Scott D. Wilson and Wayne A. Wong

Glenn Research Center, Cleveland, Ohio

NASA Glenn Research Center Support of the

Advanced Stirling Radioisotope Generator Project

NASA/TM—2015-218462

April 2015

AIAA–2014–3859
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Scott D. Wilson and Wayne A. Wong

Glenn Research Center, Cleveland, Ohio

NASA Glenn Research Center Support of the

Advanced Stirling Radioisotope Generator Project

NASA/TM—2015-218462

April 2015

AIAA–2014–3859

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

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Acknowledgments

This work is funded through the NASA Science Mission Directorate and the Radioisotope Power Systems Program 

Offi ce. The authors wish to acknowledge Randy Bowman, Dave Krause, and Eugene Shin for their commitments and 

contributions to the ASRG project. The authors would also like to thank Don Roth, Rich Rauser, Dan Gorican, and

Greg Piper for dedication to the nondestructive evaluation task.

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NASA/TM—2015-218462 1 

NASA Glenn Research Center Support of the Advanced Stirling 

Radioisotope Generator Project 

Scott D. Wilson and Wayne A. Wong 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

A high-efficiency radioisotope power system was being developed for long-duration NASA space 

science missions. The U.S. Department of Energy (DOE) managed a flight contract with Lockheed 

Martin Space Systems Company to build Advanced Stirling Radioisotope Generators (ASRGs), with 

support from NASA Glenn Research Center. DOE initiated termination of that contract in late 2013, 

primarily due to budget constraints. Sunpower, Inc., held two parallel contracts to produce Advanced 

Stirling Convertors (ASCs), one with Lockheed Martin to produce ASC–F flight units, and one with 

Glenn for the production of ASC–E3 engineering unit “pathfinders” that are built to the flight design. In 

support of those contracts, Glenn provided testing, materials expertise, Government-furnished equipment, 

inspection capabilities, and related data products to Lockheed Martin and Sunpower. The technical 

support included material evaluations, component tests, convertor characterization, and technology 

transfer. Material evaluations and component tests were performed on various ASC components in order 

to assess potential life-limiting mechanisms and provide data for reliability models. Convertor level tests 

were conducted to characterize performance under operating conditions that are representative of various 

mission conditions. Despite termination of the ASRG flight development contract, NASA continues to 

recognize the importance of high-efficiency ASC power conversion for Radioisotope Power Systems 

(RPS) and continues investment in the technology, including the continuation of the ASC–E3 contract. 

This paper describes key Government support for the ASRG project and future tests to be used to provide 

data for ongoing reliability assessments. 

Nomenclature 

ACU Advanced Stirling Convertor Controller Unit 

ASC Advanced Stirling Convertor 

ASC–E Advanced Stirling Convertor Engineering design #1 

ASC–E2 Advanced Stirling Convertor Engineering design #2 

ASC–E3 Advanced Stirling Convertor Engineering design #3 

ASC–F Advanced Stirling Convertor Fight design 

ASRG Advanced Stirling Radioisotope Generator 

CPH Common Performance Hardware 

CT computed tomography 

DOE Department of Energy 

FEA finite element analysis 

FLDT Fast Linear Displacement Transducer 

GCF gigacycle fatigue 

GHA Generator Housing Assembly 

MTTF mean time to failure 

NDI nondestructive inspection 
NASA/TM—2015-218462 2 

RGA residual gas analyzer 

RPS Radioisotope Power Systems 

SRG–110 Stirling Radioisotope Generator 110 W 

Introduction 

The Advanced Stirling Radioisotope Generator (ASRG) was being developed for long-duration space 

science missions by the U.S. Department of Energy (DOE) and Lockheed Martin Space Systems 

Company. In late 2013, the ASRG flight contract began termination largely due to budget restrictions. 

Prior to termination, the majority of tasks related to assessing reliability were completed with only longterm tasks continuing, including materials aging tests and convertor extended operation. Such support 

included performing thermal, structural, and dynamic analysis, providing materials expertise, equipment, 

and unique inspection capabilities, and performing reliability tests on components, convertors, and 

generator systems. Glenn started extended Stirling convertor tests in 1999 and has since conducted 

operational evaluations in support of the Stirling Radioisotope Generator 110 W (SRG–110) and ASRG 

projects. For the ASRG project, convertor production consisted of two parallel contracts with Sunpower, 

Inc., one managed by Lockheed Martin to produce the ASC–F flight units and the other managed by 

Glenn to produce ASC–E3 engineering units that served as pathfinders by being built to the flight 

specification and documentation. The first four ASC–E3s were delivered by Sunpower to Glenn between 

November 2012 and August 2013. 

