In the last chapter, the turbojet engine was discussed in detail. Historically, commercial aviation began toward the end of World War II with propeller-driven planes. It soon became apparent that the flight speeds of the propeller planes were limited to low subsonic values. The use of the gas turbine engine in combination with a propulsion nozzle, i.e., the turbojet engine, allowed higher flight speeds to be realized. Although still used for military applications, turbojets lack the efficiency required for commercial applications. This led to the evolution of the turbofan engine.