Dong Shihong "Don't call him Dong for short: Professor Zhang." Hello! Thank you very much for being interviewed by us. Knowing that you are a research expert in infrared stealth technology, we plan to educate you on some questions about aircraft stealth. First of all, can you tell us what are the ways for aircraft to achieve invisibility?

Professor Zhang (hereinafter referred to as Zhang): The stealth technology of aircraft is mainly reflected in the control of target signals such as radar and infrared. Aircraft using radar stealth are easy to identify from their appearance. This type of aircraft mainly reduces the radar reflection cross section by optimizing the appearance design. For example, the American F-117A, the entire aircraft is almost composed of straight lines and planes, which concentrates the radar reflection waves. Several narrow beams in the horizontal plane prevent enemy radar from getting enough continuous echo signals to determine whether it is a real target. Coupled with the special stealth design of the engine air intake, tail nozzle, canopy seams, landing gear and other parts, it makes it appear on the radar screen. L shows a smaller signal than a bird would show on a radar screen. In addition, American F-22, B-2, "Comanche" and other aircraft have adopted "plastic surgery". The radar reflection cross section is extremely small.

Dong: Radar invisibility is closely related to the aerodynamic layout of the aircraft, and the aerodynamic layout directly affects the maneuverability of the aircraft. How do aircraft designers reconcile this contradiction?

Zhang: You can’t have your cake and eat it too. If an aircraft wants to have true stealth capabilities, it must lose some of its mobility; if it wants to have true super maneuverability, it must lose some of its stealth capabilities. . Therefore, designers must determine which design to choose based on its specific combat use. In addition, a compromise approach can also be adopted, which has both a certain degree of stealth capability and a certain degree of mobility.

Dong: In your opinion, which one is more important, mobility or stealth, in future air combat?

Zhang: For combat aircraft, both maneuverability and stealth are important. As for which performance is more important, it must be weighed based on the combat mission of the aircraft and the combat environment it faces.

Dong: Compared with European and American aircraft, most Russian aircraft are bulky and have no stealth design, so their radar reflection cross sections are generally larger. However, it is said that Russia is preparing to use plasma stealth technology on its 1.44-class aircraft. What are your thoughts on plasma cloaking technology?

Zhang: It is said that plasma stealth technology is very effective in reducing the radar scattering area of ??aircraft, especially for long waves. However, plasma stealth technology also has some disadvantages of its own, such as increasing the weight of airborne equipment and reducing stealth effectiveness due to the greater composite impact with air during low-altitude flight. I personally believe that plasma stealth technology and other stealth technologies will complement each other and achieve better stealth effects.

Dong: What does infrared invisibility mean?

Zhang: Infrared stealth mainly refers to suppressing and weakening the infrared radiation energy of the target, thereby making it impossible for enemy missiles to detect it.

Dong: It seems that we need to figure out infrared invisibility. We also have to figure out what is going on with infrared radiation.

Zhang: Yes. Theoretically, infrared radiation is essentially thermal radiation with a wavelength ranging from 0.76 to 100 microns. As long as the temperature of the object surface is higher than absolute zero (-273 degrees), there will always be energy continuously flowing from the object surface to the object surface. This thermal radiation phenomenon is released externally. Therefore, infrared radiation is an inherent characteristic of various military targets, especially aircraft, rockets and other aircraft that have strong infrared radiation sources. Infrared radiation is often divided into four regions according to wavelength, namely near infrared (wavelength range 0.76-3 microns), mid-infrared (wavelength range 3-6 microns), mid-far infrared (wavelength range 6-20 microns) and far infrared (wavelength range 6-20 microns) Range 20-100 microns). The first three areas all contain wavelength bands that are relatively transparent to the atmosphere, namely three atmospheric windows of 2-26 microns, 3-5 microns and 8-14 microns. The so-called infrared atmospheric window means that for the transmission of infrared radiation, only the infrared radiation in the band corresponding to the atmospheric window can be transmitted, while in the band outside the atmospheric window, a considerable part of the infrared radiation is difficult to penetrate the atmosphere. , this is because some components of the atmosphere have a strong absorption effect on infrared radiation in certain bands, and at the same time, the scattering of various particles suspended in the atmosphere also causes infrared radiation to attenuate during the transmission process. Therefore, in the infrared guidance device, the working band of the infrared detector must fall within the atmospheric window.

Dong: In modern warfare, the main threat to aircraft is various types of missiles. So does it mean that an infrared-guided missile can shoot down a plane as long as its operating band is within the atmospheric window?

