Saturday, October 9, 2010

Thrust Vector Control

Thrust vector control, or thrust vectoring, is the capacity of a jet engine to shift the direction of the engine thrust to control the angular velocity of an aircraft or rocket. In ballistic missiles that fly outside the atmosphere thrust vector control is the primary means of orientation control.

Thrust vectoring was originally conceived to provide upward vertical thrust as a means to give aircraft vertical or short takeoff and landing ability. Subsequently, it was realized that using vectored thrust in combat situations enabled aircraft to perform various maneuvers not available to conventional-engined planes. To perform turns, aircraft that use no thrust vectoring must rely on only aerodynamic control surfaces, such as ailerons or flaps; craft with vectoring must still use control surfaces, but to a lesser extent.

Operational vectored thrust aircraft use turbofans with rotating nozzles or vanes to deflect the exhaust stream. This method can successfully deflect thrust through as much as 90 degrees, relative to the aircraft centerline. However, the engine must be sized for vertical lift, rather than normal flight, which results in a weight penalty. Afterburning (or Plenum Chamber Burning, PCB, in the bypass stream) is difficult to incorporate and is impractical for take-off and landing thrust vectoring, because the very hot exhaust can damage runway surfaces. Without afterburning it is hard to reach supersonic flight speeds. The best known example of thrust vectoring is the Rolls-Royce Pegasus engine used in the Hawker Siddeley Harrier, as well as in the AV-8B Harrier II variant.

Widespread use of thrust vectoring for enhanced maneuverability in Western production-model fighter aircraft would have to wait until the 21st century, and the deployment of the Lockheed Martin F-22 Raptor fifth-generation jet fighter, with its afterburning, thrust-vectoring Pratt & Whitney F119 turbofan.

Thrust vector control for many liquid rockets is achieved by gimballing the rocket engine. This often involves moving the entire combustion chamber and outer engine bell, or even the entire engine assembly including the related fuel and oxidizer pumps. Such a system was used on the Saturn V and is employed on the space shuttle.

Thrust vectoring on an F-22