The high temperatures created on the absorber panels preclude use of aluminum or even stainless steel. There are some high temperature alloys which can be made of lunar materials and which can be used for construction of the panel skins and even the pipes. Croloy (2.25% Cr - 1% Mo) can be used up to about 830 K and other heat resistant steel alloys (25% Cr - 12% Ni or 25% Cr - 20% Ni) have been exposed up to 1810 K. Nickel alloys such as Rene 41 (10% Co, 10% Mo, .005% B, plus Ni, Al, Ti, and Fe) are mostly made of lunar materials and are used in high temperature devices such as aircraft engines and re-entry vehicles. It was assumed that 50% of the high temperature absorber and 10% of the low temperature absorber would require non-lunar materials. The absorber mass estimates are shown in Table 2.4-5. TABLE 2.4-5 HEAT ABSORBER MASS FOR 9 GW BRAYTON CYCLE 2.4.3.4 Turbomachinery The high temperature of helium going through the turbine is tough to accommodate. This Brayton cycle also requires large diameter tube discs. Experiments with high temperature super alloys such as Astroloy or INCO 608 and refractory alloys such as molybdenum TZM have been conducted to overcome such problems. Refractory materials such as silicon carbide have also been suggested for making turbine blades facing high temperatures. The compressor, however, faces lower temperatures and doesn’t require high temperature materials like the turbine. A good portion of the pipes also do not face the highest temperature of the cycle since the working fluid reaches the peak temperature when entering the turbine. Overall a good portion of the turbomachinery can be made of lunar materials with good strength and resistance to higher temperatures (such as Croloy, stainless steel, or Ni alloys). It is estimated that 75% of the high temperature turbomachinery and 10% of the low temperature turbomachinery will require non-lunar materials. The turbomachinery mass estimate for each cycle is shown in Table 2.4-6. TABLE 2.4-6 TURBQMACHINERY MA°S FOR 9 GW BRAYTON CYCLE
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