David J. Anderson

Glenn Research Center, Cleveland, Ohio

NASA Radioisotope Power Conversion

Technology NRA Overview

NASA/TM—2005-213981

November 2005
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David J. Anderson

Glenn Research Center, Cleveland, Ohio

NASA Radioisotope Power Conversion

Technology NRA Overview

NASA/TM—2005-213981

November 2005

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

Space Technology and Applications International Forum (STAIF–2005)

sponsored by the University of New Mexico’s Institute for

Space and Nuclear Power Studies (UNM–ISNPS)

Albuquerque, New Mexico, February 13–17, 2005
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, Project Prometheus Program. The author would

like to acknowledge the assistance of Jean-Pierre Fleurial and Jeff Snyder, Jet Propulsion Laboratory; and Roy Tew,

Wayne Wong, and Thomas Kerslake, NASA Glenn Research Center, and the rest of the RPCT NRA contract and

technical monitors for their inputs and review of 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—2005-213981 1 

NASA Radioisotope Power Conversion 

Technology NRA Overview 

David J. Anderson 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The focus of the National Aeronautics and Space Administration’s (NASA) Radioisotope Power Systems (RPS) 

Development program is aimed at developing nuclear power and technologies that would improve the effectiveness 

of space science missions. The Radioisotope Power Conversion Technology (RPCT) NASA Research 

Announcement (NRA) is an important mechanism through which research and technology activities are supported in 

the Advanced Power Conversion Research and Technology project of the Advanced Radioisotope Power Systems 

Development program. The purpose of the RPCT NRA is to advance the development of radioisotope power 

conversion technologies to provide higher efficiencies and specific powers than existing systems. These advances 

would enable a factor of 2 to 4 decrease in the amount of fuel and a reduction of waste heat required to generate 

electrical power, and thus could result in more cost effective science missions for NASA. 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 100 We

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, a summary of the power conversion 

technologies approaches being pursued, and a brief digest of first year accomplishments. 

Radioisotope Power Conversion Technology (RPCT) Overview 

The objective of the National Aeronautics and Space Administration’s (NASA) Radioisotope Power Systems 

(RPS) Development program is to develop nuclear power conversion technologies that would improve the 

effectiveness of space science missions. The NASA Research Announcement (NRA 02-OSS-01, “Research 

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

an important mechanism through which research and technology activities are funded under the Advanced Power 

Conversion Research and Technology project of the Advanced Radioisotope Power Systems Development program. 

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

provide higher efficiencies and specific powers than existing systems. These advances would enable a factor of 2 to 

4 decrease in the amount of fuel and a reduction of waste heat required to generate electrical power, and thus could 

result in more cost effective science missions for NASA. 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 100 We 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, a summary of the power conversion technologies approaches being pursued, and a 

brief digest of first year accomplishments. 

Summary of the Need and Future of Advanced RPS in Space 

Radioisotope power systems are capable of producing decades of both heat and electricity under the harsh deep 

space conditions where the sunlight is too faint to power a spacecraft. NASA and the U.S. Department of Energy 

(DOE) (and its predecessors) have used radioisotope power systems reliably in space exploration for more than 
NASA/TM—2005-213981 2 

35 years to serve this purpose (see Table 1). To date, the United States has launched 25 missions involving 44 

Radioisotope Thermoelectric Generators (RTGs). RTGs have demonstrated continuous power for 30-plus years, and 

have been proven safe, reliable and maintenance free (DOE, 2002). 

