Salvatore Oriti and Scott Wilson

Glenn Research Center, Cleveland, Ohio

Advanced Stirling Convertor (ASC-E2) Performance 

Testing at NASA Glenn Research Center

NASA/TM—2011-217102

November 2011

NETS–2011–3496
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Salvatore Oriti and Scott Wilson

Glenn Research Center, Cleveland, Ohio

Advanced Stirling Convertor (ASC-E2) Performance 

Testing at NASA Glenn Research Center

NASA/TM—2011-217102

November 2011

NETS–2011–3496

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Nuclear and Emerging Technologies for Space (NETS–2011)

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

and the AIAA

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Acknowledgments

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-217102 1

Advanced Stirling Convertor (ASC-E2) Performance Testing at 

NASA Glenn Research Center

Salvatore Oriti and Scott Wilson

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Abstract

The National Aeronautics and Space Administration (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, four pairs of ASCs capable of operating to 850 °C and 

designated with the model number ASC-E2, were delivered by Sunpower of Athens, Ohio, to GRC in 

2010. The ASC-E2s underwent a series of tests that included workmanship vibration testing, performance 

mapping, and extended operation. Workmanship vibration testing was performed following fabrication of 

each convertor to verify proper hardware build. Performance mapping consisted of operating each 

convertor at various conditions representing the range expected during a mission. Included were 

conditions representing beginning-of-mission (BOM), end-of-mission (EOM), and fueling. This same 

series of tests was performed by Sunpower prior to ASC-E2 delivery. The data generated during the GRC 

test were compared to performance before delivery. Extended operation consisted of a 500-hr period of 

operation with conditions maintained at the BOM point. This was performed to demonstrate steady 

convertor performance following performance mapping. Following this initial 500-hr period, the ASCE2s will continue extended operation, controller development and special durability testing, during which 

the goal is to accumulate tens of thousands of hours of operation. Data collected during extended 

operation will support reliability analysis. Performance data from these tests is summarized in this paper.

Nomenclature

AC Alternating Current

ASC Advanced Stirling Convertor

ASRG Advanced Stirling Radioisotope Generator

BOM Beginning of Mission

CSAF Cold-Side Adapter Flange

DC Direct current

EMI Electromagnetic Interference

EOM End of Mission

GPHS General Purpose Heat Source

GRC Glenn Research Center

NASA National Aeronautics and Space Administration

PID Proportional Integral Derivative

RTG Radioisotope Thermoelectric Generators

TDC Technology Demonstration Convertor

UPS Uninterruptible Power Supply
NASA/TM—2011-217102 2

Introduction

The Thermal Energy Conversion Branch at the National Aeronautics and Space Administration 

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

Radioisotope Generator (ASRG) (Refs. 1 and 2). Use of Stirling technology quadruples the efficiency of 

thermal-to-electrical power conversion when compared to Radioisotope Thermoelectric Generators (RTGs), 

which significantly reduces the amount of radioisotope fuel required for a given power level (Ref. 3). 

Demonstrating reliability of the ASC is one of the components vital to ensuring reliability of the ASRG.

GRC is providing life and reliability data to the ASRG project in a multitude of areas, including 

structural dynamics, electromagnetic compatibility (EMC), high-temperature materials, and organics

(Ref. 4). GRC is also conducting extended operation life testing at the convertor level. To date, a total of 

38 Stirling convertors have been operated at GRC in support of a space radioisotope power system

development, with a total cumulative runtime exceeding 358,000 hr (Ref. 1). Two units in particular, 

Technology Demonstration Convertors (TDCs) #13 and #14 have each operated for over 54,700 hr. 

Amongst these 38 convertors are four pairs of ASC-E2s (e.g., Figure 1). The ASC-E2 model is the latest 

design from Sunpower, Inc. of Athens, Ohio, and is physically representative of the units that will be 

integrated into the flight unit generator. The ASC-E2 uses MarM-247 for the heater head material and is 

capable of continuous operation at 850 °C with a life no less than 17 years. During flight, the operating

temperature ratio is expected to range from 3.1 to 3.6.

Figure 1.—Example ASC-E2 upon delivery to GRC.

