Heat powered ECS concepts using a Rankine or Stirling power cycle to drive a vapor compression refrigeration cycle were evaluated for future fighter aircraft. Arrangements with separated power cycle and refrigeration cycle working fluids were considered, as were arrangements combining these cycles via a common working fluid.Promising heat sources and working fluids for these concepts were identified. Haste heat sources in the propulsion system and airframe were compared with regard to heat capacity, temperature level, and collection system design complexity. A screening process, which distinguished between the requirements imposed by the power cycle and refrigeration cycle, was developed to select promising working fluids.Concepts were evaluated using various system arrangements, heat sources, and working fluids to minimize the ECS-related TOGW penalty. The most attractive heat powered ECS concept was found to be a configuration using a Rankine power cycle integrated with the refrigeration cycle through a common working fluid, Freon R-11. However, the predicted TOGW penalty for this concept is still 23% higher than for a conventionally (electric motor) powered vapor compression refrigeration cycle concept.