Accelerated life tests were performed for Advanced Stirling Convertor (ASC) components, including 

the heater head, planar spring, fasteners, various organic materials, and power feedthrough. Convertor 

testing characterizes the response to external random vibration, overstress conditions intended to operate 

in the design margins in order to stress the convertor without failure, and nominal conditions intended to 

evaluate performance over long durations. Nondestructive inspection (NDI) of the heater head assembly, 

a critical component of the ASC, has been performed to provide a unique in-process inspection capability. 

Analysis results have been provided as inputs for verification of certain ASC specification requirements. 

The ASC–E3 started production with the goal being built to the same design and processing 

documentation and the same flight ASC product specification. The first two pairs of ASC–E3s were 

planned to be process pathfinders, followed by two pairs of “flight-like” convertors. The first two pairs of 

ASC–E3s were used to reduce risk and validate design and processing changes prior to implementation 

on the ASC–F convertors. The last two pairs of ASC–E3s were being built using the flight-approved 

documents and are representative of the flight convertors. The third pair of convertors, shown in Figure 1, 

should be delivered this fall and the last pair by early next year (Ref. 1). 

Figure 1.—Completed ASC–E3 #5. 
NASA/TM—2015-218462 3 

Glenn’s technical support activities focus on lowering risk and addressing reliability for the long-life 

application. Establishing life and reliability of Stirling convertors of this general type, based on 

noncontacting operation and the elimination of wear mechanisms by design, prove challenging. While 

components can be tested at accelerated conditions, the entire operating Stirling convertor cannot. The 

ASRG reliability effort had been formulated to be multifaceted, because the statistical test durations to 

meet life requirements are more than the 17-year ASC life requirement. It makes use of classic techniques 

such as failure modes, effects, and criticality analysis; reliability block diagrams; and fault tree analyses. 

It also includes probabilistic techniques that are applied to many of the key components such as the heater 

head. Extended operation of convertors provides further data regarding any potential aging effects. The 

approach follows a sequence of (1) identifying risks and potential failure modes, (2) characterizing the 

potential failure modes, (3) mitigating the risks associated with each failure mode to an acceptable level, 

and (4) accepting those risks after mitigation steps are complete (Refs. 2 and 3). 

Material and Component Evaluation 

Material evaluations have been performed on various components in the ASC in order to predict 

potential life-limiting mechanisms or provide reliability models with more accurate input data. The heater 

head assembly is one such key component in the ASC. This thin-walled component has had numerous 

evaluations and analyses performed, including uniaxial creep testing of thick and thin specimens of the 

high-temperature superalloy 247-LC. Glenn materials and structures personnel have developed an approach 

to characterize the long-term durability of the heater head, including both deterministic and probabilistic 

methods. Other evaluations performed include gigacycle tests of the material used for the displacer planar 

spring, ductility testing of the fasteners, and evaluation of the multiple organic materials used in the cold end 

of the convertor. Organics accelerated testing characterized material limits or functionality based on 

moderately higher temperatures over time in different environments. In situ outgassing assessment under 

gamma radiation, compatibility of off-gassed organics, thermal stability, and mechanical integrity of epoxies 

were performed. Some of the ASC component activities are shown in Figure 2. 

Figure 2.—ASC with accelerated component testing performed for the ASRG project. 
NASA/TM—2015-218462 4 

Heater Head 

A significant focus of Glenn structural analysis has been on the heater head, due to the limited 

availability of 247-LC material properties. In order to enable a heater head design with a high probability 

of meeting the requirement of the 17-year design life, Glenn structural engineers developed guidelines 

and performed characterization tests on material coupons, intermediate machined components, and final 

machined subassemblies. The test and analysis campaign resulted in positive margins for throughthickness average creep strain, limits for local creep strain, and the time at the onset of tertiary creep 