NASA has identified a number of potential missions that can best or only be undertaken using radioisotope 

power and/or heat sources. Future potential planetary missions such as Titan Explorer, Europa Lander, or Mars 

Landers require RPS that are mass and volume efficient, and that can work reliably for long periods of time in 

vacuum and planetary atmospheres. (NASA, 2002) The future potential missions also require a technology that can 

be scalable from a few watts to a few hundred watts as requirements vary from mission to mission. To enable the 

next ambitious steps in exploration of our Solar System using safe, cost effective spacecraft, NASA and DOE are 

developing advanced, high-efficiency radioisotope power convertors (NASA, 2003) (see Table 1). Several 

radioisotope power conversion technologies for space applications are under investigation through RPCT NRA 

contracts. Safety and reliability are key factors in determining the possible value of each technology in space 

missions. These technologies 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 Pu-238 than 

current RTGs typically use. This makes these new space power technologies highly attractive due to lower weight 

and less radioactive material for the same power output. In addition, these power conversion designs must minimize 

electromagnetic interference and vibration to enable scientific experiments. 

TABLE 1.—PAST, PRESENT, AND FUTURE OF RADIOISOTOPE POWER SYSTEMS 

Past Present Under Development RPCT NRA 

Application 

SNAP-27 

(Apollo)

SNAP-19 

(Viking, Pioneer) 

MHW RTG 

(Voyager)

GPHS RTG 

(Galileo, Cassini)

Multi-Mission 

RTG (MMRTG)

Stirling 

Radioisotope 

Generator

(SRG)

TBD 

Power 

Conversion 

Approach 

Thermoelectrics Thermoelectrics Thermoelectrics Stirling 

Free-Piston Stirling 

Turbo-Brayton 

Segmented Thermoelectrics 

Thermophotovoltaics 

Specific Power 

(We/kg) 3 to 4 5.4 3 to 4 3.3 to 4 5 to 10 

System 

Efficiency 6 percent 7 percent 6 percent 23 percent 12 to 35 percent 

Power (We) SNAP19 ~40

MHW ~160 285 >110 >110 ~100Class 

Scalable 

Life (years) ---- >15 10 to 14 (Space) 

>3 (Mars) 

10 to 14 (Space)

>3 (Mars) >14 Years 

Environment Space Space Multi-Mission Multi-Mission Multi-Mission 

Summary of How an RPS Works 

A radioisotope power system converts the heat emitted from the decay of the radioactive isotope plutonium238, which is unusable for nuclear weapons, into the electricity required to power a spacecraft. These power systems 

are ideal for spacecrafts that are too far from the sun to effectively use solar energy conversion, such as, the Galileo 

mission to Jupiter and the recently launched Cassini mission to Saturn. In addition to radioisotope power systems, 

radioisotope heater units (RHUs) use the heat generated by plutonium-238 to keep a spacecraft’s instruments warm 

and within their designed operating temperatures (DOE, 1990). 

Heat is supplied to the convertor from a DOE General Purpose Heat Source (GPHS) module, containing 

approximately 600 grams of Plutonium dioxide that produces about 250 watts of thermal power. However, only a 

portion of the heat energy supplied to the power convertor is converted into electricity. The remainder is rejected as 

waste heat and is typically used by the spacecraft to maintain key electronics and instruments within their operating 

temperature range (see Fig. 1). NASA is responsible for RPS convertor technology development up to component 
NASA/TM—2005-213981 3 

Figure 1.—Anatomy of a Radioisotope Power System. 

TABLE 2.—THE GPHS/RTG PERFORMANCE PARAMETERS AS SPECIFIED IN THE RPCT NRA 

Performance Parameters for GPHS/RTG 

System Efficiency 7 percent 

Power Conversion Device Efficiency 8 percent 

Specific Power 5 We/kg (Beginning of Life, BOL) 

Specific Power 

4 We/kg (BOL+10 years) 

In addition to offering advancements over State of Practice, it is also required 

that non-milliwatt RPS have an output voltage of 28VDC. 

and/or breadboard validation in a relevant environment (Technology Readiness Level TRL 5). And DOE is 

responsible for generator system development beyond TRL 5. The RPCT NRA technology development is inside 

the dashed line in Figure 1. 

