CYCLIC METHYLSILOXANES AS WORKING FLUIDS FOR SPACE POWER CYCLES

The potential merits of cyclic polymethylsiloxanes, particularly those conventionally denominated D4 and Ds, as working fluids for space pow er cycles are discussed The attractive technical characteristics of th ese substances which are fully nontoxic, moderately flammable, and sta ble at high temperature are presented. Some experimental results on va por pressure and on thermal stability are reported. A maximum operatin g temperature of about 400-degrees-C appears achievable. A comprehensi ve thermodynamic analysis comparing siloxanes with other classes of hi gh temperature fluids is performed. The peculiar characters of siloxan e cycles are found to be: a good overall efficiency achieved through a massive regeneration, a moderate expansion work, and an abundant volu me flow at turbine exhaust. A number of two-stage turbines for two pow er levels (i.e., 30 and 5 kW) were designed using an appropriate optim ization program. The resulting main features of such expanders were a satisfactory efficiency, a low rotating and peripheral speed, and a co mparatively large wheel diameter These characteristics seem of particu lar interest for low capacity systems where, with other fluids, turbin es tend to be impractically small and fast rotating and where a high l evel of regeneration becomes more acceptable. In considering for the s ake of comparison the thermodynamic performance of many classes of org anic fluids, it becomes apparent that the full potential of organic po wer cycles in view of the variety of future needs has not yet been tho roughly investigated.