(Refs. 2 and 3). The various successful heater head tests performed included 

1. Benchmark creep tests of fully machined heater heads to verify long-term creep behavior at 

1.4 times the peak pressure and a localized head temperature of 861 C (above the current 760 C 

operating temperature) 

2. Lateral load testing of a complete heater head assembly to simulate high loading conditions at axial 

and lateral load conditions 

3. Material characterization to verify fracture toughness, fracture crack growth rate, and fracture 

threshold 

4. Permeation testing to verify no leakage through thin wall specimens at 850 C with up to 6 percent 

strain of the material (significantly above anticipated lifetime creep) 

Secondary Locking Mechanism 

A test program was developed to evaluate the fastener locking mechanisms for the ASC threaded 

fastener joints and various Loctite liquid locking compounds. The requirements were based on the NASA 

specifications, NASA–STD–5017, “Design and Development Requirements for Mechanisms,” and 

NASA–STD–5020, “Requirements for Threaded Fastening Systems in Spaceflight Hardware,” and the 

final report from NASA Engineering and Safety Center (NESC) investigations, NESC–RP–04–092, 

“Performance Characterization of Loctite® 242 and 271 Liquid Locking Compounds (LLCs) as a 

Secondary Locking Feature for International Space Station (ISS) Fasteners.” The test effort consisted of 

three phases, including the determination of baseline control properties, evaluation of installation 

variables and processes, and evaluation of cure kinetics and thermal stability. Procedures and test 

conditions were developed to identify processing sensitivities of the liquid locking compounds, factoring 

in test specimen fabrication, environmental thermal aging, torque strength, and other thermal and 

structural parameters. The test matrix was evaluated in terms of cure state, thermal stability, and fastener 

locking integrity. The designs of torque test specimens followed the actual fastener joint configurations as 

close as possible. In order to evaluate torque, a high-resolution torque testing system was developed and 

certified at NASA Glenn. The effort successfully tested 720 torque test specimens at various 

temperatures, up to 150 °C for the ASC fastener joints. The results demonstrated positive margins in the 

ASC fastener joint designs. 

Gigacycle Spring Material 

The planar spring, attached to the displacer, undergoes roughly 55 billion stress cycles during the 

17-year design life. Previously, overstressed accelerated life testing on fully processed displacer planar 

springs were completed with no failures. To further verify fatigue endurance of the spring material, Glenn 

experts directed a high-cycle accelerated fatigue test effort at the University of Akron, Akron, Ohio. The 

gigacycle fatigue (GCF) test methodology was designed to subject cantilever beam specimens processed 

similarly to displacer planar springs in a high-frequency resonant mode. The effort successfully designed 

and manufactured stable specimen fixtures, established test procedures, and developed system safety 

interlocks and monitoring for long-term tests using the electrodynamic shaker. The effort successfully 

characterized the number of stress cycles endured by the specimens over a stress range of 540 to 850 MPa. 
NASA/TM—2015-218462 5 

The results showed that fatigue cracks initiated and propagated at stress levels of 775 MPa and above, 

while tests below this level were runouts, with no fatigue cracks evident up to 20 billion cycles. The highcycle fatigue testing further demonstrated the reliability of the displacer planar spring materials. 

Synergistic Durability Aging Tests on Organics 

The combined effects of irradiation, thermal exposure, and synergistic organic outgassing interactions 

on stability and durability of ASC organics were assessed with a simulated thermal aging test. This test 

involved all ASC organic materials, which were preconditioned with various irradiation exposure 

conditions under either gamma or reactor neutron radiation from the previous NASA Glenn tasks in order 

to simulate irradiation from the radioisotope heat source and potential mission environments. Aging 

temperature was determined to be 130 °C based on the maximum alternator qualification temperature. 

The test vessels holding organic samples in either coupon or actual part configurations were pressurized 

and sealed with the ultra-high-purity helium to create a representative ASC internal gas environment. 

Short- and longer-term aging tests were initiated simultaneously. After these synergistic durability aging 

exposures, the resulting gas including generated outgas species from the organics were carefully 

analyzed. Additionally, systematic residual property characterizations were performed on the material 

specimens to determine potential changes by comparing directly with those of various unaged control 

specimens. The results were also compared with the property changes from the isolated thermal aging 

tests of individual organics at the similar temperature range but under a controlled inert gas environment 

in order to further ascertain the effects of the synergistic durability aging. The 5- and 16-month aging tests 

results suggest that ASC organics will continue to function reliably for the intended life. 

