Edward J. Lewandowski

Glenn Research Center, Cleveland, Ohio

Gary S. Bolotin

Jet Propulsion Laboratory, Pasadena, California

Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Test Program for Stirling Radioisotope Generator 

Hardware at NASA Glenn Research Center

NASA/TM—2015-218458

February 2015

AIAA–2014–3964
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Edward J. Lewandowski

Glenn Research Center, Cleveland, Ohio

Gary S. Bolotin

Jet Propulsion Laboratory, Pasadena, California

Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Test Program for Stirling Radioisotope Generator 

Hardware at NASA Glenn Research Center

NASA/TM—2015-218458

February 2015

AIAA–2014–3964

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

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cosponsored by AIAA, ASME, SAE, ASEE

Cleveland, Ohio, July 28–30, 2014
Acknowledgments

The authors wish to acknowledge all of the members of the Advanced Stirling Radioisotope Generator (ASRG) Integrated Federal 

Project team who designed, analyzed, and procured the hardware being put under test at NASA Glenn Research Center. This work 

was funded with the support of the NASA Science Mission Directorate and the Radioisotope Power Systems Program Of¿ ce. The 

research described in this paper was carried out at NASA, and the Jet Propulsion Laboratory, California Institute of Technology, 

under a contract with NASA. Any opinions, ¿ ndings, conclusions, or recommendations expressed in this article are those of the 

authors and do not necessarily reÀ ect the views of NASA.

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subject to revision as analysis proceeds.
NASA/TM—2015-218458 1 

Test Program for Stirling Radioisotope Generator Hardware at 

NASA Glenn Research Center 

Edward J. Lewandowski 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Gary S. Bolotin 

Jet Propulsion Laboratory 

Pasadena, California 91109 

Salvatore M. Oriti 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Stirling-based energy conversion technology has demonstrated the potential of high efficiency and 

low mass power systems for future space missions. This capability is beneficial, if not essential, to 

making certain deep space missions possible. Significant progress was made developing the Advanced 

Stirling Radioisotope Generator (ASRG), a 140-W radioisotope power system. A variety of flight-like 

hardware, including Stirling convertors, controllers, and housings, was designed and built under the 

ASRG flight development project. To support future Stirling-based power system development NASA 

has proposals that, if funded, will allow this hardware to go on test at the NASA Glenn Research Center. 

While future flight hardware may not be identical to the hardware developed under the ASRG flight 

development project, many components will likely be similar, and system architectures may have heritage 

to ASRG. Thus, the importance of testing the ASRG hardware to the development of future Stirling-based 

power systems cannot be understated. This proposed testing will include performance testing, extended 

operation to establish an extensive reliability database, and characterization testing to quantify subsystem 

and system performance and better understand system interfaces. This paper details this proposed test 

program for Stirling radioisotope generator hardware at NASA Glenn. It explains the rationale behind the 

proposed tests and how these tests will meet the stated objectives. 

Introduction 

Stirling-based energy conversion technology has demonstrated the potential of high efficiency and 

low mass power systems for future space missions. This capability is beneficial, if not essential, to 

making certain deep space missions possible. Significant progress was made developing the Advanced 

Stirling Radioisotope Generator (ASRG), a 140-W radioisotope power system. A variety of flight-like 

hardware, including Stirling convertors, controllers, and housings, was designed and built under the 

ASRG flight development project, but saw only limited testing, and then only at the component and 

subsystem level. Under proposed funding, this hardware will go on test at the NASA Glenn Research 

Center to support future Stirling-based power system development. While future flight hardware may not 

be identical to the hardware developed under the ASRG flight development project, many components 

will likely be similar, and system architectures may have heritage to ASRG. We are proposing to continue 

the testing the ASRG hardware to support the development of future Stirling-based power systems. This 

testing will include performance testing, extended operation to establish an extensive reliability database, 
NASA/TM—2015-218458 2 

and characterization testing to quantify subsystem and system performance and better understand system 

interfaces. This paper details the proposed test program for Stirling radioisotope generator hardware that 

is already underway at NASA Glenn. It explains the rationale behind the tests being conducted and how 

these tests will further advance the technology to meet the objectives stated in this paper. 

