A method and system for delivering pressurized propellants to a rocket engine, that has significant advantages over the current state-of-the-art. One of the propellants (24), the 'pressurizing propellant', is at least partially vaporized and the vapor (30) is in pressure communicating relationship with other propellants (44) on board the rocket-propelled vehicle. This vapor pressure pressurizes the propellants to a sufficient degree that they can be charged directly to the rocket engine (42), or the pressure may be boosted through pumps (43), if required. Moreover, the pressurized vapor (30) may be used in other applications on board the vehicle, such as orbital adjustment, attitude control, station keeping, and the like. In several embodiments, the propellants are contained in variable volume reservoirs (13), exemplified by bladders and diaphragms. These variable volume reservoirs are preferably not subjected to tensile stresses when expanded, and are preferably designed for controlled volume reduction, when being drained of propellant. In some embodiments, the propellant reservoirs are contained in a single housing (10), and in other embodiments the pressurizing propellant may be contained in a separate tank (10a, 10b), as long as pressure communicating relationship (50, 52), is maintained between the propellants. Also provided is a method of refueling one vehicle with liquid propellant from another vehicle, using pressure from a more volatile propellant onboard the supply vehicle.