Since many of the concepts are not well defined and we have no specific satellite to serve as a guide for power system integration, we cannot determine system volumes at launch and during operation. Instead, we present areas of the radiators required to dump waste heat as a general indication of those volumes. Area estimates include radiators for the power conversion and power conditioning subsystems but not the payload. We have optimized component performance within the constraint of minimizing overall power system mass for all of the more massive components. Our mass and area results may not be completely representative of real systems because many design details are unknown at this time. They should however provide good relative comparisons among the various concepts. Special care was taken to treat the SP-100 Thermoelectric system in identical fashion to the other concepts since it has received more design funding and has therefore identified many additional contributors to total system mass. We believe our approach is reasonable because our results agree well with those obtained from the SP-100 program for the SP-100 thermoelectric and innovative SP-100 concepts. There are other important issues such as safety, reliability, launch costs, development costs and schedules, etc. and we allude to some of these to point out that mass and radiator areas are only a subset of the total that must be considered in the final selection process. Based on our mass and area estimates, there is no compelling reason to choose one nuclear power system technology over another at power levels below 40 kW(e) until requirements become more firmly established. Differences in volume at launch and during operation may be more important than mass differences at the lower power levels. However, differences among power system masses become more significant as the required power increases further and further beyond the 40 to 60 kW(e) range. Satellite designers must understand how the total system mass, volume at launch, and volume during operation change with the amount of power required by different satellites. These are major factors in determining cost and operational capabilities and the power system's contribution to satellite mass and volume must be well understood. Since the required technology is not available, any proposed U.S. reactor will require many years for development, and the costs will be significant. This means the technology selected should be capable of satisfying U.S. operational needs for 30 or 40 years to amortize the costs. During this time, new civilian and military missions could evolve and the required power levels are not well known this far in the future. As a result, space reactors should have the flexibility to meet different power requirements in successive designs without excessive increases in specific mass (i. e. mass per unit of electrical power) or volume. Since our results show that the various concepts differ greatly in their ability to provide such flexibility, realistic appraisals of the range of future power requirements are essential to choosing the correct reactor technologies for space missions. Even then, other power system characteristics may be equally important. This is especially true if, as we expect, the power system will account for only 10 to 20% of the satellite's total mass and 25 to 50% of its area.
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