Why and How Should High-Speed Aircraft Electronics Be Liquid Cooled?

Based on weight penality comparisons of ultimate heat sinks for electronic equipment cooling, the use of expendable evaporants and fuel is indicated for high supersonic flight. Centralized ram air should be the alternate coolant during subsonic flight. System integration is best accomplished with a recirculating liquid transfer system, which is relatively easy to control and which is characterized by small pumping power, line size, and heat gain from high temperature environments. Because of these features close temperature control of dispersed components and cooling of remote highpower units are best achieved by liquid coupling, regardless of the type of ultimate heat sink. Part temperature rises in high voltage equipment can be minimized by use of dielectric liquids. Although this permits some reduction in ultimnate coolant weight penalty, the reduction is usually not great enough to offset the equipment weight increases that are due to liquid filling. Vapor-filled or air-filled units with minimal liquid contents and liquid transport to part surfaces by capillary action or mechanical means are superior. Electronic assemblies that are to be series cooled in sealed liquid transfer systems should be designed for conduction, forced air convection, or radiation heat transfer from the parts, and high power units should have integral liquid cooled heat exchangers, placed in separate transfer system branches. Internal heat transfer in such units may be attained by conduction through flexible metal or rubber jackets and electrical insulators, by air convection in standard modules, and by liquid film cooling.