"Precision Guide" Science Kaleidoscope (56) How to "cool down" the tail nozzle of a fighter jet

The targets of air-to-air missiles are mainly various combat aircraft. The infrared radiation sources of aircraft mainly include three parts: engine tail nozzle, exhaust airflow and skin. The solar radiation reflected by the aircraft skin at a specific angle is also an important radiation source.

The higher the surface temperature of an object, the shorter the peak wavelength of the infrared radiation energy. For the infrared radiation source of an aircraft, the temperature of the engine tail nozzle is the highest, the temperature of the exhaust gas flow is the second, and the temperature of the skin is the lowest. Therefore, the peak wavelength of infrared radiation from the tail nozzle is the shortest, while the peak wavelength of infrared radiation from the skin is the longest.

The infrared guidance system of air-to-air missiles uses the difference in infrared radiation between the target and the background to detect the target, track the target and measure information.

The engine tail nozzle is a strong infrared radiation source, with a general temperature range of 400℃-1000℃. The radiation intensity of the hottest part of the tail nozzle is generally concentrated in the short-wave infrared and medium-wave infrared bands. In order to meet the challenge of infrared air-to-air missiles, some combat aircraft have adopted infrared stealth technology, mainly through heat suppression to reduce the intensity of infrared radiation, such as using fuselage structural parts to block the tail nozzle at a larger observation angle: the US F-117 fighter uses a narrow slit dual tail nozzle, the US B-2 bomber uses a "beaver tail" shaped tail nozzle, and the US F-22 stealth fighter and F-35 stealth fighter all use butterfly tails to block the tail nozzle.

The exhaust flow is another major infrared radiation source, with a temperature of about 85% of the exhaust nozzle. The infrared radiation of the exhaust flow is mainly concentrated in the mid-infrared band. In order to deal with infrared air-to-air missiles, the main heat suppression method used by combat aircraft is to introduce cold air into the exhaust flow to reduce the exhaust flow temperature.

Skin radiation is generated by aerodynamic heating and local heat conduction of the engine. The faster the flight speed, the higher the skin temperature. Skin radiation is generally strongest in the long-wave band. (Source: National University of Defense Technology "Science Popularization China" co-construction base)