A turbojet engine is a type of jet engine that uses a simple design to produce thrust. It works by drawing in air through an inlet, compressing it with a series of blades, and then mixing it with fuel. The mixture is ignited, causing a rapid expansion of gases that are expelled out the back, propelling the aircraft forward. Turbojet engines are known for their high speeds and efficiency at high altitudes. These engines were widely used in early military and commercial aircraft, such as the Boeing 707 and the McDonnell Douglas F-4 Phantom II. However, they have largely been replaced by more efficient engines, like turbofan engines, which provide better fuel economy and lower noise levels.