The gas turbine is much older than the piston (reciprocating) engine. Hero used a gas turbine (steam) to drive a toy merry-go-round in 130 B.C. A patent for a gas turbine was granted to John Barber in England in 1791, and many were developed for various applications in the nineteenth century. Jets of hot gas impinge against turbine blades to spin the turbine which, in turn, performs some useful function such as driving a generator, compressor, pump, or other mechanical device. In an aircraft turbine engine, the turbine drives a compressor, fan, and sometimes a propeller, depending on the engine design. The most usual turbine engine is the turbojet where the turbine drives a compressor. Figure 68 is a sectional view of a turbojet, with the various components labeled. Air entering the inlet is compressed by the axial flow compressor having multiple stages. A liquid is sometimes injected in the compressor of high-performance turbojets as a coolant. The main fuel is injected and burned in the combustors in the annular space between the outer shell and the inner "spool". The hot combustion gases drive the turbine, shown in two stages, and in the after section additional fuel may be injected and burned to increase gas temperature and provide more thrust. The hot exhaust gases expand through the nozzle to produce thrust.
Thrust augmentation with an afterburner is for high performance military turbojets; afterburners are not used on commercial turbojets, where low specific fuel consumption is a major factor. Figure 69 shows the growth of thrust of turbojet engines from the early 1940s until 1952.