Edward J. Lewandowski

Glenn Research Center, Cleveland, Ohio

Dennis Hill

Lockheed Martin Space Systems Company, King of Prussia, Pennsylvania

Natural Convection Cooling of the Advanced

Stirling Radioisotope Generator Engineering Unit

NASA/TM—2011-217101

November 2011

NETS–2011–3609
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Edward J. Lewandowski

Glenn Research Center, Cleveland, Ohio

Dennis Hill

Lockheed Martin Space Systems Company, King of Prussia, Pennsylvania

Natural Convection Cooling of the Advanced

Stirling Radioisotope Generator Engineering Unit

NASA/TM—2011-217101

November 2011

NETS–2011–3609

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Nuclear and Emerging Technologies for Space (NETS-2011)

cosponsored by ANS Aerospace Nuclear Science and Technology Division,

the ANS Trinity Section, and the AIAA

Albuquerque, New Mexico, February 7–10, 2011
Acknowledgments

This work is funded through the National Aeronautics and Space Administration (NASA) Science Mission Directorate, the 

Radioisotope Power Systems Program Offi ce, and the U.S. Department of Energy. Any opinions, fi ndings, conclusions, or 

recommendations expressed in this article are those of the authors and do not necessarily refl ect the views of NASA. The authors 

wish to acknowledge the many people who supported ASRG EU testing in the Stirling Research Laboratory at the NASA Glenn 

Research Center and at Lockheed Martin, Valley Forge. The authors further wish to acknowledge Amy Delisa and Amar Setty 

for thermal analysis results.

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NASA/TM—2011-217101 1 

Natural Convection Cooling of the Advanced Stirling 

Radioisotope Generator Engineering Unit 

Edward J. Lewandowski 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Dennis Hill 

Lockheed Martin Space Systems Company 

King of Prussia, Pennsylvania 19406 

Abstract 

After fueling and prior to launch, the Advanced Stirling Radioisotope Generator (ASRG) will be 

stored for a period of time then moved to the launch pad for integration with the space probe and 

mounting on the launch vehicle. During this time, which could be as long as 3 years, the ASRG will 

operate continuously with heat rejected from the housing and fins. Typically, the generator will be cooled 

by forced convection using fans. During some of the ground operations, maintaining forced convection 

may add significant complexity, so allowing natural convection may simplify operations. A test was 

conducted on the ASRG Engineering Unit (EU) to quantify temperatures and operating parameters with 

natural convection only and determine if the EU could be safely operated in such an environment. The 

results show that with natural convection cooling the ASRG EU Stirling convertor pressure vessel 

temperatures and other parameters had significant margins while the EU was operated for several days in 

this configuration. Additionally, an update is provided on ASRG EU testing at NASA Glenn Research 

Center, where the ASRG EU has operated for over 16,000 hr and underwent extensive testing. 

Introduction 

The Advanced Stirling Radioisotope Generator (ASRG) is being developed for flight by Lockheed 

Martin under contract to the Department of Energy (DOE). In addition to designing the generator itself, 

all of the processes for fueling, handling, storing, spacecraft integration, and launch need to be planned in 

sufficient detail taking into account all generator operating parameters. The period from generator fueling 

to launch can be as long as 3 years and includes numerous activities performed while the generator 

continues to operate, generating electric power and rejecting heat to the environment. Typically, the 

generator will be cooled by forced convection using fans. During some of the ground operations, 

maintaining uninterrupted forced convection may add significant complexity, allowing natural convection 

to simplify some operations. Examples are unpacking and inspecting the generator after shipment, onsite 

transportation, spacecraft integration, and space vehicle integration before forced air can be supplied in 

the fairing. During natural convection cooling the heat transfer from the generator housing and fins to the 

ambient air is much less efficient, and as a result the generator housing temperatures rise to maintain the 

same heat flux. While the ASRG is designed to operate over a wide temperature range, temperatures of 

the Stirling convertor pressure vessel (PV) cannot exceed 115 °C set by the magnet material in the 

alternator (Ref. 1). As some operations must assume temporary exposure to the Florida summer heat, 

worst-case thermal analysis predictions using conservative assumptions for natural convection heat 

transfer predicted minimal temperature margin for operation in these conditions. Test results can be used 

to correlate models and analysis replacing conservative assumptions with hard data and providing added 

confidence to finalize procedures. 

