The ramjet engine relies on forward velocity to compress inlet air for the combustion process. In order to operate efficiently, a ramjet must be traveling at several times the speed of sound. At the present time, there is little demand for commercial or military transport in this flight regime, so the primary application for ramjets is in military single-use missiles. There are a number of systems around the world in various stages of development ranging from exploratory research to production. The scramjet engine is a close cousin, with the differentiating factor being the combustion process speed—subsonic for a ramjet and supersonic for a scramjet.