rack calibration and another helium 

$
	
*	

|*`X

	


	at which 

time the piston amplitude was adjusted to return to 4.20 mm. 

60

65

70

75

80

85

90

95

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Power Output at Alternator Termlinals (Watts)

Time (Years)

Sage prediction for 

1x10-6 std CC / sec

Sage prediction for 

5x10-6 std CC / sec

Sage Prediction for 

9x10-6 std CC / sec

Measured values for June 30,2010 - June 21, 2012

Figure 9.—Sage predicted power output at the alternator terminals over time for various leak rates 

compared to measured data. 
NASA/TM—2013-216543 14 

Extended Operation of ASCE2s #5 and #6 

The third pair of ASCE2 units, #5 and #6, was delivered to Glenn in July 2010. These convertors 

were previously discussed in Oriti (2011) and Oriti and Wilson (2011). A plot of convertor performance 

is shown in Figure 10. These convertors were operated as a pair in the dual-opposed vertical orientation 

until 7184 hr. Between 0 and 1194 hr, checkout testing and performance mapping were completed. 

Extended operation, at the baseline |*`X condition as defined in the E2 product specification, began 

immediately after performance mapping. Between 1194 and 1914 hr, an increase in power output was 

observed and has been attributed to changes in fluid concentration in the cold-end circulator. This is 

because a specific mixture of glycol and water was required to achieve the desired temperatures during 

performance mapping. During extended operation the water slowly evaporated, altering the fluid viscosity 

and thermal characteristics. At the 1914 hr mark, adjustments were made to the circulator fluid 

temperature and concentration to maintain the |*`X condition. Between 2202 and 2323 hr, an offnominal test was performed during which the alternator housing temperature was varied. Between 2323 

and 2833 hr, the convertors were operated at the end-of-mission (EOM) LR condition, which resulted in a 

lower power output, as expected. Near 2480 hr, the ASCE2 #5 heat source began to fail and completely 

failed at 2665 hr, which resulted in the fluctuations in power output near that time. Between 2883 and 

4233 hr, operation at the high rejection temperature was explored for both BOM and EOM conditions to 

ascertain its effect on convertor performance. During this time, the convertor performance was less 

steady, which has been attributed to the previously described variability in rejection temperature control. 

At high rejection temperature, the circulator bath requires more frequent adjustments to maintain steady 

conditions. Several manual adjustments were made around the 3019 hr of operation. Near 3823 hr, a large 

disturbance in ambient temperature resulted in the observed performance variation. Between 4233 and 

6776 hr, the operating condition was maintained at the baseline |*`X condition. Near 6460 hr, the 

heat sources on both convertors began to fail. The dip in power near 6600 hr has been attributed to 

simultaneous heater cartridge failures in both heat sources. At 6776 hr, the hot-end temperature was 

lowered from 842 to 760 °C to extend heater life. Between 6776 and 7208 hr, alternator housing 

temperatures were varied and rejection temperatures for both EOM and BOM conditions were explored to 

determine their effect on convertor performance and measure forces transmitted through the CSAF. At 

7208 hr, ASCE2 #6 was removed from operation for an inspection. Further detail is given on this later in 

the paper. At this same time the test rack was calibrated, a new heat source was installed on ASCE2 #5, 

and #5 was returned to operation as a single convertor. From 7208 to 9507 hr, a modified E3 |*`X 

condition was maintained and the convertor produced approximately 78 We at operating temperatures of 

750 °C hot end, 35 °C cold end, and 46 °C alternator housing. From 9507 to 10 671 hr, off-nominal tests 

were performed to explore a new method for characterizing thermal insulation losses and to simulate 

convertor performance during expected thermal transients at launch. Operation was then returned to the 

modified E3 |*`X condition and remained at this condition until 11 947 hr with the only exception 

being an off-nominal test performed around 11 346 hr. During this period, the increase in power output 

has been attributed to the previously described fluid concentration changes in the cold-end circulator. 

More off-nominal tests were performed between 11 947 and 12 590 hr to support exploration of simulated 

launch transients and convertor performance determination. Operation then returned to the modified E3 

|*`X condition and remained at this condition until 14 609 hr. An increase in power output was 

observed between 12 590 and 13 773 hr and has been attributed to variability in the rejection temperature 

control. Adjustments were made to the circulator bath to correct this variability at 13 773 hr. Between 

14 609 and 14 686 hr, an off-nominal test was performed where the E3 minimum qualification CSAF and 

alternator housings were reached, the test rack was calibrated, and ASCE2 #4 was paired with #5 in a 

dual-opposed vertical configuration. In future continued extended operation, ASCE2s #4 and #5 will 

operate as a pair at the |*`X condition per the E3 product specification. 
NASA/TM—2013-216543 15 

Figure 10.—Advanced Stirling Convertor (ASC)E2s #5 and #6 performance through 14 650 and 7185 hr of 

operation, respectively. 

E2 Convertors Removed From Extended Operation 

Since 2011, three E2 convertors, #2, #3, and #6, have been removed from operation for inspection. 

ASCE2s #2 and #6 operated at Glenn for 2751 and 7184 hr, respectively. Over the operational life of E2 

#2, two power variations of ~1 W were observed. Over the operation life of E2 #6, numerous instances of 

power variations were observed, primarily during the first 1000 hr. During initial performance mapping, 

slow oscillations in power of ~6 W were observed. After 100 hr of operation, the power variations 

appeared more like a noisy signal with variations of 1 to 2 W. From 2400 through the remainder of 

operation life, no more power variations were observed. Because of the differing magnitude and number 

of performance variations, these two units were selected for inspection. The findings of these inspections 

have driven corrective actions and other design and process improvements for implementation in the E3s. 

