Edward J. Lewandowski, Jeffrey G. Schreiber, and Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

David W. Meer

Sest, Inc., Middleburg Heights, Ohio

Michael H. Brace and Gina Dugala

Glenn Research Center, Cleveland, Ohio

Design of a Facility to Test the Advanced Stirling 

Radioisotope Generator Engineering Unit

NASA/TM—2010-216742

August 2010
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Edward J. Lewandowski, Jeffrey G. Schreiber, and Salvatore M. Oriti

Glenn Research Center, Cleveland, Ohio

David W. Meer

Sest, Inc., Middleburg Heights, Ohio

Michael H. Brace and Gina Dugala

Glenn Research Center, Cleveland, Ohio

Design of a Facility to Test the Advanced Stirling 

Radioisotope Generator Engineering Unit

NASA/TM—2010-216742

August 2010

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Seventh International Energy Conversion Engineering Conference (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

Denver, Colorado, August 2–5, 2009
Acknowledgments

This work is funded through the NASA Science Mission Directorate. 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 the National Aeronautics and Space 

Administration. The authors wish to acknowledge the many people who supported the design and build-up of the Advanced 

Stirling Radioisotope Generator (ASRG) Engineering Unit (EU) test facility in the Stirling Research Laboratory at the NASA 

Glenn Research Center, including those at Lockheed Martin in Valley Forge and Denver who guided us in Advanced Stirling 

ASRG EU control and operation.

Available from

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7115 Standard Drive

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National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://gltrs.grc.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2010-216742 1 

Design of a Facility to Test the Advanced Stirling Radioisotope 

Generator Engineering Unit 

Edward J. Lewandowski, Jeffrey G. Schreiber, and Salvatore M. Oriti 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

David W. Meer 

Sest, Inc. 

Middleburg Heights, Ohio 44130 

Michael H. Brace and Gina Dugala 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The Advanced Stirling Radioisotope Generator (ASRG), a high efficiency generator, is being 

considered for space missions. An engineering unit, the ASRG engineering unit (EU), was designed and 

fabricated by Lockheed Martin under contract to the Department of Energy. This unit is currently under 

extended operation test at the NASA Glenn Research Center (GRC) to generate performance data and 

validate the life and reliability predictions for the generator and the Stirling convertors. 

A special test facility was designed and built for the ASRG EU. This paper summarizes details of the 

test facility design, including the mechanical mounting, heat-rejection system, argon system, control 

systems, and maintenance. The effort proceeded from requirements definition through design, analysis, 

build, and test. Initial testing and facility performance results are discussed. 

Nomenclature 

ACU ASC Control Unit 

ASC Advanced Stirling Convertor 

ASRG EU Advanced Stirling Radioisotope Generator Engineering Unit 

CSAF cold-side adapter flange 

EHS electric heat source 

GPHS General Purpose Heat Source 

GRC Glenn Research Center 

LM Lockheed Martin 

NASA National Aeronautics and Space Administration 

UPS uninterruptible power supply

Introduction 

The Advanced Stirling Radioisotope Generator (ASRG) is a high-efficiency generator being 

developed for potential use on space missions. The ASRG system efficiency of 28 to 32 percent would 

reduce the amount of radioisotope required for a given power level by a factor of four in comparison to 

that for radioisotope thermoelectric generators. Its high specific power (7 We/kg or greater (Ref. 1)) 

enables certain missions and applications. 
NASA/TM—2010-216742 2 

A key step in the flight development process was the design and fabrication of an ASRG engineering 

unit (EU), by system integrator Lockheed Martin Space Systems Company (LM) under contract to the 

Department of Energy (Ref. 1). The generator assembly was completed in December 2007 and controller 

integration followed in March 2008. This unit underwent a series of system-level tests to qualification 

level thermal and dynamic environments at LM. These included thermal balance, thermal performance, 

mechanical disturbance, sine transient, random vibration, simulated pyrotechnic shock, and 

electromagnetic interference (Ref. 2). Following an internal inspection, LM delivered the ASRG EU to 

the NASA Glenn Research Center (GRC) on August 28, 2008, for extended operation. The EU underwent 

inspection followed by integration into a test facility specially designed for the EU. The ASRG EU began 

extended operation on November 6, 2008. This test is intended to demonstrate extended operation of an 

integrated system, to monitor for trends in Stirling convertor performance, and to provide additional data 

from long-term operation of Stirling convertors. The EU has operated almost 5,000 hr to date at NASA 

GRC (Ref. 3). 