Heat collector

Heat input

Cold-Side Adapter Flange 

(CSAF)

Heat rejection

Alternator output 

lead wires

Welded heater head

assembly

Pinched fill tube

Welded alternator 

housing
NASA/TM—2011-217102 3

ASC-E2s #1 through #8 arrived at GRC between February and August 2010. The first three pairs

(#1 through #6) were hermetically sealed by welding pressure vessel joints and pinching the helium fill 

tubes. These convertors have completed or are undergoing a standard test sequence including

workmanship vibration testing, insulation loss characterization, performance mapping, and extended 

operation. Workmanship vibration testing was performed following convertor final hermetic sealing and 

prior to delivery to GRC. The purpose of this test was to demonstrate proper fabrication by subjecting 

each convertor to 6.8 grms vibration with a frequency range of 20 to 2000 kHz, per NASA-STD-7001.

Following the workmanship vibration tests, the convertors underwent final performance testing at 

Sunpower. During this test, the convertor performance was measured over a range of operating conditions 

representing various mission scenarios, as identified in the ASC-E2 product specification.

Following delivery to GRC and installation onto the test stands, the convertors underwent thermal 

insulation loss characterization, which generated data to enable calculation of net conversion efficiency. 

Following this, the performance mapping test was executed, which was a duplication of the final 

performance test performed by Sunpower. This test provided data to compare convertor performance 

before and after delivery to ensure the hardware remained unaltered during transit. The data also served 

the purpose of establishing a baseline for convertor performance at GRC, as well as comparison to the 

ASC-E2 product specification. The convertors then underwent at least 500 hr of continuous operation 

during which the operating condition was maintained at the beginning-of-mission (BOM) low-rejection

temperature point. This test provided performance data for demonstrating stable convertor operation by 

comparing the initial and final performance over this time span. 

The latter two ASC-E2s, #7 and #8, were fabricated with removable, non-hermetic pressure vessels, 

as these convertors will undergo durability testing, permitting disassembly and inspection of internal

components following each test. As such, the test sequence is different than the first three convertor pairs.

These convertors did not undergo workmanship vibration testing, but will still undergo insulation loss 

characterization and performance mapping prior to execution of the durability tests. Following 

completion of the durability tests, the convertors will be returned to Sunpower to complete hermetic

processing, after which workmanship vibration and final performance testing will be performed.

GRC ASC-E2 Test Setup Overview

A diagram showing component interconnections of the current GRC test station design for ASC-E2 

operation is shown in Figure 2. The convertor is heated by an electric heat source, which is driven by a 

programmable power supply. This power supply is controlled by a proportional-integral-derivative (PID) 

loop, which can use either hot-end temperature or heater power for feedback. The cold-end temperature is

controlled by a laboratory circulator, which flows fluid through coolant collars attached to the cold-side 

adapter flange (CSAF). A second circulator may be used to flow fluid through a heat exchanger around 

the alternator housing. Inside the test rack, a tuning capacitance is connected in series with the alternator 

output. A Pearson coil is used to measure alternator current. If operating a pair of convertors, the other 

convertor would be connected electrically in parallel between the capacitance and load. The alternator 

connector voltage and current signal from the Pearson coil are connected to a power meter. The load 

consists of three 1-Ω resistors connected in parallel, producing 1/3 Ω. An alternatring current (AC) power 

supply was connected to this load through a step-down transformer. The AC power supply controls the 

voltage and frequency across the load, simulating an AC bus for convertor control. This enables 

independent control of piston amplitude and operating frequency. A failsafe protection circuit can apply 

an emergency load if an overstroke condition is sensed. Power is provided to the test rack and circulators 

by an uninterruptible power supply (UPS).
NASA/TM—2011-217102 4

Figure 2.—GRC ASC-E2 test setup overview.

The test setup containing ASC-E2 #1 is shown in Figure 3. ASC-E2 #1 and #2 were not delivered as a 

pair, so the ability to operate a single convertor with a ballast mass was implemented. The heat input 

hardware consisted of a spring-loaded electric heat source. Watlow HT Firerod cartridge heaters were 

chosen for their high maximum use temperature rating of 982 °C. A ceramic stud made of Cotronics 

Rescor 902 was used to transfer the force of the springs and load the heat source onto the heat collector. A 

0.015-inch-thick alumina disk was placed between the heat source and convertor heat collector to prevent 

diffusion bonding of the two nickel-based components. Volumes inside the insulation box not occupied 

by microporous insulation were filled with silica blanket insulation (Thermal Ceramics Kaowool). Brazed 

assemblies that form coolant passages with inlet and outlet tubes were attached to the CSAF for cold-end 

temperature control. Kapton surface heaters were attached to the alternator housing and a coil of tubing 

was wound around the alternator housing to effect a heat exchanger. Microporous insulation (Zircar 

Microsil) was installed around the heat source and heater head. Note that the section view does not show 

the tubing connecting to the CSAF coolant collars, nor the tubing around the alternator housing. Also not 

shown in the section view are the alternator housing surface heaters.

