The Stirling radioisotope generator (SRG) is based on a Stirling engine powered by a large radioisotope heater unit. The hot end of the Stirling converter reaches high temperature and heated helium drives the piston, heat being rejected at the cold end of the engine. A generator or alternator converts the motion into electricity. This Stirling converter produces about four times as much electric power from the plutonium fuel than a radioisotope thermoelectric generator (RTG). The Stirling generators have been extensively tested but have not yet been deployed on actual missions.
Stirling engine development began at NASA Glenn in the early 1970s (then NASA Lewis.) The Space Demonstrator Engine (or SPDE) was the earliest 12.5 kWe per cylinder engine that was designed, built and tested. A later engine of this size, the Component Test Power Converter (or CTPC), used a "Starfish" heat-pipe heater head, instead of the pumped-loop used by the SPDE. In the 1992-93 time period, this work was stopped due to the termination of the related SP-100 nuclear power system work and NASA's new emphasis on "better, faster, cheaper" systems and missions.
A current effort is based on a 55-watt electric converter. The thermal power source for this system is the General Purpose Heat Source (GPHS). Each GPHS contains four iridium-clad Pu-238 fuel pellets, stands 5 cm tall, 10 cm square and weighs 1.44 kg. The hot end of the Stirling converter reaches 650°C and heated helium drives a free piston reciprocating in a linear alternator, heat being rejected at the cold end of the engine. The AC is then converted to 55 watts DC. Thus each SRG will utilise two Stirling converter units with about 500 watts of thermal power supplied by two GPHS units and will deliver 100-120 watts of electric power.
   | Stirling Converter Technologies at NASA Glenn Research Center - Stirling Convertor Technologies for Stirling Radioisotope Generator |
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