Wayne A. Wong and Peggy A. Cornell

Glenn Research Center, Cleveland, Ohio

Advanced Stirling Convertor (ASC) Technology 

Maturation in Preparation for Flight

NASA/TM—2012-217207

August 2012

NETS–2011–3492
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Wayne A. Wong and Peggy A. Cornell

Glenn Research Center, Cleveland, Ohio

Advanced Stirling Convertor (ASC) Technology 

Maturation in Preparation for Flight

NASA/TM—2012-217207

August 2012

NETS–2011–3492

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Nuclear and Emerging Technologies for Space (NETS-2011)

cosponsored by the ANS Aerospace Nuclear Science and Technology Division, the ANS Trinity Section, 

and the AIAA

Albuquerque, New Mexico, February 7–10, 2011
Acknowledgments

The work described in this paper was performed for the Science Mission Directorate (SMD) and the Radioisotope Power System 

(RPS) Program, which provided funding for these projects. 

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217207 1 

Advanced Stirling Convertor (ASC) Technology Maturation 

in Preparation for Flight 

Wayne A. Wong and Peggy A. Cornell 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The Advanced Stirling Convertor (ASC) is being developed by an integrated team of Sunpower and 

National Aeronautics and Space Administration’s (NASA’s) Glenn Research Center (GRC). The ASC 

development, funded by NASA’s Science Mission Directorate, started as a technology development effort 

in 2003 and has since evolved through progressive convertor builds and successful testing to demonstrate 

high conversion efficiency, low mass, and capability to meet long-life Radioisotope Power System (RPS) 

requirements. The technology has been adopted by the Department of Energy and Lockheed Martin Space 

Systems Company’s Advanced Stirling Radioisotope Generator (ASRG), which has been selected for 

potential flight demonstration on Discovery 12. This paper provides an overview of the status of ASC 

development including the most recent ASC–E2 convertors that have been delivered to GRC and an 

introduction to the ASC–E3 and ASC flight convertors that Sunpower will build next. The paper also 

describes the technology maturation and support tasks being conducted at GRC to support ASC and 

ASRG development in the areas of convertor and generator extended operation, high-temperature 

materials, heater head life assessment, organics, nondestructive inspection, spring fatigue testing, and 

other reliability verification tasks. 

Introduction 

National Aeronautics and Space Administration (NASA) requires a next-generation Radioisotope 

Power System (RPS) that minimizes the use of the very limited plutonium fuel for a variety of projected 

missions that cannot be performed with photovoltaic power systems. The National Research Council’s 

Radioisotope Power Systems (RPS) Committee (2009) recommended that “NASA and DOE should 

complete the development of the Advanced Stirling Radioisotope Generator (ASRG) with all deliberate 

speed...put final-design ASRGs on life test as soon as possible (to demonstrate reliability on the ground) 

and pursue an early opportunity for operating an ASRG in space (e.g., on Discovery 12).” This 

recommendation resulted in part from the continued advancements of the Advanced Stirling Convertor 

(ASC), two of which are used on each ASRG to efficiently convert the heat from the plutonium source 

into electrical power. Sunpower, Inc., has been developing the ASC since 2003 under contract to NASA 

Glenn Research Center (GRC) and with technical support from GRC (Wong, 2006; Shaltens, 2007). 

The key technologies in the ASC that enable high efficiency and low mass are the hydrostatic gas 

bearings, a moving-magnet linear alternator, high-frequency operation, high-temperature hot-end 

materials and fabrication processes, and high-temperature high-porosity regenerators. Initially, the ASC 

was a technology development project with a goal of demonstrating high conversion efficiency, which 

was accomplished with the initial non-hermetic Frequency Test Bed (FTB) and ASC–1 laboratory 

prototype convertors that demonstrated, respectively, 36 percent efficiency (80 We, 650 °C/30 °C) and 

38 percent efficiency (88 We, 650 °C/90 °C) (Wood, 2007; Wong, 2008). Sunpower then developed 

hermetic sealing and processing techniques while building the ASC–0 and ASC–1HS convertors that had 

IN718 (for operation up to 650 °C) and MarM-247 (for operation up to 850 °C) heater heads, respectively 

