Liquid rocket propellants need special storage and handling – the most appropriate methods depending on the type of fuel and oxidizer used. On most launch vehicles the fuel and oxidizer tanks are stacked vertically, with the fuel tank on top so that its greater density shifts the center of gravity forward and thereby increases the vehicle's stability. Typically, the space above the remaining propellant is pressurized by external gas lines at the top of each tank. This prevents a vacuum from developing and keeps the propellant flowing smoothly. The gas pressurization lines may use inert gas from a separate tank, or products from the engine itself. Cryogenic propellants have to be carefully insulated and, prior to launch, recirculated through an umbilical to an external cooler. Many tanks, and all cryogenic tanks, have a bleed valve to allow high pressure gases to escape. Because cryogenic tanks aboard spacecraft cannot store propellants for long, they are usually limited to launch vehicles.