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

Glenn Research Center, Cleveland, Ohio

Status of NASA’s Advanced Radioisotope Power

Conversion Technology Research and Development

NASA/TM—2006-214243

November 2006
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Wayne A. Wong, David J. Anderson, Karen L. Tuttle, and Roy C. Tew

Glenn Research Center, Cleveland, Ohio

Status of NASA’s Advanced Radioisotope Power

Conversion Technology Research and Development

NASA/TM—2006-214243

November 2006

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Space Technology and Applications International Forum (STAIF–2006)

sponsored by the University of New Mexico’s Institute for Space

and Nuclear Power Studies (UNM-ISNPS)

Albuquerque, New Mexico, February 12–16, 2006
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 acknowledge the technical achievements by the respective

contractor teams, and the contributions of the NASA contract and technical monitors.

Available from

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7121 Standard Drive

Hanover, MD 21076–1320

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

Trade names and trademarks are used in this report for identification

only. Their usage does not constitute an official endorsement,

either expressed or implied, by the National Aeronautics and

Space Administration.

Level of Review: This material has been technically reviewed by technical management.
NASA/TM—2006-214243 1

Status of NASA’s Advanced Radioisotope Power Conversion 

Technology Research and Development 

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

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

NASA’s Advanced Radioisotope Power Systems (RPS) development program is funding the advancement of 

next generation power conversion technologies that will enable future missions that have requirements that can not 

be met by either the ubiquitous photovoltaic systems or by current Radioisotope Power Systems (RPS). 

Requirements of advanced radioisotope power systems include high efficiency and high specific power 

(watts/kilogram) in order to meet mission requirements with less radioisotope fuel and lower mass. Other Advanced 

RPS development goals include long-life, reliability, and scalability so that these systems can meet requirements for 

a variety of future space applications including continual operation surface missions, outer-planetary missions, and 

solar probe. This paper provides an update on the Radioisotope Power Conversion Technology Project which 

awarded ten Phase I contracts for research and development of a variety of power conversion technologies 

consisting of Brayton, Stirling, thermoelectrics, and thermophotovoltaics. Three of the contracts continue during the 

current Phase II in the areas of thermoelectric and Stirling power conversion. The accomplishments to date of the 

contractors, project plans, and status will be summarized. 

Introduction 

NASA has a history of successful space flight missions that depended on radioisotope fueled power systems. 

These Radioisotope Power Systems (RPS) use the heat generated from the decay of radioisotope material, and 

convert the heat into useful electrical power. RPS is most attractive in applications where photovoltaics are not 

optimal such as for deep space applications where the solar flux is too low or for extended surface applications such 

as those on Mars or the Moon where the day/night cycle, settling of dust and life requirements limit the usefulness of 

photovoltaics. Past RPS systems utilizing thermoelectric power conversion have proven to be highly reliable, longlived designs. The U.S. has flown twenty-one space missions that have successfully used Radioisotope 

Thermoelectric Generators (RTG), which incorporate General Purpose Heat Source (GPHS) modules. While a 

proven reliable design, the GPHS-RTG has a relatively low power conversion efficiency of about 7 percent, a BOM 

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

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

(RPS) Program is to develop power systems and technologies that will enable or improve the effectiveness of future 

space science missions. The NASA Science Mission Directorate, recognizing the need for reestablishing and 

improving RPS, is currently developing two flight systems, the Multi-Mission RTG (MMRTG) and the Stirling 

Radioisotope Generator (SRG) (Schmidt, Wiley, and Richardson, 2005). NASA anticipates future mission 

requirements that go beyond the capabilities of MMRTG and SRG requiring advanced RPSs that offer better 

performance and higher specific power. This paper describes NASA’s Radioisotope Power Conversion Technology 

(RPCT) Project, the purpose of which is to improve future RPS by developing conversion technologies applicable to 

these systems that provide higher efficiency and specific power. The paper provides background on the RPCT 

Project, and describes in greater detail the current status. 

