Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Test Hardware Design for Flightlike Operation of 

Advanced Stirling Convertors (ASC-E3)

NASA/TM—2012-217726

November 2012

AIAA–2012–4254
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Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Test Hardware Design for Flightlike Operation of 

Advanced Stirling Convertors (ASC-E3)

NASA/TM—2012-217726

November 2012

AIAA–2012–4254

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

10th International Energy Conversion Engineering Conference (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

Atlanta, Georgia, July 30–August 1, 2012
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NASA/TM—2012-217726 1

Test Hardware Design for Flightlike Operation of 

Advanced Stirling Convertors (ASC-E3)

Salvatore M. Oriti

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135 

Abstract

NASA Glenn Research Center (GRC) has been supporting development of the Advanced Stirling 

Radioisotope Generator (ASRG) since 2006. A key element of the ASRG project is providing life, 

reliability, and performance testing of the Advanced Stirling Convertor (ASC). For this purpose, the 

Thermal Energy Conversion branch at GRC has been conducting extended operation of a multitude of 

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

thousands of hours) simultaneously on multiple units to build a life and reliability database. The test 

hardware for operation of these convertors was designed to permit in-air investigative testing, such as 

performance mapping over a range of environmental conditions. With this, there was no requirement to 

accurately emulate the flight hardware. For the upcoming ASC-E3 units, the decision has been made to 

assemble the convertors into a flight-like configuration. This means the convertors will be arranged in the 

dual-opposed configuration in a housing that represents the fit, form, and thermal function of the ASRG. 

The goal of this effort is to enable system level tests that could not be performed with the traditional test 

hardware at GRC. This offers the opportunity to perform these system-level tests much earlier in the 

ASRG flight development, as they would normally not be performed until fabrication of the qualification 

unit. This paper discusses the requirements, process, and results of this flight-like hardware design 

activity.

Nomenclature

ASC Advanced Stirling Convertor 

ASRG Advanced Stirling Radioisotope Generator 

BOM beginning of mission

DOE Department of Energy

EOM end of mission

EMI electromagnetic interference

EU engineering unit 

GRC NASA Glenn Research Center

GPHS General Purpose Heat Source

IV&V independent verification and validation

LMCT Lockheed Martin Coherent Technologies 

LMSS Lockheed Martin Space Systems

LR low rejection

HR high rejection

SRG-110 Stirling Radioisotope Generator, 110-We 

TDC Technology Demonstration Convertor 
NASA/TM—2012-217726 2

Figure 1.—ASRG flight unit. 

Introduction

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

supporting the development of the Advanced Stirling Radioisotope Generator (ASRG). The ASRG 

(Figure 1) has been identified for use on two of the three candidate Discovery 12 missions (Ref. 1). A key 

element of the ASRG project is providing life, reliability, and performance data for Sunpower’s 

Advanced Stirling Convertor (ASC). For this purpose, GRC has established a laboratory that is 

conducting extended operation of several ASCs (Ref. 2). The goal of this effort is to generate 

performance data over a sufficiently long period of time (tens of thousands of hours) to support 

probabilistic reliability analyses. As of May 2012, a total of 214,000 hr have been accumulated on 24 

convertors. This hardware set spans the history of Sunpower’s ASC technology development, and 

includes developmental units such as ASC-0s, ASC-Es (engineering unit convertors), and the most 

recent ASC-E2s.

The next set of hardware, deemed ASC-E3, will also be hermetically sealed and capable of 850 °C 

hot-end temperature operation, similar to the ASC-E2. The convertors are being produced to the flight 

unit drawings and quality practices, and as such will exercise the fabrication process planned for the 

ASRG flight unit convertors. However, they will not be installed in any qualification or flight level 

generator. Instead, they will be used for extended operation and controller testing. In the past, GRC 

extended operation test articles have not been designed to emulate the flight unit hardware, but instead 

provide the capability to simulate a range of operating conditions. An example of an ASC-E2 test station 

is shown in Figure 2. For the ASC-E3s, the decision has been made to tailor the extended operation 

hardware such that it emulates the ASRG flight unit. This provides the opportunity for executing systemlevel tests that would normally not be possible until the production of the ASRG qualification unit. In 

effect, the activity will integrate the high-value ASC-E3 hardware into assemblies that will each provide 

the same thermal functionality as an ASRG engineering unit (EU). The controller will not initially be 

included in the ASC-E3 flightlike configuration at GRC. The option exists for future use of the controller. 
NASA/TM—2012-217726 3

Figure 2.—Example ASC-E2 test station at GRC.

Design Guidelines

The test hardware designed for ASC-E2 operation differs from a flight generator in several ways.

