thermionic cell in electrical series will be removed from operational status. Therefore, there is no advantage in having more than one cesium reservoir per series string. From a reliability standpoint, therefore, it would be highly advantageous to have at least one or possibly even two redundant series strings. This translates to low output bus voltages, which should be transformed to the standard spacecraft voltage by singly or even doubly redundant power conditioning units. In practical terms, if there are about 100 thermionic fuel elements with 200 cells, a bus voltage of 10 V should allow about 10 strings. This bus voltage is considered adequate for boosting to the 100 V range with state-of-the-art DC to DC MOSFET power conditioning techniques. Confidence requirement of 0.98 for the cesium system (regarding open circuit failures) with one redundant series string requires an individual cesium reservoir confidence of 99%. With two redundancies, the requirement is 94%. Assuming a confidence requirement of 0.98 for the thermionic subsystem (regarding short circuit failures), 10 redundant elements (10%) results in a TFE confidence requirement of about 95%. Conversely, the TFE confidence requirement can easily be improved by adding additional redundancy. For example, a redundancy of 13 TFEs instead of 10 improves the thermionic subsystem confidence to 99.4%. It should be noted that individual TFE component reliability and confidence is very likely a function of temperature. Qualitatively, it can be expected that adding redundant components and operating at reduced temperature would combine synergistically. As mentioned previously, the perceived need to achieve high reliability and confidence require that very extensive testing be accomplished to statistically verify the component reliability and confidence characteristics. Adding additional redundancy is attractive because it reduces testing requirements. Indeed, if life testing shows that TFEs are unable to meet the very ambitious reliability goals suggested, additional
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