(Wilson, 2008). Based on these successful performance demonstrations, NASA Headquarters provided 

direction to accelerate ASC technology development to transition towards flight implementation. In 

response, the ASC project was modified to include direct technical support from GRC focused on high- 
NASA/TM—2012-217207 2 

Figure 1.—Sunpower ASC convertor evolution from technology development to flight. 

temperature materials, structures, organic materials, magnet aging, launch vibration simulation, 

electromagnetic interference (EMI) characterization, modeling, and reliability assessment, all aimed at 

reducing risk and enhancing reliability of the ASC. Additionally, the Department of Energy (DOE) and 

Lockheed Martin Space Systems Company (LMSSC) adopted the ASC as part of the ASRG. To 

transition the ASC from technology development to flight, the general plan illustrated in Figure 1 was 

developed to advance the technology with several progressive convertor designs. Completion of the 

technology development and transition to flight are part of Sunpower’s development contract with GRC, 

whereas flight hardware development and production are part of Sunpower’s subcontract to the DOE and 

Lockheed Martin ASRG contract. Each successive build includes design, processing, and hardware 

refinements, and in some cases newly defined requirements. To expedite demonstration of the ASRG 

system, Sunpower and GRC developed the ASC–E and worked collaboratively with Lockheed Martin to 

develop the product specification requirements for the ASC, including generator interfaces and 

preliminary flight requirements. In addition to some design changes, the ASC–E build, which uses an 

IN718 heater head, also required greater quality assurance rigor, documentation, and the incorporation of 

additional processing and testing including a workmanship-level vibration test of each completed 

hermetic convertor performed at GRC. In October 2007, two ASC–E convertors plus a spare were 

delivered on schedule as Government Furnished Equipment to DOE and Lockheed Martin for integration 

onto the ASRG Engineering Unit. 

While the ASC–E convertors represented a major advancement of the technology, the ASC required 

further development to prepare for flight. It was recognized that, due to evolving requirements, 

refinement of the design, and a need to continue to mature the quality system at Sunpower, two additional 

generations of ASC convertors were needed. The ASC–E2 design would be built under a new formal 

Quality Management System, and would implement several reliability- or flight-requirements-driven 

design changes. The ASC–E2 convertors were recently completed and delivered by Sunpower to GRC. 

After completing the ASC–E2, Sunpower is now developing the ASC–E3 that is intended to conclude 

the technology transition phase by completing limited design refinements and production development 

and demonstrating repeatability. The original intent as illustrated in Figure 1 was that Phase A.—

Technology Development and Phase B.—Refinement and Transition would have been completed by 
NASA/TM—2012-217207 3 

Figure 2.—Sunpower advanced Stirling convertor (ASC–E3/–F) with external interfaces. 

Sunpower under contract to GRC prior to handoff to Lockheed Martin for Phase C.—Flight Production. 

In reality, the technology maturation plan has been compressed and Sunpower has recently started 

developing the ASC–E3 convertors for GRC and is, in parallel, developing the ASC–F flight convertors 

for Lockheed Martin. The intention is that the ASC–E3 and ASC–F convertors would be identical and 

will be built to the same design documentation. An illustration of the latest ASC–E3/–F is shown in 

Figure 2, with callouts for the primary exterior parts including the interfaces with the ASRG generator 

such as the heat collector that interfaces with the General Purpose Heat Source (GPHS), the Cold-Side 

Adapter Flange (CSAF) that bolts to the radiator generator housing, the power feedthrough, the piston 

sensor, and a flange on the pressure vessel that allows the structural mounting of a pair of ASC convertors 

back-to-back onto a common interconnect tube. 

To date, Sunpower has delivered under the GRC development contract 22 ASC convertors with 

multiple units of each of the following designs: ASC–1, ASC–0, ASC–1HS, ASC–E, and ASC–E2. Of 

these convertors, many of which are on extended 24-hr/7-day week operation at GRC, a total of over 

140,000 hr of operation have been accumulated as of December 2010, including the leading pair of 

ASC–0 convertors that each have over 22,000 hr, and another pair of ASC–0s that accumulated 15,000 hr 

in a thermal vacuum environment. Also, a pair of ASC–E convertors is currently operating in the ASRG 

Engineering Unit and has accumulated over 13,000 hr at GRC. 

