lunar materials is very precise, neither temperature cycle has a clear cost advantage over the other. 1.1.1.5 Rankine Cycle Two working fluids, potassium and steam, were evaluated for the Rankine engine. Each fluid was evaluated with two temperature ranges. The more promising specific mass estimates are shown in Figures 1.1-3 and 1.1-4. The high temperature steam cycle was found to require least non-lunar material among the Rankine designs, but the Rankine cycle requires much more non- lunar material than any other option. 1.1.1.6 Stirling Cycle Performance of the Stirling engine was estimated using the goals of a current NASA demonstration project. As shown in Figures 1.1-3 and 1.1-4, its specific mass is rather high. However, the Stirling requires little non-lunar material because of its low operating temperatures. The state of Stirling engine technology is advancing rapidly, so it should not be ruled out for SPS applications. 1.1.1.7 Concentrating Reflectors Standard technology for large reflectors in space is vapor deposited aluminum on facets of Kapton plastic. A similar design which uses vapor deposited aluminum on facets of aluminum foil was selected. This requires virtually no non-lunar material. 1.1.1.8 Radiator Systems Three radiator concepts which might be used to dissipate heat from active cooling systems are the heat pipe radiator (HPR), the liquid droplet radiator (LDR), and the moving belt radiator (MBR), The heat pipe radiator was selected for all active cooling needs in this study because it represents least technological risk and requires little non-lunar material. The moving belt radiator may have even lower non-lunar material requirements, but was considered too risky to use in this study. Nonetheless, the MBR concept is promising and deserves further study. 1.1.2 Structural Material Aluminum was found to be the best structural material for this study because it is well understood, requires few non-lunar alloying agents, has good structural properties, and has relatively low mass per unit strength. Passive thermal control systems should suffice to keep eclipse-related thermal expansion within acceptable bounds.
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