A technology development program is underway at NASA Glenn Research Center to evaluate the performance of heat rejection systems for lunar surface power. This technology development includes heat rejection systems utilizing titanium-water heat pipes as well as the design, fabrication, and testing of Radiator Demonstration Units. NASA Glenn Research Center procured three RDU panels of similar construction, utlizing slightly different materials and heat pipe wick configuration. All panels were flat, rigid, and employed a honeycomb sandwich construction with three titanium-water heat pipes embedded at an interval.
High thermal conductivity graphite fiber-polymer matrix composite face sheets were bonded to the aluminum honeycomb core utilizing an adhesive layer. Polymer matrix composites offer the promise of reducing mass and increasing the performance of future heat rejection systems. Within the honeycomb core, titanium heat pipes are adhesively bonded to Poco graphite foam saddles which are bonded to the polymer matrix composite face sheets. The saddles are constructed such that the high thermal conductivity orientation is placed from heat pipe to face sheet. Testing of the radiator demonstration units includes infrared thermography, thermal vacuum exposure, and thermal vacuum exposure with a simulated heat pipe failure.
Steady state performance data at different operating temperatures help to identify heat transfer and thermal resistance characteristics. Cryogenic thermal vacuum exposure with solar simulation utilizing quartz lamps can be used for lunar simulation testing. Heat pipe performance is limited by a number of factors: 1) Evaporator dry out, in which heat entering the heat pipe vaporizes the working fluid at a rate faster than the working fluid is being returned from the condenser. Essentially, the evaporator dry out process is starving the evaporator of working fluid. Ideal heat pipe operation occurs prior to the evaporator dry out. 2) Entrainment, in which vapor traveling down the length of the heat pipe can reach sonic values and entrain a portion of the liquid in the condenser section. This factor decreases working fluid return to the evaporator. Identifying the amount of working fluid to use when charging the heat pipe is critical in eliminating entrainment.