Advanced RPS designs will interface with GPHS modules, or a de-rated GPHS. For the nominal, 100 We level 

RPS effort, the proposed power conversion technologies will offer advancements over the State-of-Practice 

GPHS/Radioisotope Thermoelectric Generator (RTG) performance parameters (see Table 2) as specified in the 

RRCT NRA (NASA, 2002). A de-rated GPHS can be used for scaling to low power levels (less than the nominal 

100 We). While the milliwatt Radioisotope Power Systems must interface with either one or more Radioisotope 

Heater Units (RHUs) or with a de-rated GPHS module. Descriptions of the GPHS and RHU can be found at 

http://nuclear.gov/ under "Program Offices," "Space and Defense Power Systems." 

Advanced RPS Project Goal 

The goal of the Advanced RPS Project is to advance, via research and technology development, promising 

power conversion technologies for potential application in RPS beyond current 1st-generation Multi-mission 

Radioisotope Thermoelectric Generators (MMRTG) and Stirling Radioisotope Generators (SRG) that are being 

developed for potential use on the 2009 Mars Science Laboratory (MSL) mission. 

• Viability is characterized by safety, performance (power conversion efficiency and specific power (We/Kg), 

reliability and long life (>14 yrs with well understood degradation), weight and volume, development risk, 

scalability, operational capability, and system integration 

• Success depends on providing a significant improvement over state-of-practice RPS and meeting the needs 

of future mission designers

Radioisotope Power Conversion Technology (RPCT) Areas 

The objective of the Radioisotope Power Conversion Technology (RPCT) NASA Research Announcement 

(NRA) is to advance the development of radioisotope power conversion technologies to provide higher efficiencies 

and specific powers than existing systems. The RPCT NRA selected advanced RPS power conversion technology 

Radiator

Heat 

Source

Electrical

Power

High Temp

Waste 

Heat

Low Temp 

Power 

Conversion

Generator

GPHS 

Pu-238 Thermal 

Source 

Spacecraft 
NASA/TM—2005-213981 4 

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

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

power conversion research (NASA, 2002). 

Innovative RPS Power 

Conversion Research

• Research will be conducted on a broad range of innovative space application RPS 

power conversion technology concepts, for the development of very low Technology 

Readiness Level (TRL) research activities. At the end of the 3 years, it is expected 

that the technology will be developed to a TRL level of 2-3 (see Table 3 below). 

RPS Power Conversion 

Technology 

Development 

• Development will be conducted on advance RPS technologies from a basic/applied 

research level to verification of power conversion device and/or breadboard 

validation. At the 3 years, it is expected that the technologies will be at TRL 4-5. 

Milliwatt/Multi-watt 

RPS (mWRPS) Power 

Conversion Research

• Research will be conducted on advanced power conversion options for 

milliwatt/multi-watt-level power converters compatible with heat sources designed 

for space applications. At the end of the 3 years, it is expected that the technology 

will be developed to a TRL level of 2-3. 

TABLE 3.—THE NASA TECHNOLOGY READINESS LEVELS 

TRL Accomplishment 

5 Component and/or breadboard validation in relevant environment. 

4 Component and/or breadboard validation in laboratory environment. 

3 Analytical and experimental critical function and/or characteristic proof of concept. 

2 Technology concept and/or application formulated. 

1 Basic principles observed and reported. 

Introduction to the RPCT Power Conversion Technologies

Four power conversion technologies are being explored under RPCT: dynamic technologies like Brayton and 

Stirling, and static technologies like thermoelectric (TE) and thermophotovoltaic (TPV). The dynamic systems have 

moving parts that transform heat into mechanical energy, which in turn is used to generate electricity. The dynamic 

isotope power systems are more efficient (~20 to 35 percent) in converting heat into electricity than the static power 

conversion technologies. While the static systems have a benefit of having no moving parts, they also have lower 

efficiencies (~10 to 20 percent) than the dynamic systems. Both the advanced static and dynamic power conversion 

approaches have projections for specific power of between 5 to 10 We/kg. All four power conversion approaches use 

heat from the GPHS to heat the hot side of the device, and the cold of space (or an atmosphere) to produce a low 

temperature at the cold side of the device. 