Nomenclature 

ASC Advanced Stirling Convertor 

ASRG Advanced Stirling Radioisotope Generator 

BOM Beginning of Mission 

CCB Configuration Control Board 

CSAF Cold-Side Adapter Flange 

CT Computed Tomography 

DOE Department of Energy 

EMI Electromagnetic Interference 
NASA/TM—2012-217207 4 

EOM End of Mission 

ERB Engineering Review Board 

FEA Finite Element Analysis 

FTB Frequency Test Bed 

GPHS General Purpose Heat Source 

GRC Glenn Research Center 

ILS Internal Limit Sensor 

LMSSC Lockheed Martin Space Systems Company 

NASA National Aeronautics and Space Administration 

NRA NASA Research Announcement

RPS Radioisotope Power System 

ASC–E2 Convertor 

The ASC–E2 is based on the ASC–E with several design changes that were determined by trade 

studies conducted by the GRC, Sunpower, and Lockheed Martin team. The changes included a new heater 

head design that uses MarM-247 rather than the IN718 used on ASC–E to increase reliability margin and 

offer higher temperature operation and improve performance. Another development task under the 

ASC–E2 build was the development of an electron beam (E-Beam) weld to bond the convertor to the 

CSAF for heat rejection and structural mounting to the generator. Additionally, to improve reliability of 

the convertor, a new Internal Limit Sensor (ILS) for piston stroke monitoring was selected for 

implementation to eliminate two feedthroughs in the pressure vessel and improve robustness of the piston 

assembly. Beyond these design changes, an emphasis of the ASC–E2 build was to increase the design 

rigor and to develop a new quality management system to prepare Sunpower as an eventual hardware 

supplier for flight nuclear systems. The ASC–E2 build was kicked off in November 2007, shortly after 

delivery of the ASC–E convertors. The Preliminary Design Review was held in January 2008, and the 

Critical Design Review was held July 2008. Some of the accomplishments of the ASC–E2 project are 

highlighted below. 

Heater Head Assembly Development 

One of the accomplishments of the ASC–E2 build was the development of a new heater head 

assembly that uses MarM-247 with the capability of operating up to 850 °C. The design of the heater head 

assembly takes into account thermal and structural interfaces with the GPHS and generator, launch load 

requirements, and life and reliability requirements. For the ASC–E2, a new team of vendors led by Refrac 

Systems was selected to produce the new heater head assembly and displacer assembly. The heater head 

assembly’s functions include efficient transfer of heat from the GPHS into the convertor helium working 

gas, hermetic containment of the pressurized helium, structural support of the convertor under flight load 

conditions, and resistance to creep deformation while at high temperatures while maintaining high 

reliability over the 17-year life. Development of the heater head assembly, shown in Figure 3, was 

completed through a collaborative process with Lockheed Martin defining flight requirements, Sunpower 

leading the design, GRC leading the materials evaluation and reliability and life assessment, and Refrac 

providing manufacturing and processing expertise. After completion of trade studies, and several design 

iterations, the design of the ASC–E2 heater head assembly was completed using a composite of highstrength MarM-247 and high-thermal-conductivity nickel. Demonstration of the many processes needed 

to complete this long lead component was completed with a variety of hardware trials, culminating in a 

representative ASC–E2 trial heater head. Additional accomplishments related to the heater head are 

described below in section “GRC Technical Support.” 
NASA/TM—2012-217207 5 

Figure 3.—ASC–E2 production heater head assemblies. 

