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Space Applications
Published in D.M. Rowe, CRC Handbook of Thermoelectrics, 2018
The U.S. began studying the use of NPS in the late 1940s and early 1950s. By the mid- to late 1950s the U.S. had active programs to develop both space radioisotope and space reactor power sources. The first known actual use of a NPS on a spacecraft came in 1961 with the launch of the small SNAP-3B radioisotope thermoelectric generator (RTG) by the U.S. (SNAP is an acronym for Systems for Nuclear Auxiliary Power. All odd-numbered SNAP power sources used radioisotope fuel and all even-numbered SNAP power sources used nuclear fission reactors.) In total, as shown in Table 1, the U.S. has launched 41 RTGs and one reactor to provide power for 25 space systems. (Thirty-eight of these NPS on 22 space systems are still in space or on other planetary bodies. Four U.S. RTGs have returned to Earth in one form or another because of spacecraft or launch vehicle malfunctions.) The U.S. has also used small radioisotope heater units (RHUs) on some of its RTG-powered science missions and on the Apollo 11 science package. All of the U.S. RTGs have used 238Pu as the source of heat because of its long half-life (87.8 years) and its comparatively low level of radiation emission. The only U.S. space reactor flown used 235U as the fuel.' The former Soviet Union has reportedly placed at least 35 reactor-powered and two RTG-powered satellites in orbit and placed at least two RHU-heated rovers on the Moon. In addition the former Soviet Union has reportedly had at least six re-entries of NPS (two of radioisotope units and four of reactors).4
Development of a Novel Miniature Power Converter for Low-Power Radioisotope Heat Sources: Numerical and Experimental Results
Published in Nuclear Technology, 2021
Francisco I. Valentín, Gregory Daines
Future NASA missions will benefit from miniature thermal-to-electric energy conversion technology with higher efficiency and power density. These small-scale converters are required to transform the thermal energy from LWRHUs [i.e., radioisotope heater units (RHUs)] into on-demand electricity essential to space exploration probes, unmanned surface rovers, small landers, small satellites, and similar small-scale systems operating in darkness.9 In response, Creare is developing a modified Stirling free-piston converter (MSC) that promises a high-efficiency and simple-to-manufacture system in an extremely compact enclosure. The primary challenge of the proposed project is the design and fabrication of high-efficiency convertors of extremely small size. The primary loss mechanisms in miniature converters are generally different from those encountered at conventional sizes.10 For miniaturized systems, defining the fractional contribution of frictional losses, leakage, parasitic heat losses, and potential dead volumes becomes increasingly important to a system’s power density as its design volume is reduced.