The test rack was designed and built to accommodate two convertors, but is also capable of operating 

a single unit. The path taken by the alternator current to the load was designed to minimize connections. 

Where possible, an uninterrupted length of wire was used along the power path. Where connections were 

necessary, solder joints were preferred.

Each convertor was instrumented for temperature measurements with four hot-end thermocouples, 

two CSAF thermocouples, one CSAF thermistor, and one alternator housing thermocouple. The heat 

source and coolant temperatures were also measured using thermocouples.

The product specification specifies input temperature using the term “acceptor temperature.” This is 

the component in the heater head that transfers heat to the working gas. Since the hot-end thermocouples 

could not be located to directly measure this temperature, a correction was made. The acceptor 

temperature was defined as the average of the four hot-end thermocouple readings plus 7.5 °C. This offset 

was determined by Sunpower using a thermal finite element model. Similarly, a correction was made for 

the rejector; the component that extracts heat from the working gas. In this case, the rejector temperature 

was defined as the average of the two CSAF thermocouple readings plus 3.5 °C.

The power supplied to the heaters was measured using a Yokogawa WT230 power meter. Alternator 

output (voltage, current, power, power factor, and frequency) was also measured by a Yokogawa WT230 

power meter. The alternator currents were measured using an external sensor, namely a model 411 

Pearson coil. The piston amplitude was measured using the convertor’s internal limit sensor (ILS).
NASA/TM—2011-217102 5

Heat source spring 

loading mech.

150 lbs.

Ceramic stud

Cotronics Rescor 902

Heat source

Oxidized, 8 HT Firerods

Shaft collar

Connecting rod

120 lbs. mass

Helical compression spring

Lower adapter plate

Table top

Upper adapter plate Shim washers

Bulkhead

Insulation Housing

CSAF

Microporous 

Insulation

Zircar Microsil 2

Interconnect Tube

HT Firerods

Figure 3.—Completed ASC-E2 single-convertor test article (left), and section view (right).

ASC-E2 #2

The first hermetically sealed MarM-247 convertors delivered by Sunpower were designated with the 

serial numbers 1 and 2. The goal was to deliver these convertors as a pair to GRC, but because of a 

manufacturing delay, #1 was delivered after to #2. #2 completed workmanship vibration testing near the 

end of January 2010, and was delivered to GRC on February 5, 2010. All installation and checkout tasks 

were completed on #2 by the end of February. #2 was setup in the single vertical configuration with its 

heater head in the upwards direction.

Thermal insulation loss characterization began on March 5. The method for characterizing insulation 

losses has been revised since the first ASC-E2 test and will be described in subsequent sections. For #2, a 

stall load was connected to prevent convertor operation while the hot and cold ends of the convertor were 

maintained at various temperatures. The hot-end temperature was varied from 550 to 850 °C, while the 

cold-end temperature was varied from 35 to 90 °C. The heater power required to maintain steady state at 

each condition was measured. From each heater power, the calculated conduction and radiation heat 

transfer down the convertor heater head was subtracted. This difference represents the thermal losses 

through the insulation at each condition. With these data, a correlation was developed relating heat source 

and hot-end temperature to insulation losses. Net heat input was then defined as heater power minus 

insulation losses. A non-operating convertor, such as that when the stall load is engaged, does not draw as 

much thermal power from the heat source as an operating convertor. Because of this, the heat source to

hot-end temperature difference realized during thermal loss characterization is smaller than that realized 

during actual convertor operation. To maintain a given hot-end temperature, the heat source will rise to a 

higher temperature on an operating convertor than a non-operating convertor. Since the thermal loss 

characterization was performed on a non-operating convertor, this effect had to be accounted for in order 

to appropriately calculate insulation losses when operating, even at the same hot-end temperature. To do 

this, a thermal finite element model of ASC-E2 #2, its heat source, and its insulation was developed to 

determine the relative dependence of insulation loss on both the hot-end and heat source temperatures. 
NASA/TM—2011-217102 6

The method for both executing the thermal loss characterization procedure and post processing the data 

has since been modified to improve accuracy of the net heat input calculation. As a result, the net heat 

input for all ASC-E2s is still being reviewed.

