Thrust vectoring, also known asthrust vector control (TVC), is the ability of anaircraft,rocket or other vehicle to manipulate the direction of thethrust from itsengine(s) or motor(s) tocontrol theattitude orangular velocity of the vehicle.[1][2][3]
Inrocketry andballistic missiles that fly outside the atmosphere, aerodynamiccontrol surfaces are ineffective, so thrust vectoring is the primary means ofattitude control. Exhaust vanes andgimbaled engines were used in the 1930s byRobert Goddard.
For aircraft, the method was originally envisaged to provide upward vertical thrust as a means to give aircraft vertical (VTOL) or short (STOL) 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 aerodynamic control surfaces only, such asailerons orelevator; aircraft with vectoring must still use control surfaces, but to a lesser extent.
In missile literature originating from Russian sources, thrust vectoring is referred to asgas-dynamic steering orgas-dynamic control.[4]
Nominally, theline of action of the thrust vector of arocket nozzle passes through the vehicle'scentre of mass, generating zero nettorque about the mass centre. It is possible to generatepitch and yaw moments by deflecting the main rocket thrust vector so that it does not pass through the mass centre. Because the line of action is generally oriented nearly parallel to theroll axis, roll control usually requires the use of two or more separately hinged nozzles or a separate system altogether, such asfins, or vanes in the exhaust plume of the rocket engine, deflecting the main thrust. Thrust vector control (TVC) is only possible when the propulsion system is creating thrust; separate mechanisms are required for attitude andflight path control during other stages of flight.
Thrust vectoring can be achieved by four basic means:[5][6]
Thrust vectoring for manyliquid rockets is achieved bygimbaling the wholeengine. This involves moving the entirecombustion chamber and outer engine bell as on theTitan II's twin first-stage motors, or even the entire engine assembly including the relatedfuel andoxidizer pumps. TheSaturn V and theSpace Shuttle used gimbaled engines.[5]
A later method developed forsolid propellantballistic missiles achieves thrust vectoring by deflecting only thenozzle of the rocket using electric actuators orhydraulic cylinders. The nozzle is attached to the missile via aball joint with a hole in the centre, or a flexible seal made of a thermally resistant material, the latter generally requiring moretorque and a higher power actuation system. TheTrident C4 andD5 systems are controlled via hydraulically actuated nozzle. TheSTS SRBs used gimbaled nozzles.[7]
Another method of thrust vectoring used onsolid propellantballistic missiles is liquid injection, in which therocket nozzle is fixed, however a fluid is introduced into theexhaust flow from injectors mounted around the aft end of the missile. If the liquid is injected on only one side of the missile, it modifies that side of the exhaust plume, resulting in different thrust on that side thus an asymmetric net force on the missile. This was the control system used on theMinuteman II and the earlySLBMs of theUnited States Navy.
An effect similar to thrust vectoring can be produced with multiplevernier thrusters, small auxiliary combustion chambers which lack their own turbopumps and can gimbal on one axis. These were used on theAtlas andR-7 missiles and are still used on theSoyuz rocket, which is descended from the R-7, but are seldom used on new designs due to their complexity and weight. These are distinct fromreaction control system thrusters, which are fixed and independent rocket engines used for maneuvering in space.
One of the earliest methods of thrust vectoring in rocket engines was to place vanes in the engine's exhaust stream. These exhaust vanes or jet vanes allow the thrust to be deflected without moving any parts of the engine, but reduce the rocket's efficiency. They have the benefit of allowing roll control with only a single engine, which nozzle gimbaling does not. TheV-2 used graphite exhaust vanes and aerodynamic vanes, as did theRedstone, derived from the V-2. The Sapphire and Nexo rockets of the amateur groupCopenhagen Suborbitals provide a modern example of jet vanes. Jet vanes must be made of a refractory material or actively cooled to prevent them from melting. Sapphire used solid copper vanes for copper's high heat capacity and thermal conductivity, and Nexo used graphite for its high melting point, but unless actively cooled, jet vanes will undergo significant erosion. This, combined with jet vanes' inefficiency, mostly precludes their use in new rockets.
Some smaller sized atmospheric tacticalmissiles, such as theAIM-9X Sidewinder, eschewflight control surfaces and instead use mechanical vanes to deflect rocket motor exhaust to one side.
By using mechanical vanes to deflect the exhaust of the missile's rocket motor, a missile can steer itself even shortly after being launched (when the missile is moving slowly, before it has reached a high speed). This is because even though the missile is moving at a low speed, the rocket motor's exhaust has a high enough speed to provide sufficient forces on the mechanical vanes. Thus, thrust vectoring can reduce a missile's minimum range. For example, anti-tank missiles such as theEryx and thePARS 3 LR use thrust vectoring for this reason.[8]
Some other projectiles that use thrust-vectoring:
Most currently operational vectored thrust aircraft useturbofans with rotatingnozzles or vanes to deflect the exhaust stream. This method allows designs to deflect thrust through as much as 90 degrees relative to the aircraft centreline. If an aircraft uses thrust vectoring for VTOL operations 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. A PCB engine, theBristol Siddeley BS100, was cancelled in 1965.
Tiltrotor aircraft vector thrust via rotatingturboprop enginenacelles. The mechanical complexities of this design are quite troublesome, including twisting flexible internal components anddriveshaft power transfer between engines. Most current tiltrotor designs feature two rotors in a side-by-side configuration. If such a craft is flown in a way where it enters avortex ring state, one of the rotors will always enter slightly before the other, causing the aircraft to perform a drastic and unplanned roll.