 
NASA/TM—2011-217101 2 

A test was conducted on the ASRG Engineering Unit (EU) to quantify temperatures and operating 

parameters with natural convection only. The EU was designed and fabricated by Lockheed Martin. It 

underwent a series of system-level tests to qualification-level thermal and dynamic environments at 

Lockheed Martin and was delivered to the NASA Glenn Research Center (GRC) on August 28, 2008, for 

extended operation. The EU underwent inspection at GRC followed by integration into a test facility 

specially designed for it. The natural convection cooling test was conducted in this facility (Ref. 2). 

Nomenclature 

ASC Advanced Stirling Convertor 

ASRG Advanced Stirling Radioisotope Generator 

BOM Beginning of Mission 

CSAF cold-side adapter flange 

DOE Department of Energy 

EDU Engineering Development Unit 

EU Engineering Unit 

GPHS General Purpose Heat Source 

GRC Glenn Research Center 

HVAC heating, ventilation, and air conditioning 

PV pressure vessel 

Test Configuration 

The natural convection test was conducted using the ASRG EU under Engineering Development Unit 

(EDU) 1 Advanced Stirling Convertors (ASCs) Controller Unit (ACU) control. The EDU 1 ACU controls 

and synchronizes the two ASCs using power electronics. The ASRG EU was mounted vertically on a 

rigid test table in GRC’s Stirling Research Laboratory with the outboard end pointing up (Figs. 1 and 2). 

The outboard convertor, ASC A, is ASC–E #2 and the inboard convertor, ASC B, is ASC–E #3. The heat 

sources consist of cartridge heaters in a nickel block controlled to a fixed heat input. 

During most of the ASRG EU testing at GRC, heat rejection from the EU has been accomplished by 

circulating cooled air around the EU in a Lexan cage (Fig. 2). Auxiliary fans were incorporated to 

improve heat transfer through forced convection from the ASRG EU housing to the air. 

For the natural convection test, the air around the ASRG EU was made as quiescent as possible. The 

air duct above the EU was blocked off with foam. The ASRG EU is located in a partitioned area in a 

corner of the Stirling Research Lab. The partitioned area is 3.4 by 4.9 m with a 2.9-m drop ceiling above. 

The bottom 0.79 m of the partition is an open grille, and there is a 0.74 m gap between the top of the 

partition and the drop ceiling, leaving a 1.4-m Lexan barrier. The heating, ventilation, and air 

conditioning (HVAC) unit for the room was located outside the partitioned area, but created strong air 

currents that carried into the partitioned area. To prevent these currents from affecting the test, plastic 

sheeting was taped over the grille and between the top of the partition wall and the drop ceiling (Fig. 3). 

After installing the plastic sheeting there still was a 0.81 m gap to the windows (left side of Fig. 3), as the 

drop ceiling did not extend all the way back to the windows. But this was on the side away from the air 

currents and likely had minimal effect inside the partitioned area. 
NASA/TM—2011-217101 3 

Figure 3.—Partitioned test area with plastic sheeting to block air currents. 

 

Figure 1.—ASRG EU in test facility at NASA GRC’s 

Stirling Research Laboratory. 

Figure 2.—ASRG EU in the test facility; auxiliary fans 

and overhead ventilation duct. 
NASA/TM—2011-217101 4 

Rejected Heat Load 

This test was conducted with a rejected heat load that was slightly higher than is expected for the ASRG 

flight unit. Power into the heat sources was 252.9 and 266.0 W for ASC A and ASC B, respectively. Electric 

power from the alternators was 61.0 and 62.9 W. The difference between heat input to the ASRG EU and 

electric power output is the total amount of heat rejected through insulation loss, rejection at the convertor 

cold end, and other losses. Ultimately, the difference of 395 W is conveyed to the ASRG EU housing. 