A thermoelectric (TE) power convertor is a static energy conversion system that turns heat directly into 

electricity utilizing the Seebeck effect. A thermoelectric couple is produced when the junctions of a circuit formed 

from two dissimilar materials (such as n- and p-type semiconductors) are connected electrically in series but 

thermally in parallel and the junctions are maintained at different temperatures; the open circuit voltage developed is 

proportional to the combined Seebeck coefficient of the two materials and the temperature differential across the 

junctions. A TE convertor contains many thermoelectric couples (or thermocouples). Such thermoelectric couples 

are typically arrayed in a series/parallel configuration to produce the desired output power characteristics. The 

efficiency and therefore power output is a function of the temperature of each junction and thermoelectric material 

properties. High conversion efficiency requires TE materials that possess a combination of low thermal 

conductivity, low electrical resistivity and large Seebeck coefficient values. 

Thermophotovoltaic (TPV) energy conversion is a static process whereby infrared photons, emitted by a hot 

surface, are converted to electrical energy by a photovoltaic (PV) device. TPV convertors use advanced PV cells, 

spectrally-tuned to optimize the conversion of the emitted photon flux. Spectral control, including selective emitter, 

PV cell, and filter characteristics, are key to high performance. 

A Free-piston Stirling engine operates on the Stirling thermodynamic cycle and has two reciprocating 

components, the displacer and the piston. The displacer shuttles the working gas, typically Helium, between the hot 
NASA/TM—2005-213981 5 

expansion space, and the cold compression space through a series of heat exchangers. The regenerator improves the 

efficiency of the cycle by storing heat as the hot gas flows toward the cooler, and heats the cool gas as it flows 

toward the heater conserving energy. The pressure waves created by piston and displacer motion, and the heating 

and cooling of the gas, cause sustained piston and displacer reciprocating motions (free-piston designs can provide 

self-starting oscillations, once an adequate temperature differential is achieved). A linear alternator attached to the 

piston converts this mechanical motion into electrical power. 

A Brayton power converter consists of a turbine, compressor, alternator and separate heat exchangers for the 

heat source and the waste heat rejection. An additional heat exchanger, called a recuperator, is often used to transfer 

heat within the cycle and improve cycle efficiency. An inert gas, typically a mixture of helium and xenon, is used as 

the working fluid. The working fluid undergoes a thermodynamic cycle; the fluid is heated in the heat source heat 

exchanger, expanded through the turbine, passed through a gas cooler (rejecting waste heat) and pressurized by the 

compressor before re-entering the heat source heat exchanger to be heated again. A rotary alternator attached to the 

turbine shaft produces alternating current (AC) electrical output. 

The Radioisotope Power Conversion Technology (RPCT) Efforts 

Ten RPCT NRA contracts were awarded in the areas of Thermoelectrics, Thermophotovoltaic, Stirling, and 

Brayton power conversion technologies in the research areas identified in Table 4. Each NRA contract is divided 

into 3 one-year periods. For each of the ten RPCT NRA contracts, Phase I accomplishments, as presented by the 

contractors, are summarized below, along with a short description of their project 

TABLE 4.—RPCT NRA NUMBER AND DISTRIBUTION OF CONTRACTS PER FOCUS AREA 

Focus Areas Nominal ~100 We-scale applications 

(1 to 100 We scalable) 

Milliwatt-scale (mW) applications 

(~10 to 100 mWe) 

Research

TRL ≤ 3 Proof of concept

1 Thermoelectric (TE)

1 Thermophotovoltaic (TPV)

1 Stirling 

2 Thermoelectric (TE)

Development

TRL ≤ 5 Component/ breadboard in 

relevant environment

1 Thermoelectric (TE)

2 Thermophotovoltaic (TPV)

1 Stirling 

1 Brayton 

None funded 

Thermoelectrics (TE) 

