Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Performance Measurement of 

Advanced Stirling Convertors (ASC–E3)

NASA/TM—2013-216564

December 2013

	




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Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

Performance Measurement of 

Advanced Stirling Convertors (ASC–E3)

NASA/TM—2013-216564

December 2013

	




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NASA/TM—2013-216564 1 

Performance Measurement of 

Ad	


	E3) 

Salvatore M. Oriti 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

NASA Glenn Research Center 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 data for the Advanced Stirling Convertor (ASC). The latest version of the 

ASC (>E3, to represent the third cycle of engineering model test hardware) is of a design identical to 

the forthcoming flight convertors. For this generation of hardware, a joint Sunpower, Inc., and Glenn 

effort was initiated to improve and standardize the test support hardware. After this effort was completed, 

t"'>E3 units was produced by Sunpower and then delivered to Glenn in December 

2012. Glenn has begun operation of these units, including performance verification, which examined the 

data from various tests to validate the convertor performance to the product specification. Other tests 

included detailed performance mapping that encompassed the wide range of operating conditions that will 

exist during a mission. These convertors will be transferred to Lockheed Martin for controller checkout 

testing. The results of this latest convertor performance verification activity are summarized in this paper. 

Nomenclature 

AC alternating current 

ASC Advanced Stirling Convertor 

ASRG Advanced Stirling Radioisotope Generator 

BOM beginning of mission 

DOE Department of Energy 

EOM end of mission 

EU engineering unit 

GPHS General Purpose Heat Source 

IV&V independent verification and validation 

LMSS Lockheed Martin Space Systems 

LR low rejection 

HR high rejection 

SRL Stirling Research Laboratory 

Introduction 

The Thermal Energy Conversion Branch at NASA Glenn Research Center has been supporting the 

development of the Advanced Stirling Radioisotope Generator (ASRG). The ASRG (Figure 1) was 

identified for 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 Sunpower’s Advanced Stirling 

Convertor (ASC). For this purpose, NASA Glenn 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. 

As of May 2013, a total of 269 000 hr have been accumulated on 27 convertors. This hardware set spans 

the history of Sunpower’s ASC technology development, and includes developmental units (>0s), 
NASA/TM—2013-216564 2 

Figure 1.—Advanced Stirling Radioisotope Generator (ASRG) flight unit. (Image courtesy of Lockheed Martin 

Space Systems Corp.) 

engineering units (>Es and >E2s), and t">E3s. Other facets of the Glenn test 

include independent measurement of the convertor’s performance to its specification, and durability tests 

>E2s to demonstrate margin in the convertor design. 

The latest set of hardware, >E3, are being produced to the flight unit drawings and quality 

practices, and as such will exercise the fabrication process planned for the ASRG flight unit convertors. 

With this, ">E3s are the most flightlike units that have ever been put on test. However, they will 

not be installed in a qualification or flight generator. The E3 design represents the culmination of the ASC 

development effort. It includes design features such as hermetic sealing, a lower current alternator, and 

the capability for a hot-end operating temperature up to 850 °C. ">E3s are planned to 

be built. 
"'!>E3 #1 and #2 were delivered to Glenn in November 2012. The remaining 

units are at various stages of the fabrication process. The Glenn test sequence for E3 operation consists of 

four stages: (1) Receipt/Inspection/Installation, (2) Insulation Loss Characterization, (3) Performance 

Mapping, and (4) Extended Operation. The receipt stage consists of a procedure to inspect the shipment 

and set up the convertors on the test stand. The insulation loss stage consists of a test to calibrate the 

thermal losses through the insulation package, and enable calculation of net heat input. The independent 

verification of convertor performance occurs during the performance mapping stage. During this test, the 

convertors are operated at the points contained in the ASC product specification. Prior to delivery, 

Sunpower also measured performance at these points. E3 #1 and #2 have completed the first three 

stages of this sequence. These units were earmarked for controller checkout testing at Lockheed Martin. 