ASCE2 #3 operated at Glenn for 841 hr prior to being shipped to Lockheed Martin Coherent 

Technologies for controller development testing. After review of the data, it was found that ASCE2 #3 

experienced power variations of up to 2 W during performance mapping. Performance mapping occurred 

early in the operational life of the convertor and no performance variations were observed after 

performance mapping. During controller development tests, ASCE2 #3 developed an anomalous sound 

for which the root cause is unknown. When opened for inspection, ASCE2 #3 exhibited many of the 

same symptoms observed in the other E2s for which corrective actions and other design and process 

improvements had already been developed and implemented for E3s. 

0 2,500 5,000 7,500 10,000 12,500 15,000 0

 10

 20

 30

 40

 50

 60

 70

 80

 90

100

Hours of Operation

Power Output (We)

ASC-E2 #5 & #6 Performance Data

140 : Checkout Testing879 : Checkout Complete, Initiated Perf Map1218 : Initiated Extended Operation at BOM-LR1938 : Operating Point Adjustments2226 : Alt Housing Temp Adjustment2504 : #5 Heat Source Begins Failing2857 : Moved to EOM HR3216 : Shutdown/Restart BOM-HR3510 : Shutdown/Restart BOM-HR3849 : Temporary AC Loss4257 : Moved back to BOM-LR5530 : increased aux htr on both to maintain 61 deg c5980 : turned on fans. 5981 : reduced aux htr on #56460 : Heater Begins to fail6776 : Lowered hot-end temp/strain gauge test7208 : Moved to E3 BOM-LR; end #6 operation; #5 continued alone7900 : Ambient Disturbance9507 : Begin specific tests10671 : Returned to E3 BOM-LR11346 : Specific test, back to E3 BOM-LR11947 : Continuing specific tests12590 : Returned to E3 BOM-LR13773 : Cold-end Circ Adjustment14612 : E2 #5 paired with E2 #4, performance check

ASC-E2 #6

ASC-E2 #5
NASA/TM—2013-216543 16 

Conclusion 

Reliability is a crucial requirement of radioisotope generators for space applications, which are 

required to have an operational life of 17 years once fueled. NASA Glenn Research Center has been 

operating Stirling convertors in 24-hr/7-day unattended operation for the purpose of generating a 

reliability database in support of the Advanced Stirling Radioisotope Generator (ASRG) project. 

Performance data are continuously recorded, and analyzed to investigate long-term performance trends. 

To date, Glenn has operated 39 convertors with a cumulative runtime exceeding 530 000 hr. Currently, 11 

convertors are operating continuously with the intention of increasing that number when the E3s arrive. 

Long-term data collected and analyzed to date have indicated steady performance when operating 

conditions are maintained with exceptions of Advanced Stirling Convertor (ASC!E #1, which 

experienced a permanent power loss of 7-We, and ASCE2 #1, which has a known helium leak. Most 

disruptions to performance have been attributed to variability in support hardware and facility conditions. 

Power variations observed on the ASC units, both short term and long term, have been investigated and 

used to drive design and process improvements that will be implemented in future convertors. 

References 

Cornell, Peggy A., et al. (2009): Stirling Convertor Extended Operation Testing and Data Analysis at 

Glenn Research Center. Proceedings of the 7th International Energy Conversion Engineering 

Conference (NASA/TM—2010-216231), Denver, CO. 

Lewandowski, Edward J.; and Schreiber, Jeffrey G. (2008): Extended Operation Testing of Stirling 

Convertors in Support of Stirling Radioisotope Power System Development. Proceedings of the 6th 

International Energy Conversion Engineering Conference (AIAA 20085791 and NASA/TM—2009-

215481), Cleveland, OH, 2008. 

Oriti, Salvatore M. (2011): Extended Operation of Stirling Convertors at NASA Glenn Research Center. 

Proceedings of the 9th International Energy Conversion Engineering Conference (AIAA 20115726 

and NASA/TM—2012-217289), San Diego, CA. 

Oriti, Salvatore; and Wilson, Scott (2011): Advanced Stirling Convertor (ASCE2) Performance Testing 

at NASA Glenn Research Center. Proceedings of the Nuclear and Emerging Technologies for Space 

Conference, NETS 20113496 (NASA/ TM—2011-217102), Albuquerque, NM. 

Roth, M.; and Schreiber, J.G. (2004): Extended Operation of Stirling Convertors. Proceedings of the 2nd 

International Energy Conversion Engineering Conference (AIAA 20045508 and NASA/TM—2004-

213388), Providence, RI. 

Schreiber, Jeffrey G. (2006a): Summary of Stirling Convertor Testing at NASA Glenn Research Center. 

Proceedings of the 4th International Energy Conversion Engineering Conference (AIAA 20064061 

and NASA/TM—2006-214429), San Diego, CA. 

Schreiber, Jeffrey G. (2006b): Developmental Considerations on the Free-Piston Stirling Power 

Convertor for Use in Space. Proceedings of the 4th International Energy Conversion Engineering 

Conference (AIAA 20064015 and NASA/TM—2007-214805), San Diego, CA. 

Wong, Wayne A.; and Cornell, Peggy A. (2011): Advanced Stirling Convertor (ASC) Technology 

Maturation in Preparation for Flight. Proceedings of the Nuclear and Emerging Technologies for 

Space Conference, NETS 20113492 (NASA/TM—2012-217207), Albuquerque, NM.