The ASRG flight unit rejected heat load is estimated knowing the Beginning of Mission (BOM) 

General Purpose Heat Source (GPHS) module heat output is between 244 and 258 W (Ref. 1). The 

electric heat input to the convertors during ground operations will be less than BOM in vacuum due to 

increased insulation losses from the argon cover gas inside the ASRG. For example, with nominal fuel 

loading of 250 W, if the alternator electrical output is only 112 W, the flight heat load would be 250 W × 

2 – 112 W = 388 W. 

In addition, the ASRG EU has the controller mounted on one side, decreasing effectiveness of that 

side whereas the flight controller will be mounted remotely on the spacecraft. The controller also 

generates some heat from internal electrical losses. Housing temperatures on the controller side were 

generally a degree or more higher than temperatures on other sides of the ASRG EU. 

Test Results 

Although this test focused on natural convection cooling, data were taken under other conditions for 

comparison. Four test cases were studied: 

1. Forced convection cooling with ASRG EU inside cage, vertical orientation 

2. Forced convection cooling with ASRG EU in ambient air, vertical orientation 

3. Natural convection cooling with ASRG EU in ambient air, vertical orientation 

4. Natural convection cooling with ASRG EU in ambient air, horizontal orientation, not steady-state 

Data were recorded during the transitions from one test case to the next, providing a clear indication 

of relative changes and quantifying thermal response times. Table I chronicles the steps taken in 

transitioning from test case 1 (forced convection in cage) to test case 2 (forced convection in ambient air) 

to test case 3 (natural convection). Case 4 is a natural convection test briefly run at Lockheed Martin with 

the ASRG EU in the horizontal orientation. The test was performed at the end of an EMI test in an 

anechoic chamber. Due to the necessity to eliminate all outside sources of electrical noise, normal air 

ventilation in the 15- by 8-m and 8-m-high chamber is limited, resulting in a very quiescent environment. 

The generator was operated for about 65 min with cooling fans turned off, and temperatures had not 

reached steady state when the test was terminated. 

TABLE I.—TEST SEQUENCE OF EVENTS 

Date Time Hours 

(Fig. 4) 

Event 

21 Sept 10 17:30 0 Blocked HVAC duct above ASRG EU with foam. Test case 1.

22 Sept 10 10:24:08 0.0 Popped open top and duct door on cage around ASRG EU; fans off 

22 Sept 10 10:24:26 0.005 Turned on fans to maintain cooling 

22 Sept 10 10:45:38 0.358 Turn off fans. Install four thermocouples near ASRG EU housing 

22 Sept 10 10:52:30 0.473 Turn on fans; begin taking data with forced convection. Test case 2.

22 Sept 10 13:09:52 2.762 Turn off fans to begin natural convection cooling 

22 Sept 10 17:30 7.098 Put up plastic sheeting over door to partition 

23 Sept 10 10:00 to 

10:45 

23.598 to 

24.348 

Install plastic sheeting around top of partition 

23 Sept 10 13:00 to 

13:45 

26.598 to 

27.348 

Install plastic sheeting around base of partition. Test case 3.

23 Sept 10 19:22 32.964 Open plastic sheeting over door and around a portion of base of partition 

27 Sept 10 10:59 to 

11:30

 Remove all plastic from around partition; HVAC duct remains blocked 
NASA/TM—2011-217101 5 