The Teledyne Energy Systems/Boeing Segmented Thermoelectric Generator development effort evaluates the 

combination of a number of proven state-of-practice telluride materials in a segmented couple to take advantage of 

the high efficiencies available from Bi-Te materials at low temperatures with the demonstrated PbTe and TAGS 

performance at higher temperatures. This approach moves the heat source clad material well away from its high 

temperature limits for long term operation. First year achievements include: 

• A Phase I baseline BiTe/PbTe - BiTe/TAGS/PbSnTe compact module/mini-couple (4 by 4 in. with 108 

couples) design was completed, and projecting 20 We at 5V using single GPHS with BOL system 

efficiency of 8 percent (Thot = 538 °C, Tcold = 121 °C) 

• Demonstrated co-hot pressing of TAGS-85 and PbSnTe, and other advanced thermoelectric materials 

development efforts making progress 

The MIT/Boston College/JPL Si-Ge Nanocomposites in Thermoelectrics research effort is researching a novel 

approach for using nano-structured materials to improve significantly the performance of bulk thermoelectric 

materials. The use of SiGe-based nano-components to reduce phonon transport promises not to adversely affect the 

electrical properties. It thus has a strong chance to increase the conversion efficiency from the current 8 percent 

state-of-the-art to 14 to 16 percent. First year achievements include: 
NASA/TM—2005-213981 6 

• Successful synthesis of Si and Ge nanowires and nanoparticles 

• Successfully achieved bulk sample size of nano Si micro Ge composite 

• Nanostructures offer promise for increasing ZT (TE Figure-of-Merit), which leads to increase efficiency 

• Very significant reduction in thermal conductivity of Si-Ge nanocomposites 

• Developed first model for the thermal conductivity of 2D periodic nanocomposites 

Milli-Watt Thermoelectrics (mW) 

The Teledyne Energy Systems/Research Triangle Institute Superlattice Thermoelectric Power Systems research 

effort is to develop and test a cascaded thin-film Superlattice BiTe and PbTe/TAGS-PbSnTe thermoelectric 

converter module. This arrangement is expected to improve power conversion efficiency from 4 percent to 

12 percent and reduce plutonium requirements by 50 to 70 percent. Teledyne Energy Systems is leading the effort 

with Research Triangle Institute (RTI) as an important team member. First year achievements include: 

• Developed a co-hot pressing process for fabrication of segmented TAGS-PbSbTe 

• Identified and evaluated several commercial ceramic adhesive and low temperature glass formulations for 

bonding thermoelectric material slices 

• Fabricated several linear array multicouples 

• Superlattice Cold Stage device: developed and fabricated high performance Superlattice unicouple and 

multicouple with high aspect (L/A) ratio elements with superior performance 

The Hi-Z Technology MultiWatt Quantum Well Thermoelectric research effort investigates use of materials 

employing Quantum Well (QW) technology, which could increase thermoelectric conversion efficiencies to more 

than 25 percent. The objective is to develop QW materials from their current proof-of-principle status to a prototype 

level for higher temperature space power systems. 30 to 50-micron thick multilayered QW films will be deposited 

onto 5-micron thick Si substrates via a sputtering process. First year achievements include: 

• Hi-Z obtained a new Sputtering machine with ~15 cm diameter samples and planetary expansion system 

• Conducted thermal aging of both Si/SiGe and B4C/B9C QW structures, showing very little change over 

time even at 600 °C 

• Modeling work in progress 

• QW Materials have been fabricated on a thick (~200 micron) substrate (goal is on a 5 micron substrate) 

Thermophotovoltaic (TPV) 

The Essential Research, Inc./University of Notre Dame Very High Efficiency (Thermophotovoltaic) TPV 

Converter research effort investigates application of non-planar TPV cells, which may yield a 50 percent increase in 

conversion efficiency over state-of-the-art planar TPV technology. For a band gap of 0.6eV, and an assumed 

20 percent planar device conversion efficiency, the proposed non-planar cell with sub-micron size p/n dot-junctions 

may yield conversion efficiencies of almost 30 percent, a significant increase over traditional thermoelectrics. 

