Leakage of propellants such as oxygen and hydrogen in an enclosed environment in the atmosphere can lead to fires or explosions. For storage tanks on reusable launch vehicles such as the space shuttle where propellant is to be stored in the cargo bay of the vehicle, the storage tank should be filled while the vehicle is at a safe altitude to avoid the dangers associated with propellant leakage. The invention a process for filling propellant tanks at safe altitudes where if there is propellant leakage ignition will not occur. The process involves launching the vehicle with an inert gas in the storage tanks, dumping the inert gas and filling the tanks in the cargo bay from the main propellant tanks during the launch once the vehicle is at a safe altitude. The propellant can be dumped overboard and the storage tanks filled with an inert gas in case of a mission abort if the vehicle needs to make an emergency landing.