SNAP-3B The SNAP-3B RTGs, which were developed out of the SNAP-3 program, were used to provide 2.7-We of power to radio transmitters and other electronic equipment aboard the US Navy's Transit 4A and Transit 4B navigation satellites. The SNAP-3B RTGs also were flown to prove the practicability of radioisotope power sources in space [2, 3], Prior to the use of NPS, continuous electrical power had been obtained by solar arrays and nickel-cadmium (NiCd) batteries. Concern over possible degradation of solar cells in the inner Van Allen belt and battery breakdown from repeated chargedischarge cycles had led the Navy to fly RTGs [3]. Each 2.1-kg SNAP-3B RTG contained 27 spring-loaded, series-connected pairs of PbTe thermoelectric elements operating at a hot junction temperature of about 783 K and a cold-junction temperature of about 366 K. Each radioisotope heat source provided about 52.5 Wt. The design life was 5 years. Figure 2 shows an assembled SNAP-3B and Fig. 3 shows the first mounting of NPS to a spacecraft in 1961. At the time Transit 4A, which is shown in Fig. 4, had the longest operating life of any satellite launched by the US—over 15 years. The RTG on Transit 4B was still operating 10 years after launch when the last signals were received [2-5]. SNAP-9A The SNAP-9A RTGs, as shown in Figs 5 and 6, were built to provide all of the electrical power for the Navy Transit 5BN navigation satellites. In fact, Transit 5BN-1, which was launched in 1963 and is shown in Fig. 7, was the first satellite to get all of its power from an RTG. The RTG approach was selected because RTGs are
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