A densifier is provided which in one embodiment can simultaneously densify two cryogenic liquids at different temperatures. The densifier has an oscillatory power source for generating oscillatory power and a two stage pulse tube refrigerator. The oscillatory power source can be a thermoacoustic heat engine or a mechanical oscillatory power source such as a linear flexure bearing compressor. The first stage densifies a first cryogenic liquid to a first cryogenic temperature, and the second stage densifies a second cryogenic liquid to a second, lower cryogenic temperature. A densified propellant management system also is provided which has a densifier for simultaneously densifying two cryogenic liquids at different temperatures, and a cryogenic temperature probe for measuring the temperature gradient in a cryogenic liquid.