Following the controller testing, the convertors will be returned to Glenn and then begin extended 

operation. Extended operation will consist of continuous operation to achieve the aforementioned runtime 

in support of reliability analyses. Accumul'">E3 units is most important, as 

these are most representative of the ASCs that will be used in the flight generator. 
NASA/TM—2013-216564 3 

Test Methodology 

The goal of the Glenn testing effort is to provide convertor performance data, both for product 

verification and demonstration of dynamic conversion for space applications. The Stirling Research 

Laboratory (SRL) at Glenn serves this purpose. The SRL came into existence in 2000 and has been 

evolving and adapting to the project needs ever since. The lab currently consists of 15 test stations each 

capable of operating a pair of convertors. A typical test station is shown in Figure 2. One of these test 

stations includes a thermal vacuum facility for space-like environmental testing. Each test station includes 

data system software capable of monitoring all convertor operating parameters. The software includes 

autonomous functionality, and can safely shut down operation should an alarm condition arise. Hardware 

safeties are also in place, should the software malfunction. 

An overall schematic for convertor operation is also shown in Figure 2. The purpose of the test support 

hardware is to achieve the wide range of operating conditions that a convertor might experience during a 

mission. An electric heat source is used to simulate the thermal energy input of radioisotope fuel. An 

electric heat source was chosen because the gross thermal energy input can be measured easily. The heat 

source is pressed against the convertor with a spring and the preload is adjusted to match the flight unit 

design. The cold end of the convertor is controlled with a fluid loop and a heat exchanger attached 

directly to the rejection flange of the convertor. In the flight generator, the convertor rejects heat to the 

housing of the generator which acts as a radiator. Thus, in flight the cold-end temperature of the convertor 

is a function of the environment of the mission. With a fluid loop, the convertor cold-end temperature can 

be manually controlled. The power output from the convertor’s alternator is taken from two feedthroughs. 

The details of the convertor instrumentation can be found in Figure 3. This is the scheme used to 

measure convertor performance. The heat input to the convertor is controlled by a feedback loop. Either 

hot-end temperature or gross heat input can be used as the feedback variable. Hot-end temperature control 

allows the user to achieve a desired temperature quickly. However, the radioisotope fuel is a constant 

thermal power source that degrades according to its half-life. When performance points are recorded, the 

Figure 2.—Typical Stirling Research Laboratory (SRL) test station for a pair of Advanced Stirling Convertor 

(ASCE3s). 

Circulators

Cold-end temp control

2 Convertors

Side-by-side

Vertical orientation

Test Rack

Data acquisition

Monitoring

Operating point control

Electric Heat 

Source

Cold End

Hot End

Alternator

Fluid Flow

Electrical Output

Thermal Input
NASA/TM—2013-216564 4 

hot-end control mode is transitioned to constant heater power to make the operating point as flightlike as 

possible. The power into the electric heat source is measured by a power meter, which also measures 

heater voltage and current. The voltage measurement point is taken as close to the convertor as possible. 

This is done so that line losses between the power supply and heater are not counted as thermal input into 

the system. Key temperature measurements are made at four points: heat source, hot end, cold end, and 

alternator. The heat source and hot-end temperature measurements are made using thermocouple probes. 

The cold-end and alternator housing temperature measurements are made with thermocouple probes and 

thermistors. The net heat input is a calculation based on analysis. It is a function of the gross thermal 

input and the operating temperatures. The insulation package thermal losses are calibrated during the 

aforementioned insulation loss characterization test. The process for correlation insulation losses to the 

test article temperatures is documented in previous publications by Wilson (Ref. 3). Stable operation of 

the convertor is maintained by connection of the alternator to an alternating current (AC) bus. The AC bus 

consists of a tuning capacitor, a load resistor, and an AC power supply in parallel with the resistor. This 

method permits simultaneous control of the convertor’s piston amplitude and frequency of operation. The 

power output of the convertor is measured using a power meter, which also reports voltage, current, and 

power factor. The voltage measurement point is made as close to the convertor feedthroughs as possible 

so as to not count the lead wire losses. The alternator current measurement is made with a Pearson coil. 

