Pulse Jet

300px, Diagram of a pulsejet

A pulsejet engine (or pulse jet) is a type of jet engine in which combustion occurs in pulses. A pulsejet engine can be made with few or no moving parts, and is capable of running statically (i.e. it does not need to have air forced into its inlet, typically by forward motion). The best known example may be the Argus As 109-014 used to propel Nazi Germany's V-1 flying bomb.

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

There are two main types of pulsejet engines, both of which use resonant combustion and harness the expanding combustion products to form a pulsating exhaust jet that produces thrust intermittently. The first is known as a valved or traditional pulsejet and it has a set of one-way valves through which the incoming air passes. When the air-fuel is ignited, these valves slam shut, which means that the hot gases can only leave through the engine's tailpipe, thus creating forward thrust. The second type of pulsejet is known as the valveless pulsejet. Technically the term for this engine is the acoustic-type pulsejet, or aerodynamically valved pulsejet.

One notable line of research of pulsejet engines includes the pulse detonation engine, which involves repeated detonations in the engine, and which can potentially give high compression and reasonably good efficiency.

History

left, Ramón Casanova and the pulsejet engine he constructed and patented in 1917

Russian inventor and retired artillery officer Nikolaj Afanasievich Teleshov patented a steam pulsejet engine in 1867 while Swedish inventor Martin Wiberg also has a claim to having invented the first pulsejet, in Sweden, but details are unclear.

The first working pulsejet was patented in 1906 by Russian engineer V.V. Karavodin, who completed a working model in 1907.
The French inventor Georges Marconnet patented his valveless pulsejet engine in 1908, and Ramón Casanova, in Ripoll, Spain patented a pulsejet in Barcelona in 1917, having constructed one beginning in 1913. Robert Goddard invented a pulsejet engine in 1931, and demonstrated it on a jet-propelled bicycle.
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.George Mindling, Robert Bolton: ''US Airforce Tactical Missiles:1949–1969: The Pioneers'', Lulu.com, 200: . pp6-31

In 1909, Georges Marconnet developed the first pulsating combustor without valves. It was the grandfather of all valveless pulsejets. The valveless pulsejet was experimented with by the French propulsion research group SNECMA (Société Nationale d'Étude et de Construction de Moteurs d'Aviation ), in the late 1940s.

The valveless pulsejet's first widespread use was the Dutch drone Aviolanda AT-21

Argus As 109-014

In 1934, Georg Hans Madelung and Munich-based Paul Schmidt proposed to the German Air Ministry a "flying bomb" powered by Schmidt's pulsejet. Madelung co-invented the ribbon parachute, a device used to stabilise the V-1 in its terminal dive. Schmidt's prototype bomb failed to meet German Air Ministry specifications, especially owing to poor accuracy, range and high cost. The original Schmidt design had the pulsejet placed in a fuselage like a modern jet fighter, unlike the eventual V-1, which had the engine placed above the warhead and fuselage.

The Argus Company began work based on Schmidt's work. Other German manufacturers working on similar pulsejets and flying bombs were The Askania Company, Robert Lusser of Fieseler, Dr. Fritz Gosslau of Argus and the Siemens company, which were all combined to work on the V-1.

With Schmidt now working for Argus, the pulsejet was perfected and was officially known by its RLM designation as the Argus As 109-014. The first unpowered drop occurred at Peenemünde on 28 October 1942 and the first powered flight on 10 December 1942.

The pulsejet was evaluated to be an excellent balance of cost and function: a simple design that performed well for minimal cost. 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. Although it generated insufficient thrust for takeoff, the V-1's resonant jet could operate while stationary on the launch ramp. The simple resonant design based on the ratio (8.7:1) of the diameter to the length of the exhaust pipe functioned to perpetuate the combustion cycle, and attained stable resonance frequency at 43 cycles per second. The engine produced of static thrust and approximately in flight.

Ignition in the As 014 was provided by a single automotive spark plug, mounted approximately behind the front-mounted valve array. The spark only operated for the start sequence for the engine; the Argus As 014, like all pulsejets, did not require ignition coils or magnetos for ignition — the ignition source being the tail of the preceding fireball during the run. The engine casing did not provide sufficient heat to cause diesel-type ignition of the fuel, as there is insignificant compression within a pulsejet engine.

The Argus As 014 valve array was based on a shutter system that operated at the 43 to 45 cycles-per-second frequency of the engine.

Three air nozzles in the front of the Argus As 014 were connected to an external high pressure source to start the engine. The fuel used for ignition was acetylene, with the technicians having to place a baffle of wood or cardboard in the exhaust pipe to stop the acetylene diffusing before complete ignition. Once the engine ignited and minimum operating temperature was attained, external hoses and connectors were removed.

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