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A lift jet is a form of propulsion system for aircraft that does not require the traditional principle of aerodynamic lift to increase its elevation, which traditionally occurs through air pressure changes as a vehicle moves horizontally through the atmosphere. The unique propulsion of lift jet vehicles allows them to be aerostatic, which means that they can rise vertically from the ground without having to generate lift by moving horizontally at any significant velocity. Aircraft based on aerostatic designs are broken down into two camps: lighter-than-air vehicles such as balloons and dirigibles that rely on the principle of buoyancy for lift, and heavier-than-air vehicles like the Harrier jump jet or vertical take-off and landing (VTOL) aircraft that use traditional jet engines or turboprop engines to rise vertically from a stationary starting position.
Most traditional commercial and military aircraft utilize aerodynamics to create lift as they move horizontal to the ground. This is done by shaping the body of the aircraft so that air takes longer to traverse upper surfaces than lower ones on the body of the craft, creating a pressure differential around the vehicle that provides atmospheric lift. While this is a reliable method of generating lift, it requires stable atmospheric currents for safe travel and relatively long take-off and landing runways. The aircraft needs to reach a minimum velocity necessary for the lift and thrust of its horizontally-placed engines to counteract the effects of gravity. Planes that use a lift jet system consume more fuel to counteract gravity, but require no runways and can take off like helicopters, yet maneuver like fixed-wing aircraft once they are airborne.
Using the lift jet principle in heavier-than-air vehicles gives them a hybrid nature and level of performance between that of traditional aircraft, helicopters, and lighter-than-air vehicles. The main feature that most VTOL aircraft have in common is that their engines swivel once airborne to provide horizontal thrust. Some designs have airframes with additional horizontally-built, fixed propulsion systems to carry the vehicles forward once they are aloft.
Several types of lift jet vehicles have been built since the 1950s, but most were later seen as impractical. The Soviet Yakovlev-38, MIG-23, and Sukhoi-24 were successful designs, but had high fuel consumption and limited payload and range capabilities, which led to their limited use. The UK-based Harrier or Jump Jet has proven to be the most enduring heavier-than-air lift jet vehicle built as of 2011.
The Harrier consists of four models which were deployed as of 2010 by the allied North-Atlantic Treaty Organization (NATO) nations of the UK, US, Spain, and Italy. The lift jet engines on the aircraft are turbofan engines that generate thrust by pulling external air in through a fan assembly, heating it with ignited fuel, and exhausting it vertically downward through a jet exhaust nozzle during take-off to generate thrust. As of 2011, the Harriers were being phased out and replaced by a more modern version of VTOL aircraft designed in the US, known as the F-35B or Joint Strike Fighter, and the UK is expected to adopt an updated F-35C version of the aircraft by 2020.
Technically, a lift jet engine can use one of several propulsion systems including turboprop, turbofan, or turbojet engines. A turboprop engine has an external propeller, and aircraft as the US V-22 Osprey is a VTOL aircraft that uses such engines, though jet propulsion only plays a minor role in their lift capacity. Turbojets can be used as a lift jet as well, and have all of the components of a turbofan engine like that on the Harrier, with the added ability that they actively compress incoming air for increased thrust, and are most efficient above speeds twice that of the speed of sound, or Mach 2.