Zachary D. Williams and Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Advanced Stirling Convertor (ASC–E2)

Characterization Testing

NASA/TM—2012-217750

December 2012
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Zachary D. Williams and Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Advanced Stirling Convertor (ASC–E2)

Characterization Testing

NASA/TM—2012-217750

December 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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NASA/TM—2012-217750 1 

Advanced Stirling Convertor (ASC–E2) Characterization Testing 

Zachary D. Williams and Salvatore M. Oriti 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Testing has been conducted on Advanced Stirling Convertors (ASCs)–E2 at NASA Glenn Research 

Center in support of the Advanced Stirling Radioisotope Generator (ASRG) project. This testing has been 

conducted to understand sensitivities of convertor parameters due to environmental and operational 

changes during operation of the ASRG in missions to space. This paper summarizes test results and 

explains the operation of the ASRG during space missions. 

Introduction 

As a part of the continued support of the Advanced Stirling Radioisotope Generator (ASRG) project’s 

path to flight, Advanced Stirling Convertors (ASCs) from Sunpower, Inc., are on test at NASA Glenn 

Research Center’s (GRC’s) Stirling Research Lab (SRL). The latest convertors manufactured and 

currently under test are the ASC–E2 convertors. Testing is being conducted for extended life reliability 

and parameter characterization for system integration. To fully understand effects of convertor 

performance, parameter variation tests need to be conducted. 

This paper outlines two tests that were conducted on ASC–E2 convertors. The first test was to support 

controller development. This test was called the AC bus variation test and was conducted to help 

understand the sensitivities of key parameters due to AC bus voltage changes. A better understanding of 

the sensitivities of these parameters will help quantify tolerances of the controller and help finalize 

control strategies. 

The second series of tests was a simulation of argon venting of an ASRG during launch. This testing 

was to simulate the effect of ascent through the atmosphere during launch of the ASRG and quantify the 

effects. This series of tests was conducted in three phases. Each of these phases simulated different events 

that the convertor would undergo throughout the mission. 

Nomenclature 

ASC Advanced Stirling Convertor 

ASRG Advanced Stirling Radioisotope Generator 

BOM HR Beginning of Mission High Reject temperature 

BOM LR Beginning of Mission Low Reject temperature 

EU Engineering Unit 

GRC Glenn Research Center 

SRL Stirling Research Lab 

Wth Watts thermal 

Test Setup 

The ASC–E2 convertors were tested on a test stand that was designed specifically for ASC–E2 

testing. These test stands are shown in Figures 1 and 2. Further details can be found in Reference 1. 
NASA/TM—2012-217750 2 

 

AC Bus Voltage Variation Test 

The AC bus voltage variation test was conducted in the SRL at GRC using ASC–E2 #1. AC bus 

voltage is analogous to a key control input to the ASRG. The purpose of this test was to characterize 

convertor response to AC bus voltage variation. 

During testing, the convertor was operating at ASC–E2 specification Beginning of Mission Low 

Reject (BOM LR) conditions (Ref. 2) in a single-vertical setup with heater head up. The convertor was 

being controlled with constant heat input. More information on test stations in the SRL can be found in 

Reference 2. 

The test was terminated when the compact electrical heat source failed. After failure, the compact 

heat source was replaced with an HT FIREROD (Watlow) heat source, and the first four points of the test 

were repeated. Further details on the HT FIREROD heat source can be found in Reference 3. 

The test was conducted by adjusting the AC bus voltage in incremental steps while observing hot-end 

temperatures with other convertor parameters held constant. The AC bus voltage directly controls the 

piston amplitude of the convertor. An increase in AC bus voltage results in an increase in piston 

amplitude. During the test, the AC bus voltage was increased by an increment every day and then the 

system was allowed at least 8 h to reach a steady-state point. 

Data was gathered using a LabVIEW (National Instruments) data acquisition system. Most 

parameters were sampled at 2000 Hz and averaged over 1 sec. The averaged data, called 2-sec data, was 

saved every 2 sec. For steady-state data, the 2-sec data was averaged over a 5 min time span and was 

saved as a point and is called 5-min average data. 

