an annular segment, in order to conform more closely to the heater head cylinder and thus increase isotope packing density as shown in Fig. 6(b). This is desirable to reduce size and heat source/heater head insulation mass. Figure 7 depicts heat source/heater head and engine detail corresponding to a dual engine installation. When the engine is running heat continuously evolving from the decaying isotope is transferred to the working fluid in the hot end, and removed via the engine low temperature loop which is physically in close proximity to the hot end. Borrowing from earlier practice the heat source/aeroshell/heater head is enclosed by multiple layers of metallic foil insulation which will melt and provide a radiative path from the heater head if the engine cooling loop fails. Further definition, however,
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