Essential Research plans to employ a proprietary technology to produce dot junction geometries (which have been 

demonstrated to be highly efficient in silicon solar cells) at sub-micron sizes in defect free, lattice mismatched 

InGaAs on InP substrates. First year achievements include: 

• Successfully completed materials and microprocessing development 

• Demonstrated 18.5 percent improvement in AM0 (space environment) efficiency of a floating junction cell 

over a reference planar cell 

• However, initial results showed a 1 percent decrease in Voc and a 50 percent increase in dark 

saturation current density (J0). The goal was to produce a dot junction cell with an increase in Voc 

and an order of magnitude less J0 than with an “identical” planar junction cell. 
NASA/TM—2005-213981 7 

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

Advanced TPV Development effort involves the development of an advanced TPV concept that includes high 

temperature selective emitters, filters and advanced lattice-mismatched InGaAs PV cells. Creare expects to 

demonstrate a system conversion efficiency of 15 to 20 percent, a specific power of 10 to 15 We/kg, and a TRL of 5. 

The starting point is the 20 percent efficiency already demonstrated by one of the team members, and their 

confidence level for further improvement is high. First year achievements include: 

• Phase I baseline converter achieved 19 percent efficiency (includes tandem plasma-dielectric filter with a 

4×4 InGaAs array) with Temitter at 1077 °C and Tcell at 27 °C 

• Plasma sprayed tungsten emitter fabrication demonstrated 

• NRL Damage Dose Displacement method used to assess neutron degradation using protons 

• Facility prepared for materials evaporation and deposition testing 

The EDTEK Inc./University of Houston TPV Radioisotope Power Conversion Technology 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 noncontaminating emitter, and a unique scheme for thermal management that will allow a reduction in total system 

mass by about a factor of 3. If successful, the projected system efficiency could achieve a maximum of 23 percent. 

First year achievements include: 

• Phase I baseline converter achieved 20 percent efficiency (includes FSS filter, prism, and GaSb array) with 

Temitter at 1050 °C and Tcell at 18 °C 

• Single-layer filter model has been completed 

• Aperture Array Lithography demonstrated for FSS bandpass filter fabrication 

• Installation of new nano-positioning stage for precise and more efficient filter fabrication 

Stirling and Brayton 

The Cleveland State University/University of Minnesota/Gedeon Associates/Sunpower/STC Stirling Convertor 

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 will address the significant performance 

improvements achieved through better understanding of regenerator flow fields and manifold effects. It will also 

utilize these results to work with Stirling designers and micro-fabricators to build an advanced regenerator that 

achieves the potential gains. First year achievements include: 

• Developed a microscale regenerator design based on state of the art analytic and computational tools, 

projecting a 6 to 9 percent increase in power for a Stirling engine 

• Improved structure for higher reliability, which permits use of progressive materials for strength and 

corrosion resistance 

• Selected a manufacturing vendor and a microfabrication technique 

• Completed EDM tools for fabricating micro regenerator which meet specifications 

• Designed a Large-Scale Mock-up (LSMU) and began fabrication 

The Sunpower/Boeing Rocketdyne/Cleveland State University/University of Minnesota Advanced Stirling 

Convertor development effort will develop a Stirling converter that has considerably improved performance 

characteristics over current systems. The key technologies from this effort are hydrostatic gas bearings, a moving 

magnet linear alternator, a high temperature heater head, and a controller with active power factor correction. The 

proposed effort will demonstrate a converter that is superior to the Stirling Radioisotope Generator (SRG) unit that 

DOE and NASA are currently developing for flight. The efficiency of the proposed unit is 32 percent (compared to 

23 percent for the SRG), and the convertor specific power is nearly 100 We/kg (compared to 15 We/kg for the SRG). 

