Abstract: An expandable heat rejection system for radiating heat from a source of heat on a spacecraft or like vehicle is described and comprises a fluid heat exchange medium in operative circulating heat exchange relationship with the source for absorbing heat by evaporation of a liquid phase or the medium, an elongate inflatable sleeve defining an expandable and collapsible volume closed at a first end and including an inlet and outlet at a second end and further defining a condensation surface on the inner surface of the sleeve and a heat radiating surface on the outer surface of the sleeve, a reel attachable to the vehicle and operatively attached to the first end of the sleeve for selectively winding and unwinding the sleeve in response to pressure generated within the system.