Figure 1 shows a diagram of the ASRG, as designed for flight by Lockheed Martin, the system 

integrator under contract to the Department of Energy. The ASRG contains two Advanced Stirling 

Convertors (ASCs) secured together with an interconnect tube. A General Purpose Heat Source (GPHS) 

module, held against each ASC heat collector, provides the heat input. The cold-side adapter flanges 

(CSAFs) conduct heat rejected from the convertors through the beryllium housing and fins, for radiation 

in a vacuum environment or convection to air. During ground operations argon fills the housing, sealed 

using o-rings and gaskets. A gas management valve allows access to the argon. A pressure relief device is 

provided to vent the argon during launch as the surrounding air pressure approaches the vacuum of space, 

improving effectiveness of the insulation surrounding the heat source. The controller is remotely mounted 

in a location determined by the mission and connected electrically to the generator housing assembly 

(GHA) via cables. Connectors on the housing and controller provide electrical interfaces to the 

alternators, sensors, power input and output, control, and telemetry. The GHA is secured to a spacecraft 

interface or support via four mounting tabs on one end of the GHA. 

Test Objectives 

The ASRG flight development project was terminated in late 2013, before assembly and test of the 

ASRG QU had begun. While extensive analysis, modeling, and simulation had been completed at that 

point, a test of the as-built and integrated hardware had not been completed. Without this testing, the 

accuracy of system analysis and simulation cannot be confirmed, models cannot be validated, secondary 

effects and interactions would not be observed, and proof of many concepts of the ASRG, a complex 

dynamic power system with an active controller, would remain unproven and undemonstrated. 

Some subsequent testing has been completed by Lockheed Martin of an ASC Controller Unit (ACU) 

controller, a pair of ASCíE3 convertors, and the flight electrical ground support equipment (Ref. 1). This 

test provided insight and quantification of some ASCíACU system-level behavior, although results are 

limited because the ASCs were not in a flight-like environment in a GHA, and tests were not run to steady 

state as the system was not operated 24 hours a day, 7 days a week, but was started up and shut down on a 

daily basis. 
NASA/TM—2015-218458 3 

NASA Glenn has contracted with Lockheed Martin to complete two engineering-level ACUs 

(Engineering Development Unit (EDU) 4.0 and 4.1) based on the flight ACU design. These ACUs will be 

delivered to NASA Glenn in late 2014. 

NASA has proposed plans to continue the advancement of Stirling-based power system technology 

for deep space missions and for human exploration. Stirling-based thermal to electrical power conversion 

is being considered for higher power radioisotope applications (approximately 350 W to 1 kW) and for 

fission-based power systems 1 kW and above. While these power systems would likely have differences 

with ASRG, it is very likely that components will be similar. It is possible that a higher power 

radioisotope power system could comprise several ASCs. NASA is currently developing a 12-kW freepiston Stirling convertor for fission applications that bear similarities to the ASC design. 

During the development of the ASRG system architecture, many complex interactions and behaviors 

were analyzed. A future Stirling power system’s architecture may benefit from ASRG heritage by either 

adopting a similar architecture or improving on lessons learned from the ASRG experience. The systemlevel nuances uncovered during ASRG development, analysis, and testing can inform future Stirlingbased power systems. 

Invariably, almost every test of a system integrated for the first time uncovers some new findings. A 

thorough and rigorous test of the flight-like ASRG system presents an opportunity to uncover findings, 

quantify behavior, and validate system models. 

It must also be noted that while demonstrating an integrated power system can be technically 

straightforward, successfully taking a system to flight, with the concomitant rigor and thoroughness, is a 

formidable task. The ASRG system had completed most of its flight system development. The proposed 

testing of a system at such an advanced level of development presents a singular opportunity for the 

advancement of future NASA power systems. 

Also, since it may be years before a Stirling-based power system is flown, there is an opportunity to 

gather long-life performance data on flight-caliber Stirling convertors. The lack of long-life convertor 

data was one of the concerns with ASRG, and this concern could be at least partially addressed by starting 

tests now on ASRG hardware. 

Knowledge gained from testing of ASRG hardware could be readily applied to the advancement of 

Stirling power systems in a variety of ways and to varying degrees. Recognizing this opportunity and the 

potential direction of Stirling power system development, three objectives have been identified for testing 

of ASRG hardware in support of Stirling technology advancement. 

A first objective of the proposed testing is to demonstrate steady-state and dynamic performance of an 

integrated Stirling system converting heat to conditioned spacecraft bus power. This includes 

benchmarking convertor, controller, and generator system performance while simulating flight-like 

conditions under typical mission operation scenarios, including beginning of mission, end of mission, and 

diurnal Martian environmental testing. Tests will quantify efficiency, power, and power quality, shortand long-term convertor and controller stability, temperature sensitivity of the convertor, and direct 

current (dc) bus voltage sensitivity of the controller (Ref. 2). Data from these tests will be used to validate 

various system, subsystem, and component models that have been developed. No data exists yet for a 

flight-like integrated ASRG converting heat to conditioned spacecraft bus power, and the hardware from 

the ASRG flight development project presents an opportunity to fill that void. 

