Variable Conductance Heat Pipe for a Lunar Variable Thermal Link

Abstract

The Anchor Node Mission for the International Lunar Network (ILN) has a Warm Electronics Box (WEB) and a battery, both of which must be maintained in a fairly narrow temperature range using a variable thermal conductance link. During the lunar day, heat must be transferred from the WEB to a radiator as efficiently as possible. During the night, heat transfer from the WEB must be minimized to keep the electronics and batteries warm with minimal power, even with a very low (100 K) heat sink. Three different variable thermal links were identified that could perform this function: 1. A mini-loop heat pipe (LHP), 2. A mini-LHP with a thermal control valve, or 3. A Variable Conductance Heat Pipe (VCHP) with a hybrid wick. The mini-LHP has the highest Technology Readiness Level (TRL), but requires electrical power to shut-down during the 14-day lunar night, with a significant penalty in battery mass. The VCHP incorporates three novel features in order to achieve the design targets of the ILN program. The first is a hybrid wick, which allows the VCHP to operate with an adverse tilt in the evaporator. The second is locating the reservoir near the evaporator, rather than near the condenser, to prevent the reservoir temperature from dropping during the lunar night. Third, a bimetallic adiabatic section is used to minimize heat losses due to conduction when the VCHP is shut down. Testing included 1. Freeze/thaw, 2. Simulated lunar performance, with an adverse evaporator elevation, 3. Performance with a 2.54 mm (0.1 inch) adverse elevation, both for normal operation, and to demonstrate diode behavior when the condenser was heated. All of the tests were successful; however, the power with the heat pipe level was slightly lower than expected, probably due to problems with the hybrid wick interface.