While the housing was made to aesthetically resemble the flight generator, the housing is not used as the 

heat rejection device for the Stirling cycle. The ASC-E2 test stations at GRC utilize fluid circulators 

coupled to a rejection device attached directly to the cold end of the convertor to effect a cold-end 

temperature control method. Notice in Figure 2 the presence of fluid plumbing that enters the center of the 

test article. This was necessary to achieve precise, independent temperature control of the cold end and 

alternator housing of the convertors, which was necessary for performance mapping at several different 

operating scenarios expected during a mission, as well as qualification temperatures. On the flight unit, 

the cold-end temperature is a function of the sink temperature, and the conduction path provided by the 

cold-side adapter flange (CSAF) on the convertor and the ASRG housing. Also, the mechanical coupling 

of the two dual-opposed convertors was not designed to match the ASRG. Instead, the coupling was 

achieved by four connecting rods between the two insulation containments, in addition to the interconnect 

tube joining the two convertor alternator housings. Also, the current hardware was not designed to 

provide a sealed inert argon environment around the convertors, which is achieved in the flight design. 

For the design of GRC’s ASC-E3 extended operation hardware, the guiding principle was to duplicate 

all the design features of the flight unit where possible. The design concept for GRC’s flight-like housing 

is shown in Figure 3. Notice that all external features have been duplicated. The Lockheed Martin 

drawings for the ASRG flight unit in the electrically-heated configuration were used for this effort. The 

outer surface will be identical to the ASRG flight unit, including the length of the fins. The same 

electrical feedthroughs will be used to pass power and signals through the housing. Some design features 

were sacrificed to reduce cost and complexity. These deviations include the use of aluminum for the 

housing instead of beryllium, and a different electric heat source to emulate the radioisotope fuel. Other 

less-significant deviations are as follows. The pressure relief device was eliminated since this hardware 

will not experience ascent through the atmosphere. Instead, a gas port will be located where the relief 

device tube would normally enter the housing. A remotely-located shunt dissipator unit will be utilized in 

lieu of one that attaches to the housing. However, the option for attachment of a shunt remains, should the 

lieu of one that attaches to the housing. However, the option for attachment of a shunt remains, should the 

ASC-E2

ASC-E2

Cold end 

fluid plumbing

Circulators

Test Rack
NASA/TM—2012-217726 4

Figure 3.—Concept for extended operation of ASC-E3s in flightlike configuration. 

Convertors will be installed in housing that provides cooling via conduction (top). 

Housing outer surface will be cooled by forced convection.

decision be made to install one at a later time. A type of white paint has been chosen in lieu of the surface 

coating of the flight unit. This paint will be thermally equivalent, as it will achieve the same emissivity, 

but will be simpler and less costly to apply, and have less restrictive handling procedures. The GRC 

design also includes auxiliary temperature control devices, shown as the blue blocks with fluid fittings in 

Figure 3. These can be attached to the housing rib surfaces as needed, if certain tests require temperature 

control not achievable with forced-air convection alone. The auxiliary temperature control blocks also 

serve the purpose of increasing thermal mass of the test article as will be described in a later section. The 

test article will be capable of operation without these auxiliary temperature control blocks. Internally, but 

not shown in the figure, the stud that applies the preload to the heat source was simplified to a single 

ceramic piece. The details of the GRC-designed electric heat source will be discussed in a later section.

These guidelines were formed with a focus on duplicating the thermal behavior of the ASRG. No 

requirements were set for duplicating mass or dynamic characteristics.

Aluminum Versus Beryllium Housing

Aluminum 6061 was chosen for the GRC flight-like housing because of its low cost, ease of 

machining, and wide availability. Comparatively, beryllium is expensive and requires machining safety 

practices beyond that necessary for aluminum. Aluminum has a sufficiently high thermal conductivity to 

make it useful for this purpose, but has a higher density than beryllium. This means an aluminum housing 

will be higher mass than the ASRG flight unit. This was deemed acceptable, as the primary purpose of 

this design is to emulate the thermal behavior of the ASRG flight unit. No tests are planned that will 

require the same total system mass. With this decision, the effect of aluminum in place of beryllium 

needed to be quantified. Table I summarizes the key material properties useful for this purpose. 

External features duplicated 

using flight unit drawings

Electrically heated config

Fins, 12.7 cm length

Cooling fans positioned to 

flow air onto four corners

(Indicated by blue arrows)

Auxiliary reject 

temp. control
NASA/TM—2012-217726 5

TABLE I.—APPLICABLE MATERIAL PROPERTY 

DIFFERENCES BETWEEN ALUMINUM AND BERYLLIUM. 

