David J. Anderson, Wayne A. Wong, and Karen L. Tuttle

Glenn Research Center, Cleveland, Ohio

An Overview and Status of NASA's

Radioisotope Power Conversion

Technology NRA

NASA/TM—2005-213980

November 2005

AIAA–2005–5713
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David J. Anderson, Wayne A. Wong, and Karen L. Tuttle

Glenn Research Center, Cleveland, Ohio

An Overview and Status of NASA's

Radioisotope Power Conversion

Technology NRA

NASA/TM—2005-213980

November 2005

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

Third International Energy Conversion Engineering Conference

sponsored by the American Institute of Aeronautics and Astronautics

San Francisco, California, August 15–18, 2005

AIAA–2005–5713
Acknowledgments

The results and findings presented here are based on work funded by the National Aeronautics and Space

Administration (NASA) under the Radioisotope Power Conversion Technology NASA Research Announcement

NRA 02–OSS–01 issued through the Science Mission Directorate. The authors would also like to acknowledge the

hard work of the respective contractor teams, and the NASA contract and technical monitors and appreciate their

help in contributing information for, or reviewing this paper.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

This report contains preliminary

findings, subject to revision as

analysis proceeds.

Available electronically at http://gltrs.grc.nasa.gov
NASA/TM—2005-213980 1 

An Overview and Status of NASA’s Radioisotope 

Power Conversion Technology NRA 

David J. Anderson, Wayne A. Wong, and Karen L. Tuttle 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

NASA’s Advanced Radioisotope Power Systems (RPS) development program is developing next generation 

radioisotope power conversion technologies that will enable future missions that have requirements that can not be 

met by either photovoltaic systems or by current Radioisotope Power System (RPS) technology. The Advanced 

Power Conversion Research and Technology project of the Advanced RPS development program is funding 

research and technology activities through the NASA Research Announcement (NRA) 02-OSS-01, “Research 

Opportunities in Space Science 2002” entitled “Radioisotope Power Conversion Technology” (RPCT), 13 August 

2002. The objective of the RPCT NRA is to advance the development of radioisotope power conversion 

technologies to provide significant improvements over the state-of-practice General Purpose Heat 

Source/Radioisotope Thermoelectric Generator by providing significantly higher efficiency to reduce the number of 

radioisotope fuel modules, and increase specific power (watts/kilogram). Other Advanced RPS goals include safety, 

long-life, reliability, scalability, multi-mission capability, resistance to radiation, and minimal interference with the 

scientific payload. These advances would enable a factor of 2 to 4 decrease in the amount of fuel required to 

generate electrical power. The RPCT NRA selected advanced RPS power conversion technology research and 

development proposals in the following three areas: innovative RPS power conversion research, RPS power 

conversion technology development in a nominal 100We scale; and, milliwatt/multi-watt RPS (mWRPS) power 

conversion research. Ten RPCT NRA contracts were awarded in 2003 in the areas of Brayton, Stirling, 

thermoelectric (TE), and thermophotovoltaic (TPV) power conversion technologies. This paper will provide an 

overview of the RPCT NRA, and a brief summary of accomplishments over the first 18 months but focusing on 

advancements made over the last 6 months. 

I. Introduction

Radioisotope fueled power systems have a long history of successful National Aeronautics and Space 

Administration (NASA) space flight missions. Radioisotope Power Systems (RPS) are capable of producing 

electricity for decades under the harsh conditions of deep space. An RPS converts the heat generated from the 

natural decay of radioisotope material into useful electrical power. They are most attractive in deep space 

applications where the solar flux is too low, or for extended planetary applications such as that on Mars where the 

day/night cycle, settling of dust, and long-life requirements limit the usefulness of photovoltaics.

1

NASA has used RPS reliably in space exploration for more than 35 years. The current state-of-practice RPS 

used on NASA missions is the Radioisotope Thermoelectric Generator (RTG), which incorporates DOE’s General 

Purpose Heat Source (GPHS) modules. The GPHS-RTG, which uses Silicon-Germanium thermoelectric (TE) power 

conversion, was utilized on NASA missions Galileo, Ulysses, and Cassini. While the GPHS-RTG is a proven, 

reliable design, it has a relatively low power conversion efficiency of about 7%, a Beginning-of-Mission (BOM) 

system specific power of about 5 W/kg, and is limited to vacuum environment applications only.2

Current RPS development efforts, which include the Multi-Mission Radioisotope Thermoelectric Generator 

(MMRTG) and the Stirling Radioisotope Generator (SRG), are aimed at re-establishing the production of an RPS 

since the last of the already produced RTGs will be utilized on nearer term missions. The MMRTG and SRG efforts 

are also focusing on improving performance, and allowing multi-mission applications, while maintaining reliability 

for long life. A summary of the MMRTG and SRG programs can be found in Ref. 3. 

