to directly reject heat to space at essentially a constant temperature. The indirect cycle avoids the zero-g condensing radiator by using a secondary fluid Toop with an intermediate heat exchanger. Many Type-A Rankine cycles use superheated vapor at the turbine inlet to increase turbine efficiency by eliminating liquid particles in the turbine. Erosion of turbine blades is largely caused by liquid particles striking the leading edge of the blades. For application of the Rankine cycle to the SPS, four main technology areas must be carefully considered: (1) Materials problems due to corrosion at high temperatures;, (2) turbine-blade erosion; (3) thermal design considerations and stability of compact once-through boilers, (4) pump technology. While considerable progress has been made in the field of high-temperature materials over the years, corrosion in the presence of liquid metals is still a major problem, and it is not presently expected that corrosion rates can be minimized over the next several years to meet the 30-year life requirement and make the high-temperature liquid metal Rankine cycle system a viable candidate for the SPS. 2.512 The Brayton Cycle Because of problems such as those discussed above, space power system designers turned to single-phase working fluid cycles which could provide a net usable output power after satisfying compression penalties. This work led to the closed Brayton cycle as the promising gas cycle for space applications, as discussed earlier. 1 A thermodynamic diagram for the various processes in the closed-cycle Brayton system is shown in Figure IV-B-l-c-4a. After expanding polytropically through the turbine, the working gas then exchanges heat with fluid leaving the compressor in the recuperator (recuperation increases cycle efficiency) before entering a waste-heat exchanger. After rejection of energy to a heat sink, the working fluid is then compressed in stages, with intercooling between stages, before flowing through the low-temperature side of the recuperator and on to the heat source. A schematic of the system is shown in Figure IV-B-l-c-4b. The following discussion of the characteristics of the closed Brayton cycle system is presented to give the reader a basic understanding of this system before evaluating the Boeing concept.' In addition to a long and successful development history and a continually increasing "state of technology," the closed Brayton cycle is attractive for extended space missions such as the SPS for two reasons. The inert gas working fluid (1) precludes internal system corrosion, thus alleviating the peak cycle temperature limitations from the aspect of chemical combinatorial effects of the working fluid with component materials; (2) eliminates problems associated with two-phase fluids such as turbine erosion, pump cavitation, and low-gravity boiling and condensing. Furthermore, gas bearing technology is well-developed for Brayton systems,
RkJQdWJsaXNoZXIy MTU5NjU0Mg==