A second objective of the proposed testing is to validate convertor, controller, convertor-controller 

interface, controller-spacecraft bus interface, and system-level requirements. This activity will inform 

future Stirling power system requirement development and requirements flowdown and improve 

requirements definition, especially with the complex interfaces in Stirling power systems. Both internal 

interfaces and interfaces to the spacecraft are considered. ASRG system and subsystem requirements 

should be a good reference point for developing requirements for any future Stirling-based power system. 

Validating some of the more critical interfaces and requirements improves their value to future 

requirements development efforts. 

As an example, one of the more complex interfaces is the electrical interface between the convertor 

and controller. This interface has bidirectional energy flow, and the peak voltage and current can be more 
NASA/TM—2015-218458 4 

than double nominal levels during extreme dynamic events such as launch vibration. Controller and 

convertor requirements need to capture not just the requirements during steady-state nominal operation, 

but also requirements for any condition within the allowable operating envelope (i.e., corner cases) or 

different combinations of events. 

The third objective of the proposed testing is to continue extended operation of highest pedigree 

ASCs to establish a database of component and convertor performance and reliability. Having high hours 

of reliable operation for Stirling convertors fills an important gap in the data. This data can be used to 

validate component models, which can be the basis for models of future Stirling-based power system 

components. Demonstrated long-life extended operation builds confidence that any major failure modes 

have been identified and addressed. 

Available Hardware 

While much testing of Stirling convertors has been conducted over the past decade, limited systemlevel testing has been done. To date, the highest fidelity generator system testing was conducted with the 

ASRG Engineering Unit (EU). After completing a series of system-level tests to qualification-level 

thermal and dynamic environments at Lockheed Martin, the ASRG EU ran for over 33,000 hr at NASA 

Glenn. This included 15,378 hr of operation with a first-generation controller called the EDU 1. The 

ASRG EU was taken off of test in 2013 for disassembly and inspection. 

To take the ASRG to flight, many changes were made to the convertor and controller designs since 

the ASRG EU was built in 2007. It will be important to any future Stirling-based flight project to be able 

to leverage flight-like pedigree system-level test heritage. To that end, a high-fidelity engineering unit, the 

ASRG EU2, is being assembled at NASA Glenn (Fig. 2). The ASRG EU2 will consist of the first pair of 

ASCíE3 convertors, Lockheed Martin’s EDU 4 controller (a fourth-generation controller), and an 

aluminum flight-like housing (Ref. 3). Table I summarizes the most important differences between the 

ASRG EU, the ASRG EU2, and the ASRG QU as designed, to show how the EU2 is significantly 

different from the EU and much closer to the QU. 
NASA/TM—2015-218458 5 

TABLE I.—COMPARISON OF ADVANCED STIRLING RADIOISOTOPE GENERATOR (ASRG) SYSTEM HARDWARE 

Detail ASRG EU ASRG EU2 ASRG Qualification 

Unit (QU) design, 

electrically heated 

Comment 

Status Built in 2007 and tested Completing assembly Designed, components 

built, but not assembled 

Convertors Advanced Stirling 

Convertor (ASC)íE #2 

and #3 

ASCíE3 #1 and #2 ASCíF #1 and #2 ASCíE3 and ASCíF 

convertors are built to the 

same drawings and 

specifications and 

same/similar processes. 

Convertor 

design 

- Inconel 718 heater 

head 

- 650 °C max hot-end 

temperature 

- High- and low-voltage 

alternator 

- Internal piston position 

sensor 

- 247LC heater head 

- 840 °C maximum 

hot-end temperature 

- Lower current and 

higher voltage 

alternator 

- External piston 

position sensor 

- Numerous design 

details changed from 

ASCíE 

- Numerous process 

changes from ASCíE 

- Same as ASCíE3 #1 

and #2, but assembled in 

clean room 

Electric 

heat source 

Tested at Lockheed 

Martin with General 

Purpose Heat Source 

(GPHS)-like heat 

source; tested at Glenn 

with round nickel block 

heat source 

Molybdenum heat 

source with same 

thermal mass and 

external dimensions as 

GPHS 

GPHS-like electric heat 

source 

Thermal 

insulation 

Early design Improved design; uses 

insulation intended for 

QU 

Same as EU2 

Generator 

housing 

material 

Beryllium Aluminum, with most 

QU design changes 

plus some 

simplifications to 

reduce cost 

Beryllium, with design 

changes from EU 

EU2 aluminum housing is 

thermally equivalent to QU 

housing. Thicknesses 

adjusted to compensate for 

thermal conductivity 

differences. 

Fins 2.5-in.-long beryllium 

fins 

5-in.-long aluminum 

fins 

5-in.-long beryllium fins EU2 aluminum fins are 

thermally equivalent to QU 

fins. 

