Pulsejet

A pulsejet engine can be made with few[1] or no moving parts,[2][3][4] and is capable of running statically (that is, it does not need to have air forced into its inlet, typically by forward motion).

Pulsejet engines are a lightweight form of jet propulsion, but usually have a poor compression ratio, and hence give a low specific impulse.

[5] When the fuel mix is ignited, the valves close, which means that the heated gases can only leave through the engine's tailpipe, thus creating forward thrust.

[7] Engineer Paul Schmidt pioneered a more efficient design based on modification of the intake valves (or flaps), earning him government support from the German Air Ministry in 1933.

Schmidt's prototype bomb was rejected by the German Air Ministry as they were uninterested in it from a tactical perspective and assessed it as being technically dubious.

[8] It would run on any grade of petroleum and the ignition shutter system was not intended to last beyond the V-1's normal operational flight life of one hour.

[8] Ignition in the As 014 was provided by a single automotive spark plug, mounted approximately 75 cm (30 in) behind the front-mounted valve array.

[11] Three air nozzles in the front of the Argus As 014 were connected to an external high pressure source to start the engine.

The V-1, being a cruise missile, lacked landing gear, instead the Argus As 014 was launched on an inclined ramp powered by a piston-driven steam catapult.

Steam power to fire the piston was generated by the violent exothermic chemical reaction created when hydrogen peroxide and potassium permanganate (termed T-Stoff and Z-Stoff) are combined.

Pulsejet engines, being cheap and easy to construct, were the obvious choice for the V-1's designers, given the Germans' materials shortages and overstretched industry at that stage of the war.

The only other uses of the pulsejet that reached the hardware stage in Nazi Germany were the Messerschmitt Me 328 and an experimental Einpersonenfluggerät project for the German Heer.

The result was the creation of the JB-2 Loon, with the airframe built by Republic Aviation, and the Argus As 014 reproduction pulsejet powerplant, known by its PJ31 American designation, being made by the Ford Motor Company.

General Hap Arnold of the United States Army Air Forces was concerned that this weapon could be built of steel and wood, in 2000 man hours and approximate cost of US$600 (equivalent to $10,565 in 2023).

[9] However, pulsejets are used on a large scale as industrial drying systems, and there has been a resurgence in studying these engines for applications such as high-output heating, biomass conversion, and alternative energy systems, as pulsejets can run on almost anything that burns, including particulate fuels such as sawdust or coal powder.

In providing power to helicopter rotors, pulsejets have the advantage over turbine or piston engines of not producing torque upon the fuselage since they don't apply force to the shaft, but push the tips.

The duct acts as an annular wing, which evens out the pulsating thrust, by harnessing aerodynamic forces in the pulsejet exhaust.

The duct, typically called an augmentor, can significantly increase the thrust of a pulsejet with no additional fuel consumption.

When fuel ignites, the increased temperature and pressure push hot gasses out of the device, creating thrust.

Valved pulsejet engines use a mechanical valve to control the flow of expanding exhaust, forcing the hot gas to go out of the back of the engine through the tailpipe only, and allow fresh air and more fuel to enter through the intake as the inertia of the escaping exhaust creates a partial vacuum for a fraction of a second after each detonation.

Fuel, as a gas or atomized liquid spray, is either mixed with the air in the intake or directly injected into the combustion chamber.

Starting the engine usually requires forced air and an ignition source, such as a spark plug, for the fuel-air mix.

The inertial reaction of this gas flow causes the engine to provide thrust, this force being used to propel an airframe or a rotor blade.

This causes atomized fuel at the rear of the combustion chamber to "flash" as it comes in contact with the hot gases of the preceding column of gas—this resulting flash "slams" the reed-valves shut or in the case of valveless designs, stops the flow of fuel until a vacuum is formed and the cycle repeats.

The intake tube takes in air and mixes it with fuel to combust, and also controls the expulsion of exhaust gas, like a valve, limiting the flow but not stopping it altogether.

While some valveless engines are known for being extremely fuel-hungry, other designs use significantly less fuel than a valved pulsejet, and a properly designed system with advanced components and techniques can rival or exceed the fuel efficiency of small turbojet engines[citation needed].

The engines are difficult to integrate into commercial crewed aircraft designs because of noise and vibration, though they excel on the smaller-scale uncrewed vehicles.

Most PDE research programs use pulsejet engines for testing ideas early in the design phase.