Zhang: Of course not. It is only a prerequisite that the working band is within the atmospheric window. If you want to bring down the plane, you must also look at the missile detector's ability to detect the infrared radiation energy of the plane. From the perspective of the development of infrared guided missiles, the first generation of infrared guided missiles developed in the 1950s (such as the American "Sidewinder" AIM-gB, the British "Sky Flash", etc.) all used uncooled lead sulfide detectors. , the working band is 1.3 microns, and the action range is short. The action range for MiG-29 and other targets at an altitude of 10-15 kilometers is generally 7-8 kilometers, and it can only detect the infrared of the aircraft's jet engine tail nozzle. radiation.

The second generation of infrared guided missiles (after 1967) expanded the omnidirectional attack capability and can attack the target from the front or side of the target, such as the American "Sidewinder" AIM-gL, the French "Matra" R550, etc., this type The infrared guidance system of missiles generally uses refrigerated indium antimonide photosensitive elements, with an operating band of 3-5 microns. The main detection target is the high-temperature gas tail flame emitted from the engine tail nozzle. The third-generation infrared guidance missile uses an infrared focal plane. The imaging guidance of the array detector, the detection element is antimony, cadmium and mercury, can detect infrared radiation of 8-14 microns. Typical products include the American AGM-65D 'Maverick' air-to-ground missile, AGM-114A 'Helfa' air-to-ground missile, etc. Infrared imaging guidance technology has the ability to autonomously search, capture, identify and track targets in various complex tactical environments. It represents the development trend of contemporary infrared guidance technology. Infrared imaging detection truly realizes an omnidirectional attack on aircraft, because this The aerodynamic heating radiation of the fuselage skin will become a radiation source that cannot be ignored. The aerodynamic heating radiation of the fuselage skin is characterized by a large radiation area and a small radiation contrast with the environment. It uses point source hot spot guidance as the first mode. Second-generation missiles pose almost no threat to them, but the thermal radiation distribution image caused by the slight temperature difference or self-radiation rate between the target and the background is very suitable for infrared imaging detection as infrared guidance technology changes from point source guidance to imaging guidance. Rapid development, as well as the continuous improvement of aircraft thrust-to-weight ratio and speed, make the infrared radiation intensity of the aircraft propulsion system and the infrared radiation signal of the fuselage skin pose an increasingly prominent threat to the survivability of the aircraft in a war environment. Against this background, the development of infrared countermeasures has become increasingly fierce.

Dong: What are the methods of infrared countermeasures?

Zhang: There are two types of infrared countermeasures: active and passive. The purpose of active countermeasures is to actively interfere with, deceive or destroy the infrared guidance system of the incoming missile, so that the missile cannot hit the target. The main means include infrared decoy bombs, infrared jammers and high-energy lasers. The first two are already used on some aircraft. High-energy lasers have the ability to burn out infrared guidance seekers, but they can only be used on ground equipment and have not yet reached the practical stage of airborne countermeasures. The purpose of passive countermeasures is to suppress and weaken the infrared radiation energy of the target. The enemy missile cannot or can only detect the target at the closest possible distance, making it difficult for the missile to maneuver and attack within a short distance, and the effect of passive confrontation ultimately depends on the level of infrared radiation energy.

Dong: What measures can be taken to reduce infrared radiation?

Zhang: The method of suppressing aircraft infrared radiation generally includes three aspects: first, changing the infrared radiation characteristics of the target, mainly changing the infrared radiation of the target. Radiation band, the infrared radiation band of the aircraft is outside the response band of the infrared guided missile, thereby disabling the enemy's infrared detector, so as to achieve the purpose of infrared stealth. For example, using new composite materials or using active materials on the outside of the aircraft body. Selective paint coatings can change the infrared radiation characteristics of the target, radiating most of the radiated energy away from the "inclusive zone" outside the atmospheric window. The second is to reduce the infrared radiation intensity of the target, mainly by reducing the temperature and emissivity of the target to reduce the radiation power of the aircraft. For exposed solid walls, air cooling can be used to reduce the temperature. For gas tail flames, cold air is introduced for mixing, which can not only achieve the purpose of cooling, but also dilute the carbon dioxide concentration and reduce the gas emissivity. The third method is to adjust the transmission process of infrared radiation. It mainly adopts certain technical means in the structure to change the direction of infrared radiation. For example, specially designed infrared baffles can be used to block the infrared radiation of the nozzle and change the exhaust nozzle. The shape and direction of the tube, etc.

Dong: What are the main aircraft with infrared stealth capabilities currently? How do they achieve infrared invisibility?