Electron Beam Weld Development for CSAF Bond 

For the ASC–E2, an E-Beam weld was developed to bond the CSAF to the convertor, specifically to 

the external rejector on the transition assembly. E-Beam weld was selected as a result of a trade study that 

evaluated the advantages of a metallurgical bond in comparison to the shrink fit used on the earlier 

ASC–E design. The E-Beam weld is unique from other welding processes due to its high aspect ratio of 

weld depth to weld width, as well as its relatively low energy resulting in minimal thermal distortion and 

minimal heating of the components. The functional requirements of this CSAF weld are to transfer reject 

heat from the convertor through the rejection assembly to the ASRG housing radiator and to help support 

the structural loads transmitted between the convertor and the ASRG. The E-Beam weld development 

effort for the ASC–E2 was completed through optimization of the weld conditions and penetration depth, 

minimization of weld distortion, and weld structural requirements verification. Four sets of weld trial 

articles varying in geometry from simple trial parts to fully representative ASC–E2 production parts were 

used to complete the E-Beam weld development. Successful welds were verified at penetration depths 

greater than the required E-Beam weld depth. Post-weld inspection and assessment of the welds 

determined that weld distortion results were acceptable and that they did not negatively affect fit or 

functionality. The structural requirements were verified to have been met through analysis by GRC. 

Driven by budget and schedule considerations for the ASC–E2, a simpler single-material CSAF was used. 

The E-Beam weld details have been provided to Lockheed Martin who is currently qualifying the E-Beam 

weld for the flight CSAF design that includes material to enhance thermal conductivity. 

Materials “Master Buy” 

Considerable effort is going into evaluating the reliability and performance of the ASC at the 

convertor level, component level, and basic material property level. To ensure consistency of materials, to 

minimize batch-to-batch variation, to ensure relevance of the data being gathered to the flight convertors, 

and to ensure continued availability of materials for the ASC–E2 and future builds, a “master buy” of 

critical materials was purchased and placed under bonded storage at Sunpower. These materials, some of 

which have a long delivery time and do not have a shelf life, are planned to be used for convertors to be 

built in the foreseeable future including the ASC–E3 and ASC–F. Materials include MarM-247, Udimet 

720, magnets, regenerator material, fasteners, and feedthroughs. 
NASA/TM—2012-217207 6 

ASC–E2 Configuration Management, Processing, and Testing 

Configuration management of the ASC–E2 was the responsibility of the ASC–E2 Engineering 

Review Board (ERB) that was led by GRC and included membership from Sunpower and Lockheed 

Martin. All design documentation, process documents, and any nonconformances were reviewed and 

approved by the ERB. ASC–E2 assembly techniques and processes were based on heritage ASC–E and 

other Sunpower convertor and cryocooler builds. Convertor processing was performed at Sunpower in a 

clean assembly area utilizing flow benches, dedicated tools, and climate control. The standard 

subassembly and assembly vacuum bakeout processes were performed to remove the bulk of any 

potential outgassing contaminants from organics as well as any residual contaminants from atmospheric 

exposure. Purge and fill processing with ultra-pure helium further ensured that the working gas and 

internal surfaces of the convertor had minimal contamination. Convertor checkout tests were performed 

immediately before and after each critical process to confirm the process did not adversely affect 

performance (Wilson, 2008). Additional tests were performed throughout the build cycle to ensure 

integrity of the convertors and to verify performance requirements. Convertor Characterization Testing 

was performed early in the assembly and production phase to assess general characterization of the 

convertor operating behavior. Workmanship vibration testing, completed at NASA GRC, was performed 

after each hermetically sealed convertor was completed to confirm build quality and structural integrity. 

Verification of convertor performance prior to delivery was achieved with the Convertor Final 

Performance Test, which operated at Beginning-of-Mission (BOM) and End-of-Mission (EOM) 

conditions defined in the ASC–E2 Product Specification. 

GRC Technical Support 

In addition to managing the Sunpower ASC development contract, GRC has also made significant 

technical contributions to the ASC. For the ASC–E2 build, GRC directly supported Sunpower by 

providing the sintered oxidation-resistant regenerators, supporting design and analysis of the heater head 

and structural analysis of the convertor, conducting MarM-247 casting preparations, conducting 

workmanship vibration testing of each completed convertor, and maintaining configuration control of the 

design through the ERB. Further, GRC conducts analyses and tests to reduce risk and address the 

reliability of the ASC for use on applications that require a 17-year operating life. GRC has assembled a 

laboratory including autonomous test stands for conducting 24-hr/7-day week extended operation testing 

of multiple Sunpower ASC convertors, an essential part of establishing long life and reliability. 