NASA anticipates future mission requirements that go beyond the capabilities of MMRTG and SRG requiring 

advanced RPS that offer better performance and higher specific power. NASA has identified a number of potential 

missions that would require more advanced radioisotope generators. Future potential planetary missions require RPS 

that can work reliably for long periods of time in both vacuum and planetary atmospheres. As power requirements 
NASA/TM—2005-213980 2 

vary from mission to mission, future potential missions would also require a technology that can be scalable from a 

few watts to a few hundred watts. NASA and DOE are developing advanced, high-efficiency radioisotope power 

convertors to enable the next ambitious steps in exploration of our solar system using safe and cost effective 

spacecraft.4

II. Radioisotope Power Conversion Technology Objectives 

The objective of the NASA RPS development program is to develop power conversion technologies that will 

enable future missions that have requirements that cannot be met by either photovoltaic systems or by current RPS. 

Advanced RPS performance goals include improvement over the state-of-practice GPHS-RTG including 

significantly higher efficiency to reduce the number of GPHS modules and higher specific power. Other general 

advanced RPS goals include safety, long-life (14 years, with well understood degradation), reliability, scalability, 

multi-mission capability (vacuum and atmosphere), resistance to radiation (from the GPHS or potential mission 

environments), and minimal interference with the scientific payload. 

To that end, the NASA Research Announcement (NRA) 02-OSS-01, “Research Opportunities in Space Science 

2002” entitled “Radioisotope Power Conversion Technology” (RPCT), 13 August 2002 was initiated to advance the 

development of radioisotope power conversion technologies to provide higher efficiencies and specific powers than 

existing systems.4

 The RPCT NRA contracts are investigating several radioisotope power conversion technologies 

for space applications that involve both more efficient conversion of heat into electricity and reduced system mass. 

The higher efficiencies of these new technologies mean that future spacecraft may require less fuel than current 

RTGs typically use. Advances made under the RPCT NRA would decrease the amount of radioisotope fuel required 

by a factor of two to four and reduce the waste heat generated to produce electrical power, and thus could result in 

more cost effective science missions for NASA. This makes the development of these new space power technologies 

highly attractive. 

III. RPCT Contract Organization 

The RPCT NRA resulted in advanced RPS power conversion technology research and development efforts in 

the following three areas: 2,4 

1) Research on 100 We class RPS power conversion technologies 

2) Development of 100 We class RPS power conversion technologies 

3) Research on Milliwatt/multi-watt RPS (mWRPS) power conversion 

Ten RPCT NRA contracts were initially awarded in the areas of Brayton, Stirling, thermoelectric (TE), and 

thermophotovoltaic (TPV) power conversion technologies. The initial selections include five larger development 

contracts using more mature technology (Technology Readiness Level (TRL) 3 to 5) and five smaller research 

contracts using less mature technology (TRL 1 to 3). Ref. 2 and 4 provide more details on the RPCT NRA, a 

summary of how an RPS works, a brief description of the RPCT NRA research areas, and a brief summary of the 

four power conversion technologies being pursued. 

Each RPCT NRA contract has a period of performance of up to three years, divided into three one-year phases, 

with options to continue the following phase after the conclusion of the prior phase. Phase I started with ten 

contracts in the summer/fall of calendar year 2003. In the July/August 2004 timeframe, an annual review was 

conducted of all ten Phase I RPCT NRA contracts. A decision was made to continue seven of the ten contracts into 

Phase II, and to let an eighth contract complete a major Phase I module fabrication and test task. Phase II started in 

the November 2004 to January 2005 timeframe. However, due to a severe budget reduction in January of 2005, four 

of the Phase II contracts had to be stopped and only three of the Phase II contracts ultimately continued. Table 1 

shows the changes to the Phase-to-Phase RPCT NRA contract portfolio. 
NASA/TM—2005-213980 3 

