Abstract: A cold vapor pressurization system for outer space vehicles that is used prior to and during engine burn to pressurize its fuel tank and oxidizer tank to produce the required engines net positive suction pressure (NPSP). The engine start system uses reaction control system (RCS) hydrazine to pass through a gas generator where it is disassociated. The by-products are then introduced into the hot side of the propellant heat exchangers. An oxidizer feed system and a fuel feed system pass through the respective cold sides of the heat exchangers to vaporize the propellants which are then used to produce the required engine NPSP for the oxidizer tank and fuel tank respectively. This system could also be used to provide engine NPSP during engine burn.