Additionally, GRC continues testing and analyses that are specific to the Sunpower ASC design including 

investigation of high-temperature materials, structural life assessment, organic materials, modeling, 

system dynamic modeling, magnet evaluation, electromagnetic interference (EMI) characterization, 

launch vibration testing, and reliability assessment. 

ASC Reliability Assessment 

During the initial development phase of the ASC, Sunpower maintained a task to address ASC 

reliability with GRC providing technical support based in past experience. As the project has shifted 

towards system integration and flight, LMSSC has taken on the leadership for overall reliability of the 

ASRG generator including the ASC. GRC and Sunpower continue to play key roles in the efforts 

coordinated through the ASRG Reliability Working Group, whose intent is to mitigate risk and maximize 

reliability in meeting the 17-year-life requirement for the ASRG (Schreiber, 2009). Support in the ASC 

and ASRG life and reliability effort is ongoing through a variety of support tasks at GRC. 
NASA/TM—2012-217207 7 

Heater Head Life Assessment 

NASA GRC is responsible for the reliability and creep life assessment of the ASC heater head. The 

combined analytical and experimental assessment undertaken by GRC ensures high reliability of the 

heater head for its 17-year design life. This effort includes a series of uniaxial creep tests on MarM-247 

samples that comprise the same chemistry, microstructure, and heat treatment processing as the heater 

head itself (Krause, 2008). Finite Element Analysis (FEA) to predict mean value creep strain was then 

performed, followed by a probabilistic analysis. The final phase of the heater head life assessment effort 

is to validate the preceding methodology with accelerated benchmark creep testing. Over 9400 hr of 

accelerated testing has been completed to date on an ASC–E2 bare shell heater head test article measuring 

creep at high pressure and temperature to generate substantial creep strains in short time durations. This 

ongoing testing directly supports the use of the MarM-247 at high temperatures for improved convertor 

efficiency. Accelerated testing and analysis will continue until tertiary creep is evident by higher creep 

rates. Testing is about to commence on an ASC–E2 “full-up” heater head assembly with heat collector 

while an ASC–E3 full-up heater head prototypical test will initiate in fall 2011. 

Organics 

A variety of organic materials are used in the ASC convertors as adhesives, potting compounds, 

electrical insulation, and structural bonding. The ASC–E2 has 11 types of organics with most residing in 

the area of the linear alternator (Schreiber, 2009). Recent advancements made in support of ASC and 

ASRG development involve the thermal stability assessment of organic materials, the evaluation of thread 

lockers and fasteners, and radiation testing to include component gamma irradiation, gamma in situ 

outgassing, reactor neutron irradiation, and the combined effects of radiation and thermal exposure. 

Radiation testing was conducted at either the coupon/component level or the subsystem hardware level, 

and is critical to the ASC to verify that the organic materials are not susceptible to radiation-induced 

degradation from the GPHS or the surrounding space environment during flight. Additional support tasks 

include epoxy tests to assess thermal aging behavior and magnet bond strength and continued evaluation 

of adhesives and thread lockers. 

Heater Head Non-Destructive Inspection/Evaluation (NDI/NDE) 

GRC is developing a very high resolution non-destructive inspection (NDI) method capable of 

detecting and characterizing surface and volumetric flaws within the heater head. After assessing several 

NDI methods, it was determined that Microfocus X-Ray Computed Tomography (CT) would be the most 

applicable for inspection and evaluation of the ASC heater head primarily due to the resolution and 

sensitivity capabilities of the CT system that is suited for inspection of the heater head, especially in the 

thin wall section. This technique employs tomography created by computer processing. Digital geometry 

processing is used to generate a three-dimensional image of an object from a large series of twodimensional x-ray images taken around a single axis of rotation (Herman, 2009). While still in 

development, the CT trials using a representative MarM target with manufactured flaws indicates the 

inspection resolution needed to screen out critical flaw sizes is achievable. GRC is currently optimizing 

the procedure of post processing the CT data, and this method is expected to be used on the forthcoming 

ASC–E3 heater heads and potentially the flight heater heads as well. 

