Abstract: Thermal energy transfer apparatus having a controllably variable thermal conductance comprises a heat pipe having a capillary wick structure therein, a circulating supply of working fluid which is controlled by a heater, and a thermostatic heater control which is responsive to the temperature of the hot end of the heat pipe. As working fluid condenses it is removed from the heat pipe. Liquid working fluid is recirculated into the heat pipe in discrete quantities by vapor bubble injection under the control of the heater and the heater control. If no fluid is injected into the heat pipe, the thermal conductance will decrease as condensate is removed. If fluid is injected into the heat pipe, thermal conductance will increase as fluid is supplied to the heat pipe.