Following insulation loss characterization, #2 underwent performance mapping. The goal of this test 

was to duplicate the operating conditions achieved by Sunpower during the #2 final performance test. To 

accomplish this, the hot-end, CSAF, and alternator housing thermocouples were used as controlled 

parameters for repeating temperature conditions. Since too much uncertainty existed in the measurement

of the piston amplitude as calculated from the ILS waveform, the alternator output voltage was instead 

used to attain the approximatley same piston amplitude. The piston amplitude was calculated using the 

ILS signal and recorded in the data, but it was not used by the operator to set the convertor operating 

point. Also, uncertainty existed in the method of calculating net heat input both in the Sunpower and GRC 

test setups, so the net heat input was not used to control operation either. The hot-end thermocouples were 

adjusted using constant temperature control. No constant heater power method was used for this test. The 

CSAF thermocouple readings were adjusted by means of the cold-end circulator setpoint. The alternator 

housing thermocouple reading was adjusted by means of the alternator housing circulator setpoint. For 

some conditions, additional heat was applied to the alternator housing via the surface heaters. The 

alternator voltage was adjusted by means of the AC power supply voltage setpoint.

The results of the performance map are shown in Table 2. Also listed are the results from the 

Sunpower final performance test and the values from the product specification for comparison. All

controlled temperatures were reproduced within 2 °C. The power output (at the connector) measured

during GRC operation was within 1.5 percent of that measured during Sunpower operation for all 

conditions. The power output at the connector during GRC operation was greater than operation at 

Sunpower for the cases of Fueling, BOM High, and EOM High. For the other cases (BOM Low and EOM 

Low), power output was lower. These discrepancies may be explained by differences in piston amplitude 

or differences in achieving steady-state. The alternator connector voltage readout fluctuated by at least

0.1 Vrms, thus duplicating each alternator connector voltage in the Sunpower data set had some inherent 

imprecision. Some of the voltages differed by 0.2 Vrms (e.g., BOM Low). Since alternator output voltage 

is proportional to piston amplitude at a given operating condition, a lower voltage indicates lower piston 

amplitude. The power at the connector measured during GRC operation at the EOM Low condition was 

0.6 percent (0.4 We) less than the product specification requirement. At the same conditions, the 

calculated power at the feedthrough terminals was 0.1 We lower than the specification. However, this was 

most likely due to the aforementioned imprecision in reproducing the operating conditions. At this point,

the connector voltage was 1.2 percent lower than Sunpower’s. Accordingly, the alternator current was

0.7 percent lower. Had the voltage been reproduced more precisely, the power output would have met the 

power output specification. Performance at all other operating conditions met or exceeded the product 

specification for power output.

Performance mapping was completed on April 15, and operation then transitioned to extended mode, 

during which the goal was to accumulate 500 hr at the BOM Low condition. #2 actually accumulated over 

2400 hr of operation at this condition before it was manually shut down. Performance mapping and the 

majority of extended operation was at a hot-end temperature of 850 °C, which makes #2 the convertor 

with the most time at full temperature to date. However, future convertor operation will accumulate tens 

of thousands of hours of operation, for which the heat source is still yet to be proven capable. During #2’s 

extended operation, other tests were performed at the request of Lockheed Martin that deviated from the 

BOM Low condition. There was no 500-hr span of operation where BOM Low was uninterrupted. The #2 

power output during one particular 500-hr span of operation (May 4 to 25) is shown in Figure 4. The 

periods during which other tests were performed are shaded in the figure. The data in the shaded region 

can be ignored and do not represent changes in convertor performance, but rather manual adjustments to 

the operating conditions. The data outside the shaded region show negligible change in convertor power 

output from beginning to end of the 500-hr time span.
NASA/TM—2011-217102 7

Figure 4.—ASC-E2 #2 power output over a 500-hr window of operation at BOM low.

Following extended operation, #2 was shut down and paired with #1 for electromagnetic interference 

(EMI) characterization. EMI testing was completed by mid-August. As of January 2011, #2 has 

accumulated over 2700 hr of runtime. #2 is scheduled to continue extended operation.

ASC-E2 #1

The second ASC-E2 (serial number 1) completed workmanship vibration testing on April 8, 2010, 

and was delivered to GRC on April 22. Installation of #1 onto the test station was completed by May 28.

