Gas turbine

Main article: Turboprop

A turboprop gas turbine engine consists of an intake, compressor, combustor, turbine, and a
propelling nozzle, which provide power from a shaft through a reduction gearing to the propeller.
The propelling nozzle provides a relatively small proportion of the thrust generated by a turboprop.

Electric motor

Solar Impulse 1, a solar-powered aircraft with electric motors.

Main article: Electric motor

An electric aircraft runs on electric motors with electricity coming from fuel cells, solar cells,
ultracapacitors, power beaming,[41] or batteries. Currently, flying electric aircraft are mostly
experimental prototypes, including manned and unmanned aerial vehicles, but there are some
production models on the market.[42]

Jet

Main article: Jet engine

Jet aircraft are propelled by jet engines, which are used because the aerodynamic limitations of
propellers do not apply to jet propulsion. These engines are much more powerful than a
reciprocating engine for a given size or weight and are comparatively quiet and work well at higher
altitude. Variants of the jet engine include the ramjet and the scramjet, which rely on high airspeed
and intake geometry to compress the combustion air, prior to the introduction and ignition of fuel.
Rocket motors provide thrust by burning a fuel with an oxidizer and expelling gas through a nozzle.

Turbofan

Most jet aircraft use turbofan jet engines, which employ a gas turbine to drive a ducted fan, which
accelerates air around the turbine to provide thrust in addition to that which is accelerated through
the turbine. The ratio of air passing around the turbine to that passing through is called the by-pass
ratio.[43] They represent a compromise between turbojet (with no bypass) and turboprop forms of
aircraft propulsion (primarily powered with bypass air).[44]

Subsonic aircraft, such as airliners, employ high by-pass jet engines for fuel efficiency. Supersonic
aircraft, such as jet fighters, use low-bypass turbofans. However at supersonic speeds, the air
entering the engine must be decelerated to a subsonic speed and then re-accelerated back to
supersonic speeds after combustion. An afterburner may be used on combat aircraft to increase
power for short periods of time by injecting fuel directly into the hot exhaust gases. Many jet aircraft
also use thrust reversers to slow down after landing.[44]
Ramjet

Main article: Ramjet

Artist's concept of X-43A with scramjet attached to the underside

A ramjet is a form of jet engine that contains no major moving parts and can be particularly useful in
applications requiring a small and simple engine for high-speed use, such as with missiles. Ramjets
require forward motion before they can generate thrust and so are often used in conjunction with
other forms of propulsion, or with an external means of achieving sufficient speed. The Lockheed D-
21 was a Mach 3+ ramjet-powered reconnaissance drone that was launched from a parent aircraft. A
ramjet uses the vehicle's forward motion to force air through the engine without resorting to
turbines or vanes. Fuel is added and ignited, which heats and expands the air to provide thrust.[45]

Scramjet

Main article: Scramjet

A scramjet is a specialized ramjet that uses internal supersonic airflow to compress, combine with
fuel, combust and accelerate the exhaust to provide thrust. The engine operates at supersonic
speeds only. The NASA X-43, an experimental unmanned scramjet, set a world speed record in 2004
for a jet-powered aircraft with a speed of Mach 9.7, nearly 12,100 kilometers per hour (7,500
mph).[46]

Rocket

Main article: Rocket engine

Bell X-1 in flight, 1947

Whereas jet aircraft use the atmosphere both as a source of oxidant and of mass to accelerate
reactively behind the aircraft, rocket aircraft carry the oxidizer on board and accelerate the burned
fuel and oxidizer backwards as the sole source of mass for reaction. Liquid fuel and oxidizer may be
pumped into a combustion chamber or a solid fuel with oxidizer may burn in the fuel chamber.
Whether liquid or solid-fueled, the hot gas is accelerated through a nozzle.[47]

In World War II, the Germans deployed the Me 163 Komet rocket-powered aircraft. The first plane to
break the sound barrier in level flight was a rocket plane – the Bell X-1 in 1948. The North American
X-15 broke many speed and altitude records in the 1960s and pioneered engineering concepts for
later aircraft and spacecraft. Military transport aircraft may employ rocket-assisted take offs for short-
field situations. Otherwise, rocket aircraft include spaceplanes, like SpaceShipTwo, for travel beyond
the Earth's atmosphere and sport aircraft developed for the short-lived Rocket Racing League.

Design and manufacture

SR-71 at Lockheed Skunk Works

Assembly line of the SR-71 Blackbird at Skunk Works, Lockheed Martin's Advanced Development
Programs (ADP).

Most airplanes are constructed by companies with the objective of producing them in quantity for
customers. The design and planning process, including safety tests, can last up to four years for small
turboprops or longer for larger planes.

During this process, the objectives and design specifications of the aircraft are established. First the
construction company uses drawings and equations, simulations, wind tunnel tests and experience
to predict the behavior of the aircraft. Computers are used by companies to draw, plan and do initial
simulations of the aircraft. Small models and mockups of all or certain parts of the plane are then
tested in wind tunnels to verify its aerodynamics.