Loop heat pipes (LHPs) can provide variable thermal conductance needed to maintain electronics and batteries on Lunar/Martian rovers/landers within desired temperature range. During lunar day, the LHP transfers the heat to the radiator for rejection. During the fourteen-day-long lunar night, the sink temperature drops, lowering LHP and WEB/battery temperatures. Without a variable thermal link, the LHP will continue to remove heat during the night, cooling the electronics/batteries to unacceptably low temperatures. For spacecraft applications, a small heater is typically attached to the LHP reservoir to shutoff the LHP, preventing excessive cooling of the WEB/battery temperatures. The battery mass penalty to shutoff the LHP through the 14-day-long lunar night is large and must be avoided. This project will develop a LHP incorporating a passive thermal control valve, eliminating the shut-off power requirements. The valve will be installed in the vapor line to selectively route vapor to the radiator during daytime and directly to the compensation chamber (bypassing the radiator) during night, thereby maintaining electronics/battery temperatures. Phase I will demonstrate the feasibility of utilizing the thermal control valve as a varying thermal link. Phase II will fabricate and test a LHP with a thermal control valve, bringing the technology to TRL 6.