The thermal insulation loss characterization method was revised for #1. In this case, a thermal barrier was 

placed between the heat source and the convertor to effect the same temperature difference that exists 

when the convertor is operating. The goal of this effort was to raise the heat source to the same 

temperature as that when operating a convertor at 850 °C and improve the accuracy of the thermal loss 

characterization. From previous operational data, it was known that the heat source temperature reaches 

as high as 1000 °C to operate a convertor at 850 °C and maximum power flow. However, as of date the 

caclulatio of net heat input is still being reviewed. Following thermal loss characterization, the thermal 

barrier was removed and replaced by the diffusion-preventing alumina disk.

#1 began performance mapping on June 17, 2010. The test procedure was the same as that used for 

#2. The results of the performance mapping test are listed in Table 2. All controlled temperatures were 

reproduced within 2 °C of the target. The power output (at the connector) measured during operation at 

GRC was between 1.4 and 6.3 percent greater than that measured during Sunpower operation for all 

conditions. However, the alternator leads were shortened after delivery to GRC. The lead length was 

reduced from 36 to 28 in. It has been estimated that because of this, the lead resistance was reduced from 

42 to 32.7 mΩ. Thus, a higher power output at the connector would be measured in the case of the shorter 

leads. A more accurate comparison of power output could be made by comparing the calculated 

feedthrough power, which neglects the power loss through the alternator leads. Using this, the convertor 

power output measured during GRC testing was within 2.5 percent of Sunpower’s values in all cases 

Other Tests
NASA/TM—2011-217102 8

Figure 5.—ASC-E2 #1 power output over a 500-hr window of operation at BOM low.

except EOM Low. For EOM Low, the feedthrough power measured at GRC was 5.2 percent greater. This 

outlier could be explained by the fact that the alternator output voltage was not matched as precisely as 

the other operating points. For EOM Low, the alternator connector voltage was 0.7 percent higher, and 

the piston amplitude was 1.7 percent higher. For all other operating points, the alternator connector 

voltage was matched to Sunpower’s values within 0.5 percent. The convertor power output met or 

exceeded the product specification requirement for all operating conditions.

Performance mapping of #1 was concluded on July 1 and extended operation at BOM Low was 

initiated the same day. Operation was continuous until a manual shut down was initiated on July 26, at 

which time the cumulative runtime totaled 889 hr. A plot of #1’s power output during a 500-hr window of 

extended operation is shown in Figure 5. Manual adjustments were made to maintain the operating 

condition setpoints at hour markers 213, 238, and 477. The step changes seen at these points do not 

represent changes in convertor performance. These adjustments were made to account for changes in heat 

source hardware and high-temperature insulation during the first 200 hr of extended operation, and 

following a set of supplementary tests. Between the hours of 266 and 453 (shaded in the plot), other tests 

were performed. The data here do not represent convertor operation at BOM Low. The data show steady 

convertor output outside the shaded region and in between manual operating condition adjustments.

Following extended operation, #1 was shut down and paired with #2 for EMI testing. #1 has since 

then begun operation with a “compact” heat source that does not protrude through the insulation like the 

HT Firerod cartridges. The goal of this is to reduce the losses from the heater to the environment and to 

obtain a more accurate calculation for net heat input. Following this activity, #1 is scheduled to be paired 

with #2 and continue extended operation in the dual-opposed horizontal configuration. As of January 

2011, #1 has accumulated 2600 hr of runtime.

Other tests
NASA/TM—2011-217102 9

ASC-E2 #3 and #4

The second pair of ASC-E2 convertors, #3 and #4, was delivered to GRC on April 28, 2010. Since 

the convertors were delivered together, they were assembled in the dual-opposed vertical configuration

(Figure 6). This configuration was chosen to emulate the orientation of the ASRG EU currently 

undergoing extended operation. Shown in the figure is the final convertor assembly for operation. As 

such, all convertor surfaces are shown insulated. Thermal insulation loss characterization was completed 

in the same fashion as that described for #1. The thermal barrier was used to elevate the heat source 

temperature to simulate an operating temperature profile. Performance mapping was performed from

July 12 through 17. A total of 10 operating points were achieved even though there are seven operating 

conditions called out in the product specification. This is because in the dual-opposed configuration with 

a single AC bus controller, some points could not be achieved on both convertors simultaneously. In these 

cases, the conditions were achieved for one convertor, while deviating slightly on the other, and vice 

versa. The performance data gathered at GRC matched well with Sunpower’s data (Table 2). As was true 

for the first two ASC-E2s, some points differ due to imprecision associated with duplicating operation 

based on alternator connector voltage, and data acquisition methods.

