In a Stirling radioisotope system, heat must continually be removed from the General Purpose Heat Source (GPHS) modules to maintain the modules and surrounding insulation at acceptable temperatures. The Stirling converter normally provides this cooling. An alkali-metal Variable Conductance Heat Pipe (VCHP) has been developed to provide back-up cooling, allowing multiple stops and restarts of the Stirling convertor. Unlike standard VCHPs which maintain a relatively constant temperature, this VCHP has two different heat rejection surfaces. During normal operation, heat is transferred to the Stirling convertor heater head. When the Stirling convertor is stopped, the VCHP temperature increases by 30°C, and the gas front is pushed back, allowing the heat from the GPHS to be rejected to the Cold Side Adapter Flange (CSAF) using a low-mass, carbon-carbon radiator. The 880°C temperature when the Stirling convertor is stopped is high enough to avoid risking standard ASRG operation, but low enough to save most of the heater head life. The Haynes 230/sodium VCHP was successfully tested with a turn-on ΔT of 30°C in three orientations: horizontal, gravity-aided, and against gravity.