First year achievements include: 
NASA/TM—2005-213981 8 

• The Phase I Frequency Test-Bed (FTB) convertor achieved the highest efficiency 36 percent (engine + 

alternator) for any free-piston Stirling = 54 percent of Carnot (Thot = 650 °C, Tcold = 30 °C, Thot/Tcold = 3) 

• The Advanced Stirling Convertor ASC-1 (Phase II, bolted flange design) design has been initiated, and is 

projecting ~ 40 percent efficiency (engine + alternator) = 60 percent of Carnot (Thot= 850 °C, Tcold = 90 °C, 

Thot/Tcold = 3) 

• Identified high temperature heater head materials to allow operation at 850 °C and developed joining and 

processing techniques (Reference SRG at 650 °C) 

• Conducted reliability assessment to identify key elements which are now part of a reliability test plan 

The Creare Inc./Ball Aerospace/Boeing Rocketdyne/Jackson & Tull Turbo-Brayton Power System (TBPS) 

development will develop a high-efficiency (25 to 36 percent), low-mass (6.7 kg) turbo-Brayton power conversion 

system. Key to this effort is application of micro-machining and fabrication techniques to build a very small, 

compact turbine-alternator that can operate at 1200K. The effort draws on the talents of a multidisciplinary team that 

will design, fabricate and test a representative converter system. First year achievements include: 

• Completed converter design with projected efficiency of >20 percent (Thot = 777 °C, Tcold = 59 °C) 

• Identified Turbo-Alternator-Compressor materials and fabricated miniature rotor including turbine, 

compressor, shaft, and magnet 

• Demonstrated 200 start/stop cycles using spin test rig without wear 

• Designed silicon heat exchanger recuperator, fabricated components, and developed subscale recuperator 

for proof-of-concept testing. An annular recuperator concept has also been designed. 

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 the ten 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. 

References 

Department of Energy, Space Radioisotope Power Systems, “Advanced Radioisotope Power Systems Fact Sheet,” 

(2002) http://www.ne.doe.gov/space/arpsfact.pdf, accessed November 2004. 

Department of Energy`, Office of Nuclear Energy, Science, and Technology, “Nuclear Power in Space,” DOE/NE0071, (1990) http://www.ne.doe.gov/pubs/npspace.pdf, accessed November 2004. 

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-OSS-01-rpct/index.html, accessed November 2004. 

NASA, “NASA VISION MISSIONS Nuclear Systems Program Office Project Prometheus,” (2003) 

http://spacescience.nasa.gov/missions/npsfactsheet.pdf, accessed November 2004. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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14

NASA Radioisotope Power Conversion Technology NRA Overview

David J. Anderson

Stirling cycle; Thermoelectric generators; Brayton cycle; Thermoelectricity;

Space missions; Thermophotovoltaic conversion; Energy conversion efficiency;

Power convertors; Regenerators

Unclassified -Unlimited

Subject Categories: 20 and 44

Prepared for the Space Technology and Applications International Forum (STAIF–2005) sponsored by the University of

New Mexico's Institute for Space and Nuclear Power Studies (UNM–ISNPS), Albuquerque, New Mexico, February

13–17, 2005. Responsible person, David J. Anderson, e-mail: David.J.Anderson@nasa.gov, organization code PS,

216–433–8709.

The focus of the National Aeronautics and Space Administration’s (NASA) Radioisotope Power Systems (RPS)

Development program is aimed at developing nuclear power and technologies that would improve the effectiveness of

space science missions. The Radioisotope Power Conversion Technology (RPCT) NASA Research Announcement (NRA)

is an important mechanism through which research and technology activities are supported in the Advanced Power

Conversion Research and Technology project of the Advanced Radioisotope Power Systems Development program. The

purpose of the RPCT NRA is to advance the development of radioisotope power conversion technologies to provide higher

efficiencies and specific powers than existing systems. These advances would enable a factor of two to four decrease in the

amount of fuel and a reduction of waste heat required to generate electrical power, and thus could result in more cost

effective science missions for NASA. 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 100 We 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, a

summary of the power conversion technologies approaches being pursued, and a brief digest of first year accomplishments.