Electrical 

shunts 

Resistors internal to the 

housing 

Resistor bank in the 

test rack 

Shunt Dissipation Unit 

(SDU) mounted on 

outboard end of GHA 

QU-like SDU could be 

mounted on end of the EU2 

GHA if desired. 

Controller EDU 1 ACU (Ref. 4) EDU 4.0 or EDU 4.1 

ACU 

QU ACU The EDU 4.0 and 4.1 are 

functional equivalent 

predecessors to the QU 

ACU. 

 
NASA/TM—2015-218458 6 

TABLE I.—Concluded.

Detail ASRG EU ASRG EU2 ASRG Qualification 

Unit (QU) design, 

electrically heated 

Comment 

Controller 

design 

- N+1 redundant system 

architecture 

- Sunpower control 

algorithm utilizing an Hbridge circuit (Ref. 5) 

- H-bridge performs ac 

to dc boost operation 

and maintains output dc 

bus voltage 

- Multiple control loops 

- Many features and 

design details required 

for flight not 

implemented 

- Same circuits and 

board layout as QU 

- Flight equivalent 

parts 

- Boards not conformal 

coated 

- No mechanical 

staking 

- Engineering-level 

quality processing 

- Modified housing 

from QU design 

- A few features 

required for flight not 

implemented 

- EDU 4.0 has white 

wires; EDU 4.1 

replaces most white 

wires with board traces 

- N+1 redundant system 

architecture 

- Sunpower control 

algorithm utilizing an 

H-bridge circuit 

- H-bridge only 

performs ac to dc boost 

operation 

- Added dc output stage 

- Eliminated EDU 1’s 

multiple control loops 

- Significant circuit 

redesign and component 

changes from EDU 1 

EDU 4.0 and 4.1 will 

perform electrically like the 

QU ACU but cannot be 

environmentally tested or 

operated in vacuum. 

Controller 

location 

Attached to the GHA Remotely mounted Remotely mounted 

GHA paint Z–93 Dupont Imron on 

housing; Sherwin 

Williams Polane on 

fins 

Z–93 EU2 paints have similar 

emissivity to Z–93, but 

different absorptivity. EU2 

paints are more durable 

during handling. 

Assembly 

processes 

and quality 

Engineering level by 

Lockheed Martin 

Engineering level, by 

Glenn with Lockheed 

Martin support 

Flight level by 

Lockheed Martin 

Lockheed Martin support of 

EU2 assembly results in EU 

lessons learned and other 

knowledge to be applied to 

EU2. 

Given that the ASRG flight development project had completed fabrication of many of the 

components for the ASRG QU, one might ask, “Why not complete the QU instead of the EU2?” There 

are a number of reasons. An ASRG flight demonstration mission is being considered, so there is a desire 

to preserve the QU hardware for that possibility. The EU2 housing is aluminum, not beryllium, which has 

some advantages from a safety and handling standpoint. Also, the aluminum housing was designed to 

handle an internal vacuum, which allows simulation of space-like environment behavior of the insulation 

and convertors without the complexity and limitations of putting the entire GHA in a thermal vacuum 

chamber. The EU2 has some design simplifications, which reduces the time required to assemble and test. 

For example, the EU2 heat source and heat source preload assembly is less complex than the QU’s. Also, 

when hardware from the ASRG flight development became available after the project was terminated, a 

number of EU2 components were changed out and replaced with hardware that had been intended for the 

QU or flight. This includes the insulation and some of the instrumentation. 

This proposed test program does not intend to test the ASRG EU2 to the full suite of qualification 

tests. Qualification tests are mission specific, and the EU2 was not designed to be able to undergo all 

qualification tests, such as random vibration, shock, and sine transient. 

As a related point, it should be noted that while the ASRG documentation defines the system 

requirements for a fueled ASRG, the ASRG EU2 was not designed or built to meet those requirements. 

Thus, the ASRG EU2 tests will not “verify” compliance with ASRG requirements in a strict sense of the 

word. Rather, the tests will provide an indication of the ability of the ASRG design to meet certain 

requirements or quantify ASRG EU2 performance relative to certain requirements. 
NASA/TM—2015-218458 7 

ASRG EU2 Test Configurations 

The EU2 and its test facility are designed to support a variety of test configurations, enabling a wide 

variety of tests to be conducted, some of which have already been identified, and some which may be 

identified as future objectives are identified to support continued Stirling technology development. As 

with the ASRG EU, the EU2 can be operated under ac bus control or ACU control. While the EU was 

operated with the two convertors on a common ac bus, the EU2 test rack will be set up with two 

independent ac buses, which are synchronized. This configuration mimics the architecture of the ACU 

and permits the two convertors’ piston amplitudes to be set independently. 

