The composition of Inconel 718 (6) is as follows: Lunar Nickel 50 - 55% Chromium 17 - 21% Iron balance Manganese 0.35% max Silicon 0.35% max Aluminum 0.2 - 0.8% Titanium 0.65 - 1.15% Cobalt 1.00% max Non-Lunar Carbon 0.08% max Sulfur 0.015% max Copper 0.30% max Columbium + Tantalum 4.75 - 5.50% Molybdenum 2.8 -3.30% Phosphorus 0.015% max Boron 0.006% max Those in the latter group total about 9.1%. Assuming no more than 50% of the mass of the engines needs to be high strength superalloys, 4.55% of the mass will be non-lunar. Perhaps the percentage of Inconel can be reduced, but this would require detailed analysis of the structures, particularly with respect to creep failure. If the high nickel or chromium content of Inconel proves to be uneconomical with lunar materials, then the Inconel might be replaced with a less exotic alloy (perhaps requiring lower operating temperatures). Thermodynamic, mechanical, and alternator efficiencies are assumed to be 31.2%, 85.2%, and 92.4%, respectively.(3) Thus, to achieve a net 25% efficiency requires a theoretical Carnot efficiency of 50%. To achieve this Carnot efficiency with a hot side temperature of 720 K requires a maximum cold side temperature of 360 K. The limiting failure mode of the engine will probably be from creep, i.e. the metal comprising the hot side slowly stretches. Thick walls increase resistance to creep but decrease the Stirling’s thermal efficiency. An estimate of the wall thicknesses that can be used with various materials can easily be made. Based on the allowable stresses tabulated in the 1980 ASME boiler code (5) and using Mil’s pressures and dimensions, the wall thicknesses (in cm) shown in Table 2.6-2 would be required.
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