WORKING FLUIDS FOR HIGH TEMPERATURE, RANKINE CYCLE, SPACE POWER PLANTS

An analysis is presented of the relative suitability of sodium, potassium, rubidium, and cesium as working fluids in a high temperature, Rankine Cycle, space power plant. Turbine inlet temperatures of from 1800 to 2000 F with corresponding condensing temperatures of from 1240 to 1530 F are considered. The criteria by which the fluids are evaluated are the thermodynamic cycle characteristics, heat transfer and fluid friction characteristics, metallurgical compatibility, and the influence of the fluids on the design of the turbine, bearings, radiator, generator, and pump. The turbogenerator unit is thought to be the most critical component and it is found that the working fluid will determine the required number of turbine stages and will therefore establish the turbogenerator bearing arrangement. It is not known whether blade erosion will be a problem. However, since mercury and steam turbines have experienced blade erosion and since potassium has the lowest tendency to cause erosion, potassium appears to be the best over-all working fluid. If blade erosion is found to be of no consequence, then rubidium may result in fewer problems and greater reliability in the turbogenerator unit, and on this basis rubidium would then be the best choice of working fluid.