Radioisotope Power Conversion Technology (RPCT) Project 

To develop technologies applicable to advanced RPS, NASA released NASA Research Announcement (NRA) 

02-OSS-01 entitled “Radioisotope Power Conversion Technology” (RPCT) soliciting proposals for development of 

next generation power conversion technology (NASA, 2002). The objective of the RPCT Project, managed by 

NASA Glenn Research Center (GRC), is to advance the development of radioisotope power conversion technologies 

to provide higher efficiency and specific power than the state-of-practice GPHS/RTG. Other general Advanced RPS 
NASA/TM—2006-214243 2

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. The emphasis of the RPCT NRA contracts is the development 

of the power conversion technology (converting heat to electric power). Many auxiliary elements of the RPS are 

considered to be beyond the scope of this NRA, such as radiators, GPHS, insulation, etc. and are not included in this 

technology development effort. It is anticipated that future development efforts in collaboration with DOE will 

encompass development of the non-converter components and integration of these components with the power 

conversion system. 

Phase I of the RPCT Project was initiated in the summer/fall of calendar year 2003 when ten NRA contracts 

were awarded. The selections included 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). The selections included a broad range of conversion technologies including free-piston Stirling 

(Wood, et al., 2006), turbo-Brayton (Zagarola, et al., 2005), thermoelectrics (TE) (Flanders, et al., 2005), and 

thermophotovoltaics (TPV) (Crowley, et al., 2005 and Horne, et al., 2005). Each RPCT NRA contract had 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. In July/August 2004, an annual review was conducted of all ten Phase I 

RPCT NRA contracts. The Phase I review committee included personnel from NASA Headquarters, GRC, Jet 

Propulsion Laboratory (JPL), and The Department of Energy (DOE) supported by Orbital Sciences Corporation. 

Based on the results of the review, a decision was made to continue seven of the ten contracts into Phase II, Phase II 

started in the November 2004 to January 2005 timeframe. However, due to a severe RPS Program budget reduction 

in January of 2005, four more of the Phase II contracts were terminated and only three of the Phase II contracts 

continue as of this writing. Funding for Phase I of all 10 RPCT contracts totaled 12.4 million, while the reduced 

funding for the three Phase II contracts is 5.4 million. An overview at the beginning of Phase I was presented by 

Wong, (2004), the status of the Phase I accomplishments was presented by Anderson, (2005), and a status of the 

efforts which started Phase II was presented by Anderson, Wong, and Tuttle, (2005). The following provides a 

current status of the work being performed as part of the three on-going Phase II RPCT NRA contracts led by 

Sunpower, Cleveland State University, and Massachusetts Institute of Technology, respectively. 

Advanced Stirling Convertor Development 

Sunpower, Inc. (Athens, OH) is leading a team consisting of Pratt and Whitney Rocketdyne, Cleveland State 

University (CSU), University of Minnesota (UMN), and several consultants to demonstrate an Advanced Stirling 

Converter (ASC) with significant improvements over 1st-generation flight systems and to achieve converter 

efficiency of >30 percent and a system specific power of >8W/kg. The Sunpower free-piston Stirling contract is the 

only remaining higher TRL Development effort that is continuing through Phase II of the RPCT Project. The key 

technologies included in the ASC design are hydrostatic gas bearings, moving magnet linear alternators, high 

temperature heater head materials and fabrication techniques, high temperature high porosity regenerators, high 

frequency operation, and a controller with active power factor correction. The expected efficiency of the ASC is 

~40 percent (AC power out/heat in), with a design output power of 88 We AC with an engine/alternator specific 

power of about 90 We/kg making feasible a projected RPS specific power of ~8 W/kg. The charge pressure of the 

convertor is 3.5 MPa, and the frequency is 105 Hz Sunpower’s Frequency Test Bed (FTB) Stirling convertor was 

designed, fabricated, and was made operational within the first five months of Phase I to allow frequency and 

advanced component investigations. Testing of the FTB would provide guidance on the design of the ASC. The FTB 

performance has surpassed project efficiency goals having demonstrated 36 percent conversion efficiency (AC 

power out/heat in) at operating temperatures of 650 °C hot end and 30 °C cold end with output power of 80 We. 

