Pulse detonation engine
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A pulse detonation engine, or "PDE", is a type of propulsion system that has the potential to be both light and powerful and can operate from a standstill up to supersonic speeds. To date no practical PDE engine has been put into production, but several testbed engines have been built, proving the basic concept to some extent at least. In theory the design can produce an engine with an efficiency far surpassing more complex gas turbine Brayton cycle engines, but with almost no moving parts.
All regular jet engines and most rocket engines operate on the deflagration of fuel, that is, the rapid but subsonic combustion of fuel. The pulse detonation engine is a concept currently in active development to create a jet engine that operates on the supersonic detonation of fuel.
The basic operation of the PDE is similar to that of the pulse jet engine; air is mixed with fuel to create a flammable mixture that is then ignited. The resulting combustion greatly increases the pressure of the mixture, which then expands through a nozzle for thrust. To ensure that the mixture exits to the rear, thereby pushing the aircraft forward, a series of shutters are used with careful tuning of the inlet to force the air to travel in one direction only through the engine.
The main difference between a PDE and a traditional pulsejet is that the mixture does not undergo subsonic combustion but instead, supersonic detonation. In the PDE, the oxygen and fuel combination process is supersonic, effectively an explosion instead of burning. The other difference is that the shutters are replaced by more sophisticated valves. In some PDE designs from General Electric, the shutters are even removed because the process can be controlled by timing on the periodic sudden pressure drops that occur after each shock wave when the "combustion" products have been ejected in one shot.
The main side effect of the change in cycle is that the PDE is considerably more efficient (detonation => high pressure =(adiabatic)> high temperature =( The Carnot cycle )> high efficiency (less heat more noise) )[citation needed]. In the pulsejet the combustion pushes a considerable amount of the fuel/air mix (the charge) out the rear of the engine before it has had a chance to burn (thus the trail of flame seen on the V-1 flying bomb), and even while inside the engine the mixture's volume is continually changing, an inefficient way to burn fuel. In contrast the PDE deliberately uses a high-speed combustion process that burns all of the charge while it is still inside the engine at a constant volume. The maximum energy efficiency of most types of jet engines is around 30%[citation needed], a PDE can attain an efficiency theoretically near 50%[citation needed].
Another side effect, not yet demonstrated in practical use, is the cycle time. A traditional pulsejet tops out at about 250 pulses per second, but the aim of the PDE is thousands of pulses per second, so fast that it is basically continual from an engineering perspective. This should help smooth out the otherwise highly vibrational pulsejet engine -- many small pulses will create less volume than a smaller number of larger ones for the same net thrust. Unfortunately, detonations are many times louder than deflagrations.
The major difficulty with a pulse detonation engine is starting the detonation. While it is possible to start a detonation directly with a large spark, the amount of energy input is very large and is not practical for an engine. The typical solution is to use a Deflagration-to-Detonation Transition (DDT) - that is, start a high-energy deflagration, and have it accelerate down a tube to the point where it becomes fast enough to become a detonation. Alternatively the detonation can be sent around a circle and valves ensure that only the highest peak power can leak into exhaust.
This process is far more complicated than it sounds, due to the resistance the advancing wavefront encounters (similar to wave drag). DDTs occur far more readily if there are obstacles in the tube. The most widely used is the "Shchelkin spiral", which is designed to create the most useful eddies with the least resistance to the moving fuel/air/exhaust mixture. The eddies lead to the flame separating into multiple fronts, some of which go backwards and collide with other fronts, and then accelerate into fronts ahead of them.
The behavior is difficult to model and to predict, and research is ongoing. As with conventional pulsejets, there are two main types of designs: valved and valveless. Designs with valves encounter the same difficult-to-resolve wear issues encountered with their pulsejet equivalents. Valveless designs typically rely on abnormalities in the air flow to ensure a one-way flow, and are very hard to achieve a regular DDT in.
NASA maintains a research program on the PDE, which is aimed at high-speed, about mach 5, civilian transport systems. However most PDE research is military in nature, as the engine could be used to develop a new generation of high-speed, long-range reconnaissance aircraft that would fly high enough to be out of range of any current anti-aircraft defenses, while offering range considerably greater than the SR-71, which required a massive tanker support fleet to use in operation. (See Aurora aircraft)
While most research is on the high speed regime, newer designs with much higher pulse rates in the hundreds of thousands appear to work well even at subsonic speeds. Whereas traditional engine designs always include tradeoffs that limit them to a "best speed" range, the PDE appears to outperform them at all speeds. Both Pratt & Whitney and General Electric now have active PDE research programs in an attempt to commercialize the designs.
Key difficulties in pulse detonation engines are achieving DDT without requiring a tube long enough to make it impractical and drag-imposing on the aircraft; reducing the noise (often described as sounding like a jackhammer); and damping the severe vibration caused by the operation of the engine.
[edit] In science fiction
- In the "House of the Sun" by Nigel D. Findley (Paperback -- July 1995), a novel set in the Shadowrun universe, an aircraft is sighted with a "donut on a rope" exhaust trail and the Pulse Detonation Drive concept is referred to (though no more than a brief accurate footnote).
- In the movie Stealth (2005), the advanced fighters use pulse detonation engines with scramjet boosters. Incidentally, the F/A-37 Talons changed their wing shape from forward swept wings to delta wings for hypersonic flight.
- In the game UFO: Enemy Unknown the X-Com interceptor craft is said to use twin pulse-detonation engines.
- In the drama television series JAG, the Season Nine episode "The One That Got Away" (original air date October 17, 2003) features the fictional Aurora, a super-secret hypersonic aircraft under development by the CIA, that uses a pulse detonation engine.
- Dan Brown overtly mentions the technology in his book Deception Point.
- Victor Koman's novel “Kings of the High Frontier” mentions this technology.
