David W. Meer

Sest, Inc., Middleburg Heights, Ohio 

Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Advanced Stirling Convertor Durability Testing: 

Plans and Interim Results

NASA/TM—2012-217727

November 2012

AIAA–2012–4249
NASA STI Program . . . in Profile

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scientific and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Officer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major significant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of significant 

scientific and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientific 

and technical findings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scientific and 

technical findings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientific and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientific, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question to help@sti.nasa.gov

• Fax your question to the NASA STI 

Information Desk at 443–757–5803

• Phone the NASA STI Information Desk at

443–757–5802

• Write to:

 STI Information Desk

 NASA Center for AeroSpace Information

 7115 Standard Drive

 Hanover, MD 21076–1320
David W. Meer

Sest, Inc., Middleburg Heights, Ohio 

Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Advanced Stirling Convertor Durability Testing: 

Plans and Interim Results

NASA/TM—2012-217727

November 2012

AIAA–2012–4249

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
Acknowledgments

This work is funded through the NASA Science Mission Directorate. 

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identification 

only. Their usage does not constitute an official endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2012-217727 1 

Advanced Stirling Convertor Durability Testing: 

Plans and Interim Results

David W. Meer

Sest, Inc.

Middleburg Heights, Ohio 44130 

Salvatore M. Oriti

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135 

Abstract

The U.S. Department of Energy (DOE), Lockheed Martin Corporation (LM), and NASA Glenn 

Research Center (GRC) have been developing the Advanced Stirling Radioisotope Generator (ASRG) for 

use as a power system for space science missions. In support of this program, GRC has been involved in 

testing Stirling convertors, including the Advanced Stirling Convertor (ASC), for use in the ASRG. This 

testing includes electromagnetic interference/compatibility (EMI/EMC), structural dynamics, advanced 

materials, organics, and unattended extended operation. The purpose of the durability tests is to 

experimentally demonstrate the margins in the ASC design. Due to the high value of the hardware, 

previous ASC tests focused on establishing baseline performance of the convertors within the nominal 

operating conditions. The durability tests present the first planned extension of the operating conditions 

into regions beyond those intended to meet the product spec, where the possibility exists of lateral 

contact, overstroke, or over-temperature events. These tests are not intended to cause damage that would 

shorten the life of the convertors, so they can transition into extended operation at the conclusion of the 

tests. This paper describes the four tests included in the durability test sequence: 1) start/stop cycling, 2) 

exposure to constant acceleration in the lateral and axial directions, 3) random vibration at increased 

piston amplitude to induce contact events, and 4) overstroke testing to simulate potential failures during 

processing or during the mission life where contact events could occur. The paper also summarizes the 

analysis and simulation used to predict the results of each of these tests. 

Nomenclature

AC alternating current

ACU ASC Control Unit 

ASC Advanced Stirling Convertor 

ASRG Advanced Stirling Radioisotope Generator 

CSAF Cold Side Adapter Flange 

CWRU Case Western Reserve University

DOE Department of Energy

EDU Engineering Development Unit 

EU Engineering Unit 

GPHS General Purpose Heat Source

GRC NASA Glenn Research Center

ILS Internal Limit Sensor

LMSSC Lockheed Martin Space Systems Corp.

rms root mean square 

RWG Reliability Working Group 

SDL Structural Dynamics Laboratory

SDM System Dynamic Model
NASA/TM—2012-217727 2 

SRG110 110 W Stirling Radioisotope Generator 

TDC Technology Demonstration Convertor 

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 has 

been identified for potential 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 the 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. 

Due to the high value of the hardware, ongoing activity at GRC focuses on operation of hermeticallysealed ASCs at nominal operating conditions. The hermetic nature of those convertors makes inspection 

difficult. The convertors for the durability test feature a removable pressure vessel to allow for partial 

disassembly of the convertors and inspection of internal components. The purpose of the durability tests 

is to experimentally demonstrate the margins in the ASC design. The durability tests present the first 

planned extension of the operating conditions into regions beyond those intended to meet the ASC 

product specification, where the possibility exists of lateral contact, overstroke, or over-temperature 

events. These tests may operate the convertors in areas of negative margins at a reduced factor of safety, 

yet these tests are not intended to result in damage that could shorten the life of the convertors. The intent 

is that the convertors can transition into extended operation at the conclusion of the durability tests 

described in this document. ASC-E2 nos. 2 and 8 are the test articles for the durability testing. 

