for two reasons. First, they have a high temperature radiator (—1000 K), which means that it will be smaller and less massive. Second, at the higher power levels, the power conversion systems of the dynamic systems contribute a substantially higher fraction of the total system mass than at lower power levels (See Section 4.2). 4.1.5 SP-100 With Stirling Power Conversion Two SP-100/Stirling power systems are shown on Figures 4.1 to 4.3. The near-term Stirling scales much better than the SP-100 thermoelectric system. It is 32% less massive at 100 kWe and 38% less massive at 1000 kWe. The refractory Stirling version scales better than the superalloy Stirling, mainly because it has a higher heat rejection temperature, and therefore, a smaller radiator. Although the efficiencies of the SP-100/Stirling systems are 5 to 8% higher than those of the SP-100/Brayton system, they are still more massive. This is due primarily to the fact that the Stirling engines themselves are so massive. At 1000 kWe, the superalloy engine is 40% of the total power system mass and the refractory engine is 50% of the total system mass. (Note: The Stirling engine mass algorithm was obtained from the NASA/Lewis Research Center.) 4.1.6 SP-100 With Rankine Power Conversion The SP-100 with a Rankine power conversion system, which is not a near-term power system, scales better than any other power system above 25 kWe. The reasons for this are that it is a relatively high efficiency system with a small, high temperature, waste heat radiator and the Rankine turbo-alternator scales well with power level. At 300 kWe, it is less massive than the refractory Stirling and the Brayton systems by 40% and 33%, respectively. 4.2 Power System Mass Characteristics During the course of our analyses we discovered several interesting characteristics of power system masses. Three of these characteristics are discussed below. 4.2.1 Length Of The Separation Boom Figure 4.4 shows the relative mass of a 100 kWe SP-100 thermoelectric system as a function of boom length. Our calculations give a minimum system mass with a separation of 16 m. This is substantially shorter than the 22.5 m the SP-100 program uses. Also, if the separation distance is reduced to 9 m, the system mass increases by only 2.4%. This indicates that if shorter booms were required by the satellite payload that only a small mass penalty would have to be paid. 4.2.2 Power System Mass Breakdown The mass breakdown for a power system varies significantly as the power level varies (Figure 4.5). At low power levels, the reactor and shield make up the majority of the mass. A substantial contribution is also made by the balance of system, i.e., instrumentation and control, safety systems, the electrical subsystem, and the boom.
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