The overall program objective is to develop a high temperature variable conductance heat pipe (VCHP) backup radiator, and integrate it into a Stirling radioisotope power system. A VCHP is similar to a heat pipe, but has a reservoir and a controlled amount of non-condensable gas (NCG) inside the reservoir. At low power, most of the condenser is blocked by the NCG. As the power increases, more and more of the condenser becomes active. This allows a VCHP to passively control the temperature while the power varies. Aluminum/ammonia VCHPs are commonly used in spacecraft thermal control. The proposed program will extend this concept to the alkali metal heat pipes that will be required for a Stirling system. During normal operation, the VCHP will be inactive. If the Stirling engine stops working for any reason, the VCHP will passively remove the heat from the General Purpose Heat Source (GPHS) preventing the GPHS from overheating. The benefits of the VCHP radiator include increased safety and reliability, and the ability to design an optimum Stirling engine.