Hybrid Heat Pipes for High Heat Flux Spacecraft Thermal Control

Grooved aluminum/ammonia Constant Conductance Heat Pipes (CCHPs) are the standard for thermal control in zero-gravity. Unfortunately, they are limited in terms of their heat flux capability, approximately 5W/cm2. 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 (CCHP) as well as loop heat pipes (LHP). Standard CCHPs are also not suitable for Lunar and Martian Landers and Rovers, since they can only operate with a very small adverse elevation, on the order of 0.10 inch. Landers can have tilts of ±20o, while rovers can have tilts of ±45o. As a result, a wick with a higher heat flux capability and pumping capability is required. This program will develop heat pipes with both 1) Sintered wicks, and 2) Hybrid grooved and sintered wicks. Heat pipes with both wick designs will be capable of operating at heat fluxes in the ten's of watts per cm2, and in Lunar or Martian environments at large slopes against gravity. An all-sintered wick will be used when the entire heat pipe must be able to operate against gravity, while a hybrid wick will be used when the condensers will always be gravity aided on the planetary surface.

The maximum allowable evaporator heat flux in a grooved pipe is relatively low, typically 10-15 W/cm2. As spacecraft electronics increase in power and packing density, this can present a thermal control problem. Sintered wicks have much higher evaporator heat flux limits, typically four times higher than grooved wicks. The main non-NASA application will be for CCHPs that remove waste heat from high power components on commercial spacecraft.

Hybrid wick CCHPs for high power electronics on spacecraft, and NASA landers and rovers is the immediate NASA application. Conventional grooved aluminum/ammonia CCHPs are heat flux limited and are only suitable for operation in space, or when they can be engineered to operate in a gravity aided mode. The hybrid wicks developed on this program will remove this design constraint, and expand the design space to include higher heat flux operation on future missions. In addition, some vertical, grooved CCHPs have had startup problems, requiring an electric heater to generate a vapor bubble. The large number of nucleation sites in the sintered wick will eliminate this problem. Wicks developed on this program will enable the use of 3-D CCHPs for some NASA applications. Conventional grooved aluminum/ammonia heat pipes can be ground-tested with the evaporator elevated 0.10 inch above the condenser. This is only possible if the heat pipe is all in a single plane, i.e., a 2-D heat pipe. In some cases, 3-D CCHPs, with bends in multiple planes, simplify spacecraft thermal design. However, grooved 3-D pipes cannot be tested on the ground, since there is no way to keep the maximum elevation to 0.10 inch with the bends in multiple planes. With a hybrid wick, 3-D pipes can be fabricated and ground tested.

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