Argon Venting Simulation Test 

The argon venting simulation test was also conducted in the SRL at GRC but was performed using 

ASC–E2 #5, which utilizes an HT FIREROD heat source. The purpose of this test was to simulate the 

convertor environment during launch and collect data useful in developing the launch concept of 

operation strategies. 

Figure 1.—ASC–E2 #1 on 

test stand. 

Figure 2.—ASC–E2 #5 on test stand 

with test rack in background. 
NASA/TM—2012-217750 3 

Data was collected for this test using the same type of data acquisition system as the AC bus voltage 

variation test. This test began with ASC–E2 #5 operating at ASC–E3 and ASC–F specification 

BOM–HR. This point was chosen because it better represents the operating condition of a convertor in the 

payload fairing prior to launch. During this test, the convertor was controlled in constant-heat-input mode. 

After the convertor achieved steady state at this point, phase 1 of the test was started. This included 

decreasing the gross heat input by 20 W to get a baseline and allow the convertor to reach steady state. 

Once steady-state conditions were achieved, a 5-min average was recorded and the convertor was 

returned to a baseline ASC–E3 and ASC–F BOM HR operating point. Again the system was allowed to 

reach steady state, a 5-min average point was collected, and then the gross heat input was increased by 

15 Wth. The convertor was allowed to reach steady state and another 5-min average was taken for this 

point. The convertor was then returned to a baseline ASC–E3 and ASC–F BOM HR point, allowed to 

steady, and a final 5-min average point was collected to conclude phase 1. 

Phase 2 began with ASC–E2 #5 operating at ASC–E3 and ASC–F BOM HR conditions. A 5-min 

average data point was taken to establish a baseline. Subsequently, the net heat input was increased by 

30 Wth to simulate venting of argon and increased effectiveness of thermal insulation. As the hot-end 

temperature rose, the net heat input was adjusted so that it matched the hot-end temperature/net heat input 

relationship of the ASRG flight unit. Once the convertor reached a steady state at this point, a 5-min 

average was taken, and the convertor returned to baseline ASC–E3 and ASC–F BOM HR conditions. 

Phase 3 started at steady-state ASC-E3 and ASC–F BOM HR conditions. The next step in this test 

was to reduce the rejector and alternator housing temperatures by 30 C. This simulates the sink 

temperature change as the ASRG ascends from within the spacecraft fairing to the lowest expected 

radiation sink temperature of 4 K. This represents a worst-case scenario, since a spacecraft leaving 

Earth’s atmosphere sees a radiation sink temperature much higher than 4 K. As the hot-end temperature 

rose, the net heat input was adjusted to match the hot-end temperature/net heat input relationship of the 

ASRG flight unit. After this point reached steady state, a 5-min average data point was taken, and the 

system returned to baseline ASC–E3 and ASC–F BOM HR conditions. 

Test Results 

AC Bus Variation Test 

There were nine data points collected with the compact heat source. These points were plotted and 

can be found in Figure 3. Reported in the graph is 5-min average data from a time in which the convertor 

reached a steady-state operating point. 

From this data the incremental sensitivities of the hot-end temperature and piston amplitude was 

calculated by dividing the change in the parameter (e.g., change in piston amplitude) by the change in 

AC bus voltage. The incremental sensitivities that were calculated for piston amplitude and hot-end 

temperature are plotted versus hot-end temperature in Figure 4. 

 

Figure 3.—Data from AC bus voltage variation test. 

 

4.10

4.15

4.20

4.25

4.30

4.35

4.40

4.45

750

760

770

780

790

800

810

820

830

840

850

860

8.4 8.6 8.8 9.0 9.2 9.4 9.6

PistonAmplitude (mm)

Hot‐End Temperature, ̊C

AC Bus Voltage, Vrms

CompactHeat Source Hot‐end Temp Piston Amplitude

4.10

4.15

4.20

4.25

4.30

4.35

4.40

4.45

750

760

770

780

790

800

810

820

830

840

850

860

8.4 8.6 8.8 9.0 9.2 9.4 9.6

PistonAmplitude, mm

Hot‐End Temperature, ̊C

AC Bus Voltage, Vrms

HT Firerod Heat Source Hot‐end Temp Piston Amplitude
NASA/TM—2012-217750 4 

 

Figure 4.—Sensitivities plotted vs. hot-end temperature. 

