A Pulse Detonation Engine (PDE) is a type of jet engine currently being researched by various interests. The USís DARPA agency recently (in 2008) test flew an experimental craft using this technology, but the project was cancelled shortly after the initial flights due to budgetary issues.
A conventional jet engine mixes fuel and air and burns it continuously in a steady stream, the expansion of the heated gasses providing thrust. A PDE ignites the fuel-air mixture under high pressure, using powerful supersonic shockwaves funneled down specially designed metal tubes. Done thousands of times a second, these detonations can provide smooth but potent thrust for an aircraft. A PDE-powered aircraft go from low speed to hypersonic velocities very quickly, with very good handling capabilities for the latter.
The engine itself consists of one or more long tubes sectioned off into two separate combustion chambers. A pellet of mixed air and fuel is released down the tube. Quickly following on its heels, in the first combustion chamber, a primary explosion is induced with fuel, oxidizer, and a potent electrical charge. This first explosion is very much like the way a conventional jet burns fuel, called deflagration. However, the explosion is channeled into the second chamber in such a way that its supersonic shockwaves and intense heat detonate the much more potent pellet of fuel and oxidizer, creating the powerful detonation that drives the aircraft forward.
Theoretically, an aircraft powered by a PDE exclusively can achieve speeds of Mach 4 or better. A PDE can be combined with other engine cycles, such as turbines for better low-speed performance, or with rockets or scramjets for greater hypersonic speeds or even possibly insertions into space.
PDEs are very fuel efficient, from 5% to 20% over current conventional engines, as they are able to maintain near-constant-volume combustion. They can also use a variety of different fuels without extensive modification.
However, there are some technical hurdles to overcome before the technology can be adapted for widespread use. The detonation chambers and exhaust system has to be made much more durable than regular jet engines, upping the potential initial cost of any aircraft using a PDE, even after the technology matures. Especially troubling is the issue of valving. Conventional engine valves would take quite a beating from the detonations and would wear out very quickly. Some designs opt to eliminate the valves altogether, relying on the coordinated timing of the shockwaves within the engine to regulate gas flow.
There is also some concern about the airframe being able to withstand the constant acoustical pressure, which can create severe vibrations. Its is also feared the intense jackhammer-like noise from the engine may be too much for either the pilot or those close on the ground. However, the DARPA program and successful test flight went a long way toward allaying some of these concerns.
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