Heater Head Lateral Load Test 

Achieving flight status for the ASC–E2 mandates that the convertors satisfy design as well as flight 

requirements to ensure reliable operation during launch. To meet some of these requirements, GRC 

performed a series of mechanical tests simulating the operational pressure, axial mounting force, thermal 

conditions, and axial and lateral launch vehicle acceleration loadings that will be experienced by an 

ASC–E2 heater head assembly during an ASRG launch. These mechanical tests were conducted in 
NASA/TM—2012-217207 8 

Figure 4.—ASC–E2 heater head installed 

in the lateral load test setup. 

accordance with the Lockheed Martin specifications, and are collectively referred to as “lateral load tests” 

since a primary external load lateral to the heater head longitudinal axis was applied in combination with 

the other loading conditions. An ASC–E2 heater head shown in Figure 4 was used in the test to qualify 

the heater head analysis and to provide validated test results to meet launch requirements. 

To enable reliable prediction of the heater head’s structural performance, GRC completed Finite 

Element Analysis (FEA) computer modeling for the stress, strain, and deformation that will result during 

launch. The predicted force and deflection data was used during the actual test to corroborate the 

experimental results. After demonstrating that the heater head did not yield under flightlike loads, the test 

continued with increased lateral loading until the heater head yielded, establishing a lateral load capacity 

2.9 times greater than expected during launch. This test result validated the capability of the heater head 

to meet the launch load requirements with sufficient margin. The ASC–E2 heater head lateral load test is 

discussed in greater detail by Cornell (2010). 

Displacer Spring Fatigue Testing 

In order to evaluate the ASC–E2 displacer spring, GRC is performing analysis based on displacer 

spring fatigue testing as well as gigacycle testing of the spring materials. The fatigue analysis will 

quantify the fatigue strength and associated uncertainties, and enhance reliability assessment of the 

displacer spring for the long life required of the design. To date, the displacer spring fatigue testing at 

Sunpower at accelerated high stress levels has resulted in springs with over 2 billion cycles with no spring 

failures. This component-level testing of the planar spring will help validate design life by the 

extrapolation of high stress, low cycle count test results to the low stress, high cycle count design 

conditions. However, the planar spring life assessment requires further testing in the gigacycle fatigue 

range to verify and properly evaluate the processed material’s durability for the long-term life 

requirement of tens of billions of low stress cycles without degradation. This testing will be performed on 

samples with equivalent production spring processing at prototypical and near-prototypical stress levels, 

and at high frequencies for accelerated test results that simulate the required 17-year operating life. 

ASC–E2 Completion and GRC Test Plan

The first ASC–E2 convertor delivered to GRC was the ASC–E2 #2, which was completed by Sunpower 

and delivered in February 2010. The second convertor, the ASC–E2 #1, was delivered in April 2010, and 

was paired with ASC–E2 #2 to complete EMI testing in August 2010. These two convertors have currently 

accumulated nearly 4500 total hr in GRC’s Stirling Research Lab as of December 2010, and will continue 

extended operation testing to contribute to the reliability database of ASCs. 
NASA/TM—2012-217207 9 

Figure 5.—Hermetically sealed ASC–E2 #6 convertor. 

The second pair of convertors, ASC–E2 #3 and #4, which were delivered in April 2010, is the first 

pair to incorporate an E-Beam weld to bond the CSAF to the transition assembly to improve reliability of 

the joint. The E-Beam weld processing details were transferred to Lockheed Martin for verification using 

the flight-design CSAF. Performance Testing of ASC–E2 #3 and #4 at Sunpower indicated the convertors 

met the ASC Product Specification in terms of output power at the reference temperatures. GRC 

performed acceptance testing on this pair and then the convertors were provided to Lockheed Martin for 

ASRG controller developmental testing. 

ASC–E2 #5 and #6, which were fully compliant under the new Sunpower Quality Project Plan, were 

delivered to GRC on July 2010 for installation into GRC’s Stirling Research Lab. These convertors have 

recently initiated extended operation testing and have accumulated hundreds of hours to date. A photo of 

the completed hermetically sealed ASC–E2 #6 is shown in Figure 5 prior to delivery from Sunpower. 