This paper discusses details of the test facility design and highlights the special features incorporated 

for the ASRG EU. The design leveraged years of GRC experience in testing Stirling convertors in dualopposed pairs. The EU is the first Stirling generator system tested at GRC that has an integrated heat 

source, insulation, sealed housing, and controller. Special consideration was made for the mounting, heat 

rejection, argon system, controller, and other aspects of the facility. 

The ASRG EU 

Figure 1 shows a cutaway model of the ASRG EU. The EU contains two Advanced Stirling 

Convertor (ASC)-E convertors secured together through an interconnect sleeve. An electric heat source 

(EHS), held against each ASC-E heat collector, provides the heat input. The cold-side adapter flanges 

(CSAFs) conduct heat rejected from the convertors away through the beryllium housing and fins, to be 

then radiated away in a vacuum environment or convected to air. Argon fills the housing, sealed using 

various O-rings and gaskets. A gas management valve allows access to the argon. The controller is 

mounted to the outside of the housing. Connectors on the end enclosures, housing, and controller provide 

electrical interfaces to the alternators, sensors, power input and output, control, and telemetry. The EU is 

mounted via four mounting tabs on one end of the housing, bolting to a spacecraft interface or directly to 

its support. 

Advanced Stirling 

Convertors

Gas Management 

Valve Interconnect 

Sleeve

Pressure Relief 

Device

End Enclosure

Thermal 

Insulation Electric Heat 

Source

Generator 

Housing

Cold-Side Adapter 

Flange ASC Control Unit

Radiator 

Fin

Spacecraft 

Interface

Figure 1.—ASRG EU cutaway model. 
NASA/TM—2010-216742 3 

Figure 2.—ASRG EU test facility. 

Figure 2 shows the test facility. The ASRG EU is mounted on a test table through stand-offs and is 

surrounded by a Lexan cage. Ductwork, fans, heat exchangers, and plumbing to a circulator (not shown) 

for cooling all reside underneath the table. The test rack, which contains the power supplies, the 

instrumentation, the computer, and the data acquisition, control, and monitoring system, is placed to the 

left of the test table. An argon cylinder and associated gas management hardware is mounted behind the 

test table. Framed Lexan partitions surround the test facility. A facility to test two ASC-E convertors can 

be seen in the right half of the partitioned area. 

Test Facility Requirements 

The design process for the test facility began by defining the requirements. Although the EU could be 

tested in a variety of ways under various operating conditions, the extended operation test as discussed in 

the introduction focused on 24/7 steady operation for several years. To meet cost and schedule 

constraints, it was important to define the functionality necessary for extended operation. As designers 

focused on this objective, the high-level requirements became: 

 Mount in a vertical orientation, in a way that allowed monitoring of interface dynamic forces. 

Vertical orientation may work the convertor piston and displacer gas bearings harder. Several 

convertors are already under extended operation test in the horizontal orientation, so the vertical 

orientation expands the breadth of extended operation test conditions. 

 Provide two control options: (1) alternating-current (AC) bus control, and (2) ASC Control Unit 

(ACU) control. The EU would begin operation under AC bus control then transition to ACU 

control. 

 Provide two input power options for the heat source control: (1) fixed hot-end temperature mode, 

which has been used to date on all extended-operation convertor tests, and (2) fixed heat input 

mode, which simulates the General Purpose Heat Source (GPHS) module characteristics and tests 

the temperature-control feature of the ACU. 

 Remove heat from the EU fins and housing in a way that maintains a fairly constant rejection 

temperature at the convertor cold end, preferably as low as 50 °C. 

 Maintain argon pressure inside the EU within the required range. 

 Monitor various parameters throughout the system with calibrated instrumentation, as has been 

done with other convertors under extended operation, and include ACU controller parameters. 

