{{Unreferenced, date=July 2009 RollsRoyceRB108

A lift jet is a

jet engine A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, Pump-jet, water jet, and hybrid propulsion, the term ...

angled to provide an

aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...

with ''aerostatic'' (i.e. not requiring the movement of air over an

airfoil An airfoil (American English) or aerofoil (British English) is the cross-sectional shape of an object whose motion through a gas is capable of generating significant lift, such as a wing, a sail, or the blades of propeller, rotor, or turbine. ...

)

lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobil ...

, instead of (or in addition to) thrust. On a fixed-wing aircraft, lift jets may be installed as auxiliary engines, with a separate engine to provide forward thrust, or, as in the

Harrier jump jet The Harrier, informally referred to as the Harrier jump jet, is a family of jet-powered attack aircraft capable of vertical/short takeoff and landing operations (V/STOL). Named after a bird of prey, it was originally developed by British ma ...

, may be vectored in flight to provide both. Lift jets were first designed by

German German(s) may refer to: * Germany (of or related to) ** Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...

engineers during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

, but none saw operational service. Many nations had experimental programs involving such engines by the early 1950s: one typical example was Rolls-Royce Thrust Measuring Rig (TMR), nicknamed the "Flying Bedstead", which took its lift solely from engine thrust. An early dedicated lift jet was the

Rolls-Royce RB.108 The Rolls-Royce RB.108 was a British jet engine designed in the mid-1950s by Rolls-Royce specifically for use as a VTOL lift engine. It was also used to provide horizontal thrust in the Short SC.1. Design and development The RB.108 was the f ...

, first run in 1955. In the early 1960s both the

Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen national ...

and Western nations considered the possibility of lift engines as a way of providing

STOL A short takeoff and landing (STOL) aircraft is a conventional fixed-wing aircraft that has short runway requirements for takeoff and landing. Many STOL-designed aircraft also feature various arrangements for use on airstrips with harsh conditio ...

or even

VTOL A vertical take-off and landing (VTOL) aircraft is one that can take off and land vertically without relying on a runway. This classification can include a variety of types of aircraft including helicopters as well as thrust-vectoring fixed-win ...

capability to combat aircraft. The Soviets did side-by-side testing of versions of combat aircraft using variable geometry wings and lift jets (the results became the

Mikoyan MiG-23 The Mikoyan-Gurevich MiG-23 (russian: Микоян и Гуревич МиГ-23; NATO reporting name: Flogger) is a variable-geometry fighter aircraft, designed by the Mikoyan-Gurevich design bureau in the Soviet Union. It is a third-generatio ...

and

Sukhoi Su-24 The Sukhoi Su-24 ( NATO reporting name: Fencer) is a supersonic, all-weather attack aircraft developed in the Soviet Union. The aircraft has a variable-sweep wing, twin-engines and a side-by-side seating arrangement for its crew of two. It wa ...

). The problems with the auxiliary lift engine are high fuel consumption, extra weight (which is simply dead weight when the engines are not needed for lift), and taking up fuselage volume that could be used for fuel or other systems. The Soviets decided that variable-geometry wings provided comparable advantages in take-off performance without as many penalties. One of the few operational military aircraft with auxiliary lift engines was the Soviet

Yakovlev Yak-38 The Yakovlev Yak-38 (russian: Яковлев Як-38; NATO reporting name: "Forger") was the Soviet Naval Aviation's only operational VTOL strike fighter aircraft in addition to being its first operational carrier-based fixed-wing aircraft. It w ...

, a VTOL fighter used by the AVMF's small

aircraft carrier An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a ...

s, which were not large enough to support conventional fixed-wing aircraft. The Yak-38 proved disappointing in service for most of the above reasons, having limited payload and range as well as dismal handling. A novel alternative to the lift jet for auxiliary vertical thrust is employed by the STOVL Lockheed F-35B version of the U.S. Joint Strike Fighter. This does not have a jet lift engine, but a central lift fan that is powered by the main engine via a

driveshaft A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power and torque and rotation, usually used to connec ...

. When not needed, it is declutched from the engine so that it does not consume power (analogous to the

transfer case A transfer case is a part of the drivetrain of four-wheel-drive, all-wheel-drive, and other multiple powered axle vehicles. The transfer case transfers power from the transmission to the front and rear axles by means of drive shafts. It also syn ...

of a

four-wheel drive Four-wheel drive, also called 4×4 ("four by four") or 4WD, refers to a two-axled vehicle drivetrain capable of providing torque to all of its wheels simultaneously. It may be full-time or on-demand, and is typically linked via a transfer case ...

vehicle). The operational viability of this system remains to be seen, and all of the above drawbacks still apply: it is dead weight in flight, and together with the aircraft's internal weapons bays it takes up a great deal of internal space, reducing fuel and avionics load and requiring a bulkier and wider fuselage cross-section (which passes on to the A and C versions, even though they don't have a fan or V/STOL capability). It has, however, in pre-operational testing proven the ability to take off vertically and go supersonic in level flight in the same mission, a first for a combat aircraft.

See also