TABLE II.—ASRG EU COOLING TEST CASES 

Parameter 

1. Forced 

convection cooling 

in cage 

2. Forced convection 

cooling in ambient air 

3. Natural convection 

cooling in ambient air 

4. Natural convection 

cooling in ambient air 

horizontal 

ASC A ASC B ASC A ASC B ASC A ASC B ASC A ASC B 

PV temperature (°C) 66.8 66.7 61.7 62.8 92.6 90.6 a86.0 a84.9 

Hot-end temperature (°C) 616.2 612.6 614.2 610.3 628.6 635.2 611.2 604.4 

Cold-end temperature (°C) 62.2 61.8 56.9 57.9 88.9 86.6 a84.6 a83.5 

CSAF temperature (°C) 59.4 58.9 54.0 55.0 86.2 83.9 a81.9 a80.8 

Average housing temperature (°C) 42.6 40.8 37.2 37.2 68.7 64.2 a64.2 a60.8 

Piston amplitude (mm) 4.30 4.36 4.31 4.36 4.27 4.29 b b 

Alternator power (W) 63.58 65.91 64.39 66.23 60.35 62.45 62.3 61.0 

Alternator voltage (Vrms) 9.98 9.74 9.85 9.61 10.79 10.55 10.81 10.08 

Alternator current (Arms) 7.93 8.42 8.02 8.49 7.67 8.11 7.95 8.02 

Ambient temperature (°C) 22.2 22.6 25.4 c19 a

Not steady-state values; steady-state values will be higher. 

b

Adjusted to maintain hot-end temperature below 630 °C. 

c

Estimated. 

Temperatures and other parameters from the four test cases are summarized in Table II. The PV 

temperatures represent the reading from a thermocouple mounted on the PV surrounding the alternator. 

The hot-end temperature is an average of three thermocouple measurements in the heat collector. The 

cold-end temperature is an estimate of the temperature of the cold-end heat exchanger inside the 

convertor based on surface temperatures of the cold-side adapter flange (CSAF), a part of the ASC that 

conducts rejected heat to the housing. The average housing temperature is an average of four or five 

thermocouples mounted around the ASRG EU housing adjacent to the point on the housing where the 

CSAF is attached. These will be among the highest temperatures on the surface of the generator. 

The data in Table II show the maximum PV temperature reached was 92.6 °C, occurring during 

natural convection cooling in ambient air. This temperature results in a magnet temperature that is 

sufficiently below the maximum allowable. In almost all cases the temperatures on the inboard (lower) 

half of the generator (with ASC B convertor) are slightly lower than the temperatures on the outboard 

(upper) half of the generator (with ASC A convertor). The reason in the case of the vertically oriented 

generator may be that the air temperature surrounding the generator rises with height above the table as it 

picks up heat from the generator. Also, the four steel standoffs that support the ASRG EU conduct some 

heat away from the inboard end of the generator. This latter explanation may also apply when the 

generator is mounted horizontally. 

Temperatures over time are shown in Figure 4. The time history shows that temperatures actually 

dropped by about 5 °C when the cage around the ASRG EU was removed. This is perhaps due to 

improved convection from changing airflow and reduced air temperatures, especially around the top part 

of the generator, which saw air temperatures several degrees above ambient temperature with the cage in 

place. The time history also illustrates how small changes in the ambient airflow influence generator 

temperatures. Note that temperatures consistently rose each time the airflow around the test area was 

successively impeded. The data in Figure 4 also provides an indication that the ASRG EU had reached 

steady-state condition for the various test cases. 

The ASC A PV temperature was used to estimate the time response of the system when transitioning 

from forced to natural convection cooling. When fans were turned off at 2.762 hr, the ASC A PV 

temperature was 61.8 °C. Temperatures reached a quasi-steady-state condition by 7.098 hr, prior to 

plastic sheeting being put over the door to the partition, when the ASC A PV temperature was 87.5 °C. 

From the start of natural convection it took approximately 30 min to reach 63.2 percent of the steady-state 

temperature and 77 min to reach 90 percent of the steady-state temperature. 

 
NASA/TM—2011-217101 6 

Figure 4.—Temperatures during test under forced and natural convection cooling. Popped top, fans off momentarily Turned fans on Turned fans off Plastic over partition door Install plastic around top of partition Install plastic around base of partition Partially open plastic cover around partition

20

30

40

50

60

70

80

90

100

‐5 0 5 10 15 20 25 30 35 40

Time (hrs)

Time (hrs)

ASC A PV temp

ASC B PV temp

Cold end A

Cold end B

CSAF housing A

CSAF housing B

Ambient temp

Turned fans off

Figure 5.—Temperatures during transition from forced to natural convection cooling, vertical versus 

horizontal orientation.