 Operate 24/7 unattended, including safety systems to protect the ASRG EU in the event of an 

equipment failure or abnormal operation, shutting down the ASRG EU in a controlled fashion if 

necessary. 

 Achieve high uptime (greater than 85 percent). 

 Record performance data regularly. 

 Display the ASRG EU in a visually attractive way for visitors but at the same time protect the unit 

from unauthorized activity. 
NASA/TM—2010-216742 4 

Test Facility Design 

The ASRG EU test facility was based on existing extended-operation test facilities at GRC described 

in References 4 to 7. Although many features are common with existing facilities, the ASRG EU drove 

several unique features, described in detail here. 

Mechanical Mounting 

The ASRG EU is rigidly mounted to the test table as shown in Figure 3. The EU housing includes 

four mounting tabs at one end to which four standoffs are attached. These standoffs lift the EU above the 

test table enough to allow access to the electrical connectors on the end of the EU, and they bolt to a 

3/4-in- thick steel table top, with Kistler 9251A load cells inserted between the standoffs and table top. 

The table top was designed per the load cell manufacturer’s recommendations so that the sensors would 

achieve the advertised accuracy. The load cells provide dynamic measurement of the interface force 

produced by the EU in all three axes, although the force in the axial direction is the one generally of 

greatest interest. A Kistler triaxial model 8692 accelerometer is attached to one stand-off. 

This test table can be adapted to transport the ASRG EU (Fig. 4). Side rails with retractable wheels 

and jack screws can be bolted to the side of the test stand. When the ASRG EU arrived at GRC, it was 

transferred from the transport container directly to the test stand, then wheeled from the loading dock into 

position in the test facility. From there the test table was bolted into place. 

Heat Rejection System 

For previous ASC in-air extended-operation convertor test at GRC, heat was removed from the coldend heat exchanger by conduction through a flange and directly to the cooling fluid. In the case of the 

ASRG EU, heat conducted away from the cold end of the convertor through the CSAF and into the sides 

of the EU housing and the fins provides the cooling mechanism. From there the heat could have been 

rejected either into cooling tubes attached to the fins or to the air surrounding the EU. To avoid any risk 

of damage or marring of the EU surface, it was decided to use the air surrounding the EU for heat 

rejection. 

ASRG EU

Accelerometer

Load cells

Standoffs

Cooling air holes

 

Figure 3.—ASRG EU mounting to the test table. Figure 4.—Test stand transporting the ASRG EU. 
NASA/TM—2010-216742 5 

A cage made of 80/20 modular aluminum framing with Lexan sides surrounding the EU was built on 

top of the test table (Fig. 5). Air flows up out of cooling air holes in the test table (Fig. 3) and past the 

sides and fins of the EU, then it flows back down a Lexan duct to a metal duct under the table that houses 

a pair of fans and heat exchangers (Figs. 6 and 7). A circulator provides water chilled to 10 °C to the heat 

exchangers. The circulator sits just outside the partitioned area that surrounds the ASRG EU and test rack 

to reduce the heat load within the partitioned area. 

 Figure 5.—ASRG EU in Lexan cage. 

Fans

Heat exchangers

Figure 6.—Fans and heat exchangers under the test 

table. 

A

S

R

G

-

E

U

Fans

Lexan cage

Circulator

Heat 

exchanger

Duct

Auxiliary 

fans

Baffle

A

S

R

G

-

E

U

Fans

Lexan cage

Circulator

Heat 

exchanger

Duct

Auxiliary 

fans

Lexan cage top

Duct door

 Figure 7.—Schematics of the heat rejection system. 

a) Normal operation. b) Emergency shutdown mode. 
NASA/TM—2010-216742 6 

During complete loss of facility power, the system must include the capability to dissipate the stored 

thermal energy in the hottest internal components (e.g., heaters and heat collectors) to prevent other 

internal components from exceeding their allowable temperature limits during the shutdown transient. 

Overheating could be possible if the EU was in an enclosed chamber. Simply allowing natural convection 

to pull heat from the sides of the EU by venting the cage would be sufficient to keep temperatures within 

acceptable ranges. 

