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Pulse Detonation Engines: The Future of Aviation
noah_fense writes "Popular Science is running an interesting article about the race to replace the jet turbine with a more efficient source of Mach-breaking airpower: the pulse-detonation engine. It works by detonating (instead of slow burning) fuel hundreds to thousands of times a second. PDE technology is poised to make supersonic passenger flights and space travel affordable. 'Pulse detonation is a hot topic in combustion research,' says Gabriel Roy of the Office of Naval Research. 'Compared with gas turbines, the PDE has a much simpler configuration. It has the capability of going from subsonic to supersonic using less fuel, and it's thermodynamically more efficient. But there are big engineering issues--thermal fatigue, noise. It's very challenging research.'"
Yep, you should go RTFA. PDE is very explosive. The idea was first thought of in the 1930's. The article says that the Germans tried it on the V-1 rocket, but didn't succede. The article states that detonation is different from deflagration. I don't know what the internal combustion engine uses, but PDE is very complicated and has only recently been showing signs of success.
...interesting if true.
from the article:
" Imagine a tube, closed at one end and filled with a mixture of fuel and air. A spark ignites the fuel at the closed end, and a combustion reaction propagates down the tube. In deflagration?even in "fast flame" situations ordinarily called explosions?that reaction moves at tens of meters per second at most. But in detonation, a supersonic shock wave slams down the tube at thousands of meters per second, close to Mach 5, compressing and igniting fuel and air almost instantaneously in a narrow, high-pressure, heat-release zone. "
http://www.wam.umd.edu/~ojenkins/words/donuts.html
I actually did my Master's Research on a Pulsed Detonation Engine (Rocket actually, since we were providing the oxygen). It is a more efficient form of propulsion (for thermo geeks, detonation can be modeled by a constant volume Humphrey thermo cycle, rather than the constant pressure Brayton cycle and a comparison of efficiencies results in a vast improvement for the pulsed detonation engine). It certainly isn't too new as far as the idea being thrown around, but it certainly is gaining momentum as being more and more plausible. Aside from the efficiency benefits, the engine itself results in a much simpler design and weight savings rather than relying on today's complex turbomachinery. Furthermore, pulsed detonation engines offer the potential for substantiative performance increases; finally bringing hypersonic flight to within a practical reach. A detonation is different than a deflagration in basically the speed at which combustion occurs. Deflagration occurs at relatively low flame speeds on the order of 1 or 2 m/sec.; whereas, detonation is a supersonic mode of combustion. Most forms of combustion that we are familiar with today utilize the deflagrative mode. The article was accurate in stating that this technology still has a few hurdles to overcome. Primarily, the pulsed detonation engine is an unsteady flow phenomenon that requires a periodic input to control fuel injection into the detonation chamber coupled with a very large energy input to ignite the fuel and reach a critical Chapman-Jouguet velocity. Such energy input has been accomplished so far using an arc igniter, but doing so on a reliable basis at frequencies of at least 100Hz, necessary for practical use have been somewhat of a challenge thus far.
The United States Air Force Research Lab Propulsion Directorate has a pulse detonation engine program as well. Pics and story here. Apparently their engine is made mostly of off-the-shelf automotive parts. It's powered by any type of general aviation fuel (Jet-A, JP-8), and even gasoline.
ok, once again.... the V1 was a missile propelled by a jet engine, not a rocket. A rocket carries it's own oxidizer with it. The V2 was a missile propelled by a rocket.
Neither was the first successfull guided missile, and the V2 was not the first successfull Liquid-Fueled Rocket. The germans had wire-guided air-launched anti-ship missiles before either.
The PDWE has been rumored for years to be the propulsion for the fabled Aurora...this type of engine leaves "donuts on a rope" contrails behind the aircraft. The PDWE is so much different from any other engine that it's silly...First, there are few, if any, moving parts. Fuel is injected, and causes a traveling wave of combustion to move down a tube...which is reflected inside the engine, and comes back up the tube. This wave compresses fuel and air still being injected and inhaled, enough so that it detonates, instead of combusting...think of it as the "pinging" in your car engine when you have crappy fuel. But harnesses correctly (as in a diesel engine,) it's actually more efficient. So this fuel detonates, which creates a pulse which partially blows out the back, but also partially reflects back up the tube to compress more fuel. Since there are no moving parts, this can take place at a very high rate of speed...The biggest problems I've read of are starting the thing...which was supposed to be the source of low-frequency rumbles at the Groom Lake site. The tube is "tuned" to a certain speed of waves inside it, and it doesn't want to run at other speeds. And...of course...noise. The thing is capable of producing lots of power...but its operation is much like that of the German pulse-jets, which sounded like flying jackhammers. But it definately could be propulsion for the future...but not to the extent that people would dream of...
Google for German Pulse Jet Fighters...
...and he grinned, like a fox eating shit out of a wire brush.
To demonstrate the difference between a deflagration (the slow combustion you get in your auto engine or a pulsejet) and detonation (the rapid combusiton that occurs in a PDE) I like to draw the following comparison.
:-).
1. Take a can of gasoline and pour a trail on the ground as you walk along. That trail might end up being 20-30 yards long.
Above that trail there is a stoichiometric mixture (ie: a mixture capable of burning) of gasoline vapor and air -- just as you'd find inside an engine.
Now light one end of the trail and watch how long it takes for the flame to travel along to the far end.
It actually takes several seconds. That's the speed of a flame-front during deflagration.
2. Now take a very long piece of cordite or some other "high explosive" and lay it along the ground for some distance.
Then place a detonator at one end, stand well back and energize it.
The entire length of the explosive will appear to explode at once. The shockwave that propogates the explosion down the length of explosive material will travel far to quickly for you to see. Instead of taking several seconds to travel just 20-30 yards, the detonation will travel over a mile per SECOND or faster.
That's the difference in speed between deflagration and detonation.
But there's one other very important difference:
If you pour a gallon of gasoline out onto the ground and light it it will go "woof" (just like a dog
You can safely stand within just a few yards of such a deflagration without fear of being harmed.
However, if you were to *detonate* (rather than deflagrate) that same amount of gasoline it would blow you right into the middle of next week and further.
With a detonation, all the available energy is released in a very tiny fraction of a second and this generates huge pressures (thus huge thrust).
With deflagration, the energy is released far more gradually so the pressures are lower.
What's more, because deflagration is such a slow process, when the fuel is burnt inside an engine, there's far more time for the heat of combustion to be transfered to the engine itself. That means the engine will require more cooling and a greater percentage of the fuel's energy will be wasted as radiated heat rather than in producing work.
I hope that clarifies the key differences between deflagration and detonation -- and goes some way to explaining why a PDE could provide greater efficiencies than an engine that simply "burns" its fuel through deflagration.