Ramjet engine is an air jet engine, which uses oncoming airflow to decelerate after entering the engine, thus increasing the static pressure of air. It usually consists of inlet (also known as diffuser), combustion chamber and propulsion nozzle. Ramjet engine has no compressor (so it doesn't need a gas turbine), so it is also called compressor-free air jet engine.
This method of engine compressing air is to reduce the speed by relative airflow when the aircraft is flying at high speed, and convert kinetic energy into pressure energy (for example, when the intake speed is 3 times the speed of sound, the air pressure can be theoretically increased by 37 times). When the ramjet engine is working, the high-speed airflow blows towards the engine head-on, expands and decelerates in the air inlet, and after the air pressure and temperature rise, it enters the combustion chamber to be mixed with fuel oil (generally kerosene) for combustion, and the temperature rises to 2000-2200℃ or even higher. The high-temperature gas is then accelerated by the expansion of the propulsion nozzle, and is discharged from the nozzle at high speed to generate thrust. The thrust of ramjet is related to the intake speed. For example, when the intake speed is 3 times the speed of sound, the static thrust generated on the ground can exceed 200N N.
Ramjet engine has the advantages of simple structure, light weight, large thrust-to-weight ratio and low cost. However, because there is no compressor, it can not be started under static conditions, so it is not suitable for being used as a power plant of ordinary aircraft, and it is often used in conjunction with other engines to become a combined power plant. Such as the combination of ramjet engine and rocket engine, ramjet engine and turbojet engine or turbofan engine, etc. When an aircraft equipped with a combined power plant takes off, it starts the rocket engine, turbojet engine or turbofan engine, and when the flight speed is enough to make the ramjet engine work normally, it uses the ramjet engine to close the matching engine; In the landing phase, when the flight speed of the aircraft is reduced to the point where the ramjet engine cannot work normally, the matching engine is restarted. If the ramjet engine is used as the power device of the aircraft alone, the aircraft must be carried into the air by other aircraft and have a certain speed before the ramjet engine can be launched. Ramjet or combined ramjet is generally used for missiles and supersonic or subsonic target aircraft. Ramjet engines can be divided into subsonic, supersonic and hypersonic according to the scope of application.