As a solution, the cage was designed with a spring-loaded top and a door on the bottom, both held 

closed by solenoids (Fig. 7(a)). Removing power from the solenoids causes the top of the cage to pop up 

about 1 in. and the door on the bottom to drop open, allowing air to flow past the ASRG EU, driven by a 

chimney effect. 

The cooling system was designed to remove the heat coming from the ASRG EU during normal 

operation, primarily the electrical heat input less power removed through the alternators (around 400 W). 

The shunt resistors, internal to the EU and used only as an emergency load by the ACU, were not hooked 

up, and therefore the cooling system did not need to compensate for that additional load. 

Prior to installation of the EU, the cooling system was tested using power resistors to simulate the 

heat load from the EU (Fig. 8). With an input heat load of 360 W, the fluid temperature was set to 20 °C. 

A test load closer to 400 W would have accounted for higher insulation losses in the EU. Under these 

conditions, there was a 6.9 °C air temperature drop and a 0.6 °C fluid temperature drop across the heat 

exchangers, with the air temperature entering the ASRG cavity maintained at 23.9 °C. This test 

demonstrated that the system had sufficient cooling capacity to maintain a reasonable air temperature 

while removing the expected heat load. The only part of the heat rejection system not tested was the heat 

transfer from the EU housing and fins to the air. 

The cooling system test included measurement of the air-flow rates through the cooling air holes in 

the table. The front pair of holes (farthest from the fans) had roughly twice the flow rate of the back pair 

of holes. To better balance the flow, a baffle was mounted under the table to increase the flow through the 

back holes (Fig. 7). 

Figure 8.—Test of the heat rejection system. 
NASA/TM—2010-216742 7 

Figure 9.—Top view of ASRG EU showing the 

air-flow of the auxiliary fans across sides of 

the EU. 

15

25

35

45

55

65

75

‐60 0 60 120 180 240 300 360 420 480 540 600 660

Temperature (°C)

Time (sec)

ASC A cold end temp

ASC B cold end temp

ASRG housing temp at ASC A CSAF

ASRG housing temp at ASC B CSAF

Cage air temp

Air temp upstream of heat exchanger

Air temp downstream of heat exchanger

Auxiliary fans

turned off

Auxiliary fans

turned on

Figure 10.—Various temperatures when auxiliary fans were briefly turned off, along with a diagram showing 

temperature measurement locations. 

As mentioned earlier, the cooling system test did not validate the heat transfer from the EU housing 

and fins to the air. When operating the ASRG EU in room temperature air, LM had achieved sufficient 

cooling by using two large fans to blow air over the ASRG EU, enhancing convective heat transfer. This 

test facility, with smaller fans and passages, did not produce the same high air flow past the housing. So, 

in addition to circulating cooler air around the EU (the air temperature would start a few degrees above 

the coolant temperature of 10 °C), auxiliary fans were mounted next to the ASRG EU to blow air across 

the housing and fins to improve heat transfer. Note the two black fans on each side of the EU in Figure 5. 

Mounting the auxiliary fans diagonally across the EU allows each fan to blow air across two sides of the 

housing (Fig. 9). The fans were placed at elevations corresponding to the location of the CSAFs, where a 

majority of the rejected heat was being conducted. 

During checkout of the system with the ASRG EU operating near full power, the auxiliary fans were 

turned off to determine how critical they were to cooling. The plots in Figure 10 show that the effect was 

dramatic, especially on the ASC A (top) end of the EU. When the fans stopped, the housing temperature 
NASA/TM—2010-216742 8 

rose 5 °C in 8 min, and the temperatures had not leveled off when the fans started after about 8 1/2 min. 

The air temperatures around the EU dropped, indicating decreased heat transfer to the air and that 

ultimately, transferring the heat rejected by the EU would require a much larger temperature difference 

between the air and the EU housing. This test demonstrated the criticality of the auxiliary fans to 

maintaining operation at the design operating conditions. 

Argon System 

During ground operations, an argon cover gas fills the ASRG housing to prevent oxidation of the 

graphitic components of the heat source. The test facility’s argon system maintains pressure between 

5 and 9 psig whereas the EU requires a pressure between 3.3 and 15.3 psig. 