Background

There were initially six types of tests considered that could subject the convertors to over-stressed

conditions: static acceleration, random vibration, overstroke, shock, temperature, and radiation exposure.

Several discussions were held with the ASRG Reliability Working Group (RWG), led by Lockheed Martin 

Space Systems Corp. (LMSSC), to review each test in terms of risk and benefit. The discussion considered 

if the parameter being proposed in the test was or could be covered adequately in a component-level test.

The desire was for the durability tests to address aspects that are best served by convertor-level tests.

In some cases, the tests gained a slightly different purpose than was envisioned before the discussion. A 

steady- state acceleration test was considered valuable, but was contingent on finding a facility that could fit 

the project requirements in terms of cost, schedule, and technical capability. The random vibration test was 

initially intended to expose a convertor to levels exceeding qualification, but following the discussions it 

was decided that the vibration test should be used to simulate overstrokes during launch. No contacts are 

anticipated during launch, but the possibility was considered for the case of an uncharacteristically-rough 

launch environment. The overstroke test was initially intended to investigate damage to the convertor in the 

event of an unexpected loss of control. The controller has N+1 redundancy, so overstroke due to a controller 

fault would only occur after multiple failures. This could also occur if controller card switch over did not 

happen as quickly as designed, or if a connection error were made during processing on the ground. The use 

of a solid-state switch makes a contact due to switch over unlikely. Also, a safe mate procedure exists to 

minimize the possibility of a connection error during ground processing. Thus, contact due to loss of control 

is not expected at any point during a mission, but is considered for these tests as such an event would be 

outside the nominal operating conditions. A shock test was considered, but this would be better tested at the 

generator level, and no facilities were identified that could exceed the shock levels already tested at LMSSC. 

Tests at temperatures outside of the ASC-E2 product spec might be undertaken on subsequent pairs of E2s 

as a part of extended operation and were discounted. Options for radiation tests were discounted due to 

existing data on radiation tolerance of materials and other ongoing tests. After the draft of the initial 

durability test plan, a test was added to investigate repeated starting and stopping of the convertors. 
NASA/TM—2012-217727 3 

With this, the list was reduced to four tests:

1. Exposing the convertor to multiple start and stop cycles to evaluate potential Xylan (Whitford 

Corporation) wear from motion before gas bearings are fully charged. 

2. Exposing the convertor to a static g load to simulate reentry and landing on a planetary surface or 

a boost thruster during a mission. This test would investigate the response of the convertor 

moving components, as well as the possibility of Xylan wear when the static acceleration is 

applied in the lateral direction. 

3. Exposing a convertor to random vibration with above-nominal piston amplitude to create a 

number of controlled contact events. This test would also serve to evaluate potential Xylan wear 

during the random vibration exposures. 

4. Simulating a controller switchover error or connection error that would temporarily disable 

control and allow a limited number of contact events. 

The idea was adopted to incorporate a bolt-on pressure vessel into the convertors for durability testing to 

allow inspection of internal components between the different tests. Inspection was chosen in favor of a 

period of extended operation to ascertain convertor health, as this reduced the required time between tests. 

Because of hardware availability, ASC-E2s nos. 2 and 8 were chosen as the test articles for these 

durability tests.

Implementation 

The durability test plan is currently ongoing, with the first two of the four tests completed. ASC-E2 no. 8 

was used for the start/stop durability test, while ASC-E2 no. 2 was used for the static g load test. Previous 

testing of ASC-E2s nos. 2 and 8 at GRC established baseline performance. ASC-E2 no. 2 was operated for 

2,700 hr at nominal conditions and then being returned to Sunpower. At this time it was outfitted with the

removable pressure vessel. ASC-E2 no. 8 was not hermetically sealed prior to delivery, and all its operation 

thus far has been with the removable pressure vessel. Both convertors underwent an initial inspection before 

any durability test to define initial conditions of the convertor internal components for comparison later. 

This procedure consisted of removing the pressure vessel and partially disassembling the internal 

components to allow inspection of any surface markings and internal cleanliness. The GRC Stirling 

Research Laboratory (SRL) procedure, “Durability Test Convertor Inspection Procedure,” fully describes

the inspection process. Following each test, the convertor underwent a partial disassembly and inspection 

procedure to determine if any wear or damage had occurred. At the completion of the durability tests, the 

convertors will return to Sunpower for installation of the production pressure vessels and completion of the 

normal processing sequence before entering into the extended operation at GRC. 