Power Feedthrough Thermal Exposures 

The ASC flight design has two power feedthroughs and reliability of this component is essential. 

Tests were designed to subject power feedthrough test samples to thermal cycling, long-term aging, 

hermeticity testing, and structural tests. These tests were aimed at demonstrating functionality after a 

series of increased peak temperatures. For thermal cycling, the samples experienced 150 cycles at five 

temperatures up to a peak test temperature of 250 °C, twice the maximum operating temperature. Some of 

those samples were subjected to structural testing aimed at providing finite element analysis (FEA) 

models with more realistic strength properties for the pin to sealer joint present in the feedthrough. 

Additional tests subjected flight feedthroughs to low-temperature thermal cycling test, consisting of 150 

thermal cycles, ranging from 0 to 125 °C, and then long-term aging at 125 °C. This test was designed to 

demonstrate functionality after exposure to simulated minimum and maximum qualification conditions. 

All samples passed the ASC specified hermeticity requirement and demonstrated functionality after 

exposure to two times the maximum application temperature and 150 thermal cycles for the extreme 

qualification conditions. 

ASC Testing 

Convertor testing enables characterization of ASC performance using operating conditions 

representative of various reference phases of a mission. These phases include production, receipt, 

integration, launch, and cruise. Convertors can accumulate several thousand hours before entering the 

extended operation. Once in simulated cruise, they are operated unattended by test personnel for several 

years. During that extended period of operation, convertor operation is interrupted for annual facilities 

maintenance and to perform periodic checkout tests, which are used to show convertor performance has 

not degraded over time. Also, overstress conditions verify that the convertor can survive operation in 

select severe conditions and perform as expected after returning to nominal conditions. Some tests are 

 
NASA/TM—2015-218462 6 

operated in the vertical orientation while others are performed horizontally. Figure 3 shows convertors in 

single-vertical orientation and in dual-opposed horizontal, where the dynamic disturbances of operating 

convertors are canceled out. Both types of tests continue to provide input data for the life certification 

plan (Ref. 4). Operating hours for multiple convertors are accumulated and used to calculate the mean 

time to failure (MTTF) for random failures. 

Testing of Stirling convertors in the Glenn Stirling Research Laboratory was initiated in 1999 at 

Glenn and has accumulated over 660,000 hr. Of these, 316,000 hr are associated with varying generations 

of the Sunpower ASC engineering units. Table I shows only relevant convertors that are still in operation, 

as well as future ASC–E3s, which are scheduled for delivery in 2014 and 2015. 

Figure 3.—ASC–E3 test configurations. Single vertical (left) and dual-opposed horizontal (right). 

TABLE I.—ASC TEST STATUS AS OF JULY 7, 2014

ASC Location Current use Control Test setupa Next phase Hours 

E3 #1 Glenn 

Demonstration AC Bus and 

ACU 

Dual-opposed, 

vertical 

Flight-like 

GHA 

ACU 3109 

E3 #2 Glenn ACU 3104 

E3 #3 Glenn Extended 

operation AC Bus Single-vertical CPH Dual-opposed, 

horizontal 

3970 

E3 #4 Glenn Extended 

operation AC Bus Single-vertical CPH 4939 

E3 #5 

Sunpower Future 

deliveries AC Bus 

Single-vertical 

planned during 

acceptance testing 

Delivered 

in CPH 

Delivery in fall 2014 E3 #6 

E3 #7 Delivery in spring 2015 E3 #8 

E2 #4 Glenn Extended 

operation AC Bus Dual-opposed, vertical 

E2 support 

hardware 

Dual-opposed, 

vertical 

13,568 

E2 #5 Glenn 27,316 

E #2 Glenn Extended 

operation AC Bus Dual-opposed, vertical 

E support 

hardware 

Dual-opposed, 

vertical 

35,316 

E #4 Glenn 31,248 

0 #3 Glenn Extended 

operation AC Bus Single-vertical 0 support hardware Single-vertical 44,939 

a

Common Performance Hardware (CPH). 