TABLE 1.—PHASE-TO-PHASE SUMMARY OF FUNDED RPCT NRA EFFORTS 

Phase I 

Completed 

Phase I 

Completed 

Phase II 

Initiated 

Phase II 

Initiated 

Phase II 

Continued 

Phase II 

Continued 

Research 

TRL 1-3 

Development

TRL 3-5 

Research 

TRL 1-3 

Development

TRL 3-5 

Research 

TRL 1-3 

Development

TRL 3-5 

TE mW 2 0 1 0 0 0

TE 1 1 1 0 1 0

TPV 1 2 0 2 0 0

Stirling 1 1 1 1 1 1

Brayton 0 1 0 1 0 0

IV. Radioisotope Power Conversion Technology Goals and Accomplishments 

This paper will primarily focus on post RPCT Annual Review accomplishments that were presented in Ref. 2, 

and summarize other key accomplishments, if any, to date. Eight of the RPCT NRA contracts will be reported on in 

this paper. For each of the eight RPCT NRA contracts, a short description of the effort along with recent Phase II or 

on-going Phase I accomplishments will be summarized below. 

Ref. 1 discussed the Phase I 6-month status, and Ref. 2 provides a summary of Phase I RPCT Annual Review 

accomplishments. Ref. 5, 7, and 9 to 14 are status papers prepared by the RPCT contractors covering Sunpower’s 

Advanced Stirling Convertor development, Teledyne’s TE research and development, MIT's TE research, Creare’s 

Thermophotovoltaic technology development, Edtek’s Thermophotovoltaic technology development, and Creare’s 

Brayton development. The CSU’s Stirling Regenerator research was reported on in Ref. 6. 

A. Stirling 

1. Advanced Stirling Convertor Development 

The Sunpower led team consisting of Boeing Rocketdyne, Cleveland State University, University of Minnesota 

and consultants Penswick, Gedeon, Berchowitz, and Cairelli was awarded an NRA development contract to 

demonstrate an advanced Stirling converter with significant improvements over 1st-generation flight systems and to 

achieve converter efficiency of >30% and a system specific power of >8W/kg.3

 The key technologies from this 

effort are hydrostatic gas bearings, a moving magnet linear alternator, high temperature heater head materials and 

fabrication, high frequency operation, and a controller with active power factor correction. Currently, the expected 

efficiency of the Advanced Stirling Convertor (ASC) is ~40% (AC power out/heat in), and the convertor specific 

power is 90We/kg resulting in a projected RPS specific power of >8 W/kg. The ASC is designed with an advanced 

850°C heater head using Mar-M-247. Sunpower’s Frequency Test Bed (FTB) Stirling convertor was developed 

early in Phase I to allow frequency and advanced component investigations. The FTB performance has already 

surpassed project efficiency goals having demonstrated 36% conversion efficiency (AC power out/heat in) at lower 

operating temperatures of 650°C hot end and 30°C cold end during Phase I.5

 ASC-1 was designed in Phase I and 

four of these non-hermetically sealed units will be built in Phase II. Two of these units will operate in a dynamically 

opposed low vibration configuration. Launch vibration testing and component reliability testing is also planned for 

Phase II. Life testing of the ASC-1 will be initiated at the end of Phase II. Additional tasks include reliability 

studies and thermodynamic loss investigations. 

After the Phase I review, NASA directed 

Sunpower to include additional Phase II tasks that 

include a heat source thermal interface design; 

fabrication and testing of a foil regenerator; 

regenerator durability and reliability testing; 

additional heater head materials testing; EMI 

characterization and a vibration absorber 

investigation. In Phase III, four ASC-2s will be 

fabricated and tested. These low mass units will 

be hermetically-sealed and operate in dynamic 

opposition for testing and analysis.5

 Sunpower's 

major accomplishments for Phase I and early 

Phase II are as follows: Figure 1.—Sunpower FTB convertors in operation. 
NASA/TM—2005-213980 4 

1) Completed the Phase I FTB convertor and demonstrated 36% efficiency at Thot=650°C and Tcold=30°C. 

2) Completed design of the ASC-1 non-hermetically sealed units 

3) Identified high temperature materials and joining techniques for ASC-1 units to allow for 850°C operation 

for improved performance. 

4) Conducted a thorough reliability assessment to identify key elements that are now a part of a reliability test 

plan.