These temperatures represent a temperature ratio of 3, which is similar to the ASC which will operate at higher 

temperatures of 850 and 90 °C. ASC-1 (see fig. 1) was designed during Phase I and four of these non-hermetically 

sealed units will be built during the current 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. 

Phase III will involve the development of the ASC-2 which is a hermetically sealed design. The ASC is expected to 

produce 88 We AC at about 40 percent conversion efficiency assuming 220 W thermal input from the radioisotope 

heat source (minus insulation losses). 
NASA/TM—2006-214243 3

Figure 1.—Sunpower Advanced Stirling Convertor (ASC) artist concepts. 

Figure 2.—Sunpower advanced stirling 

convertor (ASC-1a) including an all 

Mar-M-247 heater head allowing 

850 °C operation. 

The ASC is designed with an advanced 850 °C heater head using Mar-M-247 which is to be provided by subcontractor Pratt and Whitney Rocketdyne. Prior to the selection of Mar-M-247, a variety of candidate high 

temperature materials was considered including materials that were being investigated and recommended by NASA 

GRC. For the 14 year life, high temperature requirements of the ASC, creep strength for the thin walled heater head 

pressure vessel is paramount. In addition to reviewing the data from creep testing conducted by NASA, the ASC 

team also conducted thin sample creep testing of Mar-M-247 during Phase I. Analysis indicates the projected 

1 percent creep of the heater head over 14 years has no impact on ASC performance. The heater head material 

processing strategy was developed which included a number of critical joints between Mar-M-247 and other 

materials including diffusion bonding to Nickel 201, and inertial welding to Inconel 718. A variety of coupon, 

joining, and processing tests were carried out during Phase I and Phase II for risk mitigation purposes in using these 

materials and processing techniques. 

The focus of the current Phase II effort is the development of the ASC-1, which is a non-hermetic design suited 

for laboratory development. The first half of Phase II included the fabrication and assembly of the ASC-1, shown in 

figure 2, which was completed and in operation by September 2005 (Wood, et al., 2006). At the time of this writing 

full temperature testing at 850 °C had not yet been conducted. Testing to date indicates that the ASC-1, which still 

requires some dynamics optimization, is operating at approximately 38 percent conversion efficiency (AC out/heat 

input). These test results provide confidence that the design predications of 88 We AC power output and ~40 percent 

conversion efficiency is achievable at 850 °C hot end and 90 °C cold end temperatures. 
NASA/TM—2006-214243 4

In addition to the development of the convertor hardware, significant effort during Phase I and Phase II also 

involved reliability studies, component testing, advanced component investigations, and thermodynamic loss 

investigations. As an example, during Phase I, displacer losses were studied both analytically via CFD by CSU and 

through component testing by Sunpower to allow a reduction in the number of baffles required in the ASC displacer. 

The impact of this work manifests as a cost savings (fewer parts), reduced fabrication risk to the hardware 

(simplifying manufacturing), and higher reliability design (lower part count) with no impact on performance. 

As part of the reliability assessment, during Phase II, a Hot Alternator Test Rig (HATR) was developed to 

characterize the ASC linear alternator performance at elevated temperatures and then to destructively test the 

alternator to identify temperature induced failure mechanisms. This ongoing work will provide valuable information 

on the margins in the alternator design and will provide input to the ongoing reliability assessment. 

The ASC regenerator continues to be an area of development during Phase II, including Large-Scale Mock-Up 

(LSMU) testing conducted by UMn which will provide data on the near wall and jetting characteristics of the 

regenerator. Processing, fabrication, and oxidation of this critical component continues to be an area of investigation 

by the Sunpower team with support from GRC, focusing on the use of high temperature oxidation resistant material. 