The GHA is designed to accommodate a wide range of ground and mission environments, making it 

easier to conduct tests while simulating the different thermal characteristics of these environments. The 

GHA can be filled with argon as would be the case during ground operations. The GHA can be evacuated 

to mimic the vacuum of space so that the insulation and convertors behave thermally as they would in 

space. Finally, the GHA can be backfilled with a Martian-like atmosphere to create internally the thermal 

characteristics found during operation on the Martian surface. 

Test Details 

In order to gain knowledge from testing of ASRG hardware that could be readily applied to the 

advancement of Stirling technology, a number of tests have been developed. These tests are to support the 

test objectives mentioned above. Table II summarizes many of the tests planned for the ASRG hardware. It 

maps the planned tests to the three test objectives and clarifies which hardware will be used for each test. 

Performance and Characterization Tests 

Over the years of ASRG development, the project team created a series of tests that can be used to 

characterize the steady-state and dynamic behavior of the ASRG. The ASRG EU2 will be put through this 

series of tests in a rigorous and controlled fashion to provide extensive data for future system 

development. These tests were conducted on the ASRG EU, and many of them are documented in 

Reference 2. The EU2 tests will be conducted with the GHA filled with argon and with an internal 

vacuum, to characterize operation both on the ground and in space. While the specific performance of a 

future Stirling power system may be different than the ASRGs, validating ASRG system-level models 

provides a measure of confidence in the methods used to predict generator steady-state and dynamic 

behavior using subsystem data. It also provides a data-based benchmark for comparison. 

ASRG EU2 Steady-State Performance Mapping 

The ASRG EU2 steady-state performance will be measured at beginning of mission and end of 

mission operating points over a range of rejection temperatures. The EU2 will be instrumented with 

power meters that measure heater power input, ac power out of the convertors and into the ACU, and dc 

power out of the ACU. So besides measuring overall generator performance, the ASC and ACU 

performance will also be quantified to measure efficiencies and losses at the subsystem level when 

operating as an integrated system. The data from ASC performance can be compared to data taken during 

ASC performance mapping (Ref. 6) to evaluate the accuracy of methods used to measure convertor 

performance in air that rely on thermal analysis (Ref. 7). The data from ACU performance can be 

compared to performance measured during ACU development (Ref. 1). 
NASA/TM—2015-218458 8 

TABLE II.—SUMMARY OF TESTS PLANNED FOR ADVANCED STIRLING 

RADIOISOTOPE GENERATOR (ASRG) HARDWARE 

Hardware 

Test 

Test objectives 

1. Demonstrate 

integrated system 

performance; 

validate models 

2. Validate 

requirements: 

system, 

subsystem, and 

interfaces 

3. Validate ASC 

reliability model 

and demonstrate 

long-life 

operation 

ASRG EU2 

ASRG EU2 steady-state performance mapping X X 

Advanced Stirling Convertor (ASC) setpoint voltage/ac 

bus voltage variation test X X 

Heat input variation test X X 

Hot-end temperature vs. efficiency test X X 

Effect of rejection temperature on performance test X X 

Disturbance force characterization test X X 

One convertor out test X X 

Martian diurnal test X X 

Various interface tests X X 

ASRG EU2 power quality tests in Radioisotope Power 

Systems, System Integration Laboratory (RSIL) X X 

ASRG EU2 extended operation X X 

ACU in RSIL 

Power quality tests X 

Multiple ASRGs on a single spacecraft bus X 

Fault propagation tests X 

ACU model validation X X 

ASC convertors 

Screening and characterization tests: 

-Piston centering 

-Natural frequency 

-ASC performance mapping 

-Horizontal operation 

-Run-in operation 

-Flight acceptance vibration 

X X X 

Extended operation X 

Periodic characterization tests during extended operation X X 

ASC Setpoint Voltage/ac Bus Voltage Variation Test 

The primary control input to the ASRG is ASC setpoint voltage. The ASC setpoint voltage is an input to 

the active power factor control algorithm in the ACU, which then sets the ac voltage to the alternator. The 

alternator ac voltage, in conjunction with convertor operating temperatures, determines the piston amplitude. 

Increasing the ASC setpoint voltage has the effect of increasing piston amplitude, although the transfer 

function contains complex transient dynamics at the system level, and the magnitude of the piston amplitude 

change resulting from an ASC setpoint voltage change is dependent on the generator operating point. When 

operating under ac bus control, the analog to ACU’s ASC setpoint voltage is the ac bus voltage. 