2. Stirling Convertor Regenerator Microfabrication Research 

The Cleveland State University (CSU) led team consisting of the University of Minnesota, Gedeon Associates, 

Sunpower, and Stirling Technologies Company (now known as Infinia Corporation) was awarded an NRA research 

contract to develop an idealized Stirling regenerator that is built using microfabrication techniques which will 

exhibit improved thermal and flow performance and to characterize these performance enhancements in hardware 

demonstration. The Stirling Regenerator Microfabrication research effort focuses on reducing Stirling regenerator 

pressure drop losses and improving structural 

reliability through use of micro-fabrication 

processes, while maintaining or improving upon 

current regenerator thermal performance. 

Regenerator design is a key consideration in 

achieving high performance with both Stirling 

and the related thermoacoustic machines. This 

effort is addressing the significant performance 

improvements achieved through better 

understanding of regenerator flow fields and 

manifold effects. Projections indicate that 

performance improvements of 6 to 9% higher 

efficiency and power may be possible.6

 CSU's 

major accomplishments for early Phase II are as 

follows: 

1) Refined regenerator concept that is 

composed of stacks of microfabricated 

disks. Each disk has involute elements 

slanted in the same direction, but are assembled with alternating involute disks in the opposite direction 

which enhances flow uniformity. Further, the position of the ring separator for the alternating disks are 

staggered to reduce the thermal conduction along the flow axis and allow radial flow mixing. Figure 2 

shows the revised involute concept. 

2) Fabrication of a nickel involute regenerator for the Sunpower oscillating flow rig is underway 

3) Fabrication of elements for Large-Scale Mock-Up testing is proceeding 

4) Conducting CFD simulations of Large-Scale Mock-Up hardware 

B. Thermoelectrics (TE) 

1. Segmented BiTe/PbTe-BiTe/TAGS/PbSnTe Thermoelectric Generators Development 

 An NRA development contract was awarded to Teledyne Energy Systems, Inc. (TESI) to improve performance 

and manufacturability of segmented BiTe with PbTe, PbSnTe and TAGS unicouples in a power converter module 

and demonstrate 10-12% efficiency and >5 W/kg specific power.3

 During Phase 1 TESI began the development of a 

thermoelectric (TE) radioisotope power 

converter with a TE efficiency goal of 10%, a 

power output of 20 We at 5V, and greater 

than 5 W/kg system specific power. TESI 

produced a baseline 20 We compact module 

sized to match a single General Purpose Heat 

Source (GPHS) module. The baseline module 

design is based on the TESI terrestrial-based 

Sentinel commercial generator design and 

uses state-of-practice BiTe segmented with 

PbTe, PbSnTe and TAGS TE unicouples. 

TESI also conducted TE material studies to 

improve performance as well as developed an 

Figure 2.—CSU improved involute regenerator concept. 

Figure 3.—TESI 20W baseline module concept. 
NASA/TM—2005-213980 5 

improved segmented TE couple model to evaluate material composition and segment sizing for a given temperature. 

Phase II and III goals were to improve the baseline converter's design and efficiency. However, as a result of a 

down-select process at the end of Phase I, NASA did not exercise the Phase II option. TESI is currently testing the 

baseline Phase I compact module to evaluate its performance under a no-cost extension of Phase I.7

 TESI's major 

accomplishments to date for are as follows: 

1) Developed a system concept based to meet 20W, 5V power goal utilizing one GPHS module and improved 

to 24W through optimization of sizing and temperatures 

2) Designed and fabricated the baseline TE compact module with mini-couples 

3) TAGS sublimation test effort started 

4) Mechanically alloyed various common thermoelectric materials 

5) Developed a viable co-hot pressing process 

6) Developed successful TAGS-80 and TAGS-85 compositional changes 

2. Si-Ge Nanocomposites for Radioisotope Power Conversion Research 

Massachusetts Institute of Technology (MIT) has teamed with Boston College (BC) and the Jet Propulsion 

Laboratory (JPL) on a thermoelectric material NRA research contract to improve the figure-of-merit (ZT) of bulk 

SiGe by investigating the development of bulk nanocomposite SiGe. This nanostructure technology promises to 

increase the material's performance by increasing the 

Seebeck coefficient and reducing the thermal 

conductivity without affecting the electrical 

conductivity. As a result, there is a net gain in the ZT. 

