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Missiles are powered by an engine, generally either a type of rocket engine or jet engine. Rockets are generally of the solid-propellant type for ease of maintenance and fast deployment, although some larger ballistic missiles use liquid-propellant rockets. Jet engines are generally used in cruise missiles, most commonly of the turbojet type, due to its relative simplicity and low frontal area. Turbofans and ramjets are the only other common forms of jet engine propulsion, although any type of engine could theoretically be used. Long-range missiles may have multiple engine stages, particularly in those launched from the surface. These stages may all be of similar types or may include a mix of engine types − for example, surface-launched cruise missiles often have a rocket booster for launching and a jet engine for sustained flight.