The ASC setpoint voltage/ac bus voltage variation test quantifies the transient and steady-state effects 

of step changes in ASC setpoint voltage or ac bus voltage. The data will be compared to calculated hotend temperature and piston amplitude sensitivities to voltage variation on the ASRG EU2, providing 

guidance for predicting such sensitivities for future Stirling systems. 
NASA/TM—2015-218458 9 

Heat Input Variation Test 

Radioisotope power systems that are fueled by Pu-238 see a 0.79 percent decrease in heat input every 

year due to the fuel decay. The heat input variation test simulates this decay and the effect on the 

generator. Decreasing the heat input results in a decrease in hot-end temperature. The planned approach to 

operating the generator as heat input decreases involves decreasing the piston amplitude in order to 

increase hot-end temperature, thereby improving conversion efficiency and increasing output power. This 

test mimics the fuel decay and the adjustment in piston amplitude in response to increase hot-end 

temperature. 

Hot-End Temperature Versus Efficiency Test 

The purpose of this test is to quantify the effect of hot-end temperature on generator efficiency at 

different operating points. There is a tradeoff between higher convertor efficiency at higher hot-end 

temperatures and higher insulation losses. With a higher hot-end temperature, the Stirling convertor by 

itself is generally more efficient. But at the same time, higher hot-end temperatures result in greater 

insulation losses. This test shows how these two effects trade off and helps characterize the optimal range 

of operation. Analysis has predicted a fairly flat efficiency curve in vacuum over a broad hot-end 

temperature range. This test will confirm the analysis. 

Effect of Rejection Temperature on Performance Test 

During a mission, a generator may experience a change in rejection temperature over a short period of 

time, for example, during a Venus gravity assist maneuver or due to a change in shading while in a high 

solar flux environment. The effect of rejection temperature on performance test will quantify how 

generator parameters change in response to just a change in the heat rejection environment. 

Special System Tests 

The ASRG EU2 presents an opportunity to conduct system-level tests that have been of special 

interest to missions. 

Disturbance Force Characterization Test 

Stirling power systems are dynamic power systems with moving parts that generate disturbance 

forces. Missions need to take these forces into account when designing spacecraft and mounting 

structures. The ASRG incorporates two convertors in an opposed configuration so that the disturbance 

force from one convertor is largely cancelled by the other. The ACU has an input, the relative phase of 

the voltage waveforms, which is one control parameter the mission can use to better cancel the 

disturbance force from each convertor. Another control parameter is to vary the relative piston amplitudes 

of the two convertors. The disturbance force characterization test will quantify the effects of these two 

approaches to minimizing disturbance force and validate the disturbance force dynamic model of the 

generator. 

One Convertor Out Test 

The ASRG is able to continue to operate with one convertor out (not producing power), although at a 

reduced output power level. In this case, the net disturbance force from the generator is higher, since the 

disturbance force from the operating convertor is no longer cancelled by the other convertor. In addition, 

there is a small secondary effect on the operating convertor due to the different thermal environment and 

the small increased oscillation of the convertor casing. The thermal change is due to the decreased 

thermal rejection from the nonoperating convertor’s cold end and the increased heat flux at the 
NASA/TM—2015-218458 10 

nonoperating convertor’s hot end. The increased casing oscillation is expected to result in a slight 

decrease in piston and displacer amplitudes, affecting hot-end temperature, and to a lesser extent, output 

power. The one convertor out test will simulate one convertor out by stalling one convertor while the 

other continues to operate. Data from this test will be used to further validate the disturbance force 

dynamic model of the generator, but now using a significantly higher input force. The secondary thermal 

and mechanical effects will be quantified to the extent possible. It may be difficult to perfectly mimic the 

thermal effects of one convertor out without modifying internal generator hardware or testing in a thermal 

vacuum environment. The test will be conducted with different GHA mounting stiffnesses ranging from a 

rigid mount to a nearly floating mount. The mounting stiffness affects the magnitude of casing oscillation, 

with higher oscillation occurring with the floating mount. 

Martian Diurnal Test 

The Martian day-night cycle would impart a cyclic variation on the ASRG rejection temperature. This 

in turn would affect the convertor operating point, piston amplitude, and hot-end temperature. 

Demonstrating the effects of the Martian diurnal cycle on the ASRG would demonstrate to mission planners 

and developers of future Stirling power systems on how these effects can affect such a power system. 

Interface Testing 

The interface between the controller and the convertor is complex, containing bidirectional energy 

flows influenced by many factors. Completely capturing and quantifying the interface requirements when 

considering all aspects of a space mission, including startup, fueling, test, integration, launch, and so 

forth, is challenging. This part of the test program will include tests to better understand interfaces for 

future power systems. Figure 3 shows the major interfaces in the ASRG. 