The goal is to demonstrate ZT of ~2 at 900K with 

efficiency of 12-14%. In addition, once perfected, 

synthesizing bulk nanocomposites is a large-scale 

manufacturing process that is needed when producing 

large numbers of thermoelectric couples. Specific areas 

of this research activity include nanostructure synthesis, 

structural characterization, thermoelectric properties 

characterization and modeling of the nanocomposites. 

Accomplishments for Phase I are listed below. Phase II 

and III activities will continue to improve the quality of 

the Si and Ge nanostructures for higher performance and 

continue with property measurements, characterization, 

and modeling.8

 The MIT/BC/JPL team accomplishments 

for the Phase I and Phase II effort included: 

1) Synthesis of high quality Si and Ge nanowires 

2) Synthesis of Si and Ge nanoparticles 

3) Synthesis of SiGe nanocomposites by induction furnace method and Plasma Pressure Compaction 

4) Developed a new heat treatment for the SiGe alloy that showed a 50% increase of ZT at room temperature 

5) Phonon Transport Modeling of two dimensional SiGe nanocomposites 

6) Modeling of material properties for enhanced materials design 

C. Milli-Watt Thermoelectrics (mW TE) 

1. Advanced Superlattice BiTe-PbTe/TAGS Milliwatt Radioisotope Power Systems Research 

An NRA research contract was awarded to Teledyne Energy Systems, Inc. (TESI) and Research Triangle 

Institute (RTI) to develop a cascaded PbTe/TAGS-Superlattice BiTe TE module for milliwatt applications and 

demonstrate module efficiency of ≥ 8% with power outputs of 50 to several 100 mW.3

 During this research effort, 

TESI and RTI set about to develop a high efficiency, two-stage cascaded thermoelectric converter for use in future 

milliwatt radioisotope power source (mWRPS) applications using one or more radioisotope heater units (RHU) as 

the heat source. The hot stage device, developed by TESI, is a miniature conventional T/E converter composed of 

over 80 T/E couples, which are glass bonded into a monolithic multicouple block. The hot stage device uses 

Teledyne’s segmented PbTe/TAGS/PbSnTe T/E device technology and is designed to produce 2/3 of the total power 

output. The cold stage device, developed by RTI, is a high efficiency superlattice BiTe based device that will 

produce approximately 1/3 of the total power output. The electrical compatibility between the high voltage-low 

current power output mode of the hot stage device operating with the low voltage-high current power output mode 

of the superlattice cold stage device is a technical challenge. While investigating a solution for this problem TESI 

designed a fully segmented PbTe/BiTe-PbSnTe/TAGS/BiTe TE module. This TE module would meet the programs 

Thermocouple

Leads

Heater

Leads

Figure 4.—MIT TE sample and measurement setup. 
NASA/TM—2005-213980 6 

technical objectives, operate between 720K-300K 

and produce end of mission power of about 55 mW 

using a single RHU with nominal 1 thermal watt 

heat output rate. The thermoelectric efficiency is 

expected to be 10.3% with a system efficiency of 

8.8%. This module would not be used in 

conjunction with a superlattice based RTI device. 

RTI completed the construction of 4 high element 

aspect ratio Superlattice devices to be tested 

independently. Figure 5 shows a cross-section of 

improved glass bonds demonstrating bond line 

uniformity.9 This contract was terminated after the 

start of Phase II due to NASA funding cuts. TESI's 

major accomplishments for Phase I and early Phase 

II are as follows:

1) Developed several material and fabricating process improvement for the quantity production of very small 

TE material slices into multicouple arrays 

2) Developed an innovative technique to assure full separation and electrical isolation of TE material slices 

after glass bonding 

3) Developed a step-grading process to form a thicker superlattice BiTe film allowing increased L/A ratios 

4) Found alternative materials to lower parasitic losses in the superlattice device 

5) Fabricated several glass-bonded linear array multicouple hot stage devices as well as a high performance 

superlattice unicouple and multicouple with results that exceed bulk BiTe material couple and module 

performance by more than 50%. 