Start/Stop Test

Table I summarizes the various operating conditions for the start/stop test. These conditions represent 

what a flight convertor will experience during processing at Sunpower and Lockheed Martin Space 

Systems Corp. (LMSSC). The cold-end and pressure vessel temperatures represent those when the 

convertor is tested at low- and high-rejection temperature conditions. The number of cycles was chosen 

based on the number of tests planned for the flight convertors, which has been estimated to be up to 150. 

Historical processing experiences of the ASRG-EU at LMSSC and the ASC-E2s at Sunpower were also 

considered. During processing of the ASRG-EU, the convertors underwent 44 start/stop cycles. During 

processing of ASC-E2s nos. 3 and 4 at Sunpower, for example, the convertors underwent 22 start/stop 

cycles. The number of cycles for the durability start/stop test was chosen to double the amount expected 

for a flight unit, hence the test plan consisted of 301 cycles. A flight convertor will undergo start/stop 

cycles in both the vertical and horizontal orientation, thus both arrangements were tested. Since the hotend temperature plays a negligible role in the operation of the gas bearings, all start/stop cycle testing was 

performed at a hot-end temperature of 23 °C. An inspection was performed following the three vertical 

orientation tests and also following the horizontal orientation test.
NASA/TM—2012-217727 4 

TABLE I.—OPERATING CONDITIONS FOR START/STOP TEST

Number of

cycles

Hot-end

temperature, 

°C

Cold-end

temperature, 

°C

Pressure vessel

temperature, 

°C

Orientation

67 23 23 23 Vertical

67 23 38 46 Vertical

67 23 52 61 Vertical

Inspection

100 23 23 23 Horizontal

Inspection

Figure 1.—Amplitude profile for start /stop test.

All of these cycles were performed using the piston amplitude profile shown in Figure 1. The ramp 

rate was designed to reach the maximum setpoint in 5 s. The piston amplitude was held at 3.5 mm for 10 s 

and then ramped down to zero in 5 s. The rest time between cycles was 10 s for vertical operation to 

prevent the piston from drifting and one minute for horizontal operation. The AC bus voltage required to 

operate at 3.5 mm at each operating condition varied and was determined at the beginning of each step of 

the testing. 

Static g Load (Centrifugal Acceleration Test) 

The second test in the durability test sequence utilized a centrifuge to expose the convertors to a 

constant acceleration load for a specified period of time. The ASRG product specification requires operation 

while exposed 18 g of acceleration. This acceleration could be transferred to the convertor in either the axial 

or lateral direction. The axial condition could occur if the ASRG were on a craft launched by a spinstabilized booster, such as the STAR48. In this case, if the ASRG were attached so that the axis of the 

convertors was in the radial direction of the spin, the convertors would experience static centrifugal 

acceleration along their piston motion axes. During a planetary decent, the convertors would experience a 

static g load in the lateral direction.

During the planning stage of this test, the anticipated static g load requirement for an ASRG flight 

was estimated at 30 g. Simulations were performed to evaluate the effect of 30 g on the convertor 
NASA/TM—2012-217727 5 

operation. The results showed that both the piston and the displacer motions shift in the direction of the 

acceleration, but still have sufficient margin in the “out” direction. The results showed that there could be 

displacer contact with the heater head dome when the acceleration is applied towards the heater head, or 

the inwards direction. Note that while a static analysis would determine the steady-state shift inward or 

outward of the piston and displacer motion, the model also takes into account the transient response of the 

system to determine if a potential problem could occur in that phase of the test. 

The specification was later altered to require only up to 18 g. The requirement also consists of a

profile of the static acceleration as a function of time. The landing profile consists of a dwell at the peak 

of 18 g acceleration for 10 s. A facility at Case Western Reserve University (CWRU) was identified for 

this test that could achieve the desired static acceleration. Figure 2 illustrates the 18 g requirement 

profiles, as well as the profile attainable by the CWRU centrifuge facility. As can be seen, the centrifuge 

is not capable of ramping quickly enough to match the ramp of the landing profile nor the ramp of the 

STAR48 profile. This means the convertor will be over exposed, which is the intent of the durability tests. 