Aluminum

6061 T6

Beryllium

(ASRG)

Density, g/cm3 2.7 1.85

Thermal conductivity, W/mK 167 187

Volumetric heat capacity, kJ/K/m3 2420 3470

Coefficient of thermal expansion, ppm/K 23 12

Aluminum alloy 6061 T6 has 12 percent lower thermal conductivity than the particular beryllium 

alloy chosen for the ASRG flight unit. To make the housing thermally equivalent, the thermal resistances 

that the rejected heat encounters were maintained by increasing the thickness of the conduction paths by 

12 percent. This included the thickness of the bulkhead that attaches to the CSAF, the wall-thickness of 

the housing, and the thickness of the fins. The changes in thickness were made such that the internal 

square dimensions were duplicated, and the extra 12 percent was added to the outer surface of the housing 

dimensions. For the purpose of duplicating transient thermal behavior, the heat capacity of the two 

materials was evaluated. Aluminum has a significantly lower volumetric heat capacity. This is true 

despite the fact that aluminum has higher mass density than beryllium. Thus, a direct material 

replacement would result in an inadequately low heat capacity of the test article, even after the additional 

12 percent thickness. The solution to this is to attach extra thermal mass to the external surface of the 

housing at axial locations corresponding to each convertor’s rejection flange. Aluminum blocks with 

dimensions 1.25 by 1.25 by 6.5 inches will constitute the required thermal mass. These same blocks will 

be used to provide the option for auxiliary temperature control via fluid flow. They will be attached to the 

ribs of the housing as shown in Figure 3. The location of these attachment points is as close as possible to 

the CSAF attachment location. With this arrangement, the overall thermal resistance will be maintained, 

because the blocks are not in the heat rejection conduction path, but the rejected heat will be absorbed by 

the extra thermal mass. This will result in a total system transient response that emulates the flight unit.

Aluminum also exhibits almost twice the thermal expansion of beryllium. To evaluate the effect of 

this, the forces on the convertor induced by differential thermal expansion of the convertor pair and the 

housing were calculated and compared to those of a beryllium housing. A linear thermal expansion model 

was created (Figure 4) to calculate the forces induced by differential expansion. Included was the 

convertor alternator housings, the convertor heater heads, the heat sources, and the housing sections. The 

stiffnesses of the CSAFs and heat source spring preload were used to then calculate forces that develop 

when the convertor and housing change temperatures. Several thermal scenarios were analyzed that 

represented the range of expected temperatures. The results of the lowest and highest sink temperature 

conditions are summarized in Table II. This table contains results of the simulation in which only the 

beryllium is substituted with aluminum. The interconnect tube in this case was the same material and 

length as the ASRG. The temperatures of the various components were calculated at these sink 

temperature scenarios, and input into the model to calculate forces. The calculations show that at the 

lowest sink temperature scenario, the induced force is 20 percent greater if using an aluminum housing. 

However, this is not of a magnitude expected to disrupt convertor operation. The convertor will be 

exposed to forces higher than those calculated here during product specification verification tests. The 

other cases, such as nominal in-air operation in a laboratory environment, fall between the two extremes 

shown in Table II. The interconnect tube material or length can be altered to skew the generated forces, 

but only one thermal condition can be made to match the ASRG. 
NASA/TM—2012-217726 6

Figure 4.—Linear thermal expansion model for calculating effect of aluminum housing on convertor mounting forces. 

TABLE II.—COMPARISON OF THERMALLY INDUCED FORCES 

ON THE CONVERTOR FOR ALUMINUM AND BERYLLIUM 

HOUSINGS AT TWO SINK TEMPERATURE ENVIRONMENTS

Thermally induced 

force on convertor, 

N

Beryllium Aluminum

At Deep Space Sink Temp –80 173

At Near Earth Sink Temp –769 –992

Electric Heat Source

The decision has been made to also deviate from the flight unit’s ground testing electric heat source. 

This was done to reduce cost and improve life expectancy for in-air operation. Building on past 

experience with high-temperature convertor operation, GRC devised the heat source concept depicted in 

