A turbojet engine is a turbine engine. The characteristic is that it relies entirely on gas flow to generate thrust. Often used to power high-speed aircraft. Fuel consumption is higher than turbofan engines. Turbojet engines are divided into two types: centrifugal and axial flow. The centrifugal engine was patented by the British Sir Frank Whittle in 1930. However, it was not until 1941 that an aircraft equipped with this engine took to the sky for the first time. Participating in World War II, the axial-flow type was born in Germany and participated in the battle in late 1945 as the power of the first practical jet fighter Me-262. Compared with centrifugal turbojet engines, axial flow engines have the advantages of small cross-section and high compression ratio. Today's turbojet engines are all axial flow engines.
I will try to put it simply, the injection direction of the general engine nozzle is fixed! In other words, there is only size and no direction. This is a scalar! The vector nozzle can change the direction of the injection, which means it has both size and direction. This is a vector!
Thrust vector technology refers to a technology that uses the thrust component generated by the deflection of the nozzle or tail jet to replace the control surface of the original aircraft or enhance the control function of the aircraft, and to control the flight of the aircraft in real time. . Its application must rely on the integrated development of computers, electronic technology, automatic control technology, engine manufacturing technology, materials and processes.
Using thrust vector technology to newly designed and modified military aircraft of the next century is indeed an effective technological breakthrough. It is very effective in stealth, drag reduction and weight reduction of fighter jets.
Thrust vector technology allows part of the engine thrust to be turned into a control force, replacing or partially replacing the control surface, thus greatly reducing the radar reflection area; no matter how high the angle of attack or how low the flight speed, the aircraft can use This part of the control force is used for maneuvering, which increases the maneuverability of the aircraft. Since the control force is directly generated and the magnitude and direction are variable, the agility of the aircraft is increased. Therefore, the vertical tail can be appropriately reduced or removed, and some other control surfaces can also be replaced. This is beneficial to reducing the detectability of the aircraft, and can also reduce the aircraft's drag and reduce its structural weight. Therefore, using thrust vector technology is the best choice to solve design conflicts. Over the years, the United States, Russia and other countries have conducted a large number of flight tests, proving that the use of thrust vector technology can indeed achieve the intended purpose.
After the Gulf War ended in April 1991, the Pentagon spent 50 billion US dollars to develop a new stealth aircraft different from the F-117, using thrust vectoring technology, so there was an aircraft that basically met the above requirements. F-22 fighter jet. Russia’s research on the application of stealth and thrust vectoring technologies includes the MiG 1.44’s use of the reaction force of airflow emitted by the engine in different directions to quickly change direction. "Jane's Defense Weekly" said in 1992 that the Russians had surpassed the F-117 and directly developed a modern supersonic attack aircraft, becoming a competitor of the F-22.
2. Technical classification and its impact on the overall performance of the aircraft
2.1 Baffles
In the mid-1970s, Wolf, an aircraft designer of the German MBB company Gang Herbers proposed controlling the direction of the engine tail jet to improve the aircraft's maneuverability. In 1985, the U.S. Defense Research Agency and MBB Corporation jointly conducted a feasibility study. In March 1990, Rockwell Corporation of the United States, Boeing Corporation, and MBB Corporation of Germany jointly developed a 3-axis thruster that can change the thrust direction at the engine tail nozzle. The test verification of the carbon fiber composite rudder surface shows that the aircraft X-31 left the factory and underwent test flights. Its rudder surface can deflect ±10° relative to the engine axis, can still operate freely when the angle of attack is 70°, and has over-stall maneuverability. .
From November 1993 to the end of 1994, a series of simulated air battles were conducted between the X-31 and F-18. The X-31 aircraft did not use thrust vectoring technology and the F/A- When 18 aircraft started air combat in parallel in the same direction, the F-18 won 12 out of 16 engagements; and when the X-31 used thrust vectoring technology, the X-31 won 64 out of 66 engagements [3]. In addition, the United States conducted tests by installing baffles on the F-14 and F-18 respectively.
Generally speaking, the baffle solution is to install 3 or 4 tail panels on the outside of the tail cowl of the aircraft that can radially rotate inward and outward, and the deflection is changed by the steering of the tail panel. The direction of the aircraft exhaust flow to achieve thrust vectoring. The characteristic of this solution is that the engine does not require any modification and is suitable for testing on existing aircraft. Its advantages are simple structure and low cost, and it has certain value as an experimental research. However, it has a large dead weight and outer dimensions, and the efficiency of thrust vector operation is low, which is detrimental to aircraft stealth and supersonic cruise. Therefore, it is only an experimental verification solution for the development of thrust vector technology.
2.2 Binary vector nozzle
The binary vector nozzle is an aircraft’s tail nozzle that can deflect in the pitch and yaw directions, allowing the aircraft to deflect in the pitch and yaw directions. An additional moment is generated perpendicular to the axis of the aircraft, thus enabling the aircraft to have thrust vector control capability. The binary vector nozzle is usually rectangular, or has four adjusting plates that can be rotated together. The types of binary vector nozzles include: binary convergent-divergent nozzle (2DCDN), pure expansion slope nozzle (SERN), binary wedge nozzle (2DWN), sliding throat nozzle (STVN) and spherical nozzle Convergence regulator nozzle (SCFN), etc.
It has been confirmed through research that the binary vector nozzle is easy to achieve thrust vectoring. In the late 1980s, two U.S. pre-development fighter jets, the YF-22/F119 and YF-23/F120, both adopted this vector nozzle.
