Pulse Jet Rebirth
Anyone back in England or Belgian back in 1944 no doubt holds fond memories of the V-1 flying bomb. It flew in with a rather pleasant buzz and landed with a blast. Without being anywhere remotely accurate, you could never count on it to land on that annoying neighbor that you hate.
Despite the best efforts of the allies, the weapon is now reemerging in the aerospace community as a groovy way to boost performance and fuel efficiency. The pulsejet that it used for propulsion is starting to show great promise in developing new engine designs.
The original pulsejet engine was a remarkably elegant design. It amounted to a standard internal combustion engine with the piston removed. It was basically a long steel tube with a series of air-intake vanes and a fuel injector at one end. At a cycle of 50 hertz, the vanes would open inducing air into the system and fuel would be added. The vanes would then close and a spark would explode the mixture out the tailpipe. While novel in engineering, it was an inefficient device.
Pratt & Whitney is currently developing the next generation of pulsejet engines that will rely on detonation instead of deflagration. Deflagration is the propagation of combustion by thermal conduction, where as with detonation it is propagated by a supersonic shockwave. This has the advantage of producing more work, but is very corrosive to a mechanical system. The new prototype engine will operate at over twice the frequency of the V-1 and could power the next-generation of cruise missiles and fighter jet afterburners.
The real jewel for Pratt & Whitney is a major upgrade of the high-bypass turbofan engine common today on airliners. Their ambition is to boost the engine efficiency by replacing part of the compressor and combustor with a cluster of pulse detonation tubes. They are already researching with a prototype connected to a A-10 Warthog engine.
Rumors persist that the alleged Aurora spy plane uses a similar propulsion system.