An improved and simplified system for densifying a cryogenic liquid for space vehicles is provided, which includes a heat exchanger (102) having heat exchange tubes (104) therein for receiving a flow of liquid from a storage tank, for example a liquid propellant (22) in a vehicle storage tank (20). The heat exchanger (102) is filled around the exchange tubes (104) with a two-component bath (112), the volume of a primary component substantially exceeding the volume of a secondary component. The secondary component has a boiling temperature that is lower than the boiling temperature of the primary component, and both are lower than the boiling temperature of the cryogenic liquid. In one example, the liquid to be densified is oxygen, the primary component is liquid nitrogen, and a secondary component is liquid hydrogen. The secondary component is preferably injected into the heat exchanger (102) in separate flows to prevent localized freezing of the first component. A manifold having a plurality of injectors may be used for introducing the second component to the heat exchanger (102). A control system (116) receives inputs from one or more sensors within the heat exchanger (102) and operates valves that control the flow of the first and second components of the heat exchange bath (112).