D. Thermophotovoltaic (TPV) 

1. Thermophotovoltaic Power Conversion Technology For Radioisotope Power Systems Development 

The Creare Inc./EMCORE Corp./ NASA GRC/Polytechnic University/Oak Ridge National Laboratory team was 

awarded an NRA development contract to demonstrate a selective emitter-based TPV power generator with a 

simulated radioisotope thermal source and a target 15-20% converter efficiency and ~15 W/kg specific power.3 This

TPV development effort involved the development of an advanced TPV concept that includes high temperature 

selective emitters, filters and advanced lattice-mismatched InGaAs PV cells. Creare's baseline converter design 

included a 4x4 InGaAs array for high power output, a tandem plasma/dielectric filter for spectral control to 

maximize conversion efficiency and tungsten 

emitters to achieve the heat balance necessary to 

meet the power input and the temperature 

constraints of the RPS. Creare expected to 

demonstrate a system conversion efficiency of 

15-20%, a specific power of 10-15We/kg, and a 

TRL of 5. The TPV array developed and tested 

under Phase I has an efficiency of 19% and more 

than 50 We for a prototypical array size of about 

100 cm2

 with an emitter temperature of 1350 K 

and a heat rejection temperature of 300K. Creare 

began work on a detailed design of a test article 

that would improve upon the baseline converter 

when NASA funding cuts required the 

termination of this contract.10,11 Creare's major 

accomplishments for Phase I and early Phase II 

are as follows:

1) Demonstrated conversion efficiency of 19% with a PV cell array in a test configuration that lacked only a 

housing and DC-DC power regulation 

2) Developed a detailed performance model that includes radiation, conduction, convection heat transfer and 

PV cell electrical performance that compares well with experimental data. 

3) InGaAs PV cells were consistently fabricated with high performance. 

4) Prepared an ultra-high vacuum facility to perform materials evaporation and deposition tests for operation 

Figure 5.—Example of TESI's glass bonded TE element. 

Figure 6.—Creare TPV test article without insulation. 
NASA/TM—2005-213980 7 

5) Determined that the GPHS heat source neutron irradiation will not affect the optical characteristics of the 

PV cells and tandem filters over the 14-year mission. Also determined that the PV cell output power would 

degrade by a factor of .8 due to the neutron irradiation over the 14-year mission. 

2) Thermophotovoltaic Radioisotope Power Conversion Technology Development 

 The EDTEK Inc./University of Houston was awarded an NRA development contract to demonstrate a TPV 

power generator employing improvements in the GaSb PV cell, IR filter housing and thermal isolation of PV array 

and achieve efficiency and specific power of ~30% and > 8 W/kg.3

 This TPV RPCT development effort involves 

development of a TPV concept that could provide a 

3 to 4-fold improvement in system efficiency over 

current thermoelectric-based radioisotope 

generators. The key technological innovations are a 

novel integrated cell front contact and concentrator 

scheme, a unique tuned Frequency Selective Surface 

(FSS) filter array, a non-contaminating emitter, 

improvements in GaSb photovoltaic cell 

technology, and a unique scheme for thermal 

management that will allow a reduction in total 

system mass by about a factor of 3. Performance 

goal is to achieve 17-23% converter efficiency, 

which is estimated to provide an RPS specific 

power of >8 W/kg.12,13 This contract was terminated 

after the start of Phase II due to NASA funding cuts. 

The Edtek/U. Houston team accomplishments for 

the Phase I and early Phase II effort included: 

1) An efficiency of 20% was demonstrated in 

Edtek’s TPV cell and prism array shown in 

Figure 7. Three of these arrays make up a 4×4 in. unit. 

2) Advanced development work was performed on 1) an integrated filter TPV cell contact and 2) advanced 

filter modeling and fabrication technologies for designing and optimizing single layer filter performances 

and to design, evaluate and fabricate multilayer filters. 

3) Fabricated large-area Aperture Array Lithography (AAL) masks, by prototyping five unique filter designs, 

and by developing a three-dimensional model that accurately predicts the behavior of the fabricated IR 

filters. 

4) Developed a concept to thermally isolate the filter and housing from the prism/cell convertor assembly. 

The filter and housing can operate at higher temperatures, and through the use of a split radiator, can reject 

waste heat using a smaller radiator (potentially by a factor of 2). 

