The high temperature steam cycle requires the least non-lunar mass (1.23 kg/kW) and has the lowest cost, assuming a 50:1 cost ratio favoring lunar materials. This cycle also has the highest efficiency among the Rankine cycles. A mass analysis for this cycle is shown in Table 2.5-3. TABLE 2.5-3 MASS ANALYSIS FOR 9 GW HIGH TEMPERATURE STEAM RANKINE CYCLE In general the potassium cycles proved to be less efficient than steam cycles. The efficiency of the potassium cycle was sacrificed to avoid the low pressure stages of the turbine from growing to unrealistic dimensions, which would have also increased the amount of non-lunar mass required. The higher heat rejection temperatures of these cycles reduce the mass of the radiators greatly. But the large mass of the turbomachinery offsets the mass reduction of the radiators. Most of the non-lunar materials used in the potassium cycles are for construction of the turbomachinery. Of the potassium cycles, the high temperature cycle shows better efficiency, lower total mass and lower non-lunar mass. The steam cycles are much more massive than the potassium cycles in spite of their higher effic_ancy for both high and low temperature ranges. The large mass of these cycles is mainly due to low heat rejection temperatures, which require large radiators. However, the relatively small turbomachinery of the steam cycles results in less non-lunar mass. The potassium Rankine cycles and the low temperature steam cycle were designed with the turbine expansion process terminating in the two-phase region. Therefore, there is no temperature potential for regeneration in
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