Thrust vectoring, also thrust vector control or TVC, is the ability of an aircraft, rocket,
or other vehicle to manipulate the direction of the thrust from its engine(s) or motor
in order 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.
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; craft with vectoring must still use control surfaces, but to a lesser extent.
In missile literature originating from Russian sources, thrust vectoring is often referred
as "gas-dynamic steering" or "gas-dynamic control".
Thrust vectoring methods Rockets and ballistic missiles
Thrust vector control is effective only while the propulsion system is creating thrust.
At other stages of flight, separate mechanisms are required for attitude and flight path
control. Nominally, the line of action of the thrust
vector of a rocket nozzle passes through the vehicle's center of mass, generating zero
net moment about the mass center. 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 center. 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 vectoring for many liquid rockets is
achieved by gimballing the rocket engine. This often 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. Such a system was
used on the Saturn V and the Space Shuttle. Another method of thrust vectoring used on
early solid propellant ballistic missiles was liquid injection, in which the rocket
nozzle is fixed, but 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 and 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.
A later method developed for solid propellant ballistic missiles achieves thrust vectoring
by deflecting the rocket nozzle using electric servomechanisms or hydraulic cylinders. The
nozzle is attached to the missile via a ball joint with a hole in the center, 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. 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 motor exhaust to one side.
Thrust vectoring is a way to reduce a missile's minimum range, before which it cannot reach
a speed high enough for its small aerodynamic surfaces to produce effective maneuver. 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
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 35 km 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.
Aircraft Most currently 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 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 2 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. But, designs have
recently been proposed, especially for Project WALRUS, where a significant portion of the
weight of the craft is supported by vectored thrust. 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. 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.
Nozzles
Thrust-Vectoring flight control is obtained through deflection of the aircraft jets into
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.
When TVFC is implemented to complement CAFC, agility and safety of the aircraft are maximized.
To implement TVFC a variety of nozzles both mechanical and fluidic may be applied. This
includes, but is certainly not limited to convergent and convergent-divergent nozzles
that may be fixed or geometrically variable. Within these aircraft nozzles, the geometry
itself may vary from two-dimensional to axisymmetric or elliptic. Thus, it is necessary to clarify
some basic definitions used in thrust-vectoring nozzle design.
Axisymmetric: Nozzles with circular exits. Conventional Aerodynamic Flight Control: Pitch,
Yaw-Pitch, Yaw-Pitch-Roll or any other combination of aircraft control through aerodynamic deflection
using rudders, flaps, elevators and/or ailerons. Converging-Diverging Nozzle: Generally found
on fighter aircraft, the jet stream is taken through a reduction in area to achieve Mach
1 and then expanded through a diverging section to achieve a supersonic speed greater than
Mach 1 before issuing forth. Converging Nozzle: Nozzle used on standard
subsonic transport and passenger jet aircraft. After the turbine, the nozzle converges to
the designed exit area where the jet stream issues forth at Mach 1 or less.
Effective Vectoring Angle: The average angle of deflection of the jet stream centerline
at any given moment in time. Fixed Nozzle: A Thrust-Vectoring Nozzle of
invariant geometry or one of variant geometry maintaining a constant geometric area ratio,
during vectoring. This will also be referred to as civil aircraft nozzles and represents
the nozzle thrust vectoring control applicable to passenger, transport, cargo and other subsonic
aircraft. Geometric Vectoring Angle: Geometric centerline
of the nozzle during vectoring. For those nozzles vectored at the geometric throat and
beyond, this can differ considerably from the effective vectoring angle.
Nozzle: Either convergent or convergent-divergent nozzles, focusing on those generally implemented
in aircraft. Pitch: Vertical directional movement of the
nozzle exit or nose of the aircraft. Roll: Circular directional movement of the
aircraft around the body axis vector. Thrust Vectoring: The deflection of the jet
away from the body-axis through the implementation of a flexible nozzle, flaps, paddles, auxiliary
fluid mechanics or similar methods. Thrust Vectoring Flight Control: Pitch, Yaw-Pitch,
Yaw-Pitch-Roll or any other combination of aircraft control through deflection of thrust
generally issuing from an air-breathing turbofan engine.
Two-Dimensional: Nozzles with square or rectangular exits.
Two-Dimensional Converging-Diverging: Square or rectangular supersonic nozzles on fighter
aircraft. Variable Nozzle: A thrust vectoring nozzle
of variable geometry maintaining a constant effective nozzle area ratio, during vectoring.