E. Brayton 

1) Advanced Turbo-Brayton Power System for Radioisotope Power Systems development 

The Creare Inc. led team composed of Ball Aerospace, Boeing Rocketdyne, and Jackson & Tull was awarded an 

NRA development contract to develop a Turbo-Brayton Power System (TBPS) and demonstrate operation of a low 

mass mini turbo-Brayton converter that would achieve an efficiency of 25-36% and specific power of 9-13 W/kg.3

Key to this effort is application of micromachining and fabrication techniques to build a 

very small, compact turbine-alternator. During the 

first year the design, analysis and demonstrations 

efforts were geared towards addressing the key 

technical challenges of extending Creare’s turboBrayton cryocooler technologies to high 

temperature operation in an RPS. To address the 

key challenges, the first year of this effort was 

focused on proof-of-concept demonstrations and 

the thermo-mechanical design of the convertor. 

The TBPS is to be integrated with two GPHS 

modules, and is predicted to produce 104 We of 

28 VDC output power, with a convertor Figure 8.—Assembled TAC Rotor-Shaft-Compressor. 

Figure 7.—Edtek TPV cell and prism array test 

apparatus with IR Bandpass filter in place. 
NASA/TM—2005-213980 8 

efficiency of ~23% and a system efficiency ~21% with a turbine inlet temperature of 1050K (1100K hot side 

interface), and a compressor inlet temperature of 332K. Figure 8 includes a photograph of a titanium alloy 

compressor, zirconia shaft, and a niobium alloy turbine fabricated using Creare’s precision fabrication capability.14

This contract was terminated after the start of Phase II due to NASA funding cuts. The key Phase I accomplishments 

for the Creare Brayton effort included: 

1) Completed material and fabrication evaluations, and the demonstration of a high temperature rotor, 

lightweight recuperator, and power conversion electronics 

2) Completed design, fabrication, and assembly of a Turbo-Alternator-Compressor (TAC), temperature cycled 

the unit to demonstrate mechanical integrity, spin tested the prototype TAC to demonstrate its dynamic 

behavior, and completed 200 start/stop cycles without noticeable degradation 

3) Completed a conceptual design of a Turbo-Brayton convertor 

4) Identified an alternate conceptual convertor design with a higher level of component integration to be a 

more flight-like package to reduce mass and improve efficiency 

V. Conclusion 

This paper described the Radioisotope Power Conversion Technology NASA Research Announcement, and its 

focus to advance the development of radioisotope power conversion technologies to provide higher efficiencies and 

specific powers than existing systems. Results from eight of RPCT NRA contracts in the areas of Brayton, Stirling, 

thermoelectric (TE), and thermophotovoltaic (TPV) power conversion technology efforts have been summarized. 

The advances from these RPCT NRA efforts will enable more cost effective science missions for NASA. 

VI. References 1

Wong, W.A., “Advanced Radioisotope Power Conversion Technology Research & Development,” AIAA-2004-5515, 

Proceedings of the 2nd International Energy Conversion Engineering Conference (IECEC 2004), Providence, RI, August 16-19, 

2004. 2

Anderson, D.J., “NASA Radioisotope Power Conversion Technology NRA Overview,” Space Technology and Applications 

International Forum (STAIF 2005), Albuquerque, NM, February 14-17, 2005. 3

Schmidt, G.R., Wiley, R.L., Richardson, R.L, “NASA’s Program for Radioisotope Power System Research and Development,” Space Technology and Applications International Forum (STAIF 2005), Albuquerque, NM, February 14-17, 2005. 4

NASA, Amendment to NASA Research Announcement NRA 02-OSS-01-RPCT, "Research Opportunities in Space Science 

2002 entitled Radioisotope Power Conversion Technology," (2002) http://research.hq.nasa.gov/code_s/nra/current/NRA-05-OSS01-rpct/index.html, accessed November 2004. 5

Wood, J.G., Carroll, C., Penswick, L.B., “Advanced 80 We Stirling Convertor Development Progress,” Space Technology 

and Applications International Forum (STAIF 2005), Albuquerque, NM, February 14-17, 2005. 6

Ibrahim, M. B., Veluri, S., Simon, T., Gedeon, D., “CFD Modeling of Surface Roughness in Laminar Flow,” AIAA-2004- 

5585, Proceedings of the 2nd International Energy Conversion Engineering Conference (IECEC 2004), Providence, RI, August 

16-19, 2004. 7

Flanders, L.A., Drinker, R.W., Heshmatpour, B., Moul, D.S., Fleurial, J.-P., Tuttle, K.L., “Improvements in Materials and 

Processes for Segmented BiTe/PbTe- BiTe/TAGS/PbSnTe Based Thermoelectric Generators,” Space Technology and 