Figure 5. The design consists of cartridge heaters brazed into a nickel 201 block which duplicates the 

outer surface geometry of the General Purpose Heat Source (GPHS) module. The heat source was 

designed to maximize life by maximizing cartridge contact area to minimize heat flux, and reducing 

thermal resistance. Brazing reduces the thermal resistance between the cartridge sheath and the nickel 

block. The effect of these tactics is to reduce the internal element temperature which ultimately governs 

life of the heater. Since the GPHS module has no electrical lead requirement, and thus no thermal 

conduction loss associated with this, the goal of this electric heat source was to minimize the conduction 

losses due to the electrical leads. The cartridge heaters will be connected electrically in parallel by spotwelded nickel bus wiring. The parallel connection between the cartridges will reside inside the thermal 

insulation. This requires only two protrusions through the insulation: one positive lead, and one negative 

lead. One disadvantage of a nickel 201 block of this geometry is that it will have significantly higher heat 

capacity than the GPHS module. The desire is for the heat source to have a similar heat capacity, so that 

the transient thermal response will be accurately emulated. Other options are being entertained that will 

more closely match the GPHS module thermal mass. Both a material and geometry change are required to 

reduce the heat capacity of the electric heat source sufficiently. A material substitution to molybdenum, 

and a geometry change to that shown in Figure 5, would achieve the desired effect. The outer surface 

geometry should be maintained so that the heat transfer from source to insulation emulates the GPHS 

module in the ASRG assembly.

Alternator 

Housing

Interconnect 

Tube 

CSAF

ASC 

(from CSAF to 

GPHS) 

GPHS

Preload 

Spring

Housing
NASA/TM—2012-217726 7

Figure 5.—Design concept of a long-life electric heat source for extended operation in the flight-like housing. 

Figure 6.—Alternative design of electric heat source 

that more closely matches heat capacity of a GPHS 

module. (Dimensions in centimeters)

Test Plans

A series of tests are planned to utilize the hardware, including independent verification and validation 

(IV&V), characterization tests, and life certification. The design of this hardware enables unique tests that 

would normally not be capable until a qualification ASRG unit. Planned tests include:

• Performance mapping with controller in air, and thermal vacuum

• Performance characterization with controller and changes in AC bus voltage, net heat input, and 

sink temperature 

• Demonstration of argon venting in thermal vacuum. 

• Quantifying effect of thermal imbalance, such as one side shaded while the other side is exposed 

to the Sun 

• Practice fueling process on fully-operable convertors 

4 TC locations

10 cartridge heaters

Brazed into block

Outboard pressure 

plate geometry

Inboard pressure 

plate geometry
NASA/TM—2012-217726 8

In addition to these tests, the convertors will undergo extended operation in the flight-like 

configuration. This mode of operation will be the same as that implemented on the other ASCs currently 

operating in the laboratory. 

Conclusion

GRC is preparing facilities for extended operation of several ASC-E3 units. A housing has been 

designed that emulates the ASRG flight generator, but is made of aluminum instead of beryllium. The 

effect of differing material properties has been accounted for to achieve similar thermal behavior as the 

flight unit. Design decisions were made to make the aluminum housing match the ASRG flight beryllium 

housing thermal conduction and heat capacity. The inside dimensions and interfaces to the convertors 

were not altered. Thicknesses that serve as conduction paths were increased by 12 percent to match 

thermal resistance. Auxiliary mass was added to match heat capacity, and provide the option for 

temperature control. The effect of higher thermal expansion of aluminum was analyzed and quantified. 

Results of this analysis showed that the effect can be tolerated. A long-life electric heat source has also 

been designed that uses relatively low-cost cartridge heaters. GRC is currently developing a research plan 

for this hardware that will include system level tests such as thermal vacuum operation in addition to 

extended operation. 

References

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

accessed November 9, 2012. 

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

AIAA-2011-5726. 

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14. ABSTRACT

NASA Glenn Research Center (GRC) has been supporting development of the Advanced Stirling Radioisotope Generator (ASRG) since 2006. A key element of the ASRG 

project is providing life, reliability, and performance testing of the Advanced Stirling Convertor (ASC). For this purpose, the Thermal Energy Conversion branch at GRC has 

been conducting extended operation of a multitude of free-piston Stirling convertors. The goal of this effort is to generate long-term performance data (tens of thousands of 

hours) simultaneously on multiple units to build a life and reliability database. The test hardware for operation of these convertors was designed to permit in-air investigative 

testing, such as performance mapping over a range of environmental conditions. With this, there was no requirement to accurately emulate the flight hardware. For the 

upcoming ASC-E3 units, the decision has been made to assemble the convertors into a flight-like configuration. This means the convertors will be arranged in the dual-opposed 

configuration in a housing that represents the fit, form, and thermal function of the ASRG. The goal of this effort is to enable system level tests that could not be performed 

with the traditional test hardware at GRC. This offers the opportunity to perform these system-level tests much earlier in the ASRG flight development, as they would normally 

not be performed until fabrication of the qualification unit. This paper discusses the requirements, process, and results of this flight-like hardware design activity.

15. SUBJECT TERMS

Radioisotope; ASRG; Stirling; Energy conversion; ASC 

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