 
NASA/TM—2015-218462 7 

Figure 4.—ASC–E3 test sequence. Tests A and B are examples of continuing periodic checkout tests used to verify 

performance over time. 

The ASC–E3 test sequence, summarized in Figure 4, enables independent performance verification, 

simulates relevant environments during the life cycle of a flight convertor within certain limits, and 

mitigates risk by providing the life certification plan with input data from nominal and overstress 

conditions. To start, each completed ASC convertor undergoes final performance testing at Sunpower 

before entering the hardware acceptance review. After approval, the convertors are delivered to Glenn for 

additional testing. In sequence 2, the ASC undergoes receipt inspection, characterization, and 

performance verification. In an effort to minimize variation between the two locations and thus minimize 

differences in the test data, the convertors are delivered in the same test setup used to verify performance 

at Sunpower, called the Common Performance Hardware (CPH). The tests also utilize similarly designed 

data acquisition systems and control racks to minimize variations in test results that are unrelated to 

convertor performance. Once approved, the test sequence will provide guidance for tests that provide data 

for life certification. 

The specified operating points were designed to simulate reference mission conditions, such as at the 

beginning and end of a mission, when the amount of available heat varies due to the natural decay of the 

radioisotope heat source. Different test conditions and configurations simulate various relevant 

environments throughout the life of a flight convertor. Horizontal operation simulates ground integration 

and fueling, random vibration testing simulates launch conditions, and extended operation is used to 

simulate a cruise phase. During the simulated cruise phase, periodic checkout tests are performed to 

monitor for any performance changes. Checkout tests include a natural frequency test to ensure the spring 

stiffness or gas spring stiffness have not changed and performance tests to ensure conversion efficiency 

has not changed. 

In February 2014, ASC–E3 #4 was exposed to external random vibration in the X-, Y-, and Z-axes, 

while operating at slightly less than full power. Flight acceptance and launch simulation vibration levels 

were performed for 1 min each test, resulting in a total of 2 min exposure in each axis. After vibration 

tests, the convertor was returned to extended operation in a single-vertical orientation and is now being 

prepared for test sequence 3, where it will undergo horizontal testing. 
NASA/TM—2015-218462 8 

Overstress Testing 

Durability tests are intended to stress convertors by operating outside limits imposed by the ASC 

specification and experimentally demonstrate the existing margins in the design. After the overstress 

testing has been completed, the convertors are intended to transition into extended operation. These tests 

subject a convertor to the possibility of lateral contact between the moving components and running 

surfaces and/or axial contact of the moving components to hard limits. Four durability tests were 

originally planned, including (1) repeated starting and stopping of the convertor to simulate ground 

processing prior to fueling, (2) exposure to constant acceleration to simulate liftoff or landing, 

(3) exposure to random vibration testing at higher piston amplitudes, inciting a number of piston and/or 

displacer contact events to simulate worst-case launch conditions, and (4) temporarily disabled control to 

simulate off-nominal card switcher events, allowing contact events over a controlled duration. The first 

two tests have been completed and the last two may be replanned due to project priorities. 

Start/Stop Testing 

In September 2011, ASC–E2 #8 was used by Glenn and Sunpower personnel to complete a total of 

300 start/stop cycles, simulating startup and shutdown over the lifetime of a typical flight unit, with an 

over-test factor of two times. The convertor was motored in horizontal and vertical orientations and in a 

range of cold-end temperatures expected of the ASC, including 200 in the vertical orientation and 100 

cycles in the horizontal orientation. During startup and shutdown, the self-pressurizing gas bearings are 

activated and deactivated, allowing brief contact between the running surfaces. This test was used to 

characterize any potential degradation on the internal convertor surfaces due to starting and stopping. 

Post-test disassembly and inspection, as well as gas bearing flow tests, indicated no evidence of wear or 

gas bearing degradation (Ref. 5). 