This is most useful during regular power meter calibration, since the power meter may be removed and 

replaced without disturbing the alternator power path. The piston motion is monitored by a position 

Figure 3.—Advanced Stirling Convertor (ASCE3) instrumentation scheme. 

7 kHz

7 kHz

Alternator 

Power Meter

Heat Distributor

Heater Power 

Meter

V+

VI

AC Bus V+Power Meter

VI Piston 

Position 

Sensor 

Processor

PID Temp Controller

DC Power Supply

Pearson Coils

Net Heat Input

(calc)

AC Power Supply

(variable freq.)

High-rate voltage 

and current 

waveform archival

Capacitor

Load

High-rate piston 

position 

waveform 

archival

VV+

Alternator Housing Temp

Surface TCs

Thermistors

Cold End Temp

TC probes

Thermistors

Hot End Temp

TC probes

Heat Source Temp

TC probes

Alternator Feedthroughs
NASA/TM—2013-216564 5 

sensor capable of resolving the 100 Hz motion. The analog waveform from this sensor is used to calculate 

piston amplitude and operating frequency. The data system is also capable of high frequency (7-kHz) 

storage of the dynamic data. The alternator voltage, alternator current, accelerometer, and piston position 

sensor waveforms can be stored continuously for analysis later. 

Improvements for E3 Test Support Hardware 

Previous test support hardware designs were found to be lacking in certain areas. The measurement of 

the convertor performance is largely dependent on the support equipment. For example, a different 

insulation package would alter the thermal losses from the hot end and require a new analysis to calculate 

net heat input. Net heat input is the most critical parameter for validating convertor performance, as it is part 

of the net conversion efficiency metric. In the past, such as during ASCE2 testing, the convertor 

performance was measured at Sunpower and later Glenn, but with different support hardware. Discrepancies 

as large as 7.7 percent existed that were attributed to the differences in the support hardware. To address 

this, an effort was initiated in November 2011 to develop a common design for the test support hardware. 

With this, the convertor would undergo final performance verification at Sunpower, then be delivered to 

Glenn without removing this support hardware. Glenn could then execute their independent performance 

measurement using the same hardware, with reduced chance of discrepancy. 

This opportunity was also used to improve several deficiencies in both the Sunpower and Glenn E2 

support hardware. These deficiencies are illustrated in Figure 4. Most notable was the relatively high 

amount of insulation losses. This was due to the type of cartridges used in the E2 electric heat source. 

These cartridges required a cooler temperature at their lead wire end, so they needed to protrude through 

the insulation. This introduced high conduction losses that needed to be accounted for via insulation loss 

characterization and the subsequent analysis. The E2 support hardware was designed to aesthetically 

resemble the ASRG flight design. As such it was square and of similar size. This made the geometry 

nonaxisymmetric, which was undesirable for the thermal modeling. It also introduced air gaps between 

insulation segments, which were also difficult to model. All these factors introduced uncertainty in the net 

heat input calculation, thus possibly corrupting the measurement of convertor net efficiency. These 

undesirable traits existed in similar form in Sunpower’s support hardware design. 

Figure 4.—Advanced Stirling Convertor (ASCE2) 

Glenn support hardware deficiencies. 

Heaters protruded 

through insulation

Air gaps (radiation 

heat transfer)

Square housing 

(non-axisymmetric)

Limited life heat 

source (~2500 hrs)
NASA/TM—2013-216564 6 

The E3 support hardware design effort moved forward with the goal of resolving these deficiencies, 

and achieving a common design that would accommodate both Sunpower’s and Glenn’s test sequences. 