0

10

20

30

40

50

60

70

80

90

100

‐0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

Temperature (°C)

Time (hrs)

ASC A PV, generator vertical

ASC B PV, generator vertical

CSAF housing A, generator vertical

CSAF housing B, generator vertical

ASC A PV, generator horizontal

ASC A PV extrapolated, generator horizontal

ASC B PV, generator horizontal

ASC B PVextrapolated, generator horizontal

CSAF housing A, generator horizontal

CSAF housing A extrapolated, generator horizontal

CSAF housing B, generator horizontal

CSAF housing B extrapolated, generator horizontal
NASA/TM—2011-217101 7 

It is expected that cooling would be less efficient with the generator horizontal since the air currents 

would not develop velocities as high as in the vertical orientation, and since a large face of the generator 

is pointed downward, limiting airflow. Figure 5 shows the temperature rise over time with natural 

convection cooling of a horizontally oriented generator and compares temperature rise over time with 

vertical versus horizontal orientation. Since the horizontal test was terminated before reaching steadystate, temperatures were extrapolated by the methodology described in Reference 3. Although the test 

conditions are not identical between the test conducted at GRC and the test conducted at Lockheed 

Martin, it can be reasonably inferred from the figure that the steady-state temperatures for a horizontally 

oriented generator would be a few degrees higher than a vertically oriented generator. 

Airflow Measurements 

To quantify to some extent the amount of forced convection and natural convection, air velocity 

measurements were made using a Model 8385A VelociCalc Plus air velocity meter. The measurements 

were made by holding the end of the probe against the ASRG EU (Fig. 6). This put the probe sensor 

about 10 mm from the ASRG EU. A small piece of foam insulation was taped to the probe tip (Fig. 6) so 

that the cool metal probe did not directly contact the warm ASRG EU housing, potentially causing 

thermal changes that would affect the air velocity reading. Air velocity measurements under forced 

convection are shown in Figure 7. The green arrow next to each air velocity reading indicates the 

orientation of the probe when the measurement was taken. In Figure 7, the yellow “8” with blue arrows 

indicate location and orientation of the auxiliary fans. Measurements were taken in line with the fans that 

are located at elevations corresponding to the locations of the convertor’s CSAFs, which conduct most of 

the rejected heat to the ASRG EU housing. Air velocities near the fans were between 2.2 and 6.6 m/s and 

decreased as the probe moved away from the fans. 

Figure 7.—Airflow measurements (m/s) under forced convection. 

Front side

0.5 0.4

6.6 2.6 1.7

0.3

6.2

2.5 1.3

2.2

Left side

0.6 2.2

0.1

0.3 2.2

0.2

2.1

2.2

Back side

0.7 2.0 1.0

0.1

Right side

0.9 2.1

0.9 1.2

1.0 2.1

0.8

4.6

1.5

1.1

4.9

 

Figure 6.—Air velocity probe held against ASRG EU housing (left); insulation on air velocity probe tip (right). 
NASA/TM—2011-217101 8 

Under natural convection the air velocity measurements 

were read on the right side only. The other three sides had 

obstructions, which interfered to varying degrees with airflow, 

so the readings on the right side were deemed the most 

representative. The readings in Figure 8 show that air velocities 

were lowest at the bottom of the ASRG EU and reached 0.21 to 

0.30 m/s at the top of the ASRG EU. 

Thermal Imaging 

An infrared (IR) camera, Fluke Ti45, was used to record 

thermal images of the ASRG EU to characterize temperature 

distribution on the housing and fins in both forced and natural 

convection conditions. IR images of the ASRG EU are 

compared in Figures 9 and 10. Under forced convection, 

housing temperatures are hottest near where heat is rejected 

internally at the CSAFs, then fall off quickly as one moves 

away from the area. In contrast, for the natural convection case, 

the temperature of the center portion of the generator is closer 

to that of the peak temperature and the peak temperature itself 

is 30 °C higher than the forced convection case. This illustrates 

how forced air cooling greatly enhances convection heat 

transfer. 