Constant Acceleration 

In May 2012, Glenn completed the constant acceleration tests using ASC–E2 #2 at Case Western 

Reserve University’s centrifuge facility (Ref. 5). To prepare for the test, convertor rework included 

installing a removable pressure vessel to enable interim inspections of the internal running surfaces and 

installation of more accurate piston and displacer Fast Linear Displacement Transducer (FLDT) position 

sensors. The test was envisioned to simulate the static acceleration force anticipated during launch from a 

boost thruster or from reentry and landing. However, the spin-up time of the centrifuge facility took 

significantly longer than the specified flight durations, resulting in an over-test condition for exposure 

time to elevated levels of acceleration. To simulate various mounting orientations, testing was carried out 

with a constant acceleration force imparted in the axial direction toward the heater head, in the axial 

direction away from the heater head, and in the lateral direction, the latter of which is expected to be the 

most challenging orientation. Test objectives included characterization of convertor performance when 

exposed to 18 g constant acceleration profile and evaluation of the effects on the convertor Xylan 

(Whitford) surfaces when the forces from lateral acceleration exceed the gas bearing load capacity. The 

axial tests were performed before the lateral tests at an acceleration of 18 g. The acceleration was 

increased for lateral tests: 8, 12, and 18 g were applied between inspection of the running surfaces. After 

testing was completed, review of the performance data showed differences within the margin of 

measurement error and inconsequential minor “polishing” of the surfaces was noted confirming that 

lateral contact did occur as expected but did not result in degradation of the Xylan surfaces. 

 
NASA/TM—2015-218462 9 

ASC Piston Contacts During Random Vibration 

Random vibration tests are used to expose test articles to the harsh environment of launch. As 

vibration levels increase, the mean position of the moving components vary randomly and the piston 

amplitude can increase for a limited number of cycles before returning to the nominal mean position and 

amplitude. These excursions could result in piston and/or displacer contact events. Testing had already 

started on this effort and was broken into three parts. Most of the characterization testing and inspections 

had been completed prior to ASRG termination. Future plans could include finishing this test. 

ASC Piston Contacts During Out-of-Control Event 

An out-of-control event is a duration where control of the convertor is temporarily disabled and could 

allow a limited number of contact events. Earlier analysis results suggest such an event could happen during 

controller card switchover (Ref. 3). Demonstrations of the Lockheed Martin controller switchover 

functionality indicates proper performance without loss of control. Analysis was also used to determine the 

velocities at which permanent component deformation would occur, in order to establish test limits to avoid 

damage to the convertor and enable extended operation. Plans were under development to simulate this 

condition and enable quantification of contact events, but they were put on hold after ASRG termination. 

Thermal Cycling 

Overstress testing can also be achieved by thermal cycling particular components to simulate 

conditions anticipated in application. Lockheed Martin recommended the next two test aspects and had 

adopted them into the ASRG life test approach. To learn about temperature effects on performance, the 

cold-end temperatures would be cycled between end-of-mission low reject and a high reject point to 

simulate extreme environments on alternator components. Plans are being evaluated for implementing a 

version of these overstress conditions into ongoing ASC–E3 performance tests. It is important to point out 

that these operating conditions do not exceed conditions experienced during Sunpower production tests. 

Increased Piston Amplitude 

Running at slightly higher piston amplitude provides additional means of overstress and was also 

adopted into the Lockheed Martin’s ASRG life test approach. This accelerates any wear on moving parts 

and could be implemented into extended operation. Similar to thermal cycling, plans are also being 

evaluated for potential implementation, however, a limit has not been selected. A likely candidate would 

be higher amplitudes experienced during production testing.

Government-Provided Inspection, Analysis, and Equipment 

Glenn provided a variety of unique capabilities from 2010 to 2012, including performing in-process 

inspection of flight heater heads, providing convertor performance predictions, and providing test support 

personnel, equipment, and convertors to enable key tests. The in-process inspection was performed as a 

screening technique on all ASC–E3 and ASC–F heater heads to ensure they passed hermeticity 

requirements prior to installation on a convertor. There is the possibility of casting defects to allow the 

working gas to leak through at rates that exceed specified requirements (Ref. 6), because the heater heads 

are fabricated from cast material. Such leakage would decrease the charge pressure and degrade 

performance over the life of the mission. The first inspection technique used Glenn’s x-ray Microfocus 

Computed Tomography (CT) system, able to identify casting imperfections in the thin wall and taper 

region of the superalloy cylinder of the heater head. The second method is the high-temperature leak test, 

designed to detect helium leaks caused by through-thickness defects that were too thin for detection using 

Microfocus CT x-ray. When the NDI effort was completed in May 2013, a total of 34 heater head 
NASA/TM—2015-218462 10 

assemblies had been processed at Glenn. These efforts were completed to ensure that the heater heads 

selected for use on flight were of the utmost quality, able to maintain hermeticity, and meet the thermal 

and structural requirements of the ASC. 