Figure 5 ""Y>]
"Y

for axisymmetric finite element modeling. The insulation segments were designed to eliminate line-ofsight gaps. A layer of compressible insulation was used as a “gap filler” where spaces were not filled by 

the solid insulation. The heat source was revised to minimize parasitic conduction losses, as well as 

improve life. The conduction losses were reduced by changing the cartridge type to a unit that can reside 

entirely inside the insulation package. A method for connecting the cartridges in parallel was developed, 

so that only two main leads exited through the insulation. This alone reduced the insulation losses by 

about 50 percent when compared to the E2-style heat source. The life of the heat source was improved 

relative to the E2 version. The cartridges were designed with more contact area to reduce heat flux. They 

were also procured with a centerless-ground outer diameter option. The pockets in the nickel block of the 

heat source were accurately bored to match the cartridge diameter, giving a better fit than those of the E2 

heat source. These design features were implemented to reduce the heat flux through the cartridge and 

maximize the thermal conductance from the cartridge. These parameters are most closely coupled to the 

heater life as they work to minimize the element temperature. 

Additional instrumentation was also integrated into the E3 support hardware design that did not exist 

in the E2 hardware. The thermal modeling effort necessary for calculating net heat input relies on a 

multitude of temperature measurements throughout the insulation package. During the E2 test effort, 

these thermocouples were added after test hardware was assembled, and the placement was inaccurate 

and sometimes undesirable. Provisions for these thermocouples were included from the beginning of the 

E3 support hardware design effort. An example of these auxiliary thermocouples is shown in Figure 6. 

The number and placement of the thermocouples was decided based on the previous modeling effort 

undertaken during E2 testing. These measurements provide the critical boundary conditions that are used 

 

Figure 5.—ASCE3 support hardware improvements relative to E2 design. 

Circular form factor

Heater lead wires

(small conduction path)

Compressible insulation
NASA/TM—2013-216564 7 

Figure 6.—Auxiliary thermocouple placement in Advanced Stirling Convertor (ASCE3) insulation package. 

in the finite element model for calculating net heat input. The thermocouples were embedded at the 

desired locations in the insulation prior to assembly. They were routed along the isotherm for a large 

portion of their length. This is a commonly used method to retrieve the most accurate temperature 

reading, as the conductance of the probe does not corrupt the temperature measurement if it resides along 

an isotherm. The insulation package was designed in such a way that the assembly sequence would permit 

filling of all the voids with the “gap-filling” insulation. 

Data Archival and Processing 

The data system of the E3 test stations comprises all the knowledge learned over the past decade of 

SRL operation. A schematic of the data collection and processing method is shown in Figure 7. Each test 

station is connected via Ethernet to a central data server. The previously described performance 

measurements are made by the instruments in the test rack. The data system software handles all the 

communication to the various instruments, and devices in the test rack. The software also performs 

calculations based on the measured parameters. Examples of calculated parameters include neat heat 

input, heater resistance, or piston amplitude. The data system also has functionality for an “electronic 

event log.” The user may enter a comment as operation is progressing. The data system automatically 

timestamps the entry and stores it in a local database on the test rack. 

There are three rates for data storage: 2-sec 5-min averages and dynamic data (7 kHz). The 2-sec data 

consists of measurements recorded once every 2-sec window. A rolling cache of 2-sec data is stored on 

the test rack computer in a database. The data system software also creates a 5-min average of the 2-sec 

data at a rate specified by the user. By default, a 5-min-average data point is recorded once per hour. The 

5-min-average points are stored locally on the test rack computer in a file. Each new point is appended to 

the file. Each day, an automated program on the data server copies the previous 24-hr of 2-sec data, and 

the 5-min-average file to a permanent storage location on the server. The user-entered event log is also 

copied and stored on the server. The 2-sec data is used to examine performance on the order of a few 

days, or for closer examination of transients. The 5-min-average data is most useful for examining a 

lengthy span of time, such as thousands of hours to confirm long-term steady performance. The dynamic 

signals, such as alternator voltage and piston motion, are also recorded by the data system software. The 