A mounting fixture was designed to interface the convertor with the CWRU centrifuge and permit 

multiple orientations of the convertor (Figure 3). The centrifuge testing consisted of these steps: 

• 18 g axial, heater head inwards 

• 18 g axial, heater head outwards 

• 8 g lateral, heater head down 

• Inspection 

• 12 g lateral, heater head down 

• Inspection 

• 18g lateral, heater head down, rotated 90 ° in fixture, stalled convertor after 10 s of 18 g 

• Inspection 

Since the spin-up portion of the test was so much slower than the required profile, the decision was made 

to stall the convertor after the 10 s at18 g was completed. Operating during the latter half, when the 

centrifuge is slowing down, would double the static acceleration exposure for this stage. The spin-up 

portion plus the 10 s of 18 g still exceed the area under the landing profile sufficiently to serve the 

purpose of this durability test. 

Figure 2.—Graph of static acceleration profiles for the flight requirement the profile 

attainable by the CWRU facility.

0 100 200 300 400 500 600 700 800 900 1000 0

2

4

6

8

10

12

14

16

18

20

Time (s)

Acceleration (g)

ASRG Landing Load Spec

CWRU Centrifuge

STAR48 Accel
NASA/TM—2012-217727 6 

Figure 3.—Centrifugal acceleration test: photo of rotating arm/bucket (upper) and video 

frame from 18 g lateral test with heater head down (lower). 

Contact During Launch Test

Figure 4 presents results from earlier vibration testing to qualification levels using ASC-E no.1 under 

control of the LMSSC engineering development unit (EDU) 1 ASC control unit (ACU). This test was 

conducted in March 2009. The figure shows the maximum piston position excursions as a function of 

vibration level in grms. In this case, the convertor was operated at a nominal piston amplitude of 4.25 mm 

with a hot-end temperature of 650 °C and a cold-end temperature of 50 °C. During this test, both the in 

(toward the heater head) and out (away from the heater head) safety limits were set 0.5 mm away from the 

hard stops. As the plot shows, the safety limit was exceeded once during testing at the qualification level.

Note that the input spectra used during this test was shaped based on the results of the dynamic test to 

qualification level of the ASRG Engineering Unit (EU) conducted at LMSSC in the spring of 2008. For 

details of the overall vibration test approach for ASC units, see Reference 3. 

Rotating 

Bucket Axis of 

rotation

Radius of 

rotation
NASA/TM—2012-217727 7 

Figure 4.—Piston amplitude excursions as a function of vibration input level.

The first step in specifying details for this test utilized the GRC-developed system dynamic model 

(SDM) to predict vibration level or operational amplitude required to lead to contact events. Since 

qualification level vibration with the shaped input spectra already approaches the maximum capabilities 

of the shakers in the GRC’s Structural Dynamics Laboratory (SDL), the most likely adjustment would 

involve an increase in piston amplitude during operation. Due to the random nature of the vibration 

testing, some adjustment during the test is likely. 

Besides position sensors incorporated into the ASCs, several alternative methods for determining the 

actual number of contact events are under investigation, including acoustic sensors attached to the 

convertor, microphones located near the convertor, and additional high-frequency accelerometers. This 

test began in June 2012. The steps consist of the following:

• Vibration exposure at workmanship level in all three axes

• Inspection to evaluate workmanship vibe effect on Xylan surfaces

• Lateral vibration exposure at qualification level

• Inspection 

• Axial vibration exposure at qualification level, with increased piston amplitude 

• Inspection 

The first stage, workmanship-level vibration, has been completed. The inspection for this stage is 

currently being completed. 
NASA/TM—2012-217727 8 

Overstroke Test

The goal of the fourth durability test is to simulate possible contact events during processing or 

during the mission life. An off-nominal switchover within the ACU from one control card to another 

could result in an overstroke. At the present time, the estimated switchover time due to relay closure is 

1 ms. Within this timespan, SDM simulations have shown the possibility of collisions, depending upon 

the point in the cycle at which the switchover is initiated. Analysis has been performed to determine the

contact velocities at which permanent component deformation will occur. These will be used to develop 

the test plan, during which the piston amplitude will be increased until overstroking occurs. The goal is to 

prevent permanent deformation by staying below the calculated threshold velocities. 

Test Results

Start/Stop Test

The convertor was disassembled, inspected, and evaluated following the vertical orientation tests, and 

also following the horizontal orientation tests. The primary purpose of the start/stop test was to 

characterize the anticipated wear on the Xylan surfaces of the convertor by doubling the number of 

start/stop cycles seen by a typical flight convertor. No noticeable wear resulted after either the vertical or

horizontal tests. Based on the photographs and visual inspection by a Sunpower technician experienced in 

the assembly, disassembly, and production of ASCs, the Xylan surfaces did not exhibit any detectable 

change as a result of the start/stop cycles. Some light buffing was noted on the piston, on the side facing 

downward during the horizontal testing. In addition, the lint-free cloths used to wipe the piston and 

displacer as part of the disassembly process showed no signs of any Xylan or other foreign materials.