Microfocus CT X-ray 

The Glenn Microfocus CT system is considered to be one of the best in the United States and is 

ideally suited for this application (Ref. 7). Able to detect defects as small as 11 m, the state-of-the-art 

capability is able to screen out any defects that are larger than the crucial flaw size of the head. 

Microfocus CT creates a three-dimensional model of the inspected object by computer processing and 

reconstructing a very large series of high-resolution two-dimensional x-ray images taken of the object 

around a single axis of rotation. To determine accuracy, a probability of detection study was conducted 

using a precisely machined calibration standard made from an actual ASC thin-walled heater head 

containing an array of target features with varying width and depth. 

High-Temperature Helium Leak Testing 

The Glenn setup, shown in Figure 5, was originally developed to verify that the convertor’s helium 

working gas would not permeate through the walls at representative pressures and temperatures. The test 

setup consists of a vacuum chamber used to create the high vacuum environment, which avoids oxidation 

during the test and allows the use of a residual gas analyzer (RGA) to detect leaking helium. It contains a 

tantalum heating element to impart the 850 C maximum hot-end temperature (nominal ASRG hot-end 

temperature is 760 C) and a mounting flange, which cools the other end of the heater head to around 

200 C. To enable a seal that is also able to meet the required leak rate, a very tight crushed metallic seal 

 

Figure 5.—High-Temperature Leak Test Facility. 
NASA/TM—2015-218462 11 

was used between the heater head assembly open end and the actively cooled mounting flange. Therefore, 

this test setup simultaneous detected any helium leakage through the heater head wall and through the 

crushed metallic seal. An RGA was used to compare the response from the tested heater head and from a 

calibrated leak standard. This resulted in a simple approach to verifying the hermeticity requirement for 

each heater head prior to production use. 

Test Support Equipment 

To support ongoing ASC–E3 production, ground support equipment was provided to Sunpower to 

enable autonomous continuous operation of convertors. Such test methods were developed at Glenn to 

enable routine continuous safe operation of the convertors. Rack components, including the Failsafe 

Protection Circuits and ASC piston sensor Buffer Circuit, prevent higher-than-desired piston amplitudes 

during facility malfunctions during unattended continuous operation. They enable safe autonomous 

shutdown while test personnel are not present. Also, long-life electric heat sources are being provided to 

enable longer duration testing without the need for more frequent teardown of the test setup. This 

minimizes disruptions in the Sunpower production schedule and post-delivery Glenn testing and further 

reduces handling of the delicate microporous insulation. 

High-Fidelity Computation 

In an effort to improve accuracy of performance predictions, a high-fidelity computational model was 

developed at Glenn to predict convertor net heat input, a key parameter needed to determine thermal-toelectric conversion efficiency. Performance predictions were calculated for the earlier ASC–E2s and 

provided to Lockheed Martin to support the ASRG Preliminary Design Review in December 2010 and 

again in 2012 to support the Final Design Review. Performance predictions were calculated for ASC–E3s 

and provided to Sunpower to aid in performance verification and for inclusion in the end item data 

packages for the first two pairs of delivered convertors. Plans are currently underway to complete the 

same calculations on the third pair of ASC–E3s, followed by an analysis to determine how close this 

high-fidelity computational method is to a lower fidelity method used at Sunpower. So far, the two 

methods have been close so it is anticipated that the high-fidelity results, which are more time consuming 

and expensive may be phased out for future ASC–E3 convertors tested in the CPH (Ref. 8).

Convertors Provided for Controller Tests 

Glenn provided Government-owned convertors and test support for ongoing controller 

characterization from 2010 to 2014. Such testing enabled Lockheed Martin to characterize various 

generations of the ASC Controller Unit (ACU), using the highest fidelity convertors as they became 

available. In August 2010, ASC–E2 Pair 2 was delivered to Lockheed Martin, mounted in a dual-opposed 

horizontal configuration. In March 2012, ASC–E2 Pair 4 was delivered in the single-vertical arrangement. 