Accomplishments and Benefits of the Sunpower Advanced Stirling Convertor development effort: 

• Phase I FTB demonstration surpassed performance goal demonstrating 36 percent conversion efficiency 

• Phase II ASC-1a completed and operational; testing to date at lower than design temperature provides 

encouraging results that design performance goals (88 We AC, 40 percent efficiency) will be met later in 

Phase II 

• Projected ASC performance and mass leads to significantly higher RPS specific power of >8 W/kg 

enabling or enhancing future mission capabilities 

• Success of this NASA sponsored technology is spinning-off into potential DOD applications 

Stirling Regenerator Microfabrication Research 

Cleveland State University (CSU, Cleveland, OH) leads this research contract to develop micro-fabricated Stirling 

regenerators that offer structural robustness and improved thermal and flow performance, and to characterize a 

prototype regenerator in an engine. Projections indicate that the microfabricated regenerator may result in 6 to 9 percent 

higher power for an optimized Stirling convertor. The benefits of the advanced regenerator are not only to improve 

performance, but also improved structural robustness, reliability, manufacturability and an ordered geometry that lends 

itself to modeling and analysis. The CSU team includes University of Minnesota, Gedeon Associates, Sunpower, 

Infinia Corp. (formerly, Stirling Technology Company), and International Mezzo Technologies. 

The Stirling Regenerator Microfabrication research effort has developed a new involute-foil matrix geometry that 

has precisely defined geometrical features (no pore scale randomness as in random fiber and wire screen materials), 

structured to reduce regenerator pressure drop while maintaining good heat transfer characteristics (Ibrahim, et al., 

2005). Fabrication techniques require fine sub-100 7m scale features in a regenerator that is of 100 mm scale. Several 

geometries were considered (lenticular, honeycomb, involute, and modified involute) as well as a host of microfabrication techniques and vendors before a unique geometry and vendor using a combined LIGA-EDM process was 

selected. The selected regenerator design is composed of a stack of hundreds of 265 7m thick foil disks, each disk 

having a series of involute patterns on them. The design utilizes disks assembled with involutes in an alternating pattern 

as shown in figure 3(a), mixing the flow resulting in improved heat transfer. 

The CSU Regenerator Microfabrication NRA Contract is making progress in three major areas: 1) fabrication of 

involute-foil regenerators, 2) Empirical and Analytical heat transfer and pressure drop characterization using LargeScale-Mock-Up (LSMU) and CFD analysis, and 3) actual-scale oscillating-flow rig testing of the involute foils. More 

details on the Phase I accomplishments can be found in Ibrahim, et al., (2005). Mezzo is currently in the process of 

fabricating Nickel involute-foil regenerators for testing in the NASA/Sunpower oscillating-flow test rig. Mezzo expects 

to complete this microfabrication effort by the end of December 2005 via a LIGA (X-ray lithography) approach. An Xray mask that allows simultaneous production of 19 microfabricated disks is shown in figure 3(b). The resulting disks 

will be used for testing in the oscillating-flow rig to experimentally determine the friction-factor and heat-transfer 

characteristics of a representative regenerator stack. If testing in the oscillating-flow rig provides positive results, the 

next phase of the effort would be to fabricate a full regenerator using stainless-steel elements for testing in a Stirling 

engine. These would be fabricated via a combined LIGA-EDM technique. Figure 3(c) shows the details of a portion of 

a stainless-steel involute foil disk that was fabricated by Mezzo via the LIGA-EDM approach as a proof of concept. To 

complete Phase II, the regenerator disks fabricated by Mezzo will be tested in the oscillating-flow test rig by 

Sunpower and analyzed by Gedeon Associates in the January to March 2006 time period. 
NASA/TM—2006-214243 5

(a) Two Involute Disks Stacked 

in Alternating Sequence. 

(b) X-ray Mask. (c) Stainless Steel Involute Foil.

Figure 3.—Design and hardware from the CSU regenerator microfabrication contract. 

 

Figure 4.—Analytical predictions from the CSU regenerator microfabrication contract. 

To determine the figure of merit (ratio of heat transfer and pressure drop characteristics) for the involute foil 

regenerator, heat-transfer measurements and CFD heat-transfer calculations will be made. Figure 4(a) shows CFD 

determined velocity contours inside the flow channels of an involute foil disk. Such velocity results were used in 

calculating friction-factors. The CFD will also provide support to the analysis of the oscillating-flow rig results. 