This will also be referred to as military aircraft nozzles and represents the nozzle
thrust vectoring control applicable to fighter and other supersonic aircraft.
Yaw: Horizontal directional movement of the aircraft or nozzle.
Methods of Nozzle Control Geometric Area Ratios – Maintaining a fixed
geometric area ratio from the throat to the exit during vectoring. The effective throat
is constricted as the vectoring angle increases. Effective Area Ratios – Maintaining a fixed
effective area ratio from the throat to the exit during vectoring. The geometric throat
is opened as the vectoring angle increases. Differential Area Ratios – Maximizing nozzle
expansion efficiency generally through predicting the optimal effective area as a function of
the mass flow rate. Methods of Thrust Vectoring
Type I – Nozzles whose baseframe mechanically is rotated before the geometrical throat.
Type II – Nozzles whose baseframe is mechanically rotated at the geometrical throat.
Type III – Nozzles whose baseframe is not rotated. Rather, the addition of mechanical
deflection post-exit vanes or paddles enables jet deflection.
Type IV – Jet deflection through counter-flowing or co-flowing auxiliary jet streams. Fluid-based
jet deflection. Operational examples
A famous 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 didn't occur until the deployment of the Lockheed Martin F-22 Raptor fifth-generation
jet fighter in 2005, with its afterburning, thrust-vectoring Pratt & Whitney F119 turbofan.
Lockheed Martin F-35 Lightning II is currently in the pre-production test and development
stage. Although this aircraft uses a conventional afterburning turbofan to facilitate supersonic
operation, the F-35B variant, developed for joint usage by the US Marine Corps, Royal
Air Force, Royal Navy, and Italian 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.
The Sukhoi Su-30 MKI, produced by India under license at Hindustan Aeronautics Limited is
in active service with the Indian Air Force, and employs 2D thrust vectoring. The 2D TVC
makes the aircraft highly maneuverable, capable of near-zero airspeed at high angles of attack
without stalling, and dynamic aerobatics at low speeds. The Su-30MKI is powered by two
Al-31FP afterburning turbofans. The TVC nozzles of the MKI are mounted 32 degrees outward
to longitudinal engine axis and can be deflected ±15 degrees in the vertical plane. This produces
a corkscrew effect, greatly enhancing the turning capability of the aircraft.
Examples of rockets and missiles which use thrust vectoring include both large systems
such as the Space Shuttle Solid Rocket Booster, S-300P surface-to-air missile, UGM-27 Polaris
nuclear ballistic missile and RT-23 ballistic missile and smaller battlefield weapons such
as Swingfire. 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 boat Dvora Mk-III, the Hamina class
missile boat and the US Navy's Littoral combat ships. A few computerized studies add thrust
vectoring to extant passenger airliners, like the Boeing 727 and 747, to prevent catastrophic
failures, while the experimental X-48C may be jet-steered in the future.
List of vectored thrust aircraft 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. For VTOL ability
Bell Model 65 Bell X-14
Bell Boeing V-22 Osprey Boeing X-32
Dornier Do 31 EWR VJ 101
Harrier Jump Jet British Aerospace Harrier II
British Aerospace Sea Harrier Hawker Siddeley Harrier
McDonnell Douglas AV-8B Harrier II
Hawker Siddeley Kestrel Hawker Siddeley P.1127
Lockheed Martin F-35B Lightning II VFW VAK 191B
Yakovlev Yak-38 Yakovlev Yak-141
For higher maneuverability Vectoring in two dimensions
McDonnell Douglas F-15 STOL/MTD Lockheed Martin F-22 Raptor
McDonnell Douglas X-36 Me 163 B experimentally used a rocket steering
paddle for the yaw axis Sukhoi Su-30MKI
Vectoring in three dimensions Sukhoi Su-35
Sukhoi Su-37 Sukhoi PAK FA
Sukhoi/HAL FGFA Mikoyan MiG-35
McDonnell Douglas F-15 ACTIVE General Dynamics F-16 VISTA
Rockwell-MBB X-31 McDonnell Douglas F-18 HARV
Mitsubishi ATD-X Other
23 class airship, a series of British, World War 1 airships
Airship Industries Skyship 600 modern airship Zeppelin NT modern, thrust–vectoring airship
See also
References
8. Wilson, Erich A., "An Introduction to Thrust-Vectored Aircraft Nozzles", ISBN 978-3-659-41265-3