The tests already described would provide data related to mechanical, thermal, and electrical interfaces 

both within the generator and between the generator, the spacecraft, and the generator’s environment. 

Additional tests may be conducted to study specific aspects of certain interfaces. For example, a test may 

be conducted to quantify core losses between the alternator and controller associated with the high 

frequency switching of the H-bridge circuit. 
NASA/TM—2015-218458 11 

Controller Testing 

Controller-only tests will focus on the electrical interface to the spacecraft. Tests will be conducted in 

the NASA Glenn Radioisotope Power Systems, System Integration Laboratory (RSIL). These tests will 

be conducted with active electronic convertor simulators in place of actual convertors to allow for ease of 

testing and to permit a wider range of inputs. 

The first series of proposed tests will assess power quality and other controller metrics. These tests 

will focus on measuring the power quality from the ACU, validation of the fault protection within the 

ACU, and testing of multiple ACUs on a spacecraft power bus, simulating multiple ASRGs. The power 

quality tests will measure the voltage ripple and stability through a variety of test conditions with both a 

battery bus and capacitive bus. Spacecraft load simulators will be varied to see the extent this variation 

has on the output power quality. Fault protection validation will validate the protection within the ACU. 

One key test here is to vary the power bus to simulate the expected power variations from events such as 

firing pyros or faults within the spacecraft. This testing would ensure that these faults or events are 

contained within the spacecraft and do not cause the ACU to switch to its spare card prematurely. When 

multiple ACUs are available, testing will be done to characterize the power quality of the system, and to 

ensure that spacecraft or ACU faults do not propagate across fault containment regions. 

Many mission configurations need to integrate multiple radioisotope generators to power the 

spacecraft when a single generator cannot provide sufficient power. The two proposed Discovery 12 

missions that used ASRG, TiME and CHopper, were both designed around two ASRGs. While the ASRG 

did not have an explicit “multiple generators on a single bus” requirement, it was expected that the design 

of the ASRG electrical interface did not preclude having multiple generators on a single bus. NASA 

Glenn is procuring two controllers from Lockheed Martin, EDU 4.0 and EDU 4.1, and will test these two 

controllers on a single bus to demonstrate this capability. This test will quantify the effect on power 

quality, and will look for effects on voltage ripple perhaps related to the fact that the controller’s clocks 

are not synchronized. It will also look for interactions among controllers as the dc bus voltage moves into 

and out of over-voltage and under-voltage conditions. 

Extended Operation Testing 

After completion of performance, demonstration, and characterization testing, the ASRG EU2 will 

continue on extended operation. Besides generating long-life data, this testing will provide an opportunity 

to quantify any performance degradation. Some potential minor degradation modes have been 

hypothesized, but for the most part have not been quantified empirically. Convertor and generator 

performance will be accurately measured periodically at certain operating points to monitor for any 

changes in performance and validate degradation models. Operating conditions during this phase will 

mimic expected flight conditions to the extent possible, including thermal environment variation and 

vacuum, with the goal of demonstrating long life in a “test as you fly” configuration. 

Convertor testing that had been planned under the ASRG project largely continues under Stirling 

technology advancement. Because demonstrating high reliability is so critical to any space power system, 

and this invariably necessitates years of testing, this work will continue. Multiple high-pedigree 

convertors will be put on test under a flight-like test profile, with the goal of accumulating a significant 

number of failure-free years of operation (Fig. 4). Extended operation data will be used to validate the 

ASC reliability model. The convertors will be monitored for small fluctuations in power as a method to 

monitor for contact of internal moving parts. Extended operation with no significant fluctuations in power 

provides evidence of noncontact operation. 

One of the biggest challenges with developing Stirling-based power systems for flight has been 

overcoming the hurdle of establishing sufficient confidence in the long-term reliability of a dynamic 

power system without long-term system-level life data. Traditional reliability methods rely on testing a 

sufficiently large number of the systems for three or four times the life to establish a statistically 

significant result for system reliability. It is not possible to accelerate life testing of Stirling-based 
NASA/TM—2015-218458 12 

systems, so to demonstrate, for example, 17-year life, would require multiple systems operating for 51 to 

68 years by traditional reliability methods. This is clearly not realistic. So other methods are employed to 

establish long-term reliability (Ref. 8). 

System Life Phases 

From the reliability standpoint, Figure 5 shows the typical three phases of a system life with 

associated hazard rate. Each phase is associated with specific failure modes and mitigation. 

Early-life phase has most of its risks associated with fabrication defects. Stringent quality assurance 

and adequate burn-in tests provide the mitigation for these infant mortality risks. We will also be relying 

on extensive production screening tests to screen for defects during this phase of our testing early in our 

test flow. 