Zhang: At present, the aircraft with infrared stealth performance are mainly American F-117A, F-22, B-2 and other aircraft. Their infrared stealth methods mainly use binary nozzle technology and tail nozzle 1. Shielding technology, internal and external airflow enhancement, exhaust duct injection mixing technology and low-emissivity coating technology, etc. The low infrared radiation binary nozzle is a high-performance exhaust nozzle that appears in advanced propulsion system technology and can meet the technical and tactical requirements of aircraft. The use of a binary nozzle with low infrared radiation is beneficial to improving the mixing structure of the tail flame and the atmosphere. The F-117A uses a binary nozzle with a large aspect ratio (15 cm high and 1.83 m wide), which greatly improves the This reduces the cooling speed of the tail flame and shortens the core length of the tail flame. In addition, the binary nozzle is integrated with the non-circular cross-section fuselage to achieve an integrated design, which is beneficial to reducing aerodynamic resistance and improving cruise capabilities. It is also easy to achieve thrust steering and thrust reversal, which enhances the aircraft's maneuverability and short-distance take-off. It has the capability of landing and is widely used in stealth aircraft. Blocking technology refers to using the rear fuselage, vertical tail, etc. to effectively block the inner cavity of the nozzle and the core of the tail flame, which can achieve good infrared suppression effects. The F-117A places the nozzle on the upper part of the fuselage, extends the rear edge of the fuselage backward, and arranges 11 exhaust deflectors inside the nozzle, making the nozzle look like a harmonica, which greatly reduces the viewing angle of infrared detection. The power of the stealth aircraft must be a turbofan engine, which uses the outer fan cold air and the inner hot airflow to implement forced mixing. At the same time, the cold air mixing effect during exhaust can be used to further reduce the temperature of the exhaust.

The F-117A is equipped with an exhaust/bypass air lobe mixer, and its inlet throat area is approximately four times that of the F/A-18 equipped with the same engine. Such a large The air inlet area can draw more cold air into the bypass ejector nozzle.

Dong: Since helicopters do not need to use the exhaust momentum of the engine to generate thrust, is it much easier to suppress infrared than fixed-wing aircraft?

Zhang: Indeed. The infrared suppression technology of helicopters has made greater progress compared with fixed-wing aircraft. So far, three generations of infrared suppressors have been developed. The first generation of infrared suppressors were used during the Vietnam War. The main purpose is to block the high-temperature radiation of hot metal parts and avoid the threat of infrared missiles in the l-3 micron band. The second-generation infrared suppressor is different from the first-generation infrared suppressor in that its purpose is to reduce the radiation of hot metal parts in the rear hemisphere at the rear of the engine. Its infrared countermeasure band range is 3-5 microns. In addition to reducing the infrared radiation of hot metal parts In addition, it is also required to reduce the temperature of the exhaust tail flame and reduce the infrared radiation of the tail flame, so that the infrared missile loses its omnidirectional attack target. Therefore, it requires using the exhaust momentum of the engine to suck in a large amount of cold air from the environment, and to strengthen uniform mixing in a very short mixing tube. For example, the "Dolphin" SA365 helicopter jointly developed by the United States and France uses a lobe nozzle. Ejection infrared suppressor. The second-generation infrared suppressor is still an add-on or optional part of the engine. It is subject to too many restrictions in shape and size, and it is very difficult to further eliminate the influence of infrared radiation from the engine exhaust device. According to the latest data analysis, the integrated infrared suppression concept using the tail fuselage designed by the American Boeing/Sikorsky Company for its new century helicopter RAH-66 "Comanche" undoubtedly represents the development direction of the third generation of infrared suppression technology. It sucks in outside air (including air from the rotor downwash) through two narrow inlets on the helicopter's spine, mixes it with the exhaust from the engine, and then uses the slit outlets on both sides of the lower belly of the tail fuselage to mix it. Gas is discharged. This integrated infrared suppression solution has at least three outstanding advantages: First, it completely blocks the high-temperature components of the engine exhaust device, which is conducive to improving the core structure of the exhaust tail flame and preventing the tail flame from attacking the fuselage wall. Heating to eliminate infrared radiation from the exhaust system to the greatest extent. Not only can it reduce infrared radiation in the 3-5 micron wavelength range to a considerable level (its infrared radiation is 25 times lower than the AH-64 "Apache" helicopter). It also has a significant inhibitory effect on infrared radiation in the wavelength range of 8-14 microns. Second, introducing the exhaust into the tail fuselage can maximize the use of the helicopter's effective space, which is beneficial to the helicopter's aerodynamic layout design. The third is to help realize the comprehensive effect of radar invisibility.

Dong: Thank you again Professor Zhang for accepting our interview.