The PDE puzzle

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Mark S.
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The PDE puzzle

Post by Mark S. » Fri Oct 29, 2004 1:51 pm

I have been trying to follow the development of the next generation jet engine, the pulse detonation engine (PDE).

Finding any detailed engine information about this subject is next to impossible. I find that interesting due to the fact that General Electric and Pratt & Whitney are feverishly trying to meet milestones set by NASA, and pouring millions into it’s development. Flight testing of a modified F-15 is set for 2005 according to NASA reports.

I recently found a conceptual model of a PDE in a research university student’s thesis. It gives some insight on the internal workings of a conceptual PDE. (general info FYI: http://www.galcit.caltech.edu/EDL/projects/pde/pde.html)

Intake flow valve used on model ??? (FYI: USPTO # 6,637,187)

Mark S.

Mark S.
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re: The PDE puzzle

Post by Mark S. » Mon Oct 17, 2005 2:16 pm

The 2 pictures are rare conceptual layouts of Pulse Detonation Engines (PDE) that I found on the internet.

A common key component for both engines is the initiator (pre-detonator). It is basically a digitally controlled igniter, that touches off the detonation or deflagration of the main fuel supply. The initiator allows a weak spark (usually a spark plug) and fuel to create a repeatable plasma discharge at a defined frequency (50hz -200hz). Depending on the shape of the engine (round or rectangular) the initiator will be an annular or a linear in style.

After some thought, why couldn’t the concept of the initiator be adapted to LARGE digitally controlled pulse jet engine. It boggles my mind on the scale and power that could be made on this type engine. The power of this type engine would only be limited to how much fuel and air you could push through it. This type configuration would also allow the physical size of a LARGE pulse jet engine to be made considerably shorter than a normal pulse jet. The down side of this type engine is the complexity and cost involved using extensive electronics, and a custom machined initiator module.

I know that this goes against the simplicity of traditional pulse jet engines, but micro-controllers are on everything now !!! Has anyone tried electronically controlling a pulse jet engine ? [/list]

Mark S.
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Bruno Ogorelec
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re: The PDE puzzle

Post by Bruno Ogorelec » Mon Oct 17, 2005 8:15 pm

Sorry, I fail to understand what role the initiator would have. Ignition? Well, a pulsejet has the ignition problem solved pretty well. Pulsejets flame out very rarely. Detonation? Well, then you have a PDE, not a pulsejet. So, what exactly do you think could be achieved?

As for your other question, yes, electronically controlled pulsejets have been built and tested with success. You need a very fast fuel injection system (not a big problem) and a very complex computer algorithm to suss out patterns in pulsating combustion chaos and respond to them (a much bigger problem, but doable by combustion experts adept at programming). You can thus establish a feedback loop that introduces order into chaotic combustion and 'tames' the pulsejet into a more regular beat than it normally has.

But, why would you want to do that? Is the erratic beat of your pulsejet making you nervous, or what?

Mike Everman
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re: The PDE puzzle

Post by Mike Everman » Mon Oct 17, 2005 9:32 pm

Detonations by definition are much harder to contain, and so up goes the duct weight... then dictating that it no longer be self aspirating, requiring forced air and on and on...

An electronically controlled pulse-jet will need many sparks throughout the CC to match the "outside-in" nature of the pjet cycle's effective flamespeed.

Don't get me wrong, that would be cool and all, but you'd still want it to all happen at the same frequency as the pjet without electronic ignition, and the duct acoustics still must support this frequency. The only advantage I can see is being able to advance the initiation slightly, which could make the difference on shorter (higher volume to length) engines.
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Mark
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re: The PDE puzzle

Post by Mark » Tue Oct 18, 2005 2:25 am

Initiation is a funny thing. I can think of nitroglycerin, how if you pour a tiny bit of it over a flame, it merely burns like gasoline. And guncotton, the stuff you shoot bullets with, burns interestingly if you light it in the open, but will detonate if confined slightly, much moreso than gunpowder. So these compounds behave peacefully, they burn vigorously but not explosively. But if you kick them with a bit more energy, they detonate, a mere difference in the way you bring about ignition.
So the idea of using a "pre-ignition" detonation to set off the main body is a common theme for most high explosives. Even ammonium nitrate plays this game. It is about as inert as you can get unless you hit it really hard. Then it will detonate.
I really liked the two schematics Mark S. posted with the PDE concepts. Anything that speeds up the ignition or burn rate enough to bring about detonation or some increase nearing detonation would most likely result in a significant improvement in thrust. Designing a working model would be a bit more complex than an ordinary pulsejet, but with the added complexity there might be an increase in thrust, if you took the initiative.
Mark
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