1

2

3

4

5

6

7

9 Example auxiliary thermocouple installed in 

microporous insulation piece

Placed along an isotherm to reduce local 

temperature disturbance

8

10

11

12
NASA/TM—2013-216564 8 

Figure 7.—Stirling Research Laboratory (SRL) data collection and processing overview. 

software is capable of sampling and stored the data at a rate of 7 kHz. This results in approximately 

1.5 GB of data per hour for the dynamic signals alone. Given this large data rate, a separate storage device 

was dedicated to the dynamic data. Each pair of convertors has an allocated 20-TB hard disk array that is 

connected locally to the test station computer. The data system stores the dynamic data directly to this 

large array rather than to its local hard drive. The dynamic data does not get transferred to the main data 

server, as this would quickly consume all the storage space on the server. The 7 kHz data is desired when 

the 2-sec data is too slow. A sample rate of 7 kHz provides 70 data points per cycle of convertor 

operation, which is far greater than the minimum required by the Nyquist theorem. These data could be 

used to examine the real-time behavior of the convertor if desired at some future time. 

Once data have been stored on the server, an analyst may use the customizable Matlab-based plotting 

scripts to examine the performance data. Both the 2-sec- and 5-min-average data can be used to generate 

time domain plots. The plotting scripts also load the electronic event log and automatically populate the 

plots with these pieces of information at the correct time axis locations. This is helpful when an analyst is 

examining the data for steady behavior. If a manual adjustment at the test station is made, it is important 

that it be made obvious on the plot. Otherwise, one might misinterpret the change in performance as 

unsteady convertor behavior. The process of maintaining an electronic event log and automatically 

populating these events on the plots streamlines the process, and allows a user to quickly identify 

performance changes due to user activity. 

Performance Measurement Test Results 

After delivery of ASCE3 #1 and #2 to Glenn in November 2012, the convertors were installed on 

the test station. The convertors were shipped to Glenn in the common test support hardware that was 

described earlier. As such, there was little inspection performed on the E3s, since the support hardware 

was not removed. This is in contrast to the E2 units, which were sent as bare convertors with no attached 

support hardware. 

Insulation loss characterization was initiated in January 2013. Data from this test were used later to 

develop an equation for insulation thermal losses as a function of the various temperatures on the 

convertor. This test showed that the E3 hardware design effort to reduce insulation losses was successful. 

For example, at the nominal operating point of 760 °C on the hot end, the insulation losses were 78 W. 

This is similar to the losses that will exist in the flight unit when filled with argon. A vacuum around and 

inside the insulation package would reduce the losses further. Previous E2 test articles had an insulation 

loss of approximately 130 W at this temperature, mostly because of the cartridge protrusion through the 

insulation. 

Test Rack

Data Server

6TB array

RAID5

2-sec data

5-min avgs

Event log

20 TB array

RAID5

Daily Transfer

Ethernet

7 kHz data

1.5 GB/hr

Matlab plotting script

Daily

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NASA/TM—2013-216564 9 

Once thermal insulation loss characterization was completed, the convertors began the performance 

mapping test. The ASC product specification contains seven operating points that the performance must 

achieve. The project expressed need for a more comprehensive performance map around these seven 

points. This “extended” performance map was initiated in February 2013. The test matrix consisted of 36 

points with a hot-end temperature ranging between 640 to 800 °C, and a cold-end temperature ranging 

from 28 to 75 °C. The performance measurements for the extended performance map agreed well 

between the Sunpower and Glenn data. In almost every case, the difference in power output was less than 

2 percent. This suggests that the effort to standardize the test support hardware was successful. Since the 

insulation was not disturbed, the measurement of convertor performance at the same operating conditions 

remained consistent between the Sunpower and Glenn cases. 

