2.4 BRATTON CYCLE 2.4.1 Introduction The results reported here are based on a design as similar as possible to that of a NASA MSFC report.(1) Conservative estimates were made for potential substitution of lunar materials. The majority of the weight for the system is in the solar collection and heat rejection systems. It is straightforward to determine where lunar materials could be substituted in these systems. It is more difficult to estimate the percentage of lunar materials for the Brayton engine because the contract report(l) includes no details on the alloys appropriate for the engine components and the weights for specific components. The NASA report information was used to calculate engine efficiency for two operating temperatures and then conservative estimates were made for potential use of lunar materials at each temperature. 2.4.2 Design Description A simple schematic of the Brayton cycle is given in Fig. 2.4-1. This single regenerative Brayton cycle uses a solar concentrator to collect the solar energy and reflect it into a cavity heat absorber. The heat is absorbed by the gas through the heat exchanger tubing lined on the heat absorber’s insulated shell. This heat transfer also helps reduce the temperature of the heat absorber itself. The hot gas then goes through the turbine, which converts some of the thermal energy to mechanical work by turning the compressor and the generator. The recuperator extracts more heat from the gas coming out of the turbine and transfers it to the cold side of the cycle. The waste heat is rejected by the radiator. Helium was chosen as the working fluid to make use of the mass statements of the cycle investigated by NASA. According to one reference (2), a 60% Xe / 40% He mixture with a molecular weight of 80 was also considered. This mixture requires fewer stages for the turbomachinery, but would require a much larger heat exchanger due to reduced thermal conductivity. The size of the turbomachinery would also have to be increased (2) which is already a problem, even with helium. Thus, pure helium was selected as the working fluid. The selected radiator system is a liquid metal radiator (LMR), i.e., a heat pipe radiator which uses sodium-potassium (NaK) as the coolant and water as the working fluid in the heat pipes. It is possible that an LDR system using lunar NaX as the working fluid would be suitable at some temperatures, but such a system was not selected due to its high technical risk.
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