A mechanical regulator with a range of 0 to 10 psig controls this pressure. Because the EU is a closed 

volume, the pressure changes primarily during startup and shutdown as internal temperatures change. A 

small orifice placed between the pressure regulator and the EU limits the gas flow rate in the unlikely 

event of a regulator failure to a rate that the pressure relief valve could handle, preventing the EU from 

possibly being overpressurized. 

The argon system includes a provision for connecting a vacuum pumping station with a residual gas 

analyzer. The gas in the EU is periodically monitored for the presence of air. Figure 11 shows a schematic 

of the argon system. 

Facility Electrical Power 

An uninterruptible power supply (UPS) provides backup power for the ASRG EU test facility so that 

a loss of building power, even momentarily, does not cause a loss of control and convertor stall. 

Furthermore, a 50-kW natural-gas-fueled backup generator provides power in the event of a loss of 

building power. In the unlikely event of loss of power from the backup generator and building power, the 

system will automatically shut down the ASRG EU. Power draw during shutdown is under 1500 VA, and 

the UPS can power the system for at least 6 hr during a controlled shutdown. 

Bottle Valve

Relief 

valve

Pressure 

Regulator

Argon

Orifice V0

V2

Sampling 

valve

RGA

Vacuum 

gauge

Vacuum 

Pumping 

Station

ASRG EU

Pressure 

Transducer

V5 V4

Gas 

Management 

Valve

Figure 11.—Argon system schematic. 
NASA/TM—2010-216742 9 

Instrumentation, Monitoring, and Safety Systems 

Extensive effort went into designing the instrumentation, monitoring, and safety systems for the test 

facility. These systems are covered in Reference 8, so they are not discussed in detail in this paper. 

Instrumentation in the ASRG EU and the test facility includes a variety of sensors to monitor performance 

of both the EU and the test systems. Some of the sensors in the ASRG EU were installed by LM and 

delivered with the unit. Where possible, multiple sensors are used to provide redundancy and verification. 

Some of the safety systems have already been discussed. The safety systems protect the ASRG EU 

from potential test system failure modes and abnormal operation and automatically take appropriate 

corrective action to shut down the ASRG EU in a controlled fashion if necessary. 

The LabVIEW (National Instruments) data system monitors critical parameters and will automatically 

notify the cognizant engineer via a cell phone text message when a parameter exceeds a programmed 

limit. This allows the engineer to take corrective action in a timely fashion, potentially avoiding a 

shutdown. 

Convertor Control for the ASCs 

The test facility supports two types of convertor control for the ASCs: AC bus control and ACU 

control. Reference 8 discusses AC bus control in detail. ASRG EU testing began at GRC with AC bus 

control in part to verify the test facility functionality before transitioning to ACU control. 

The ACU (Fig. 12), mounts to the ASRG EU housing, was designed and built by LM (Refs. 1 and 2). 

Its functions include: 

 Controlling the convertors to maximize spacraft power 

 Synchronizing the convertors to achieve dynamic balance 

 Converting AC output of the convertors to direct-current (DC) power at the spacecraft bus voltage 

 Providing telemetry on the ASRG status 

 Receiving commands to adjust ASC operating conditions 

The ACU uses power electronics to eliminate the need for the tuning capacitors used with AC bus control. 

Figure 13 shows the ASRG EU test facility implementation of ACU control. The flight design has the 

alternator leads connected directly to the power input of the ACU. The ASRG EU test facility includes 

some modifications. Wires were added to connect the alternator output to a power meter, which measures 

voltage, current, power, and power factor. A Pearson coil measures current rather than a shunt resistor to 

avoid adding impedance between the alternator and the ACU. Also, a relay allows the operator or the 

monitoring system to remove the ACU from the system and shunt the alternator output through an 

emergency load to stall the convertors. This would be done in the event of a piston overstroke or a loss of 

control of the convertors. These relays mount to a panel underneath the ASRG EU in the test table. Rather 

than mount them in the test rack, they were put as close to the EU and ACU as possible to minimize line 

impedance. 