Table II summarizes the gas bearing flows measured at the 16 bearing pads on the piston at three 

points in time: before testing, after vertical testing, and after horizontal testing. The data demonstrate that 

the testing did not significantly change the gas bearing flow. Table III compares the breakaway torques 

measured during the disassemblies to the installation torque specification. The breakaway torque values 

do not correspond exactly to the installation torque value because the fasteners were installed without 

threadlocker. However, these torque data suggest the fasteners did not loosen due to the start/stop cycling. 

In conclusion, the anticipated start/stop cycling of a convertor during a flight life cycle will not adversely 

affect the Xylan-coated surfaces within the ASC.

TABLE II.—GAS BEARING FLOWS

Bearing

no.

Pre-test,

% 

Post-vertical,

% 

Post-horizontal,

% 

O.D. 1 7.5 7.5 7.5

O.D. 2 10.5 10.0 10.5

O.D. 3 11.5 11.0 11.5

O.D. 4 9.0 9.0 9.5

O.D. 5 3.5 3.5 3.5

O.D. 6 3.5 3.5 3.5

O.D. 7 5.0 5.0 5.5

O.D. 8 5.0 5.5 5.5

I.D. 1 25.5 25.0 25.0

I.D. 2 29.0 28.5 28.0

I.D. 3 31.0 30.5 29.5

I.D. 4 28.0 28.5 28.0

I.D. 5 20.0 20.0 20.0

I.D. 6 19.5 19.5 19.5

I.D. 7 21.0 21.5 21.5

I.D. 8 21.5 22.0 21.5
NASA/TM—2012-217727 9 

TABLE III.—BREAKAWAY TORQUE

Fastener Post vertical,

lbf-in.

Post horizontal,

lbf-in.

Nominal install

specification, 

lbf-in.

Flex rod nut –7.590 –7.727 8.4

Spring fastener 1 –5.496 –5.502

6.8 Spring fastener 2 –6.579 –5.848

Spring fastener 3 –5.599 –5.623

Spring fastener 4 –6.289 –5.439

Magnet can fastener 1 –4.675 –4.980

6.4

Magnet can fastener 2 –4.447 –4.831

Magnet can fastener 3 –4.859 –5.150

Magnet can fastener 4 –4.465 –4.868

Magnet can fastener 5 –4.534 –5.003

Magnet can fastener 6 –4.703 –4.648

Centrifugal Acceleration Test

All segments of the centrifugal acceleration test were completed by May 7, 2012. During the first 

segment, 18 g in the axial direction with the heater head inwards, the convertor exhibited a temporary 

outward drift of the piston center of oscillation. The effect was large enough to approach the established 

safety limit, but did not exceed it. The convertor returned to normal operation once the centrifugal 

acceleration was removed. The convertor exhibited little response during the 18 g exposure in the axial 

direction with the heater head outwards. SDM simulations suggested the displacer would come close to 

the safety limit as it would be pushed towards the heater head, but this was not observed during the test.

The convertor was then exposed to 8 g in the lateral direction. Analysis suggested the gas bearing ability 

would be overcome once the lateral acceleration exceeded 3 g. Thus, rubbing would be expected during 

the lateral test, and it was observed. During this lateral exposure, the convertor power output was 

temporarily reduced by 18 W. This was due to a temporary reduction in piston and displacer amplitude, 

which would be expected if the static acceleration were producing cylinder contact. The convertor 

performance returned to normal once the lateral acceleration was removed. Following the 8 g lateral test, 

the convertor was transported to Sunpower, disassembled, and inspected. No signs of Xylan surface wear 

nor debris generation were observed. There was some evidence of buffing on the displacer body in line 

with the static acceleration. The convertor was then operated while exposed to 12 g of lateral acceleration. 