Meanwhile testing of a Large-Scale-Mock-Up (LSMU) of the involute foils, and CFD simulation of them, is 

ongoing at the University of Minnesota (UMN) and Cleveland State University (CSU), respectively. Figure 4(b) 

shows UMN experimental friction-factor results for the LSMU involute-foil elements compared with friction-factors 

for (1) continuous channels which have the same aspect ratio as the tested foils, (2) staggered parallel-plates, (3) 

woven-screen matrices, and (4) random-fiber matrices. The LSMU testing offers the encouraging result that, while 

the involute-foil pressure drop will not be quite as low as if the channels were continuous (instead of interrupted 

from one disk to the next), the pressure drop will be substantially less than for wire-screens and much less than for 

random fibers which represent the state-of-the art regenerators. However, the interruption of the flow path from one 

disk to the next will improve radial heat transfer and decrease axial heat transfer (both desirable changes) relative to 

that of the continuous channels. Regenerator operation is expected to be close to the low end of the Reynolds 

operating range shown in figure 4(b) (~Reynolds No. = 100). CSU CFD calculations of involute foil friction-factors 

seemed consistent with the measurements. 

(a) CFD Calculated Velocity Contours in portion 

of Involute Foil.

(b) Darcy-Weisbach Friction-Factor versus 

Reynolds Number for Different Geometries. 
NASA/TM—2006-214243 6

Accomplishments and Benefits of the CSU Stirling Regenerator Microfabrication research effort: 

• Development of an advanced regenerator offering improved structural robustness for higher reliability, 

controlled manufacturability, and improved Stirilng convertor performance (projected 6 to 9 percent power 

increase) 

• Permits use of progressive materials for strength and corrosion resistance 

• Broad benefits—technology can be used in Stirling cryocooler regenerators and microscale heat exchangers

Thermoelectric (TE) Nanocomposite Research 

Massachusetts Institute of Technology (MIT), acting as the prime contractor, has teamed with Boston College 

(BC) and the Jet Propulsion Laboratory (JPL) on this research contract to design, synthesize, and develop Si-Ge 

nanocomposites with an improved figure-of-merit (ZT) over bulk SiGe. This nanostructure technology has the 

potential to increase the material’s performance by increasing the Seebeck coefficient and reducing the thermal 

conductivity without affecting the electrical conductivity. The effect of these improvements would be to provide a 

net gain in the ZT that enhances the thermoelectric performance. The goal of this effort is to demonstrate ZT of 

~2.0 at 900 K with efficiency of 12 to 14 percent. Another goal of this contract is to synthesize bulk nanocomposites 

in a large-scale manufacturing process such that large numbers of thermoelectric couples could be produced at a 

relatively low cost. Specific tasks of this research activity include nanostructure synthesis, structural 

characterization, thermoelectric properties characterization and modeling of the nanocomposites. 

During Phase II of this effort, progress was made in three areas: 1) Synthesis and structural characterization of 

both nanoparticles and nanocomposite samples, 2) Thermoelectric property measurements of the nanocomposite 

materials, and 3) modeling and simulation of the nanoparticles and the nanocomposites they formed. Synthesis of 

nanoparticles was advanced using three different methods (see fig. 5): 1) the wet chemistry method carried out at 

Boston College, which has a high potential for scale-up for making bulk quantities, 2) the ball milling method that 

has been under development at JPL, which also has potential for scale-up, and finally 3) the inert gas condensation 

method under development at the University of Illinois at Champaign/Urbana. These nanoparticles were hot pressed 

into nanocomposite samples using a newly acquired Plasma Pressure Compaction (P2

C) apparatus at Boston 

College, and in a hot press apparatus at JPL. Mechanically strong bulk size samples of nanocomposite materials 

were produced both at JPL and at Boston College that also preserved the nanostructure of the Si and Ge. 

Nanocomposite samples were characterized at room temperature for screening and promising samples were 

then characterized at high temperatures (up to 500 K at MIT and up to 1275 K at JPL). The thermal conductivity 

measurements of nanocomposite samples produced by both the mechanical alloying (MA) and P2

C methods indicate 

significantly lower thermal conductivity than any previously reported on bulk alloys. (Lee et al., 2005) A plot of 

temperature dependence of ZT for a variety of p-type thermoelectric samples (fig. 6) shows very encouraging 

results. For temperatures up to 1050 K, the best nanocomposite samples have higher ZT (better performance 

characteristics) than that of RTG SiGe and samples from SP-100 and JIMO. 

