A system and method for cooling a first cryogenic liquid in a vessel comprises the steps of directing a second cryogenic into the first cryogenic fluid and releasing gas from the vessel. The first cryogenic fluid has a boiling point that is higher than the boiling point of the second cryogenic fluid. Directing the second cryogenic fluid into the first cryogenic fluid results in the second fluid cooling the first fluid and the second fluid vaporizing. The vaporized fluid is released as a gas from the vessel. The vessel may be a propellant tank for a space vehicle. The first and second cryogenic fluids are delivered from supplies located outside of the vehicle. The first cryogenic liquid may be oxygen and the second cryogenic liquid may be nitrogen, neon, or helium. In another aspect of the invention, the first cryogenic liquid may be hydrogen and the second cryogenic liquid may be helium.