In both of those cases, test support was provided to get the convertors integrated and operating for the 

first time using approved procedures. Lastly, ASC–E3 Pair 1 was provided in a dual-opposed horizontal 

configuration in September 2013 to enable successful demonstration of the highest fidelity ACU at that 

time, including system startup, ASC control and operation at nominal and worst-case operating 

conditions, power rectification, direct-current output power management, ACU fault management, and 

system command/telemetry (Ref. 9).

Controller support was also provided for random vibration testing on two occasions. First, ASC–E #1 

was subjected to launch simulation vibration levels while being controlled by an early ACU (EDU 1) in 

February 2009. More recently, ASC–E2 #2 and ASC–E2 #8 were controlled by a later higher fidelity 

ACU (EDU 3) during qualification-level vibration tests in July and December of 2012, successfully 

demonstrating convertor control during random vibration of the ASC. 
NASA/TM—2015-218462 12 

Conclusion 

NASA Glenn Research Center provided materials, equipment, testing and inspection services, and 

related data products to Lockheed Martin Space Systems Company and Sunpower, Inc., in support of two 

parallel contracts to deliver engineering and flight Advanced Stirling Convertors (ASCs). These technical 

support tasks include material evaluations, component tests, convertor characterization, and technology 

transfer. Material evaluations and component tests have been performed on various components in the 

ASC in order to predict potential life-limiting mechanisms and provide inputs to reliability models. 

Convertor level tests were used to characterize performance at reference mission operating conditions. 

These accomplishments are not only relevant to continued interest in 80-W ASC but also apply to future, 

potentially higher power ASC, which will utilize much of the same technology. Despite termination of the 

Advanced Stirling Radioisotope Generator (ASRG) flight development contract, NASA continues to 

recognize the importance of high-efficiency ASC power conversion for Radioisotope Power Systems 

(RPS) and continues investment in the technology, including the continuation of the ASC–E3 contract. 

The first four ASC–E3s were delivered by Sunpower to Glenn and they continue to be used for a variety 

of tests to advance the state of the technology. The last two pairs of flight-like ASC–E3 convertors are 

planned for delivery late in 2014 and early 2015. 

References 

1. Wong, W.A., et al.: Pathfinding the Flight Advanced Stirling Convertor Design with the ASC–E3. 

Proceedings of the Tenth International Energy Conversion Engineering Conference, American 

Institute for Aeronautics and Astronautics, Atlanta, GA, 2012. 

2. Schreiber, J.G.; Thieme, L.G.; and Wong, W.A.: Supporting Technology at GRC to Mitigate Risk as 

Stirling Power Conversion Transitions to Flight. Proceedings of the Sixth International Energy 

Conversion Engineering Conference (IECEC 2008), Cleveland, OH, 2008. 

3. Ha, C., et al.: Reliability Demonstration Approach for Advanced Stirling Radioisotope Generator. 

Proceedings of the Eighth International Energy Conversion Engineering Conference, American 

Institute for Aeronautics and Astronautics, Cleveland, OH, 2010. 

4. Rusick, J.J.; and Zampino, E.J.: Advanced Stirling Radioisotope Generator Life Certification Plan. 

Proceedings of Nuclear and Emerging Technologies for Space, 2013. 

5. Meer, D.W.; and Oriti, S.M.: Advanced Stirling Convertor Durability Testing: Plans and Interim 

Results. NASA/TM—2012-217727, 2012. 

6. Roth, D.J., et al.: Fine Scale Metrology on Cylindrical Structures Using X-ray Micro-computed 

Tomography. Materials Evaluation Journal, paper 03695. 

7. Schreiber, J.G.; and Thieme, L.G.: GRC Supporting Technology for NASA’s Advanced Stirling 

Radioisotope Generator (ASRG). NASA/TM—2008-215196, 2008. 

8. Wilson, S.D., et al.: Overview of Heat Addition and Efficiency Predictions for an Advanced Stirling 

Convertor. Proceedings of the Ninth International Energy Conversion Engineering Conference, 

American Institute for Aeronautics and Astronautics, 2011. 

9. Chan, J., et al.: System-Level Testing of the Advanced Stirling Radioisotope Generator Engineering 

Hardware. Proceedings of the Twelfth International Energy Conversion Engineering Conference, 

American Institute for Aeronautics and Astronautics, 2014.