High Temperature Titanium-Water and Monel-Water Heat Pipes

Space nuclear systems require large area radiators to reject the unconverted heat to space. System optimizations with Brayton cycles lead to radiators with radiator temperatures in the 400 to 550 K range. To date, nearly all space radiator systems have used aluminum/ammonia heat pipes but these components cannot function at the required temperatures. Titanium-water and Monel-water heat pipes will operate in the temperature range, but titanium and Monel cannot be extruded in the same way as aluminum to form grooved heat pipes for space radiators. A method has been developed to form heat pipes in these materials. Grooves are machined into a flat plate, than the plate is bent and welded to form a heat pipe with grooves. Titanium-water heat pipes with a 1.3 cm O.D. and a length of roughly one meter have been fabricated with 3 different groove designs. The heat pipes carried 300-400 W at temperatures of 425 and 475 K. Water life test pipes have been fabricated with commercially pure (CP) titanium, Monel K-500, Monel 400, and various titanium alloys. CP-Ti and Monel pipes now have 17,400 hours of operation. These pipes continue to operate successfully, with a small amount of gas generation in the CP-Ti pipes. Life test pipes with titanium alloys are also underway, with between 4,000 and 9,000 hours of operation.