Thrust vectoring is also used as a control mechanism forairships. An early application was the British Army airshipDelta, which first flew in 1912.[16] It was later used on HMA (His Majesty's Airship)No. 9r, a British rigid airship that first flew in 1916[17] and the twin 1930s-era U.S. Navy rigid airshipsUSSAkron andUSSMacon that were used asairborne aircraft carriers, and a similar form of thrust vectoring is also particularly valuable today for the control of modernnon-rigid airships. In this use, most of the load is usually supported bybuoyancy and vectored thrust is used to control the motion of the aircraft. The first airship that used a control system based on pressurized air wasEnrico Forlanini'sOmnia Dir in 1930s.
A design for a jet incorporating thrust vectoring was submitted in 1949 to the British Air Ministry by Percy Walwyn; Walwyn's drawings are preserved at the National Aerospace Library at Farnborough.[18] Official interest was curtailed when it was realised that the designer was a patient in a mental hospital.[citation needed]
Now being researched, Fluidic Thrust Vectoring (FTV) diverts thrust via secondaryfluidic injections.[19] Tests show that air forced into a jet engine exhaust stream can deflect thrust up to 15 degrees. Such nozzles are desirable for their lower mass and cost (up to 50% less),inertia (for faster, stronger control response), complexity (mechanically simpler, fewer or no moving parts or surfaces, less maintenance), andradar cross section forstealth. This will likely be used in manyunmanned aerial vehicle (UAVs), and 6th generationfighter aircraft.
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Thrust-vectoring flight control (TVFC) is obtained through deflection of the aircraft jets in some or all of the pitch, yaw and roll directions. In the extreme, deflection of the jets in yaw, pitch and roll creates desired forces and moments enabling complete directional control of the aircraft flight path without the implementation of the conventional aerodynamic flight controls (CAFC). TVFC can also be used to hold stationary flight in areas of the flight envelope where the main aerodynamic surfaces are stalled.[20] TVFC includes control ofSTOVL aircraft during the hover and during the transition between hover and forward speeds below 50 knots where aerodynamic surfaces are ineffective.[21]
When vectored thrust control uses a single propelling jet, as with a single-engined aircraft, the ability to produce rolling moments may not be possible. An example is an afterburning supersonic nozzle where nozzle functions are throat area, exit area, pitch vectoring and yaw vectoring. These functions are controlled by four separate actuators.[20] A simpler variant using only three actuators would not have independent exit area control.[20]
When TVFC is implemented to complement CAFC, agility and safety of the aircraft are maximized. Increased safety may occur in the event of malfunctioning CAFC as a result of battle damage.[20]
To implement TVFC a variety of nozzles both mechanical and fluidic may be applied. This includes convergent and convergent-divergent nozzles that may be fixed or geometrically variable. It also includes variable mechanisms within a fixed nozzle, such as rotating cascades[22] and rotating exit vanes.[23] Within these aircraft nozzles, the geometry itself may vary from two-dimensional (2-D) to axisymmetric or elliptic. The number of nozzles on a given aircraft to achieve TVFC can vary from one on a CTOL aircraft to a minimum of four in the case of STOVL aircraft.[21]
An example of 2D thrust vectoring is theRolls-Royce Pegasus engine used in theHawker Siddeley Harrier, as well as in theAV-8B Harrier II variant.Widespread use of thrust vectoring for enhanced maneuverability in Western production-model fighter aircraft didn't occur until the deployment of theLockheed MartinF-22 Raptor fifth-generation jet fighter in 2005, with its afterburning, 2D thrust-vectoringPratt & Whitney F119turbofan.[28]
While theLockheed Martin F-35 Lightning II uses a conventional afterburning turbofan (Pratt & Whitney F135) to facilitate supersonic operation, its F-35B variant, developed for joint usage by theUS Marine Corps,Royal Air Force,Royal Navy, andItalian Navy, also incorporates a vertically mounted, low-pressure shaft-driven remote fan, which is driven through a clutch during landing from the engine. Both the exhaust from this fan and the main engine's fan are deflected by thrust vectoring nozzles, to provide the appropriate combination of lift and propulsive thrust. It is not conceived for enhanced maneuverability in combat, only forVTOL operation, and the F-35A and F-35C do not use thrust vectoring at all.
TheSukhoi Su-30MKI, produced by India under licence atHindustan Aeronautics Limited, is in active service with theIndian Air Force. The TVC makes the aircraft highly maneuverable, capable of near-zero airspeed at high angles of attack without stalling, and dynamic aerobatics at low speeds. TheSu-30MKI is powered by twoAl-31FPafterburningturbofans. The TVC nozzles of the MKI are mounted 32 degrees outward to longitudinal engine axis (i.e. in the horizontal plane) and can be deflected ±15 degrees in the vertical plane. This produces acorkscrew effect, greatly enhancing the turning capability of the aircraft.[29]
A few computerized studies add thrust vectoring to extant passenger airliners, like the Boeing 727 and 747, to prevent catastrophic failures, while the experimentalX-48C may be jet-steered in the future.[30]
Examples of rockets and missiles[31] which use thrust vectoring include both large systems such as theSpace Shuttle Solid Rocket Booster (SRB),S-300P (SA-10)surface-to-air missile,UGM-27 Polarisnuclearballistic missile andRT-23 (SS-24) ballistic missile and smaller battlefield weapons such asSwingfire.
The principles of air thrust vectoring have been recently adapted to military sea applications in the form of fast water-jet steering that provide super-agility. Examples are the fast patrol boatDvora Mk-III, theHamina class missile boat and the US Navy'sLittoral combat ships.[30]
Thrust vectoring can convey two main benefits: VTOL/STOL, and higher maneuverability. Aircraft are usually optimized to maximally exploit one benefit, though will gain in the other.
8. Wilson, Erich A., "An Introduction to Thrust-Vectored Aircraft Nozzles",ISBN 978-3-659-41265-3
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