Operation then continued with the product specification mapping in April 2013. This consisted of 

seven points representing the boundaries of a flight unit life: beginning of mission (BOM), end of mission 

(EOM), low rejection (LR), high rejection (HR), maximum qualification, minimum qualification, and 

maximum temperature. BOM represents the onset of the mission where the thermal power output of the 

radioisotope fuel is greatest, while EOM represents some number of years later in the mission when there 

is less energy from the fuel. LR represents a deep space sink temperature, while HR represents a sink 

temperature such as low Earth orbit. The measured power output was normalized to the specification net 

heat input for each point. The independent performance measurement at Glenn showed both convertors 

met the performance specification for all seven points. 

ASCE3 #1 and #2 have each accumulated over 2300 hr of runtime during this process. The 

automated data archival and plotting systems enable examination of every 2-sec data point throughout this 

history (4.1 million data points). An example 24-hr period of convertor performance (power output at the 

feedthroughs) is shown in Figure 7. Notice first the flat-line nature of the power output. Over this 24-hr 

period the convertor performance was steady and unwavering. The width of the plot is 0.3 W, which is 

less than 1 percent of the nominal convertor output. This is deemed normal, steady convertor behavior. 

The uncertainty in the power meter is 0.47 W, but this is not indicative of the precision of the 

measurement. A quantitative assessment of the power meter precision can be obtained via examination of 

a test rack checkout that is performed prior to convertor delivery. During this checkout, an AC power 

supply is connected in place of the convertor. This exercises the connections and measurements in the test 

rack. In this case, the thickness of the alternator power plot is on the order of 0.1 W. This represents an 

experimentally determined value for the precision of the power meter, since the power flow through a 

fixed resistance will be steady. Any deviation from flat-line convertor power output with a magnitude 

1 W or greater would raise a flag for further analysis. The entire 2300 hr of 2-sec data was periodically 

reviewed for power fluctuations. Every data plot was as steady as that shown in Figure 8. A similar 

analysis may be performed by examining the 5-min-average data points. The 5-min-average plots also 

show steady behavior between each operating point adjustment. 

These convertors exhibited a 5-W difference in power output over all test points, with ASCE3 #2 

having a higher power output for the same operating conditions. This was consistent with the Sunpower 

performance measurements taken prior to delivery. This magnitude performance difference is a result of 

the variability in the convertor fabrication. A piston seal clearance could explain this variance. ASCE3 

#2 is known to have a smaller clearance. Both convertors were built to the E3 design documents, and both 

met the product specification. 
NASA/TM—2013-216564 10 

Figure 8.—Example steady 2-sec data over a 24-hr period. 

Conclusion 

NASA Glenn Research Center is supporting the advancement of the Advanced Stirling Radioisotope 

Generator (ASRG) towards flight via convertor-level performance testing. Tests include performance 

measurement and extended operation. The latest generation of Advanced Stirling Convertor (ASC) 

hardware, model E3, are being built to exercise the flight hardware build process. The first pair of 

ASCE3s was delivered to Glenn in November 2012. A new design for the convertor test support 

hardware was produced via a collaborative effort between Glenn and Sunpower, Inc. This new hardware 

addressed concerns that arose during previous experimentation with ASCE2 units. Most notably, the 

decision was made to ship the E3s in the support hardware, so that the same setup was used for 

performance measurement at Sunpower and Glenn. The new support hardware design also included 

improvements to the insulation package and heat source. Thus far, these convertors have operated for 

2300 hr each. The convertors have completed performance mapping and are being transferred to 

Lockheed Martin for controller checkout tests. 

References 

1. NASA Discovery Program Mission News: NASA Announces Three New Mission Candidates. 2011. 

http://discovery.nasa.gov/news/index.cfml?ID=1034 Accessed Sept. 23, 2013. 

2. Oriti, S.M.: Extended Operation of Stirling Convertors at NASA Glenn Research Center. AIAA

20115726, 2011.

3. Wilson, S.D.: Overview of Heat Addition and Efficiency Predictions for an Advanced Stirling 

Convertor. AIAA20115576, 2011. 

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