Investigation of Loop Heat Pipe Survival and Restart After Extreme Cold Environment Exposure

NASA plans human exploration near the South Pole of the Moon, and other locations where the environment is extremely cold. This paper reports on the heat transfer performance of a loop heat pipe exposed to extreme cold under the simulated reduced gravitational environment of the Moon. A common method of spacecraft thermal control is to use a loop heat pipe with ammonia working fluid. Typically, a small amount of heat is provided either by electrical heaters or by environmental design, such that the loop heat pipe condenser temperature never drops below the freezing point of ammonia. The concern is that a liquid-filled, frozen condenser would not re-start, or that a thawing condenser would damage the tubing due to the expansion of ammonia upon thawing. This paper reports the results of an experimental investigation of a novel approach to avoid these problems. The loop heat pipe compensation chamber is conditioned such that all the ammonia liquid is removed from the condenser and the loop heat pipe is non-operating. The condenser temperature is then reduced to below that of the ammonia freezing point. The loop heat pipe is then successfully re-started.