Posted 10-5-2004 at 01:13 AM

Thrust Vectoring

Imagine two US Air Force Jets with controls not responding, there heading right for each other, the pilots don?t have enough time to eject, there?s a mid-air explosion, and the needless death of American servicemen. About one fifth of peacetime fighter losses during the past few years were due to loss of control. Now imagine that the US has been developing the technology to prevent this for the last decade, but due to budget problems this technology was never installed on our fighters. I?m talking about a Thrust Vectoring Control (TVC). This engineering term describes the use of an engines nozzle to direct the force coming from a jet engine in different directions other then straight out the back. Besides tragic, needless deaths, this technology has a military significance for front line fighter jets. With the wars in Iraq and Afghanistan, Americans have seen the devastating power of our Air Force on Countries without a serious Air Defense network, like Israel or Great Britain. These are countries with not just a Surface to Air Missile (SAM) threat, but an Air Force that can rival ours in its current state. Thrust Vectoring is the technology that will make our fighter jets true rulers of the air, not just on bombing runs, but air-to-air combat, better know as ?Dog Fighting?.

Thrust Vectoring was first used in a trivial form on Nazi Germany?s V-2 rockets. These rockets were devastating to the Allies in WWII with their accuracy due to graphite control vanes that helped the guidance of the missile. Modern rockets, both SAMs and Air-to-Air missiles have been using thrust vectoring to increase their agility in flight, and hence make them more lethal. During the Cold War German military planers recognized the shear numbers of Soviet fighters, and believing that any war would include intense Dog Fighting, began to look for ways to even the odds. Wolfgang Herbst with the Messerschmitt-Boelkow-Blohm, now Deutsche Aerospace, Company led a team in Post-Stall engineering. Post-Stall describes a flight condition in which normal flight controls, like flaps, are no longer sufficient to maintain the flight ability of the aircraft. His team investigated new flight laws to describe the movement of an aircraft in Post-Stall flying conditions.

Why is Thrust Vectoring so important for modern day fighter jets? It?s main purpose is to provide our jets with more mobility in close encounter (within visual range) combat. During a dog fight planes lose speed and altitude to gain a high angle of attack, (AOA). These high AOA can produce a stall, or loss of flight controls in an aircraft, but a jet with thrust vectoring can have a much higher AOA without stalling. A jet with thrust vectoring can perform a maneuver called the ?helicopter? where it is in a controlled flat spin while the nose of the fighter, hence its gun, remains on its target. This added agility can also help fighters avoid deadly missile attacks that have threatened the lives of so many of our pilots in conflicts in Vietnam and Iraq. Thrust Vectoring can also be used to overcome flight limitations in bomb runs, and maintain maneuverability even with the additional bomb load. It can also shorten the takeoff distances, and reduce the ?wind-over-deck? requirement for launching aircraft from our Navy carriers. This technology can also compensate for battle damage to regular flight controls. In the future it could reducing weight and drag by getting rid of radar-reflecting surfaces. Thrust vectoring can also allow for greater thrust expansion ratios by allowing the pilot to control the exhaust exit area independently of the throat of the engine?s exhaust. These are just some of the benefits that we know can occur through this system, but until it is put in a combat arena all the benefits will not be seen.

Examples of aircraft with TVC:

2d TVC: Lockheed Martin F-22 Raptor

Sukhoi Su-37 Flanker

Three main prototypes have been developed and tested to install on existing fighter jet engines. General Electric developed the Axisymmetric vectoring Exhaust Nozzle (AVEN) in 1991 as the first retrofittable nozzle. It has 3-dimensional vectoring in the pitch (up and down), yaw (side to side), and thrust reversible directions (back and forth directions). Pratt and Whitney has engineered to prototypes the Pitch Yaw balanced Beam Nozzle (PYBBN) and the Spherical Convergent Flap Nozzle (SCFN). The PYBBN has a multiple redundancies system, which acts as a fail save in cases of emergency, and a faster response time that the GE AVEN design. The SCFN, is the newest model, and allows for a 20 degree pitch and yaw vectoring as well as engine reversibility capability. It also uses advanced materials to decrease the weight and cooling requirements of the thrust vectoring system.

There have been 2 main research programs within the United State military to investigate the uses of thrust vectoring within the last decade. The F-16 MATV (Multi-Axis Thrust-Vectoring) program uses GE?s AVEN design. Besides the added nozzle on the end of the engine a digital flight control system using post-stall laws for better guidance was added, this new system has been recognized as the key to post-stall flight. The parts for the nozzle are a vectoring ring, 3 actuators (hydraulics), multiaxis hinges (those that move in 2 or more directions), and some additional structural supports. The prototype adds an additional 400 pounds, which is offset by ballast tanks near the nose of the fighter. A drawback from this system occurs because the thrust vectoring takes place past the nozzle throat and prevents pressure fluctuation feedback to reach the engine controls. But due to the adjusting fore and aft translation of the vectoring ring the nozzle exit area is independently controlled from the nozzle throat. After in flight testing the results are very impressive. The AVEN design allowed a jet to maintain an 83 degree angle of attack compared to the standard 25 degrees. A maximum nozzle deflection angle of 15 degrees was achieved during standard military use and 17 degrees in after burner mode. Even more impressive was the results from mock engagements. After 182 one versus one and one versus two engagements the following patterns arose: offensively vector throttling allowed for a reduced time to the first shot at a target, reduced overshooting and other common mistakes, and allowed the fighter to at least threaten, if not engage, the wingman while attacking the primary target. Defensively the system allowed the jet to survive longer, and the possibility to make offensive maneuvers during the two versus one engagements, were typical jets try to escape. ?Thrust vectoring was very effective in our close-in combat evaluations,? said Lester Small, Air Force program manager for the MATV Project. (Ashley, 59)

The second major program is the F-15 ACTIVE, a joint NASA/Air Force program, using 2 PYBBN engines due to the fact that the F-15 is a dual engine fighter. The goals of this program is to find optimum nozzle setting for the maximum performance, investigate new air flow due to thrust vectoring, and find methods for reducing noise generated by the nozzles. The components include a divergent actuation system, a divergent synchronization ring, and an aft static structure. The nozzle utilizes the air in the nozzle region to balance forces on the front and rear side of the unit?s flap assembly. These tests have shown that this system?s multiple redundancy capability adds greatly to the safety factor of the fighter. The two engines off set from the center have allowed for new testing that could not be performed in the MATV project. Official results from this project haven?t been published, but everything indicates that the fighter?s success in this project rivals that of the MATV?s F-16.

Capt. Jim Henderson of the 422nd Test and Evaluation Squadron at Nellis AFB in Nevada said this about the thrust vectoring system, ?the bottom line is you have a greatly increased capability to survive and kill with this system.? (Ashley, 63) Thrust vectoring is a technology worth the money and ready for the next step; implementation in front line fighters. We are not the only country with this technology, the Russians have developed a thrust vectoring nozzle for their new age Sukhoi-35 fighter, which will become their front line fighter when their military budget and costs come together. It is imperative that the United State continue to provide the state of the art equipment for our servicemen. The new F-22 Raptor is slated to have this feature, but companies have developed, tested, and prepared nozzles to attach to current fighter jet engines. The greatest summation of the system is by Lester Small, MATV project manager, who said, ?vectored thrust in no panacea, it?s just another tool in the pilot?s toolbox.? Thrust vectoring is a tool that can save lives and allow America to continue its air dominance right now.

Reference
http://www.personal.psu.edu/users/t/m/tmb935/thrust_vectoring.html