Extended operation at BOM Low began at the conclusion of the performance mapping on July 17 and 

lasted until August 8. Data from extended operation are still being assembled, and will be covered in 

future publications. Following extended operation, the convertors were shipped to Lockheed Martin and 

are being used for controller development. As of January 2011, ASC-E2 #3 and #4 have each

accumulated 840 hr of runtime.

Figure 6.—ASC-E2 #3 and #4 assembled in the dual-opposed vertical configuration.

ASC-E2 #4 

insulation housing

CSAF coolant 

plumbing

Alternator housing 

coolant plumbing

Electrical heat source 

wiring

ASC-E2 #3 

insulation housing
NASA/TM—2011-217102 10

ASC-E2 #5 and #6

The third pair of ASC-E2 convertors, #5 and #6, completed workmanship vibration testing on July 7, 

2010 and was delivered to GRC on July 23. These convertors have completed performance mapping, 

which began in October 2010, as well as 500 hr of extended operation at BOM Low. The performance 

data for these tests are still being assembled and analyzed. As of January 2011, these convertors have 

each accumulated 1,970 hr of runtime.

ASC-E2 #7 and #8

The fourth pair of convertors, #7 and #8, was delivered to GRC on August 13, 2010. #7 was operated 

in the single vertical configuration and completed performance mapping. The performance mapping data 

are still being assembled and analyzed. This convertor is currently undergoing 500 hr of extended 

operation at BOM Low. #8 was returned to Sunpower for supplementary testing support and will be 

returned to GRC in the near future.

These convertors will also be used for durability testing, during which some components may be 

stressed beyond nominal conditions. Figure 7 shows the removable alternator housing installed on these 

convertors to allow for inspection of internal components after each test. In contrast, convertors #1 through 

#6 were fabricated with hermetically sealed alternator housings. The purpose of the durability tests is to 

experimentally demonstrate the margins that exist in the ASC-E2 design. The durability tests will subject 

the convertors to conditions beyond those intended to meet the product specification. During these tests, the 

possibility exists of axial or lateral contact, piston contact events, and over-temperature. These tests are not 

designed to result in damage that would significantly shorten convertor life, such that the convertors may 

later be hermetically sealed and continue extended operation. Table 1 summarizes the planned tests.

Figure 7.—ASC-E2 #7 and #8 with removable alternator housings for inspection after durability testing.

Heat collector

Cold-Side Adapter 

Flange (CSAF)

Removable 

alternator 

housings
NASA/TM—2011-217102 11

TABLE 1.—SUMMARY OF PLANNED ASC-E2 #7 AND #8 DURABILITY TESTING

Test description Purpose Anticipated date

Start/Stop cycling Cycle the convertor repeatedly through start/stop cycle to 

exacerbate any possible wear induced before gas bearings become 

fully functional

March 2011

Contact events during launch Simulate a limited number of contact events during launch by 

adjusting piston amplitude

April 2011

Piston overstroke Simulate a limited number of contact events with desired relative 

velocities between the piston and displacer by adjusting piston 

amplitude

May 2011

Centrifugal acceleration Expose operating convertor to 30 g static load using a centrifuge 

facility to observe response in moving components

June 2011

ASC-E2 #1 Through #4 Performance Comparison

For simplicity, the results of performance mapping for the first four convertors (#1 through #4) are

contained in Table 2. #1 and #2 were operated as single units, each in the vertical orientation with the 

heater head in the upwards direction. #3 and #4 were operated as a pair in the dual-opposed vertical 

configuration, which emulates ASRG engineering unit operation (Ref. 2). Prior to delivery, Sunpower 

operated each convertor individually at each of the points shown in the table. For comparison to 

Sunpower’s operation, the convertors were each operated at GRC by duplicating the temperatures and 

alternator connector voltages for each operating point. All temperatures were duplicated within 2 °C. The 

connector voltages were each duplicated within less than 1.4 percent, due to imprecision associated with 

adjusting alternator voltage via the aforementioned method. With this, individual convertor power output 

differed by 3 percent or less between Sunpower and GRC data, except for EOM Low on ASC-E2 #1. This 

outlier is most likely due to higher operating point duplication discrepancy. The GRC EOM Low data 

point for #1 was at a higher piston amplitude than Sunpower’s data point, resulting in a higher power 

output. Other discrepancies may be accounted for by differences in data recording and averaging 

techniques at the two laboratories. As of writing, both the method for performing thermal insulation loss 

characterization and the calculation of net heat input are being scrutinized and refined. The results of this 

effort will be summarized in future publications. The current method for calculating net heat input at 

