Voyager 1 and 2 spacecraft led to a doubling of the power requirement compared to the SNAP-27 RTGs. The MHW-RTG, which is illustrated in Fig. 16, was designed to produce over 150 We at BOM. Two MHW-RTGs were flown on each LES as shown in Fig. 17 and three MHW-RTGs were flown on each Voyager as shown in Fig. 18. Originally, Voyagers 1 and 2 were to fly past Jupiter and Saturn. The MHW-RTGs were the first US space RTGs to use SiGe as the thermoelectric material (see Fig. 19). The use of SiGe permitted higher operating temperatures and higher specific powers all within a space vacuum operating environment [19]. The MHW-RTGs on LES 8/9 continue to operate beyond the prelaunch required 5-year operational life. Similarly, the MHW-RTGs on Voyagers 1/2 continue to operate well beyond the prelaunch required 4-year operational life. Because of the outstanding performance of the Voyager RTGs NASA was able to extend the Voyager mission to include flybys of Uranus and Neptune [20]. The RTGs are performing so well that scientific data will be received into the early 21st century [12], The successful performance of the MHW-RTGs has led to the use of the SiGe technology for the high-power (285 We) general-purpose heat source RTG (GPHS- RTG), shown in Fig. 20, which is to provide power for NASA's Galileo spacecraft and the European Space Agency's Ulysses spacecraft [21]. Table II illustrates the trends in RTG technology from SNAP-3B to GPHS-RTG, showing the overall steady progress to date [2]. Reactor Power Sources By the early 1950s it was apparent that nuclear reactors offered the potential to power
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