Jet aircraft are powered by jet engines which, unlike earlier piston and turbine engines, do not rely on propellers for power and movement. As a result, jets can approach speeds and heights not previously available on propeller driven aircraft. On average, jet aircraft perform at optimal efficiency at speeds greater than Mach 0.8 and at altitudes of over 30,000-50,000 feet.
Jet engines are available in many basic types including: turbojets, turbofans, rockets, ramjets, and pulse jets which all offer a variety of utilities. However, civilian aircraft do not require the high altitude/high acceleration capabilities or inclusion of oxidizer of rockets (which are usually reserved for extreme applications such as spaceflight and ballistic missiles) and supersonic flight by non-military entities is banned by the Federal Aviation Administration. Civilian jets use airbreathing type engines (or duct engines) driven by internal combustion from gas turbines. Duct engines are labeled airbreathing because their engines derive oxygen from the surrounding atmosphere in order to keep jet fuel burning.
To produce jet propulsion, a jet engine intakes air and emits fluid backwards at a higher speed, thrusting the aircraft forwards. The first aircraft driven by jet propulsion utilized the turbojet design. However turbojet engines are being phased out in favor of newer jet design technology. The turbojet gas turbine/propelling nozzle system is less fuel efficient than turboprops (for slower aircraft) and turbofans (for faster aircraft).