 (a) (b) 

Figure 9.—ASRG EU right side thermal images: (a) forced convection, (b) natural convection. 

Figure 8.—Airflow measurements (m/s) 

under natural convection. 

Right side

0.18

0.18

0.01 0.08

0.23

0.22

0.21

0.20

0.02

0.15

0.30 0.23
NASA/TM—2011-217101 9 

 (a) (b) 

Figure 10.—ASRG EU front side thermal images: (a) forced convection, (b) natural convection. 

Figure 11.—ASRG EU natural convection cooling, IR image and analysis prediction. 

Comparison of Thermal Model and Test Results 

Figure 11 displays the IR camera image aside the temperature prediction determined by analysis prior 

to the test. The two images are very similar in that the hottest parts of the generator is shown where waste 

heat is rejected by the ASCs and temperature in the middle section of the generator is elevated. The peak 

temperature predicted by analysis is 7 °C higher than the measured temperature indicating that heat 

transfer via natural convection is more efficient than assumed in the analysis. The plan going forward is 

for the analytical model to be tuned to match the test data. Model tuning will yield more accurate heat 

transfer coefficients and allow more temperature margin to be demonstrated for operation scenarios where 

heat rejection via natural convection could be advantageous. 
NASA/TM—2011-217101 10 

ASRG EU Update 

The natural convection cooling test is just one of many tests that have been conducted on the ASRG 

EU at GRC since the generator began extended operation in 2008. A series of tests were conducted in 

2009 and 2010 to characterize generator performance and operation in response to varying parameters and 

system inputs such as piston amplitude, heat input, cold-end temperature, PV temperature and DC bus 

voltage. These tests were conducted with an AC bus controller and with two different engineering-level 

controllers developed by Lockheed Martin. Some of these test results are documented in Reference 4. 

These tests have generated data for Lockheed Martin to use to develop system-level control and 

management strategies and to update the controller design in preparation for future Qualification and 

Flight generators. 

The EU continues extended operation when it is not undergoing special tests. The purpose of the 

extended operation test is to demonstrate extended operation of an integrated system, to monitor for 

trends in generator performance, and to provide additional data from long-term operation of Stirling 

convertors. Representative convertor and generator power data from steady-state extended operation is 

shown in Figure 12. Convertor power represents the AC power measured from the convertors and input 

into the controller. Generator power is the DC power output from the controller into the DC bus. Data 

from time periods during which the generator was operated in off-nominal conditions for test purposes, 

such as the current effort, has been excluded from the plot for clarity. The ASRG EU was initially 

operated under AC bus control (Ref. 4) while the EDU 1 ACU underwent additional testing by Lockheed 

Martin. The ASRG EU began operation under EDU 1 ACU control on September 8, 2009, at 5234 hr. It 

has continued operation under ACU control since then, except for one brief period of operation under AC 

bus control for special tests. Additional details on ASRG EU testing can be found in Reference 5. 

Comparing initial and current power levels for the convertors and the generator, there has been no 

significant change in power within the accuracy of the instrumentation. The small change in convertor 

power levels at 13,512 hr is due to a change in the test rack to reduce wiring impedance. Performance will 

continue to be monitored and tracked. 

Figure 12.—ASRG EU and convertor power. 

 

0

10

20

30

40

50

60

70

80

90

100

6000 8000 10000 12000 14000 16000

Power (W)

Time (hrs)

Generator Power

ASC A - ASC-E #2

ASC B - ASC-E #3
NASA/TM—2011-217101 11 

The ASRG EU’s output power is less than the flight unit’s output for a number of reasons. The EU 

was not designed to demonstrate full performance but to provide the opportunity to test a generator as an 

integrated system. EU incorporates the ASC–E convertors with a maximum hot-end temperature of 

650 °C, not the more efficient ASC–E2 and later convertors with a maximum hot-end temperature of 

850 °C. The generator housing is filled with argon, as the flight generator will be for ground operations. 

