Thrust vectoring


Thrust vectoring, also known as thrust vector control, is the ability of an aircraft, rocket or other vehicle to manipulate the direction of the thrust from its engine or motor to control the attitude or angular velocity of the vehicle.
In rocketry and ballistic missiles that fly outside the atmosphere, aerodynamic control surfaces are ineffective, so thrust vectoring is the primary means of attitude control. Exhaust vanes and gimbaled engines were used in the 1930s by Robert Goddard.
For aircraft, the method was originally envisaged 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 aerodynamic control surfaces only, such as ailerons or elevator; 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 as gas-dynamic steering or gas-dynamic control.

Methods

Rockets and ballistic missiles

Nominally, the line of action of the thrust vector of a rocket nozzle passes through the vehicle's centre of mass, generating zero net torque about the mass centre. It is possible to generate pitch 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 the roll axis, roll control usually requires the use of two or more separately hinged nozzles or a separate system altogether, such as fins, or vanes in the exhaust plume of the rocket engine, deflecting the main thrust. Thrust vector control is only possible when the propulsion system is creating thrust; separate mechanisms are required for attitude and flight path control during other stages of flight.
Thrust vectoring can be achieved by four basic means:
  • Gimbaled engine or nozzle
  • Reactive fluid injection
  • Auxiliary "Vernier" thrusters
  • Exhaust vanes, also known as jet vanes

    Gimbaled thrust

Thrust vectoring for many liquid rockets is achieved by gimbaling the whole engine. This involves moving the entire combustion chamber and outer engine bell as on the Titan II's twin first-stage motors, or even the entire engine assembly including the related fuel and oxidizer pumps. The Saturn V and the Space Shuttle used gimbaled engines.
A later method developed for solid propellant ballistic missiles achieves thrust vectoring by deflecting only the nozzle of the rocket using electric actuators or hydraulic cylinders. The nozzle is attached to the missile via a ball joint with a hole in the centre, or a flexible seal made of a thermally resistant material, the latter generally requiring more torque and a higher power actuation system. The Trident C4 and D5 systems are controlled via hydraulically actuated nozzle. The STS SRBs used gimbaled nozzles.

Propellant injection

Another method of thrust vectoring used on solid propellant ballistic missiles is liquid injection, in which the rocket nozzle is fixed, however a fluid is introduced into the exhaust 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 the Minuteman II and the early SLBMs of the United States Navy.

Vernier thrusters

An effect similar to thrust vectoring can be produced with multiple vernier thrusters, small auxiliary combustion chambers which lack their own turbopumps and can gimbal on one axis. These were used on the Atlas and R-7 missiles and are still used on the Soyuz rocket, which is descended from the R-7, but are seldom used on new designs due to their complexity and weight. These are distinct from reaction control system thrusters, which are fixed and independent rocket engines used for maneuvering in space.

Exhaust vanes

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. The V-2 used graphite exhaust vanes and aerodynamic vanes, as did the Redstone, derived from the V-2. The Sapphire and Nexo rockets of the amateur group Copenhagen 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.

Tactical missiles and small projectiles

Some smaller sized atmospheric tactical missiles, such as the AIM-9X Sidewinder, eschew flight 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. 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 the Eryx and the PARS 3 LR use thrust vectoring for this reason.
Some other projectiles that use thrust-vectoring:
  • 9M330
  • Strix mortar round uses twelve midsection lateral thruster rockets to provide terminal course corrections
  • Advanced Air Defence missile uses jet vanes
  • Astra
  • Akash
  • BrahMos
  • MPATGM uses jet vanes
  • Pralay uses jet vanes
  • QRSAM uses jet vanes
  • NASM-SR uses jet vanes
  • AAM-5
  • Barak 8 uses jet vanes
  • A-Darter uses jet vanes
  • ASRAAM uses jet vanes
  • R-73 uses exhaust paddles
  • HQ-9 uses jet vanes
  • PL-10 uses jet vanes
  • MICA uses jet vanes
  • PARS 3 LR uses jet vanes
  • IRIS-T uses jet vanes
  • Sky Sword II uses jet vanes
  • Aster missile family combines aerodynamic control and the direct thrust vector control called "PIF-PAF"
  • AIM-9X uses four jet vanes inside the exhaust, that move as the fins move.
  • 9M96E uses a gas-dynamic control system enables maneuver at altitudes of up to 35km at forces of over 20g, which permits engagement of non-strategic ballistic missiles.
  • 9K720 Iskander is controlled during the whole flight with gas-dynamic and aerodynamic control surfaces.
  • Dongfeng subclasses/JL-2/JL-3 ballistic missiles
  • SRAAM uses jet vanes

    Aircraft

Most currently operational vectored thrust aircraft use turbofans with rotating nozzles 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 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, the Bristol Siddeley BS100, was cancelled in 1965.
Tiltrotor aircraft vector thrust via rotating turboprop engine nacelles. The mechanical complexities of this design are quite troublesome, including twisting flexible internal components and driveshaft 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 a vortex 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 for airships. An early application was the British Army airship Delta, which first flew in 1912. It was later used on HMA No. 9r, a British rigid airship that first flew in 1916 and the twin 1930s-era U.S. Navy rigid airships USS Akron and USS Macon that were used as airborne aircraft carriers, and a similar form of thrust vectoring is also particularly valuable today for the control of modern non-rigid airships. In this use, most of the load is usually supported by buoyancy and vectored thrust is used to control the motion of the aircraft. The first airship that used a control system based on pressurized air was Enrico Forlanini's Omnia 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. Official interest was curtailed when it was realised that the designer was a patient in a mental hospital.
Now being researched, Fluidic Thrust Vectoring diverts thrust via secondary fluidic injections. 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, inertia, complexity, and radar cross section for stealth. This will likely be used in many unmanned aerial vehicle, and 6th generation fighter aircraft.

Vectoring nozzles

Thrust-vectoring flight control 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. TVFC can also be used to hold stationary flight in areas of the flight envelope where the main aerodynamic surfaces are stalled. TVFC includes control of STOVL aircraft during the hover and during the transition between hover and forward speeds below 50 knots where aerodynamic surfaces are ineffective.
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. A simpler variant using only three actuators would not have independent exit area control.
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.
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 and rotating exit vanes. Within these aircraft nozzles, the geometry itself may vary from two-dimensional 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.