The fourth pair of convertors, ASC–E2 #7 and #8, were completed and delivered to GRC on August 

2010. These convertors will be used for durability testing, during which some of the components will be 

stressed beyond nominal conditions. The purpose of the durability tests is to experimentally demonstrate 

the existing margins within the ASC–E2 design. The tests will consist of: 1) repeated starting and 

stopping of the convertor to accentuate any issues associated with contact before the gas bearings that 

support the piston and displacer become fully established, 2) random vibration with above nominal piston 

amplitude to create a number of controlled contact events to simulate potential contact during launch, 3) a 

controller switchover or other event that would temporarily disable control allowing a limited number of 

contact events, and 4) exposing the convertor to a large static g-load to simulate either reentry and landing 

on a planetary surface or a boost thruster during a mission to determine how the convertor would respond. 

Unlike the preceding ASC–E2s that use a hermetically sealed pressure vessel, a removable test pressure 

vessel was installed on ASC–E2 #7 and #8 to allow for inspection of the convertors after each test (Oriti, 

2011). These tests are not intended to cause damage that would shorten the life of the convertors, as they 

will transition into extended operation at the conclusion of durability testing. 

Path Forward: ASC–E3 and ASC–F Flight UNITS 

The ASC–E3 build will take place in parallel with the ASC–F (flight) build with the initial two pairs 

of ASC–E3 units leading the flight units by months. This provides a unique opportunity to use the 

ASC–E3 as pathfinders for the design and process changes that are required for flight. To maximize 

similarity between the two builds, the ASC–E3 will be built using the flight documentation. A Joint 

Configuration Control Board (CCB) with membership from GRC, DOE, Lockheed Martin, and Sunpower 

has been formulated and has the responsibility to review and approve Sunpower ASC design 

documentation to maintain configuration control for the ASC–F build managed by DOE and Lockheed 

Martin and the GRC-managed ASC–E3 build. The ASC–E3 project was initiated in October 2010 and 
NASA/TM—2012-217207 10 

successfully passed a Long Lead Production Readiness Review for manufacture of the heater head, 

displacer, and transition assemblies. The ASC–E3 will focus on manufacturing, quality, and process 

control refinements along with design modifications to address new system integration and anticipated 

mission requirements. The goal of the project is to establish the viability of the technology for flight and 

complete the transition of the ASC from a technology development effort to a flight hardware build to 

support the ASRG. 

Conclusion 

The current Advanced Stirling Convertor (ASC)–E3 build is the culmination of the Sunpower and 

National Aeronautics and Space Administration (NASA) Glenn Research Center ASC technology 

development effort. The early convertors (Frequency Test Bed (FTB), ASC–1, ASC–0, and ASC–1HS) 

were developmental units, each of which progressively advanced the technology through design 

refinements, improved processing methods, and demonstrated feasibility, temperature capability, and 

break-through performance. Meanwhile, the ASC–E, recently completed ASC–E2 convertors, and the 

next-generation ASC–E3 convertors are part of transitioning the technology out of the development phase 

and into the flight application stage. While some limited production development is still needed and some 

technical challenges await, the current outlook is excellent for the ASC to exactly fulfill the objectives of 

the ASC project, which is to provide NASA with a power conversion technology that enables a reliable 

high-efficiency, high-specific-power Radioisotope Power System. Integrated into the Advanced Stirling 

Radioisotope Generator (ASRG), the ASC could enable NASA missions as early as 2016 while 

minimizing the fuel requirements and maximizing the specific power. 

References 

Cornell, P.A., Krause, D.L., Davis, G., Robbie, M.R., Gubics, D.A., “Lateral Load Testing of the 

Advanced Stirling Convertor (ASC-E2) Heater Head,” AIAA-2010-6692, NASA Glenn Research 

Center, Cleveland, OH, 2010. 

Herman, G. T., Fundamentals of Computerized Tomography: Image Reconstruction from Projections, 

2nd ed., Springer, 2009 

Krause, D.L., Kalluri, S., Bowman, R.R., and Shah, A.R., “Structural Benchmark Creep Testing for the 

Advanced Stirling Convertor Heater Head,” NASA/TM-2008-215435; AIAA-2008-5774, NASA 

Glenn Research Center, Cleveland, OH, 2008. 

