Lead telluride and silicon germanium type alloys have served over the past several decades as the preferred thermoelectric conversion materials for U. S. radioisotope thermoelectric generator (RTG) power systems for planetary deep space exploration missions. The Pioneer missions to Jupiter and Jupiter/Saturn and the Viking Mars Lander missions employed TAGS-2N (lead and -^ germanium telluride derivatives) power conversion devices. Since 1976, O silicon germanium (SiGe) alloys, incorporated into the unicouple device, have evolved as the thermoelectric materials of choice for U. S. RTG powered space missions. These include the U. S. Air Force Lincoln Experimental Satellites 8 & 9 for communications, in 1976, followed in 1977 by the National Aeronautics 4J and Space Administration Voyager 1 and 2 planetary missions. In 1989, advanced SiGe RTGs were used to power the Galileo exploration of Jupiter and, VJ in 1990, will be used to power the Ulysses investigation of the Sun. In addition, SiGe technology has been chosen to provide RTG power for the 1995

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The SNAP I development program was initiated in January 1957 to develop a 500-watt, turboelectric power conversion system for space applications. Superheated mercury vapor was used as the heat conversion working fluid" The conversion system was to obtain thermal energy from the decay of a radioisotope fuel such as Cerium-144. In this report, each of the major components and systems, is summarized with respect to initial design objectives, development progress to the point of program termination, results obtained from tests and, where indicated, future growth potentials Reference is made to 10 other reports which describe, in detail, the major components of this power generating system. Also included is a bibliography of Martin documented reports that are related to the power conversion system design criteria or system integration into a flight vehicle.

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