Applications International Forum (STAIF 2005), Albuquerque, NM, February 14-17, 2005. 8

Dresselhaus, M.S., Chen, G., Ren, Z.F., Fleurial, J.-P., Gogna, P., “Si-Ge Nanocomposites for Radioisotope Power 

Conversion,” presented at Direct Energy Review and Workshop, Coronado, CA, December 13-15, 2004. 9

Drinker, R.W., Heshmatpour, B., Reddy, A., Snyder, G.J., Tuttle, K.L., “Advanced Superlattice BiTe-PbTe/TAGS Milliwatt 

Radioisotope Power System,” Space Technology and Applications International Forum (STAIF 2005), Albuquerque, NM, 

February 14-17, 2005. 10Crowley, C.J., Elkouh, N.A., Murray, S.L., Chubb, D.L., “Thermophotovoltaic Converter Performance for Radioisotope 

Power Systems,” Space Technology and Applications International Forum (STAIF 2005), Albuquerque, NM, February 14-17, 

2005. 11Newman, F.D., Murray, S.L., Endicter, S.P., Aiken, D.J., Girard, G.R., Turner, M., Sharps, P.R., “Development of 

Thermophotovoltaic Devices Optimized for High Temperature Operation,” Space Technology and Applications International 

Forum (STAIF 2005), Albuquerque, NM, February 14-17, 2005. 12Horne, W.E., Morgan, M.D., Horne, W.P., Sundaram, V.S., “Frequency Selective Surface Bandpass Filters Applied to 

Radioisotope Thermophotovoltaic,” Space Technology and Applications International Forum (STAIF 2005), Albuquerque, NM, 

February 14-17, 2005. 13Xu, B., Sundaram, V.S., Morgan, M.D., Horne W.E., “Integrated Bandpass Filter Contacts for GaSb Thermophotovoltaic 

Cells,” Space Technology and Applications International Forum (STAIF 2005), Albuquerque, NM, February 14-17, 2005. 14Zagarola, M.V., Izenson, M.G., Breedlove, J.J., O'Connor, G.M., Ketchum, A.C., Jetley, R.L., Simons, J.K., “An Advanced 

Turbo-Brayton Convertor for Radioisotope Power Systems,” Space Technology and Applications International Forum (STAIF 

2005), Albuquerque, NM, February 14-17, 2005. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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14

An Overview and Status of NASA's Radioisotope Power Conversion

Technology NRA

David J. Anderson, Wayne A. Wong, and Karen L. Tuttle

Stirling cycle; Thermoelectric generators; Brayton cycle; Space missions;

Thermophotovoltaic conversion; Energy conversion efficiency; Power convertors;

Regenerators

Unclassified -Unlimited

Subject Categories: 20 and 44

Prepared for the Third International Energy Conversion Engineering Conference sponsored by the American Institute of

Aeronautics and Astronautics, San Francisco, California, August 15–18, 2005. Responsible person, David J. Anderson,

organization code PS, 216–433–8709.

NASA’s Advanced Radioisotope Power Systems (RPS) development program is developing next generation radioisotope

power conversion technologies that will enable future missions that have requirements that can not be met by either

photovoltaic systems or by current Radioisotope Power System (RPS) technology. The Advanced Power Conversion

Research and Technology project of the Advanced RPS development program is funding research and technology

activities through the NASA Research Announcement (NRA) 02–OSS–01, “Research Opportunities in Space Science

2002” entitled “Radioisotope Power Conversion Technology” (RPCT), August 13, 2002. The objective of the RPCT NRA

is to advance the development of radioisotope power conversion technologies to provide significant improvements over

the state-of-practice General Purpose Heat Source/Radioisotope Thermoelectric Generator by providing significantly

higher efficiency to reduce the number of radioisotope fuel modules, and increase specific power (watts/kilogram). Other

Advanced RPS goals include safety, long-life, reliability, scalability, multi-mission capability, resistance to radiation, and

minimal interference with the scientific payload. Ten RPCT NRA contracts were awarded in 2003 in the areas of Brayton,

Stirling, thermoelectric (TE), and thermophotovoltaic (TPV) power conversion technologies. This paper will provide an

overview of the RPCT NRA, and a brief summary of accomplishments over the first 18 months but focusing on

advancements made over the last 6 months.