The ACU receives input from the two piston position sensors and several temperature sensors. Some 

of these sensors are used as control inputs, whereas some are simply used for monitoring. 

The test facility uses a Hewlett Packard Model 6060B 300-W DC electronic load set to 28 V, 

connected across the output of the ACU, in parallel with some capacitance, to simulate the spacecraft DC 

bus load. The ACU is designed to be compatible with a bus voltage range of 22 to 36 V. At startup, the 

ACU requires power from the bus. A DC power supply provides startup power at 24 to 26 V through a 

diode so that as the ASRG EU heats up and begins to generate power, the power supply output goes to 

zero. Transducers measure power output from the ACU. With measurement of the power input to the 

ASRG EU at the heaters, the power between the alternators and the ACU, the power output of the ACU, 

and the performance and efficiency of the convertors, controller, and EU can be monitored. 
NASA/TM—2010-216742 10 

Figure 12.—ASC control unit. 

ASC 

Control 

Unit

Piston position 

sensors -

+

Power measurement

Alternator A

Temperature 

sensors

Emergency 

load

Alternator B

Power input Sensor input

Power output

Command & telemetry

ASRG EU

Data system

Shunt load

DC bus load and 

power supply

Power 

measurement

Figure 13.—Simplified schematic of an ACU controller integrated into the ASRG EU test facility. 

If the DC bus load becomes disconnected or does not absorb enough of the output power, the ACU 

sends the power to the shunt load. Although the ASRG EU includes internal shunts for this purpose, 

separate shunts were mounted in the test rack to use as the shunt load. This avoided the need to design the 

test facility to handle the heat load from the internal shunts in the unlikely event of loss of DC bus load. 

Instead the heat would be rejected in the test rack. 

The ACU command and telemetry interface utilizes RS-422 serial buses plus some analog signals. 

The National Instruments LabVIEW system handles the serial communication, allowing the operator to 

send commands and to continually monitor and display bus parameters. Analog telemetry is interfaced to 

the LabVIEW analog-to-digital modules and is processed along with other signals. 
NASA/TM—2010-216742 11 

Figure 14.—Cartridge heater heat 

source designed by GRC and 

used in the ASRG EU. 

Heat Source 

The ASRG EU was tested at LM using an EHS that was dynamically and dimensionally equivalent to 

a General Purpose Heat Source (GPHS) module (Ref. 2). This was important to obtaining meaningful 

data from tests like sine transient, random vibration, and shock tests. After internal inspection of the 

ASRG EU following LM’s tests, it was decided to replace the EHS with a cartridge heater furnished by 

GRC, shown in Figure 14. The EHS had been exposed to significant vibration during testing, and longterm performance was unknown. GRC’s extended operation test did not require a heat source dynamically 

equivalent to the GPHS module, so a drop-in replacement heat source was designed that was based on 

existing 650 °C heat sources already proven at GRC’s Stirling Research Laboratory. The heat source can 

provide up to 400 W, with a temperature rise as fast as 1 °C/sec. This heat source does not impose any 

limitation on the rate of power reduction. These features allow the ASRG EU to be started up and shut 

down more quickly than with the EHS. The thermal inertia of the GRC heat source is 36 percent less than 

that of the EHS, which reduces the hot-end temperature rise in the event of an emergency shutdown of the 

ASRG EU. This design is expected to have the higher reliability needed for extended operation tests, and 

to date has demonstrated steady performance with no change in heater resistance. 

Heat Source Control 

As mentioned previously, the test facility needed to provide two input power options for the heat source 

control: (1) fixed hot-end temperature mode, which has been used to date on all extended-operation 

convertor tests, and (2) fixed heat input mode, which simulates the GPHS module characteristics and tests 

the temperature-control feature of the ACU. The fixed hot-end temperature mode was implemented by the 

method used for other test racks at GRC (Ref. 8). For the fixed heat input mode, a controller was designed to 

monitor each heater’s power and adjust the command to each heater power supply. 

Figure 15 shows the schematic for the heat source controller. The system consists of an Onset 

TFX-11 microcontroller, a MAX532 dual channel 12-bit digital-to-analog converter (DAC), and a 

BPP-440 liquid crystal display. 
NASA/TM—2010-216742 12 

Figure 15.—Heater power controller schematic. 