With the compact heat source, the hot-end temperature sensitivity varied between –91 and –109 C/V 

and the piston amplitude sensitivity varied between 0.21 and 0.37 mm/V. These numbers are similiar to 

the numbers from the AC bus voltage variation test conducted with the ASRG EU, which had a hot-end 

temperature sensitivity of –73 to –81 C/V and a piston amplitude senstivity of 0.23 to 0.30 mm/V 

(Ref. 4). The difference between the operating points may account for some of the differences in the two 

sets of numbers. The ASRG Engineering Unit (EU) hot-end temperature was about 625 C at the 

beginning of that test and the ASC–E2 #1 hot-end temperature was at about 850 C. 

After the compact heat source failed, it was replaced with a HT–FIREROD heat source and the first 

four points of the test were repeated. The results of this repeated test are plotted in Figure 3. The 

incremental sensitivities were calculated the same way as in the first test. The data was also plotted in 

Figure 4. The hot-end incremental sensitivities of the second test ranged from –81 to –89 C/V and the 

piston amplitude sensitivity varied between 0.32 and 0.35 mm/V. These slightly higher numbers may be 

due to changing some of the support hardware for the new heat source which affected the insulation 

characteristics. The HT FIREROD heat source has greater heat loss compared to the compact heat source, 

and its heat loss versus temperature characteristics are different. These differences caused changes in hotend temperature and piston amplitude as AC bus voltage was increased or decreased. These test results 

may differ from test results for the flight ASRG, since the flight ASRG has different thermal 

characteristics. It should also be mentioned that operation in vacuum will affect system response to 

AC bus voltage changes. 

Some sources of potential error during testing include a failing heater during the first test. This could 

cause nonuniform heating of the hot-end and could possibly affect the results. Also, ASC–E2 #1 has a 

known helium leak that could affect the results and may be a cause of variability while repeating the test. 

An interesting observation during testing was the relationship between the alternator terminal voltage 

and AC bus voltage. As shown in Figure 5, as the AC bus voltage increased, the alternator terminal 

voltage decreased slightly. While this result in not intuitive, the complex relationship between convertor 

parameters can produce this result. As AC bus voltage increased, piston amplitude increased, and hot-end 

temperature decreased. Decreasing hot-end temperature decreased the convertor’s mean pressure, which 

changed the convertor’s natural frequency. This led to an increase in the alternator power factor required 

to maintain the operating frequency. As a result, the alternator terminal voltage decreased slightly for this 

particular test. 

Argon Venting Simulation Test 

Phase 1 of the argon venting simulation test consisted of operating in constant heater power mode 

then changing gross heat input by –20/+15 Wth. This was necessary to gather baseline data for acceptor 

temperature sensitivity to gross heat input changes. The acceptor temperature is measured at the hot-end 

heat exchanger inside heater head of the convertor. It is 2.5 C lower than the hot-end temperature due to 

thermocouple locations. Data for each of the points of the test was collected and is plotted in Figure 6. 

0.10

0.15

0.20

0.25

0.30

0.35

0.40

740 760 780 800 820 840 860

Piston Amplitude Sensitivity,

mm/Vrms

Hot‐End Temperature,  ̊C

PistonAmplitude Sensitivity

Compact HS HT Firerod HS

‐120

‐110

‐100

‐90

‐80

‐70

‐60

740 760 780 800 820 840 860

Hot‐End Temperature Sensitivity,

 ̊C/Vrms

Hot‐End Temperature,  ̊C

Hot‐end Temperature Sensitivity Compact HS HT Firerod HS
NASA/TM—2012-217750 5 

Figure 5.—AC bus voltage and terminal voltage plot. 

 

Figure 6.—Argon venting phase 1 results. 

