Initially these NPS were used to supplement solar power sources but gradually with the improvement of NPS technology and with the ever increasing requirements of spacecraft power (particularly for outer planet missions) NPS became the sole source of power. In a sense this was inevitable given the compact size, self-sufficiency, reliability, survivability, long lifetimes and operational flexibility of NPS. The basic NPS consists of a heat source (either a naturally decaying radioisotope or a nuclear reactor) and a converter (e.g. thermoelectric, thermionic, Brayton, Rankine, Stirling, magnetohydrodynamic) to change the thermal power into electrical power. To date the US has only used thermoelectric converters because of their proven reliability and the lack of a requirement to provide powers high enough to warrant the use of more efficient conversion systems such as turbine/alternators. The following sections provide an overview of the NPS flown by the US. This overview will serve to provide the framework for understanding the current programs under way in the US. Throughout the evolution of the US space NPS program there has been a general technology trend to improve NPS performance, efficiency, and specific power. This trend has led to improvements in the fuel and in the technology of thermoelectric materials, from the lead telluride (PbTe) used in the first five RTG concepts flown to the silicon germanium (SiGe) used in the SNAP-10A reactor and in the multi-hundred watt (MHW) RTGs and planned for use in future NPS. The performance of these NPS has clearly demonstrated that they can be safely and reliably engineered to meet a variety of space-mission requirements [2]. Radioisotope Power Sources The first SNAP, known as SNAP-1, was to use a radioisotope heat source coupled to a mercury Rankine cycle turbine/alternator. However, evolving requirements led the US toward the use of thermoelectrics such as were used on the SNAP-3B RTGs shown in Fig. 1. For this paper the various RTGs have been grouped into six basic design concepts: SNAP-3B, SNAP-9 A, SNAP-19, SNAP-27, TRANSIT-RTG and MHW- RTG. Since the focus of this paper is on providing a general historical overview the detailed power performance, which has been summarized in Bennett et al. [2], will not be repeated here.
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