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A passive cooling approach for aircraft electromechanical actuators is being developed to support the Air Force More Electric Airplane (MEA) program. A two-phase coolant is used in a reflux type cooler to transport heat from the actuator electric motor to a cool aircraft structural surface. Energy storage, in the form of phase change material, is incorporated into the cooler to provide load leveling between peak loads and low loads. Transient thermal analysis, which was used to select the best combination of reflux working fluid and phase change material, indicates that motor temperatures can be reduced by more than 50°C when using thermal energy storage.

The central thermal management system for the proposed NASA Space Station will likely employ a two-phase thermal bus to satisfy the high power and long transport distance requirements. Significant potential weight and power savings accrue from this approach. A pumped two-phase cooling loop is described that can meet the requirements while maintaining constant heat source temperatures with large power and sink temperature turndown capability. Predicted performance of the 25 kW ammonia flight conceptual design is presented along with test results from a Freon 114 test loop which confirms predicted characteristics.

Spacecraft have previously used passive cooling or pumped liquid loops to maintain proper thermal environments. Future large spacecraft, such as the Space Station, will have higher power levels and longer heat transport distances. To better meet these requirements, a two-phase thermal bus concept has evolved which includes a thermal transport loop with controlled temperature and pressure. A two-phase cooling system has the advantages over a single-phase system of lower mass flows, lower pumping power requirements, and operation at nearly isothermal conditions, since heat is transferred by evaporation and condensation. The system employs an electrically-driven pitot pump to supply a controlled flow of nearly saturated liquid to any arrangement of series or parallel evaporators.