TABLE 2.6-2 STIRLING ENGINE MINIMUM WALL THICKNESSES IN CENTIMETERS FOR VARIOUS HOT END TEMPERATURES AND MATERIALS Current developments are using Inconels or other nickel, cobalt based super alloys. For higher temperatures, tungsten - 25% rhenium or some other tungsten based super alloy are reasonable choices for small level production, but may exceed the productive capabilities of the Earth for the quantities required by an SPS program. Ceramic may be useful as an outer jacket for the engine, providing high temperature strength to hold a thin metal cylinder together. This has potential to allow very high pressures and temperatures, with efficiencies approaching 50%.(1) Mullite, a ceramic of the composition (A1203)3 (SiO2)2, may be useful for this application because it is strong at temperatures of interest and it is not into its creep range at those temperatures. Ceramic materials can be machined to fine tolerances and should be straightforward to make from lunar material. It is possible that many internal components of the engine could be fabricated from what is essentially hot pressed lunar dust. The absorber is assumed to be similar to that used in the Brayton and Rankine systems. However, it is assumed to require only 10% non-lunar materials because of the lower maximum temperature of the Stirling engine.
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