GRC yields convertor efficiencies between 32 and 38 percent across all the operating cases shown in the 

table. If examining the unit-to-unit repeatability, the available data show that the standard deviation in 

power output is less than two We for each of the operating cases shown here. An unknown portion of this 

variability is due to imprecision in repeating a given operating point.
NASA/TM—2011-217102 12

TABLE 2.—SUMMARY OF ASC-E2 #1, #2, #3, AND #4 PERFORMANCE MAPPING AT SUNPOWER AND GRC

Acceptor

Temp

Rejector

Temp

Alternator

Housing Temp Frequency Power Output

Feedthroughs

Power Output

Connector

Alt Voltage

Connector

°C

(calc)

°C

(calc)

°C

(meas)

Hz

(meas)

We

(calc)

We

(meas)

Vrms

(meas)

± 1 ± 1 ± 2 ± 0.47 ± 0.47 ± 0.04

850 98 ±2 104 ±2 102.2 ±0.2 Safe operation Safe operation 10.3 to 18.3

Sunpower 849 98 104 102.2 81.1 76.9 14.71

GRC 850 98 104 102.2 82.1 78.7 14.71

Sunpower 850 98 104 102.2 80.8 76.0 15.81

GRC 849 99 104 102.2 81.6 76.7 15.81

Sunpower 849 99 104 102.2 80.4 75.6 16.12

GRC 849 99 104 102.2 78.6 73.6 16.19

Sunpower 848 99 103 102.2 80.9 76.2 15.82

GRC 848 99 104 102.2 78.4 73.6 15.90

Standard Dev. 1 1 0 0 1.3 1.7 0.6

≤ 850 52 ±2 61 ±2 102.2 ±0.2 ≥ 84.5 ≥ 80.9 10.3 to 18.3

Sunpower 848 51 62 102.2 85.6 81.7 13.29

GRC 850 52 61 102.2 87.4 84.3 13.29

Sunpower 849 52 61 102.2 86.1 82.0 14.32

GRC 849 52 59 102.2 85.5 81.5 14.13

Sunpower 850 53 62 102.2 85.5 81.6 14.55

GRC 849 51 62 102.2 86.6 82.5 14.65

Sunpower 849 52 61 102.2 85.3 81.5 14.25

GRC 850 52 61 102.2 83.6 79.8 14.11

Standard Dev. 1 0 1 0 1.1 1.3 0.5

≤ 850 90 ±2 98 ±2 102.2 ±0.2 ≥ 79.5 ≥ 75.7 10.3 to 18.3

Sunpower 848 90 99 102.2 80.2 76.1 14.39

GRC 849 90 99 102.2 81.5 78.2 14.46

Sunpower 850 91 98 102.2 80.7 76.1 15.51

GRC 849 90 98 102.2 81.3 76.7 15.50

Sunpower 848 90 98 102.2 80.5 75.9 15.83

GRC 848 89 97 102.2 80.9 76.1 15.98

Sunpower 846 90 98 102.2 81.2 76.7 15.60

GRC 846 90 98 102.2 80.1 75.5 15.67

Standard Dev. 1 0 0 0 0.5 0.8 0.6

≤ 850 36 ±2 44 ±2 102.2 ±0.2 ≥ 75.3 ≥ 72.2 10.3 to 18.3

Sunpower 851 36 45 102.2 76.3 72.9 12.03

GRC 850 37 45 102.2 80.3 77.5 12.12

Sunpower 849 37 44 102.2 76.4 72.9 12.91

GRC 849 36 44 102.2 75.2 71.8 12.76

Sunpower 850 36 44 102.2 76.0 72.8 13.05

GRC 850 36 44 102.2 75.6 72.6 12.98

Sunpower 848 38 44 102.2 76.0 72.8 12.89

GRC 847 37 44 102.2 77.5 74.1 12.96

Standard Dev. 1 1 0 0 1.6 1.8 0.4

≤ 850 90 ±2 98±2 102.2 ±0.2 ≥ 69.2 ≥ 65.9 10.3 to 18.3

Sunpower 849 90 98 102.2 69.6 66.0 13.39

GRC 850 90 98 102.2 70.6 67.8 13.41

Sunpower 848 90 98 102.2 70.2 66.1 14.49

GRC 849 90 98 102.2 71.0 66.9 14.50

Sunpower 848 90 97 102.2 70.3 66.3 14.77

GRC 848 90 96 102.2 70.2 66.1 14.87

Sunpower 845 90 97 102.2 70.1 66.3 14.51

GRC 845 90 97 102.2 69.5 65.6 14.58

Standard Dev. 2 0 1 0 0.5 0.7 0.6

#1

#3

#4

BOM Low Reject Spec

BOM High Reject Spec

EOM Low Reject Spec

EOM High Reject Spec

#1

#2

#3

ASC-E2

#1, #2, #3, #4

Units -->

Measurement

Uncertainty -->

Fueling Spec

#2

#4

#1

#2

#3

#4

#1

#2

#3

#4

#1

#2

#3

#4
NASA/TM—2011-217102 13

Conclusions

For the purpose of generating life and reliability data for the ASRG project, NASA GRC has begun 

operating several ASC-E2 units. Table 3 summarizes the ASC-E2 activity as of January 2011. Seven of 

the eight convertors have begun operating. Over 12,000 hr of operation have been accumulated on these

seven ASC-E2s. Data indicated convertor performance at GRC and Sunpower matched well, with 