The results were similar. A temporary reduction in power output and amplitude was observed. No change 

in the state of the displacer body surface was observed upon disassembly and inspection. No new 

evidence of wear was observed. The convertor was then operated with 18 g in the lateral direction. For 

this test the convertor was installed in the test fixture rotated 90° relative to previous installations. Since 

the displacer body exhibited evidence of buffing, this decision was made so that the effect of the 18 g 

lateral acceleration could be isolated to a new area on the moving components. Also, the convertor was 

stalled following the 10 s plateau at 18 g. The ramp up and 10 s plateau was deemed sufficient exposure 

for durability purposes. While exposed to 18 g lateral acceleration, the convertor power output was 

reduced from 55 to 24 W, a reduction of approximately 30 W. The convertor was then restarted after the 

centrifuge stopped spinning, at which time it returned to its nominal performance with a power output of

55 W. The subsequent inspection showed a buffing mark on the displacer in the new area, but no debris 

was generated, similar to the buffing mark discovered during the first inspection. This suggests that the 

static acceleration was capable of producing displacer-to-cylinder contact. However, even with this 

temporary contact there is no measurable change in the convertor performance. One may then conclude 

that the convertor design is capable of the static acceleration loads anticipated during a mission.
NASA/TM—2012-217727 10

Conclusion

NASA Glenn Research Center has begun executing a series of durability tests that are intended to 

demonstrate the margins in the ASC design. A total of four tests were conceived. The tests were designed 

to overexpose the convertor without causing permanent damage to the hardware. The tests were designed 

to evaluate margins in four separate areas: potential wear due to start/stop cycles, response to static g 

loads in lateral and axial directions, potential contact events during launch vibration, and overstroke due 

to controller or operational error. Two of the four tests have been completed, and the third has begun. The 

latter two tests will take place during the remainder of 2012. The start/stop cycle test was completed in 

2011. This test showed that the multiple start/stop cycles expected on a flight convertor will have no 

negative effect on the moving components. The centrifuge test showed that the convertor is capable of

axial and lateral static g loads up to 18 g with no negative effect on the moving components. 

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.

3. Meer, David W., Hill, Dennis, and Ursic, Joseph, “Advanced Stirling Convertor Dynamic Test 

Approach and Results,” Proceedings of the 7th International Energy Conversion Engineering 

Conference, AIAA–2009–4552, Denver, CO, 2009. 

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188 

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB 

control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

01-11-2012 

2. REPORT TYPE

Technical Memorandum 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Advanced Stirling Convertor Durability Testing: Plans and Interim Results 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Meer, David, W.; Oriti, Salvatore, M. 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 138494.01.99.01 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

8. PERFORMING ORGANIZATION

 REPORT NUMBER

E-18225 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

10. SPONSORING/MONITOR'S

 ACRONYM(S)

NASA 

11. SPONSORING/MONITORING

 REPORT NUMBER

NASA/TM-2012-217727 

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited 

Subject Category: 20 

Available electronically at http://www.sti.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

The U.S. Department of Energy (DOE), Lockheed Martin Corporation (LM), and NASA Glenn Research Center (GRC) have been developing the Advanced Stirling 

Radioisotope Generator (ASRG) for use as a power system for space science missions. In support of this program, GRC has been involved in testing Stirling convertors, 

including the Advanced Stirling Convertor (ASC), for use in the ASRG. This testing includes electromagnetic interference/compatibility (EMI/EMC), structural dynamics, 

advanced materials, organics, and unattended extended operation. The purpose of the durability tests is to experimentally demonstrate the margins in the ASC design. Due to 

the high value of the hardware, previous ASC tests focused on establishing baseline performance of the convertors within the nominal operating conditions. The durability tests 

present the first planned extension of the operating conditions into regions beyond those intended to meet the product spec, where the possibility exists of lateral contact, 

overstroke, or over-temperature events. These tests are not intended to cause damage that would shorten the life of the convertors, so they can transition into extended operation 

at the conclusion of the tests. This paper describes the four tests included in the durability test sequence: 1) start/stop cycling, 2) exposure to constant acceleration in the lateral 

and axial directions, 3) random vibration at increased piston amplitude to induce contact events, and 4) overstroke testing to simulate potential failures during processing or 

during the mission life where contact events could occur. The paper also summarizes the analysis and simulation used to predict the results of each of these tests.

15. SUBJECT TERMS

ASRG; Radioisotope; ASC; Stirling 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF

 ABSTRACT

UU 

18. NUMBER

 OF 

 PAGES

18 

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email:help@sti.nasa.gov) 

a. REPORT

U 

b. ABSTRACT

U 

c. THIS 

PAGE

U 

19b. TELEPHONE NUMBER (include area code)

443-757-5802 

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18