A pulse jet engine is a very simple form of aircraft engine, falling somewhere in between true jets on the one hand and rockets on the other. The operating principle is simple: air is allowed to flow into the front end of a tube and mixed with fuel. Typically, shutters then close at the front of the tube, the fuel is ignited, and the expanding gases exit from the rear end of the tube, producing thrust; the shutters then open again, allowing fresh air into the tube, and the cycle starts again. Versions exist with no shutters or equivalent mechanical parts; these use non-linearities in air flow to achieve the same effect, for instance with a U tube mounted in front of the inlet to catch and turn around that portion of the exhaust that is sent forward. The cycle frequency is dependent on the length of the engine itself and, for a small model-type engine may be typically around 250 pulses per second -- whereas for a larger engine such as the one used on the German V1 flying bomb, the frequency was closer to 45 pulses per second.
The first (and so far only) major practical application of the pulse jet principle was in the German V1 Flying Bomb late in World War II; there are reports of pulse jets being used as part of hand-held devices in the 1950s to spread insecticide.
Pulse jets have a very high thrust-to-weight ratio and need very few moving parts, so they are cheap and easy to manufacture. However, they have poor high-altitude performance, bad fuel economy and they produce massive vibration and are also very noisy, which explains the nickname of "buzz bomb" that the V1 gained.
Pulse jets are mainly used today in model airplanes, though some experimenters continue to work on improved designs, including pulse detonation engines.
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