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Harnessing DDT (deflagration to detonation transition)

A place to ask general spud cannon related questions.
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Harnessing DDT (deflagration to detonation transition)

Unread postAuthor: ferret_guy » Wed Nov 14, 2012 2:07 pm

I was wondering what people know about DDT, I found several papers on the subject including this one which attempts to model an acetylene–air mixture in a long tube (in 2 dimensions). I thing using DDT would be the next logical step in spud cannon development.
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Unread postAuthor: mobile chernobyl » Wed Nov 14, 2012 8:29 pm

Ahh...

DDT is no foreign subject to spudfiles. To most it's honestly a situation that is best off avoided, and that is evident by the amount of military guns/launchers you see utilizing this to accelerate objects (very few if any use it actively).

There are a few of us that have desired to find a way to utilize it - the paper and many others like it have been reviewed. Devices utilizing solids to produce a detonation wave have been considered as well (see Voitenko compressor) and then there's the obvious EFP's that use flying plates.

The sudden shock pressure wave presents a challenge in pressure vessel design - not easily simulated and designed around other than the "make it really thick" approach.

In short - it's a whole different realm of design that not many here have even attempted to research, because once you read more than a paper on it and try and find previous examples of projectile accelerators using the effect of a detonation you'll find it's no easy feat. Inertia of the projectile really starts to become a design concern and often makes it necessary to have obscenely light objects being accelerated.

If you really want to see the extreme ends of projectile acceleration look up the Pascal-B nuclear test and the Marvel nuclear driven shock tube experiment!

I know myself and a few others on here would really like to one day harness DDT for a heavier projectile than those that you can read about in lab tests :wink: :wink:
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Unread postAuthor: ferret_guy » Wed Nov 14, 2012 9:33 pm

Could you point me to threads where DDT was discussed when searching for terms related to it in this site a very large number of articles come up and none of them are very helpful
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Unread postAuthor: mobile chernobyl » Thu Nov 15, 2012 12:09 am

Normally the user DYI would have replied by now to this elusively titled topic.

Basically just search for posts by DYI (there may be a lot of them however!) and he has brought the topic up too many times to count. He is very knowledgeable on the subject of gas kinetics.

short of that, this is my best offering for you because I don't feel like making a list and I think you'll learn more if you pursue it rather than be spoon fed ;)
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Unread postAuthor: ferret_guy » Thu Nov 15, 2012 12:23 am

:D thanks that actually helped a lot.
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Unread postAuthor: MrCrowley » Thu Nov 15, 2012 12:31 am

http://www.spudfiles.com/forums/questio ... 22406.html
http://www.spudfiles.com/forums/yet-mor ... 14578.html

Also, see the paper linked by me at the bottom of the first page here and D_Hall's musings about the paper on page 5:
http://www.spudfiles.com/forums/the-pip ... 13123.html

Searching "ddt" with "D_Hall" as the author was a sure way to bring up some relevant threads. Try the same but with "DYI" as the author/user.

edit:
http://www.spudfiles.com/forums/flame-p ... 13272.html
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Unread postAuthor: jimmy101 » Thu Nov 15, 2012 1:22 pm

What mobile chernobyl said. Real guns rarely use DDT so why would a spud gun benefit from it?

DDT doesn't release more energy, it just releases the energy faster, but since speed of combustion really isn't all that much of a limiting factor in real guns (and in combustion spud guns) it doesn't make much sense to add the complexity of design needed to consistently get DDT.
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Unread postAuthor: nitrogigi » Thu Nov 15, 2012 2:59 pm

I'm actually in training course in a research lab, and i made a test of "spudgun" with ddt and without ddt, at same final pressure (lots of tests needed to get it)
I filmed all this with a high speed camera at 25k pictures per second (sorry I can't get the videos out from the lab, the format is very strange)
then I made both test 15 times to have an average muzzle velocity with exactly the same conditions.

The results are quite interesting:
10 mm after the end of the barrel, it was faster with ddt
100 mm after the end, it was faster without ddt

the big problem of the ddt is that it induct to much turbulence at rear of the bullet and most of those are faster than the bullet, so it overtakes the bullet and disturb totally the aerodynamic flow in front of it

so if you want to shoot farther than the end of your nose, ddt is useless
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Unread postAuthor: al-xg » Thu Nov 15, 2012 7:55 pm

nitrogigi, I'm assuming the barrel wasn't rifled?

Edit: d'oh, I didn't read the post correctly. Nevermind :D
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Unread postAuthor: mobile chernobyl » Thu Nov 15, 2012 8:03 pm

nitrogigi - A LOT more info is needed about your experiment! This sounds pretty awesome however!

Typically with a DDT - unless your transferring the force of the shockwave to another working fluid you'd want a much shorter barrel than that of the combustion gun. I realize you were trying to do an "apples to apples" test, but DDT requires a much different gun configuration to even attempt to utilize it's unique pressure curve.

Also you said you were holding peak pressure of the reaction to be constant? that would mean you were using much less reactants in the chamber during the DDT test than that of the deflagration chamber I'm assuming?

But most of all - thanks for running the test! If you have any more spare time in this lab please contact the member DYI regarding recommended test setups to try and optimize your DDT results!
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Unread postAuthor: nitrogigi » Fri Nov 16, 2012 3:52 am

I can't do more tests because the security office came in the lab and prohibid all experiments with explosion because we don't have the adequate security equiment

the ammout of fuel was aproximativly 2/3 with ddt to have the same final pressure

what do you want to know about my experiment?

the chamber was made in a steel tube of 5 cm internal and 12cm external, 20 cm long
the barrel was a steel tube of 1m20, 8mm internal and 30mm external
loaded with h2 and air at mix 1, ignition with 2 spark plugs without ddt and with a laser heating to plasma an aluminium ball in less than a 0,2 seconds for ddt

the peak of pressure at 5cm of the end of the cannon was at 70 bars
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Unread postAuthor: mobile chernobyl » Fri Nov 16, 2012 8:49 am

Sounds interesting! You better get working on the security clearances then!
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Unread postAuthor: nitrogigi » Fri Nov 16, 2012 8:55 am

actually the big problem is that the lab is in a tunnel...
the speciality of the lab is aerodynamic so, they build it in an old tunnel to have a constant temperature for the wind tunnel
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Unread postAuthor: D_Hall » Fri Nov 16, 2012 12:22 pm

"Security equipment" for explosions? WTF?
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Unread postAuthor: MrCrowley » Fri Nov 16, 2012 3:46 pm

D_Hall wrote:"Security equipment" for explosions? WTF?
I suspect English is not his first language so it may be a case of lost in translation.
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