TABLE I.—TEST FACILITY COMPONENT REPLACEMENT INTERVALS

Component Planned replacement interval 

Cooling air fans 1 to 2 years 

Auxiliary fans 1 to 2 years 

Circulator 3 to 4 years 

UPS batteries Monitor battery temperature 

In the TFX-11 microcontroller, the control loop begins by reading the voltage and current sensors, the 

set-point potentiometers, and the fault status. It then calculates the power and resistance of each heater 

and compares the heater powers to the set points. The proportional-integral-derivative algorithm is then 

executed for both heaters and the results are limit checked and sent to the DAC. The DAC output is 

updated with the new values, which are buffered by a dual op-amp and sent to the remote voltage 

programming pins of the heater power supplies as 0 to 5 V signals. Finally, the display is updated with the 

values for the set point, the actual power, the output voltage to the heater power supplies, and the status of 

the integrator accumulator. The control loop executes at 1 Hz. If the test rack protection system detects a 

fault condition, the output and set points are immediately set to zero. 

Test Facility Maintenance 

Although the ASRG EU was designed for a life exceeding 14 years, many test facility components 

will not last that long without maintenance or replacement. Certain components will be replaced on a 

regular basis to reduce the likelihood of unscheduled downtime, as listed in Table I. 

The cooling air fans include a speed sensor. Data taken to date have shown a 1 percent decrease in fan 

speed over 4,000 hr of operation. Although the small decrease in speed is not an issue, it indicates gradual 

degradation, so the prudent approach involves replacing this low-cost item regularly rather than waiting 

for a failure. 

The circulators used in the ASRG EU test facility have been used for extended operation testing of 

other convertors in the Stirling Research Laboratory. Typically, they operate at most 4 to 5 years before 

failure. 
NASA/TM—2010-216742 13 

Improvements 

The ASRG-EU was first operated in the test facility under AC bus control on October 28, 2008, with 

no major problems. After further short-term tests, the EU began 24/7 extended operation on November 6, 

2008, and operated continuously until being shut down for a test in February 2009. Although the test 

facility has met its design requirements, a number of potential improvements were identified during 

operation. Some improvements have been implemented, and some will be implemented in the future as 

appropriate. 

Heat Rejection System 

The heat rejection system as currently implemented has limited ability to reduce convertor cold-end 

temperatures more than a few degrees from their current values, and actual cold-end temperatures are 

running 5 to 10 °C hotter than the desired operating point of 60 °C. Reducing the circulator temperature 

from its current setting of 10 to 5 °C or less would reduce the cold-end temperatures. However, this change 

would result in more condensation on the lines and elsewhere in the system. Some marginal improvements 

might be gained by insulating ductwork and better sealing the cage around the EU. The most effective 

improvement may be to increase the size of the auxiliary cooling fans to improve heat transfer from the EU 

housing to the air, reducing the temperature difference between the housing and the air. 

Components and Connections in the Alternator Load Circuit 

From the beginning of operation, the alternator power and piston amplitude showed anomalous 

behavior in that these parameters would increase slightly, sometimes very gradually and sometimes 

rapidly, and then decrease, usually rapidly back to the initial condition. Because the data suggested 

consistent convertor performance, test rack impedance variation was suspected as the root cause of the 

anomalous behavior. Simulations and analysis have shown that, for the ASC-E convertors under AC bus 

control, just a 3-mΩ increase in impedance would result in a 0.01-mm increase in piston amplitude and a 

corresponding increase in power. With the load circuit containing numerous connectors, relays, and other 

components, it is plausible that a variety of causes could result in a several milliohm change in impedance 

(see Fig. 5 of Ref. 8). A number of dynamic simulations were run, and the results showed that, for 

example, a particular change in amplitude and power observed in the test data was consistent with a 

35-mΩ change in impedance between the power meter and the load resistor. 