The 20 Wth decrease in gross heat input resulted in a 30 C decrease in acceptor temperature. The 

15 Wth increase resulted in a 20 C increase in acceptor temperature. Therefore, the sensitivity of acceptor 

temperature to gross heat input was an average of 1.4 C/Wth. 

Phase 2 consisted of simulating argon venting by increasing the convertor net heat input. This was 

accomplished by increasing the net heat input by 30 Wth (the increase in gross heat input was greater 

because of the thermal loss characteristics of the insulation package with the HT FIREROD heat source). 

Data for this phase of the test was collected and is plotted in Figure 7. The net effect of argon venting was 

an increase in the acceptor temperature by 53 C (from 756 to 809 C) due to a net heat input increase of 

29 Wth. 

Phase 3 consisted of reducing the rejector and alternator housing temperature to simulate temperature 

sink change as the generator ascends from storage in the fairing to space. As shown by the plot in 

Figure 8, the effect of reducing the rejector and alternator housing temperatures by 30 C was an increase 

of the acceptor temperature by 61 C (from 760 to 821 C). Therefore, the sensitivity of acceptor 

temperature to rejector/alternator housing temperature change is –2.0 C/C.

11.1

11.2

11.3

11.4

11.5

11.6

11.7

11.8

8.4 8.6 8.8 9.0 9.2 9.4

Terminal Voltage, Vrms

AC Bus Voltage, Vrms

720

730

740

750

760

770

780

320 325 330 335 340 345 350 355 360 365

Acceptor Temperature,  ̊C

Heater Power, Wth
NASA/TM—2012-217750 6 

Figure 7.—Argon venting phase 2 results. 

Figure 8.—Argon venting phase 3 results. 

Conclusion 

The Advanced Stirling Convertor (ASC)–E2 #1 sensitivity to AC bus voltage variation is similar to 

that of the ASC–E convertors in the Advanced Stirling Radioisotope Generator (ASRG) Engineering Unit 

(EU). This sensitivity is a useful parameter for system integration and control. 

The argon venting testing on ASC–E2 #5 helps us to understand how the convertor will react during 

the launch stage of a mission. Further testing of mission-specific conditions will help us to understand 

how the convertor will operate in space. 

Performance variation tests need to continue to be conducted on future generations of hardware as the 

technology matures to the flight configuration. These tests will help advance the ASRG toward flight by 

providing information to us to fully characterize and understand the convertors and how they will operate 

in a variety of conditions. 

750

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210 215 220 225 230 235 240 245 250

Acceptor Temperature,  ̊C

Net Heat Input, Wth

750

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830

50 60 70 80 90 100

Acceptor Temperature,  ̊C

Rejector Temperature,  ̊C
NASA/TM—2012-217750 7 

References 

1. Oriti, S.M., “Extended Operation of Stirling Convertors at NASA Glenn Research Center,” 

International Energy Conversion Engineering Conference, 2011-5726, Vol. 1, AIAA, San Diego, 

CA, 2011. 

2. Oriti, S., and Wilson, S., “Advanced Stirling Convertor (ASC–E2) Performance Testing at NASA 

Glenn Research Center,” Nuclear and Emerging Technologies for Space Conference, CP3496, ANS, 

AIAA, Albuquerque, NM, 2011. 

3. Wilson, S.D., Reid, T.V., Schifer, N.A., and Briggs, M., “Overview of Heat Addition and Efficiency 

Predictions for an Advanced Stirling Convertor,” International Energy Conversion Engineering 

Conference, 2011-5576, Vol. 1, AIAA, San Diego, CA, 2011. 

4. Lewandowski, E.J., and Schreiber, J.G., “Testing to Characterize the Advanced Stirling Radioisotope 

Generator Engineering Unit,” International Energy Conversion Engineering Conference, CP6693, 

Vol. 1, AIAA, Nashville, TN, 2010. 

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Testing has been conducted on Advanced Stirling Convertors (ASCs)-E2 at NASA Glenn Research Center in support of the Advanced 

Stirling Radioisotope Generator (ASRG) project. This testing has been conducted to understand sensitivities of convertor parameters due to 

environmental and operational changes during operation of the ASRG in missions to space. This paper summarizes test results and explains 

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