The potential merits of cyclic polymethylsiloxanes, particularly those conventionally denominated D4 and D5, as working fluids for space power cycles are discussed. The attractive technical characteristics of these substances which are fully nontoxic, moderately flammable, and stable at high temperature are presented. Some experimental results on vapor pressure and on thermal stability are reported. A maximum operating temperature of about 400°C appears achievable. A comprehensive thermodynamic analysis comparing siloxanes with other classes of high temperature fluids is performed. The peculiar characters of siloxane cycles are found to be: a good overall efficiency achieved through a massive regeneration, a moderate expansion work, and an abundant volume flow at turbine exhaust. A number of two-stage turbines for two power levels (i.e., 30 and 5 kW) were designed using an appropriate optimization program. The resulting main features of such expanders were a satisfactory efficiency, a low rotating and peripheral speed, and a comparatively large wheel diameter. These characteristics seem of particular interest for low capacity systems where, with other fluids, turbines tend to be impractically small and fast rotating and where a high level of regeneration becomes more acceptable. In considering for the sake of comparison the thermodynamic performance of many classes of organic fluids, it becomes apparent that the full potential of organic power cycles in view of the variety of future needs has not yet been thoroughly investigated.

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