The Pratt & Whitney J57 turbojet (installed in the F-100 and F-102 fighters, among others) became the JT-3D turbofan which powered the Boeing 707 or the DC-8.This produces a sharp boundary at the speed of sound, and many military engines designed for supersonic flight lost their afterburners and were fitted with a big fan to become the engines for subsonic transport aircraft. The corresponding bypass ratios today are up to 12 for subsonic engines, and less than 1 for supersonic engines. The optimum bypass ratio changes continuously, but since the drag coefficient drops after crossing Mach 1, airplanes are either designed for a maximum Mach number of 0.9 or less, or 1.6 and above. The intake and nozzle, however, are very different indeed. At very low speed, the ungeared, shrouded fan is exchanged for a geared, free spinning propeller, in other words, the jet changes to a turboprop. The slower the design speed, the bigger the bypass ratio may become. The main difference is in their bypass ratio. The core engines do not differ much - after all, the intake will make sure that air reaches the engine at a speed of Mach 0.4 to 0.5, regardless of flight speed. To increase thrust, subsonic engines try to maximize mass flow (by increasing the bypass ratio) while supersonic engines rely more on increasing the nozzle speed (by using afterburners). Thrust is air mass flow multiplied by the speed difference between flight and nozzle speed of the engine.
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