In March 2009 a number of improvements were made to connections in the load circuit to address this 

problem. After the ASRG EU was restarted, the magnitude of the anomalous behavior was reduced but 

not eliminated. In July 2009 additional improvements were made, including using a soldered bus bar for 

tuning capacitor bank connections and temporarily eliminating two relays. These test rack improvements 

appear to have resolved the anomalous behavior. A hermetically-sealed relay is currently being tested as a 

possible replacement for the suspect relays in the load circuit. 

Summary 

A test facility was designed and built for extended-operation testing of the Advanced Stirling 

Radioisotope Generator Engineering Unit (ASRG EU). The facility allows for heat input under both 

fixed-temperature and fixed-power modes. The EU can be operated with either AC bus control or ACU 

control. Safety features allow 24/7 unattended operation, with call-home features notifying engineers of 

any deviations in critical parameters. Performance to date has shown that the test facility has met its 

design requirements, although a few areas for improvement have been identified. 
NASA/TM—2010-216742 14 

References 

1. Chan, J., Wood, J.G., and Schreiber, J.G., “Development of Advanced Stirling Radioisotope 

Generator for Space Exploration,” Proceedings of Space Technology and Applications International 

Forum (STAIF 2007), edited by M.S. El-Genk, AIP Conference Proceedings 880, pp. 615–623, 2007. 

2. Chan, J., Hill, D., Hoye, T, and Leland, D., “Development of Advanced Stirling Radioisotope 

Generator for Planetary Surface and Deep Space Missions,” Proceedings of the Sixth International 

Energy Conversion Engineering Conference (IECEC 2008) American Institute for Aeronautics and 

Astronautics, 2008. 

3. Cornell, P.A., Lewandowski, E.J., Oriti, S.M., and Wilson, S.D., “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. 

4. Lewandowski, E.J., Schreiber, J.G., Wilson, S.D., Oriti, S.M., Cornell, P, and Schifer, N., “Extended 

Operation Testing of Stirling Convertors in Support of Stirling Radioisotope Power System 

Development,” Proceedings of the Sixth International Energy Conversion Engineering Conference 

(IECEC 2008) American Institute for Aeronautics and Astronautics, 2008. 

5. Oriti, S.M. and Cornell, P.A., “Processing and Preparation of Advanced Stirling Convertors for 

Extended Operation at NASA Glenn Research Center,” Proceedings of the Sixth International Energy 

Conversion Engineering Conference (IECEC 2008) American Institute for Aeronautics and 

Astronautics, 2008. 

6. Schreiber, J.G., “Summary of Stirling Convertor Testing at GRC,” Proceedings of the Fourth 

International Energy Conversion Engineering Conference (IECEC 2006) American Institute for 

Aeronautics and Astronautics, 2006. 

7. Roth, M.E., Schreiber, J.G., and Pepper, S.V., “Extended Operation of Stirling Convertors,” 

Proceedings of the Second International Energy Conversion Engineering Conference (IECEC 2004)

American Institute for Aeronautics and Astronautics, 2004; NASA/TM—2004-213388. 

8. Dugala, G., “Test Rack Development for Extended Operation of Advanced Stirling Convertors 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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Design of a Facility to Test the Advanced Stirling Radioisotope Generator Engineering Unit 

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Lewandowski, Edward, J.; Schreiber, Jeffrey, G.; Oriti, Salvatore, M.; Meer, David, W.; 

Brace, Michael, H.; Dugala, Gina 

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WBS 138494.04.01.01 

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14. ABSTRACT

The Advanced Stirling Radioisotope Generator (ASRG), a high efficiency generator, is being considered for space missions. An engineering 

unit, the ASRG engineering unit (EU), was designed and fabricated by Lockheed Martin under contract to the Department of Energy. This 

unit is currently under extended operation test at the NASA Glenn Research Center (GRC) to generate performance data and validate the life 

and reliability predictions for the generator and the Stirling convertors. A special test facility was designed and built for the ASRG EU. This 

paper summarizes details of the test facility design, including the mechanical mounting, heat-rejection system, argon system, control 

systems, and maintenance. The effort proceeded from requirements definition through design, analysis, build, and test. Initial testing and 

facility performance results are discussed. 

15. SUBJECT TERMS

Advanced Stirling Radioisotope Generator; High efficiency generators; Stirling convertors; Radioisotope power systems 

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