Hybrid Heat Pipes for High Heat Flux Applications

The thermal transport requirements for future spacecraft missions continue to increase, approaching several kilowatts. At the same time the heat acquisition areas have trended downward, thereby increasing the incident heat flux. Current incident heat flux for laser diode applications is on the order of 5-10W/cm2, although this is expected to increase towards 50W/cm2. This is a severe limitation for axial groove aluminum/ammonia constant conductance heat pipes (CCHPs). The maximum heat flux in a CCHP is set by the boiling limit, which typically start at 5 W/cm2 for axial grove wicks, and 20-30 W/cm2 for powder metal wicks. The innovation is to develop CCHPs with a hybrid wick, which has a sintered evaporator wick, and a conventional grooved adiabatic and condenser wick. These hybrid wicks can operate at higher heat fluxes, allowing the use of higher power laser diodes. They also allow the use of Variable Conductance Heat Pipes (VCHPs) in Lunar and Martian Landers and Rovers, which must operate at tilts up to 45�. Conventional aluminum/ammonia heat pipes can only operate with a 0.1 inch tilt, so they are unsuitable. Loop Heat Pipes can also operate with tilting, but they are two orders of magnitude more expensive. A hybrid grooved and sintered wick CCHP will allow operating at higher heat fluxes as compared to axial groove design and can also operate against gravity on the planetary surface, operate in space, carrying power over long distances, act as a thermosyphon on the planetary surface for Lunar and Martian landers and rovers, and demonstrate a higher transport capability than an all-sintered wick.

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