Variable Conductance Heat Pipes for Variable Thermal Links

Abstract

Variable Conductance Heat Pipes (VCHPs) for spacecraft thermal control typically have a cold-biased reservoir for the Non-Condensable Gas (NCG) at the end of the condenser. During operation, electrical heat is supplied to the reservoir to provide ±1-2°C temperature control over widely varying powers and sink temperatures. A second application for VCHPs is as a variable thermal link. Applications that can benefit from using VCHPs as variable thermal links include Lunar and Martian Landers and Rovers, Research Balloons, and Lunar and Space Fission Reactors. The applications that can benefit from variable thermal links normally have: 1. Variable system loads resulting from intermittent use, 2. Large variations in the sink temperature, and 3. Limited electrical power. Since the lowest sink temperature can be below the freezing point of the working fluid, many applications with variable thermal links also need to consider freeze/thaw and start-up from a frozen state. Fortunately, the NCG in the heat pipe also helps when the pipe is frozen, and during startup. An aluminum/ammonia VCHP was developed to act as a variable thermal link for lunar landers and rovers, passively minimizing heat losses during the lunar night, without requiring electric power to shut off. The reservoir was located near the evaporator, rather than near the condenser, to prevent the reservoir temperature from dropping during the lunar night, without requiring electrical heaters. Variable thermal links were also developed for high altitude, research balloons. Two VCHP configurations (hot and cold reservoir) were designed, fabricated and successfully tested, with methanol, toluene, and pentane as the working fluids. Both configurations provide a variable thermal link without electrical power. The warm reservoir VCHP has a 4.8oC temperature control band, while the cold reservoir control band is larger, at 21oC