The gas turbine, or jet engine, was the first alternative to the propeller as a means for providing thrust to move an aircraft through the sky. Like most engines, the jet is an internal combustion engine, which produces thrust through the classic four-stroke cycle: induction, compression, ignition and exhaust. Unlike propeller planes, which convert the superheated exhaust of internal combustion into mechanical power to drive a rotor, jet engine thrust comes directly from exhaust, ejected from the rear of the tube-shaped engine. While a propeller planes' maximum speed is around Mach 0.8, various jetplanes can reach speeds of Mach 15 or higher (see scramjet), though Mach 1.0 to 2.0 is more typical.
The world's first jet engine plane, the He 178, was designed during the late 1930s by German engineer Hans von Ohain, and flown from Marienehe aerodrome by test pilot Erich Warsitz on 27 August 1939. England flew its first jet engine plane in 1941. American-born designs were not implemented until the 1960s. The creation of the jetplane was a milestone in aviation not seen since the Wright Brothers' historic flight at Kitty Hawk.
The jet engine was originally developed to overcome the diminishing returns of propeller-based designs when the radial speed of the propellers began to approach the speed of sound. As opposed to accelerating a large mass of air a little bit, as in a propeller, the jet engine works on the principle of accelerating a smaller mass of air to a very high velocity. Instead of being mounted at the front of the plane like a propeller, a jet engine is mounted on the wings or at the rear of a plane.
The jet cycle begins when rapidly moving air is sucked into a chamber at the front of the engine, then compressed by a series of blades that move at progressively higher rpms. The intake port, because it sucks air directly from the flight path of the aircraft, is covered by a wire mesh or grille to prevent the entry of foreign objects. When the moving air has reached a high level of compression, it is combined with fuel and ignited, being ejected out the exhaust port at the rear of the jet engine. Because of the tremendous amount of heat generated, sophisticated cooling systems are required to prevent the internals of the engine from melting.
There are many different variants on the design of the jet engine, which mix different structures and techniques to create jetplanes suited to varying speeds and altitudes. These variants include the turbojet, turbofan, ramjet, scramjet, pulsejet, and several others.