The disadvantages of the binary vector nozzle are that the structure is relatively bulky and the internal flow characteristics are poor.
2.3 Axisymmetric vector nozzle
Research on thrust vector technology initially focused on binary vector nozzles. However, as the research deepened, it was discovered that although binary nozzles have many advantages, they also have shortcomings. It is also obvious that transplantation to active aircraft in particular would be quite difficult. Therefore, an axisymmetric thrust vector nozzle was developed. GE began the development of an axially symmetric thrust vectoring nozzle in the mid-1980s. The nozzle it developed consists of 3 A9/steering adjustment actuators, 4 A8/throat area adjustment actuators, and 3 adjustment rings. It consists of a supporting mechanism, a nozzle control valve and a set of heat-resistant sealing sheets.
2.4 Flow field thrust vector nozzle
The flow field thrust vector nozzle is completely different from the previous mechanically actuated thrust vector nozzles. Its main feature is that it passes through the nozzle The diffusion section introduces lateral secondary airflow (Secondary Fluid) to affect the state of the main airflow, in order to change and control the area and direction of the main airflow, and then obtain the thrust vector. Its main advantage is that it eliminates a large number of mechanical moving parts used to implement thrust vectoring, simplifies the structure, reduces the weight of the aircraft, and reduces maintenance costs.
There are many ways to realize flow field thrust vector control. The following methods are currently being studied:
1) Jet thrust vector control. The airflow is injected through one or more injection holes in the diffuser section of the nozzle, forcing the main airflow to flow against the wall on the opposite side of the injection hole, thereby generating lateral force; 2) Backflow thrust vector control. Add a jacket to the outside of the nozzle outlet section to form a counterflow cavity for reverse flow. When the main flow is required to be deflected, the suction system is started to form negative pressure, which deflects the main air flow to generate lateral force; 3) Mechanical/fluid combination thrust vector control.
One or more small rotatable pneumatic regulators with a length equivalent to 15%-35% of the diameter of the throat are installed at a distance from the throat. The rotation is controlled by a servo mechanism and can retract the pipe in a non-vector state. The wall deflects the airflow through the turbulence of the adjusting piece, generating lateral force
Among these thrust vectoring devices, the baffle solution is only used on X-31, F-14, F-18 and other aircraft. Experimental verification was done on the aircraft, which showed that thrust vector control aircraft is effective, and it was not adopted by the thrust vector technology solution developed later. The binary vector nozzle is the earliest researched and the technology is the most mature. It has been adopted by aircraft such as F-22. The research on the axisymmetric thrust vector nozzle was slightly later than the binary vector nozzle, but it developed rapidly and has been adopted by SU-35 and SU-37. Comparatively speaking, the axisymmetric vector nozzle has more superior functions than the binary vector nozzle and is more technically difficult. Therefore, the research and development focus of various countries has now shifted to the axisymmetric vector nozzle. The flow field thrust vector nozzle is still in the research and exploration stage due to its late research, and is still far from practical use, but it will be the most promising thrust vector nozzle.
3. Some tactical effects after applying thrust vector technology
After fighter jets apply thrust vector technology, the tactical effects have been greatly improved. According to the application experience and flight experience of the United States and Russia, Verify, it is indeed the case. The improvement in the tactical effect of fighter jets can be explained from several aspects:
1) Increased maneuverability and safety during takeoff and landing. Since thrust steering can be used to increase lift during takeoff and landing, the taxiing distance is greatly shortened. If thrust reverse is used, the effect is more obvious. Therefore, the requirements for the airport are reduced, making the aircraft more maneuverable. The climate requirements can also be relaxed, and the aircraft is not afraid of asymmetric icing, sudden winds, and small storms. The impact of damage to the landing gear is also reduced, and the combat effectiveness is relatively improved.
2) Enhanced penetration capability, flexibility, survivability and sudden attack due to reduced radar reflection area and increased mobility. This kind of suddenness is valuable. General John M. Loch, commander of the Air Systems Division of the U.S. Air Force, said that 80 of the pilots who were shot down in the past did not see who fired at them. The improvement in survival rate increases the confidence of pilots and can also reduce the number of fighter jets. The US Air Force plans to reduce the number of fighter jets by 35%.
3) The increased range increases the range of attack or defense. After using thrust vector technology, the reduction in rudder area can reduce resistance, reduce fuel consumption, and correspondingly increase the range. In addition, the reduction in tail weight can lead to a greater reduction in the total weight of the aircraft, which can correspondingly increase fuel. It can also extend the voyage.
4) Close combat effectiveness has been improved, opening up a new aerial combat tactic. The main reason is that the controllable angle of attack has been greatly expanded, greatly exceeding the stall angle of attack, and the nose pointing ability has been enhanced, which improves the opportunity to use weapons. And the increase in control power increases agility. A large pitch rate enables the aircraft to quickly control a large angle of attack, so that the aircraft nose can accurately stop at a position where it can intercept the target. At the same time, it can maintain and adjust the angle of attack in real time according to the desired dwell time as much as possible so that the aircraft nose can point to the target, lock on and Fire, then push the stick quickly to return the aircraft to a lower angle of attack (return and reset). Conventional aircraft are usually restricted to flying at conditions far below the stall angle of attack.
5) The air-to-ground attack performance is improved, the hit rate is improved, and the evasive action after bombing is more agile