cycle in the range 1-10 kWe. The FPSE is currently being developed by NASA under the CSTI program. Development goals include a 1300°K engine employing refractory metals at a temperature ratio of 2.0 for SP-100 application with specific weight less than 6 kg/kWe, and a 1050 K superalloy engine at a temperature ratio of 2.0 as an intermediate goal. The first machine representative of a space configuration, the Space Power Demonstrator Engine (SPDE) is the largest FPSE built and demonstrated. It has delivered 17 kWe with a (thermal to mechanical) efficiency of 22%, operating at a temperature ratio of 2.0. Although the SPDE was a developmental engine it can, with straightforward material substitutions, and replacing bolts and flanges with welds, provide a specific mass of 7.2 kg/kWe in flight configuration [10]. The Stirling engine scales favourably to lower power levels because surface area to volume ratio increases as unit size is reduced. This reduces the level of heat flux across the heater head boundary into the working fluid. At a few hundred watts it approximates the heat flux from an isotope source. Therefore the separate HSA and intermediate heat transfer loop required for a BIPS design can be eliminated, and at these unit sizes the heater head can be heated directly. Figure 6 shows a concept for direct integration of an FPSE heater head with the GPHS isotope heat source at a unit size of 500 watts (e). The GPHS blocks are arranged around the heater head circumferentially and held in place by a fusible strap assembly (FSA) as shown in Fig. 6(a). The GPHS aeroshell could also be modified in shape from a rectangular block to
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