DESIGN, ANALYSIS, AND OPTIMIZATION OF A RADIOISOTOPE THERMOPHOTOVOLTAIC (RTPV) GENERATOR, AND ITS APPLICABILITY TO AN ILLUSTRATIVE SPACE MISSION

The paper describes the results of a DOE-sponsored design study of a r adioisotope thermophotovoltaic generator (RTPV), to complement similar studies of Radioisotope Thermoelectric Generators (RTGs) and stirling Generators (RSGs) previously published by the author. To focus the de sign effort, it was decided to direct it at a specific illustrative sp ace mission, Pluto Fast Flyby (PFF). That mission, under study by the Jet Propulsion Laboratory (JPL), envisages a direct eight to nine-year night to Pluto (the only unexplored planet in the solar system), foll owed by comprehensive mapping, surface composition, and atmospheric st ructure measurements during a brief flyby of the planet and its moon C haron, and transmission of the recorded science data to Earth during a six-week post-encounter cruise. Because of Pluto's long distance from the sun (30-50 A.U.) and the mission's large energy demand, JPL has b aselined the use of a radioisotope power system for the PFF spacecraft . RTGs have been tentatively selected, because they have been successf ully flown on many space missions, and have demonstrated exceptional r eliability and durability. The only reason for exploring the applicabi lity of the far less mature RTPV systems is their potential for much h igher conversion efficiencies, which would greatly reduce the mass and cost of the required radioisotope heat source. Those attributes are p articularly important for the PFF mission, which - like all NASA missi ons under current consideration - is severely mass- and cost-limited. The paper describes the design of an RTPV system consisting of a radio isotope heat source, a thermophotovoltaic converter, and an optimized heat rejection system; and depicts its integration with the PFF spacec raft. It then describes the optical, thermal, electrical, and structur al analyses which led to that optimized design, and compares the compu ted performance of an RTPV system to that of an RTG designed for the s ame mission. Our analytical results indicated that - when fully develo ped - it could result in a 67% reduction of the heat source's mass, co st, and fuel loading, a 50% reduction of generator mass, a tripling of the power system's specific power, and a quadrupling of its efficienc y. And that even greater improvements may be achievable by combining t he RTPV radiator with the spacecraft's antenna. The paper concludes by briefly summarizing the RTPV's current technology status and assessin g its potential applicability to the PFF mission. For other power syst ems (e.g. RTGs), demonstrating their flight readiness for a long-term mission is a very time-consuming process, because of the need for long -duration performance degradation tests. But for the case of the descr ibed RTPV design, the paper lists a number of factors, primarily its c old (0 to 10 degrees C) converter temperature, that may greatly reduce the need for long-term tests to demonstrate generator lifetime. in an y event, our analytical results suggest that the RTPV generator, when developed by DOE and/or NASA, would be quite valuable not only for the Pluto mission but also for other future missions requiring small, lon g-lived, low-mass generators.