National Research Council’s Radioisotope Power Systems (RPS) Committee, “Radioisotope Power 

Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration,” available at: 

http://www.nap.edu/catalog/12653.html, 2009. 

Oriti, S.A., Wilson, S.D., “Advanced Stirling Convertor (ASC-E2) Performance Testing at NASA Glenn 

Research Center,” Paper 3496 in the proceedings of Nuclear and Emerging Technologies for Space 

(NETS 2011).

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

Power Conversion Transitions to Flight,” NASA TM-2009-215515, NASA Glenn Research Center, 

Cleveland, OH, 2009, also in the proceedings of 6th International Energy Conversion Engineering 

Conference, published by the American Institute of Aeronautics and Astronautics, Reston, VA, 2008, 

AIAA-2008-5790. 

Shaltens, R.K., and Wong, W.A., “Advanced Stirling Technology Development at NASA Glenn 

Research Center,” NASA TM-2007-214930, NASA Glenn Research Center, Cleveland, OH, 2007. 

Wilson, K., Wood, J.G., Buffalino, A., Wong, W.A., and Schreiber, J.G., “Sunpower Delivery of ASC-E 

Convertors and Progress Toward Flight,” in the proceedings of 6th International Energy Conversion 

Engineering Conference, published by the American Institute of Aeronautics and Astronautics, 

Reston, VA, 2007, AIAA-2008-5656, 2008. 
NASA/TM—2012-217207 11 

Wong, W.A., Anderson, D.J., Tuttle, K., and Tew, R.C., “Status of NASA’s Advanced Radioisotope 

Power Conversion Technology Research and Development,” NASA TM-2006-214243, NASA Glenn 

Research Center, Cleveland, OH, 2007. also in proceedings of Space Technology and Applications 

International Forum (STAIF 2006), edited by M. S. El-Genk, American Institute of Physics 813, 

Melville, New York, 2006, pp. 340-347. 

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

Development to Future Flight Product,” NASA/TM—2008-215282, NASA Glenn Research Center, 

Cleveland, OH, 2008, also in Proceedings of Space Technology and Applications International 

Forum (STAIF 2008), edited by M. S. El-Genk, American Institute of Physics 813, Melville, 

New York, 2008. 

Wood, J.G., Wilson, K., Buffalino, A., Wong, W.A., Frye, P., Matejczyk, D., Penswick, L.B., “Continued 

Development of the Advanced Stirling Convertor (ASC),” in the proceedings of 5th International 

Energy Conversion Engineering Conference, published by the American Institute of Aeronautics and 

Astronautics, Reston, VA, 2007, AIAA-2007-4704. 
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4. TITLE AND SUBTITLE

Advanced Stirling Convertor (ASC) Technology Maturation in Preparation for Flight 

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Wong, Wayne, A.; Cornell, Peggy, A. 

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WBS 138494.05.02.03 

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14. ABSTRACT

The Advanced Stirling Convertor (ASC) is being developed by an integrated team of Sunpower and National Aeronautics and Space 

Administration’s (NASA’s) Glenn Research Center (GRC). The ASC development, funded by NASA’s Science Mission Directorate, started 

as a technology development effort in 2003 and has since evolved through progressive convertor builds and successful testing to

demonstrate high conversion efficiency, low mass, and capability to meet long-life Radioisotope Power System (RPS) requirements. The 

technology has been adopted by the Department of Energy and Lockheed Martin Space Systems Company’s Advanced Stirling 

Radioisotope Generator (ASRG), which has been selected for potential flight demonstration on Discovery 12. This paper provides an 

overview of the status of ASC development including the most recent ASC-E2 convertors that have been delivered to GRC and an 

introduction to the ASC-E3 and ASC flight convertors that Sunpower will build next. The paper also describes the technology maturation 

and support tasks being conducted at GRC to support ASC and ASRG development in the areas of convertor and generator extended 

operation, high-temperature materials, heater head life assessment, organics, nondestructive inspection, spring fatigue testing, and other 

reliability verification tasks. 

15. SUBJECT TERMS

Advanced Stirling Convertor (ASC); Advanced Stirling Radioisotope Generator (ASRG); Stirling power conversion; Radioisotope 

Power System (RPS) 

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