differences less than 3 percent. Discrepancies in measured performance were accounted for by differences 

in operating conditions, and did not indicate changes in convertor performance. Data from a 500-hr 

window of operation at BOM Low were analyzed for two convertors (#1 and #2). These data showed 

steady performance over this timespan. The remaining convertor performance data will be covered in 

future publications.

TABLE 3.—SUMMARY OF ASC-E2 TESTING AT GRC AS OF JANUARY 2010

Convertor Cumulative runtime,

hr

Status

ASC-E2 #1 2600 Extended operation

ASC-E2 #2 2700 Extended operation

ASC-E2 #3 and #4 840 Controller development at Lockheed Martin

ASC-E2 #5 and #6 1970 Extended operation

ASC-E2 #7 1150 Extended operation

ASC-E2 #8 0 Supplementary test support at Sunpower

Total = 12,070 hr

References

1. Wilson, S.D., “Stirling Convertor Extended Operation Testing Overview,” International Energy 

Conversion Engineering Conference, Nashville Tennessee, 2010, AIAA–2010–6694.

2. Lewandowski, E.J., “Testing to Characterize the Advanced Stirling Radioisotope Generator 

Engineering Unit,” International Energy Conversion Engineering Conference, Nashville Tennessee, 

2010, AIAA–2010–6693.

3. ASRG User ICD, Lockheed Martin, May 2010. NASA SMD Science Office for Mission Assessments 

Discovery Program Acquisitions Discovery 2010 AO Program Library.

http://discovery.larc.nasa.gov/PDF_FILES/16ASRG_User_ICD.pdf

4. Schreiber, J.G., “Supporting Technology at GRC to Mitigate Risk as Stirling Power Conversion 

Transitions to Flight,” International Energy Conversion Engineering Conference, Cleveland Ohio, 

2008, AIAA–2008–5790.
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Advanced Stirling Convertor (ASC-E2) Performance Testing at NASA Glenn Research 

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The National Aeronautics and Space Administration (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, four pairs of ASCs capable of operating to 850 °C and 

designated with the model number ASC-E2, were delivered by Sunpower of Athens, Ohio, to GRC in 2010. The ASC-E2s underwent a 

series of tests that included workmanship vibration testing, performance mapping, and extended operation. Workmanship vibration testing 

was performed following fabrication of each convertor to verify proper hardware build. Performance mapping consisted of operating each 

convertor at various conditions representing the range expected during a mission. Included were conditions representing beginning-ofmission (BOM), end-of-mission (EOM), and fueling. This same series of tests was performed by Sunpower prior to ASC-E2 delivery. The 

data generated during the GRC test were compared to performance before delivery. Extended operation consisted of a 500-hr period of 

operation with conditions maintained at the BOM point. This was performed to demonstrate steady convertor performance following

performance mapping. Following this initial 500-hr period, the ASC-E2s will continue extended operation, controller development and 

special durability testing, during which the goal is to accumulate tens of thousands of hours of operation. Data collected during extended 

operation will support reliability analysis. Performance data from these tests is summarized in this paper. 

15. SUBJECT TERMS

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

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