The argon reduces the efficiency of the insulation, resulting in less heat into the convertors and ultimately 

less power output. The ACU circuitry has not been optimized for performance as later controller designs 

have. Finally the power path uses a smaller gauge of wire than ideal, resulting in several watts power loss 

just in the lines. 

Conclusion 

The ASRG EU was operated with natural convection cooling in quiescent ambient air to characterize 

temperatures throughout the generator in this environment. With a rejected heat load near the upper limit 

of that expected for the flight unit and with a non-optimal cooling from a controller mounted to the side of 

the generator, the ASRG EU was able to maintain acceptable operating temperatures throughout the 

generator. 

The ASRG EU continues extended operation at NASA Glenn Research Center and provides a 

platform for system-level tests in support of the ASRG flight project. 

References 

1. “ASRG User ICD,” Lockheed Martin, May 2010. NASA SMD Science Office for Mission 

Assessments Discovery Program Acquisitions Discovery 2010 AO Program Library. November 23, 

2010. http://discovery.larc.nasa.gov/dpl.html 

2. Lewandowski, E.J., Schreiber, J.G., Oriti, S.M., Meer, D.W., Brace, M.H., and Dugala, G., “Design 

of a Facility to Test the Advanced Stirling Radioisotope Generator Engineering Unit,” Proceedings of 

the Seventh International Energy Conversion Engineering Conference (IECEC 2009) American 

Institute for Aeronautics and Astronautics, 2009. 

3. Rickman, S.L. and Ungar, E.K., “A Physics-Based Temperature Stabilization Criterion for Thermal 

Testing,” 25th Aerospace Testing Seminar, Institute of Environmental Sciences and Technology, 

2009. 

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

Generator Engineering Unit,” Proceedings of the Eighth International Energy Conversion 

Engineering Conference (IECEC 2010), American Institute for Aeronautics and Astronautics, 2010. 

5. Cornell, P.A., Lewandowski, E.J., and Oriti, S.M., “Stirling Convertor Extended Operation Testing 

and Data Analysis at NASA Glenn Research Center,” Proceedings of the Seventh International 

Energy Conversion Engineering Conference (IECEC 2009), American Institute for Aeronautics and 

Astronautics, 2009. 
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Technical Memorandum 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

Natural Convection Cooling of the Advanced Stirling Radioisotope Generator Engineering 

Unit 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Lewandowski, Edward, J.; Hill, Dennis 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 138494.05.02.04.02 

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-17609 

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-2011-217101 

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

After fueling and prior to launch, the Advanced Stirling Radioisotope Generator (ASRG) will be stored for a period of time then moved to 

the launch pad for integration with the space probe and mounting on the launch vehicle. During this time, which could be as long as 3 years, 

the ASRG will operate continuously with heat rejected from the housing and fins. Typically, the generator will be cooled by forced 

convection using fans. During some of the ground operations, maintaining forced convection may add significant complexity, so allowing 

natural convection may simplify operations. A test was conducted on the ASRG Engineering Unit (EU) to quantify temperatures and

operating parameters with natural convection only and determine if the EU could be safely operated in such an environment. The results 

show that with natural convection cooling the ASRG EU Stirling convertor pressure vessel temperatures and other parameters had 

significant margins while the EU was operated for several days in this configuration. Additionally, an update is provided on ASRG EU 

testing at NASA Glenn Research Center, where the ASRG EU has operated for over 16,000 hr and underwent extensive testing.

15. SUBJECT TERMS

Stirling convertor; Stirling radioisotope generator; Cooling; Heat transfer 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF

 ABSTRACT

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18. NUMBER

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17 

19a. NAME OF RESPONSIBLE PERSON

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

a. REPORT

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b. ABSTRACT

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c. THIS 

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19b. TELEPHONE NUMBER (include area code)

443-757-5802 

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18