Mid-life phase extends the majority of the system design life. We are assuming that this area is 

dominated by random failure risks with a near-constant hazard rate. In other words, high stress conditions 

and potential failures could occur randomly. The risk mitigations of this phase consist of extensive 

component tests, redundancy features, and system life tests. For this phase, failures are considered to be 

independent of time and modeled with the exponential model. Typically, an equivalent life test using 

cumulative test duration with multiple units is acceptable. 

Assumptions underlying the ability to accumulate hours from multiple units for the useful life part of 

the curve include 

1. Random failure modes are assumed over the operational life time of multiple units 

2. Failure modes are assumed to be independent 

3. Failure rates are assumed to be constant, with exponential function model assumed 

Late-life phase is dominated with risks associated with wearout or degradation. Accelerated tests, or 

complete life test (if achievable), would ensure the design will not experience these risks prematurely. 
NASA/TM—2015-218458 13 

The application of this system life phase model to the ASCíE3 test program needs to be done 

judiciously. This means that we need to address some key questions associated with this model. The first 

is to adequately address infant mortality issues. This is done by ensuring that adequate production 

screening tests are in place. Based on previous experience and an evaluation of the screening tests, the test 

program will define the formal change of phase, from early-life to mid-life, thus ending the infant 

mortality phase. 

During the mid-life phase of testing, it is essential that the ASC design is stable with respect to time. 

This means proving that the design is not wearing during nominal operations and that any known 

degradation modes are behaving as predicted. 

Another aspect of applying this model to the test program is through the success criteria. Criteria for 

failure will need to evolve with the unit and point in test profile. These criteria will be used to determine 

the following: 

1. What counts as a statistical failure? 

2. When do we shut down the test or keep the test running? 

3. What does it take to validate a degradation model? 

Screening Tests 

Screening and characterization tests assess convertors for fabrication defects and provide baseline 

data used to monitor the convertors for changes in performance over life. The sequence of ASC 

production screening and characterization tests is shown in Figure 6. The piston centering test 

characterizes how the piston moves when a dc is applied to the alternator. 

The natural frequency test empirically determines the convertor natural frequency. Since the 

convertor natural frequency is a function of numerous parameters within the convertor, by periodically 

conducting this test during extended operation, a change in any one of many parameters can be detected 

this way (although not necessarily uniquely identified). 

The performance map test confirms convertor performance is within spec and characterizes 

performance at the beginning of life for each convertor. The performance map test is run periodically 

during extended operation to monitor the convertor for changes in performance. 

The horizontal operation test checks gas bearing performance under 1-g loading of the piston and 

displacer rod bearings. 
NASA/TM—2015-218458 14 

The flight acceptance vibration test exposes each convertor to the vibration it would see during flight 

acceptance testing and launch, in keeping with the “test as you fly” philosophy. This is performed 

sometime after the 2000 hr of run-in operation. 

Failures during this infant mortality period do not count against the reliability statistics due to 

probable cause being a manufacturing defect. Convertors that pass the 2000-hr mark with no failures 

become part of the extended life pool, with the hours of operation to that point included in the total 

accumulated hours of operation. 

Validate Reliability Model 

The ASC reliability model will be validated through the accumulation of failure-free hours of 

extended operation. This cumulative test time, although limited in sample size, can provide additional 

confidence that important randomly occurring failure mechanisms have been controlled. The proposed 

plan is to document the results from this additional testing to assure that failure mechanisms have been 

controlled effectively by the corrective actions implemented previously during ASC development. 

Figure 7 shows cumulative extended operational test time as a function of mission time and 

confidence level where mean time to failure (MTTF) is equated with mission time. The chart can be used 

as a tool to gauge how much accumulated testing is needed for a particular mission duration and desired 

 
NASA/TM—2015-218458 15 

confidence level. These confidence levels were calculated assuming an exponential distribution. The goal 

for extended life testing is to accumulate as many hours as possible with existing ASCíE3 hardware, 

ASCíE3 convertors being built, and the integrated EU2 hardware. 

Conclusion 

The Stirling radioisotope generator hardware test program at NASA Glenn Research Center plays a 

vital role in the development of Stirling-based power systems. Under the Advanced Stirling Radioisotope 

Generator (ASRG) project, significant progress was made towards the development of a flight-qualified 

radioisotope power system. The proposed test program described here will provide data on the 

performance and reliability of the ASRG hardware, at the system, subsystem, and component level, in 

support of future Stirling-based power system development. If funded, it will provide needed data on 

performance at the generator level, extended operation, Advanced Stirling Convertor Controller Unit 

characterization and validation of all the interfaces. 

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