Ramjet engines are even more powerful than normal jet engines, designed largely for supersonic craft. They function by gulping down air moving at high speed and combusting it to generate even higher thrust. Because of this, ramjets cannot operate at all at speeds of 500 400 mph or under, they require another source of propulsion to move forward, and they must always be pointed in the direction of travel. Internal ramjets can fold into the body when not in use, protecting them, but they cannot be used while stored inside. Ramjets require air to breath and function.