the reactor or energy conversion systems to operate in an unattended mode for up to ten years. Free piston Stirling engines with linear alternators must overcome problems associated with wear, creep, and material degradation over the long system lifetimes at high temperature. The flat thermionic diodes used in STAR-C, the thermoelectrics used in SP-100, and the thermionic fuel pin devices must overcome problems of long term material degradation at high operating temperatures. Brayton turbines using helium-xenon as a working fluid have not been used to any great extent but one has been built by Garrett (Ref. 17) and it is reasonable to expect that this is the most mature conversion technology. With two exceptions, the SP-100 reactor technology when it is coupled with liquid metal Rankine or refractory metal Stirling energy conversion systems, we have analyzed near-term concepts which we believe offer the greatest potential for small mass and radiator area. The exceptions were included to show the potential benefit of pursuing the two different conversion technologies for future systems. In both cases we assumed the use of advanced conversion technology while retaining the use of near-term technology for the remainder of the system (i. e. SP-100 reactor technology, control electronics, etc.). The Rankine conversion technology is further out in time because it requires two-phase flow in a micro-gravity environment and turbines and vapor separators must be developed and proven reliable for long term use with liquid metals at high temperatures. These are issues that will not be resolved for several years. The free piston Stirling engine must use refractory metals at the hot end in place of superalloys if it is to operate at 1350 K as we assumed in one of our system analyses. The superalloy version of this engine is currently under development at NASA's Lewis Research Center and should be available in the near-term. The refractory metal engine would result in a lighter system but it would not be available in the near-term if the present evolutionary approach being pursued by NASA is continued (Ref. 18). In this approach, a superalloy engine is being developed for initial testing up to 1050 K. It is being designed in such a way that, once the design is proven, refractory metals could hopefully be substituted for superalloys in certain components without redesign. Then a refractory metal engine could be put on test. This requires that test facilities be upgraded to provide a vacuum of 107 to 108 torr, and that materials fabrication and joining processes be developed. If there were sufficient demand for a refractory Stirling engine, a revolutionary approach to go directly to that engine could conceivably develop one in the near-term but at much higher development risk. For these reasons, we conclude that superalloy Stirling engines are near-term and refractory engines are far-term. 3.2.2 Reliability In a top-level systems study such as this, with most of the concepts in very preliminary form, it is not possible to adequately treat the effect of required reliability and therefore component redundancy on system mass. No system reliability analyses were done. However, we have included cursory estimates where possible by arbitrarily assuming that the probability of producing 100% of the design power level after 10 years should be 95%. Of the 5% unreliability, 2.5% was
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