In a bid for increased efficiency with speed, a development of the "turbofan" and "turboprop" known as a propfan engine was created that had an unducted fan.
A turbojet engine equipped with an afterburner is called an afterburning turbojet, whereas a turbofan engine similarly equipped is sometimes called an augmented turbofan.
Taking the principle a step further, the high-bypass turbofan is even more efficient, having typically three spools each spinning at a different speed.
Because of the implied low mean jet velocity, a high bypass ratio/low specific thrust turbofan has a high thrust lapse rate (with rising flight speed).
