Modern fighter aircraft typically mount an after-burner. Other alternatives are either a low bypass turbofan or a turbojet. The explanation behind this is that fighters typically need extra thrust to perform sharp maneuvers and fulfill its mission. The after-burner is essentially a long tailpipe into which additional fuel is sprayed directly into the hot exhaust and burned to provide extra thrust. When the after-burner is turned off, the engine performs as a basic turbojet. The exhaust velocity is increased compared to that with after-burner off because higher temperatures are involved.