During Phase II, modeling advances were also made. MIT carried out calculations of the in-plane thermal 

conductivity of nanowires and nanowire composites by solving the two-dimensional Boltzmann transport equation 

for a variety of core-shell structures. These detailed calculations contributed to the selection of nanoparticles rather 

than nanowires as the basic nano-sized structured unit to be used in preparing the nanocomposite materials. 

Modeling in combination with experimental data shows that enhancements in ZT can be achieved in nanocomposite 

materials through both a reduction in the thermal conductivity and an increase in power factor on the same samples 

and within a common temperature range. 

Accomplishments and Benefits of the MIT Thermoelectric Nanocomposites research: 

• Nanostructures offer promise for increasing ZT and conversion efficiency by more than a factor of 2 

• Synthesis of bulk nanostructured materials enable practical realization of this potential 

• Detailed modeling of bulk nanostructures and nanocomposites guides materials design and enables further 

optimization 
NASA/TM—2006-214243 7

Figure 6.—Plot of ZT versus temperature for mechanical alloy and P2

C 

hot pressed nanocomposites as compared to the NASA flight RTG 

SiGe bulk materials and a variety of JIMO test samples. 

Conclusion 

NASA has successfully used radioisotope fuel to power numerous past missions in applications where PV 

arrays are not practical. Next generation conversion technologies suitable for future RPS, with higher efficiency, 

higher specific power (W/kg), long life, high reliability, scalability, and multi-mission capability are being 

developed under contracts awarded as part of the Radioisotope Power Conversion Technology Project. Results from 

the three on-going Phase II RPCT NRA contracts in the areas of Stirling and thermoelectric (TE) power conversion 

technology have been summarized. Emphasis of these advanced conversion technology development and research 

efforts is on improving performance, increasing system specific power, and providing reliability for long life. These 

attributes allow for improved RPS systems that will require less plutonium fuel, are more cost effective, have lower 

waste heat rejections requirements, and will enable or enhance future mission capability. These advanced RPS 

systems will provide NASA with attractive power system options for future Space Science and Exploration missions 

such as outer planetary exploration and Lunar and Mars exploration missions. 

Figure 5.—Si and Ge nanoparticles produced under the MIT contract. 

(a) (b) (c) (d) 
NASA/TM—2006-214243 8

References 

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Technology and Applications International Forum (STAIF-2005), edited by M. El-Genk, AIP Conference 

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Technical Memorandum

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

November 2006

NASA TM—2006-214243

E–15486

WBS 138494.04.01.01

14

Status of NASA's Advanced Radioisotope Power Conversion Technology

Research and Development

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

Advanced radioisotope power systems; Radioisotope power conversion technology;

Energy conversion; Stirling; Thermoelectric; Regenerator

Unclassified -Unlimited

Subject Category: 44

Prepared for the Space Technology and Applications International Forum (STAIF–2006) sponsored by the University of

New Mexico’s Institute for Space and Nuclear Power Studies (UNM-ISNPS), Albuquerque, New Mexico,

February 12–16, 2006. Responsible person, Wayne A Wong, organization code RPT, 216–433–6318.

NASA’s Advanced Radioisotope Power Systems (RPS) development program is funding the advancement of next

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

either the ubiquitous photovoltaic systems or by current Radioisotope Power Systems (RPS). Requirements of advanced

radioisotope power systems include high efficiency and high specific power (watts/kilogram) in order to meet mission

requirements with less radioisotope fuel and lower mass. Other Advanced RPS development goals include long-life,

reliability, and scalability so that these systems can meet requirements for a variety of future space applications including

continual operation surface missions, outer-planetary missions, and solar probe. This paper provides an update on the

Radioisotope Power Conversion Technology Project which awarded ten Phase I contracts for research and development

of a variety of power conversion technologies consisting of Brayton, Stirling, thermoelectrics, and thermophotovoltaics.

Three of the contracts continue during the current Phase II in the areas of thermoelectric and Stirling power conversion.

The accomplishments to date of the contractors, project plans, and status will be summarized.