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DDT, has it ever happend in a hybird?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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DDT, has it ever happend in a hybird?

Unread postAuthor: drex » Wed May 07, 2008 6:14 pm

i hear all about DDT, but i have yet to find an instance where it has actually accrued in a cannon. has it ever happend?

[Clarification]
im not doubting that it is very real and dangerious.
[/Clarification]
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Unread postAuthor: DYI » Wed May 07, 2008 6:29 pm

I don't believe it has happened accidentally, but noname (who isn't active here anymore, but used to be a highly valued member) caused it by design in a 1/2" x 5' chamber at 6x. The chamber was 1/2" SCH 40 steel, and there was a noticeable bulge and cracks at the area where the DDT had ocurred.

I want to do DDT testing at some point, but right now I'm just working on avoiding it, and I'm in pretty deep in several other projects.
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Unread postAuthor: drex » Wed May 07, 2008 6:32 pm

thanks, you would'nt happen to have a photo of his test would you?
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Unread postAuthor: Sticky_Tape » Wed May 07, 2008 6:35 pm

I would like to see this bulge too.(No dirty jokes)
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Unread postAuthor: clide » Wed May 07, 2008 6:51 pm

There is no way to confirm it, but many have suspected that this failure may have been DDT
http://www.spudfiles.com/spudtech_archi ... =13&t=6095

Failure at a fairly low mixture and still the most powerful shot that the gun had.

Of course he could have just had a hairline crack in the chamber and not been getting the mixture right for previous shots.
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Unread postAuthor: DYI » Wed May 07, 2008 7:01 pm

On any large diameter, low length PVC hybrid chamber, I'm inclined to suspect fatiguing and microfractures from repeated shock loads over DDT. PVC has very low impact resistance, and even the ABS barrel on my prototype hybrid failed at only 4x (oxy/propane though). If it can rip 12" of highly shock resistant ABS pipe almost clean in half, it shouldn't have any problem with SCH 80 PVC, especially if given a few goes at it.

You're going to have to ask noname about the pictures though, if you can get a hold of him somehow.
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Unread postAuthor: drex » Wed May 07, 2008 8:29 pm

over the summer i might try and see if i cant make DDT happen. do you think that DDT wold happen in a 10 foot .25 inch thick copper chamber at 8X with one spark gap at the end of the chamber?
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Unread postAuthor: bigbob12345 » Wed May 07, 2008 8:32 pm

Yes it will most definately happen
but DDT is no joke, stand well back

and if you want to make absolutely sure there will be a DDT then use
oxy propane instead of air propane.
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Unread postAuthor: psycix » Thu May 08, 2008 7:53 am

bigbob12345 wrote:Yes it will most definately happen
but DDT is no joke, stand well back

and if you want to make absolutely sure there will be a DDT then use
oxy propane instead of air propane.


Or even hydrogen/oxy :D

It is indeed no joke to do this. This is not a cannon chamber, but its more like a pipe bomb. Get yourself a nuclear-proof bunker if you want to survive this.
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Unread postAuthor: paaiyan » Thu May 08, 2008 8:02 am

Well, technically I'm not sure if DDT can happen with oxy/hydrogen actually. It's not actually burning, it's a combination reaction.
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Unread postAuthor: SpudFarm » Thu May 08, 2008 8:34 am

well paaiyan pulse detonation engines uses hydrogen/oxy..

mabe the spudtech hybrid was weakened by the extreme recoil stresses to the chamber? if i am going to build a large scale PVC hybrid i will make a stand that allows it to be hold up by the barrel
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Unread postAuthor: paaiyan » Thu May 08, 2008 8:37 am

After reading over some material on the subject, I concede my previous point. I had never heard of this type of engine.

Carry on.
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Unread postAuthor: Fnord » Thu May 08, 2008 8:45 am

It is indeed no joke to do this. This is not a cannon chamber, but its more like a pipe bomb. Get yourself a nuclear-proof bunker if you want to survive this.



Gun powder has thousands of times the energy density of typical propane mixes. I wouldn't want to be standing by it when it happens, but it's not likely to be as spectacular and destructive as everyone has made it out to be.

I mean, when ddt happens in a car engine, it will weaken it and eventually ruin the engine unless you fix the problem, but it's not like your engine block explodes.
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Unread postAuthor: paaiyan » Thu May 08, 2008 8:47 am

_Fnord wrote:
It is indeed no joke to do this. This is not a cannon chamber, but its more like a pipe bomb. Get yourself a nuclear-proof bunker if you want to survive this.



Gun powder has thousands of times the energy density of typical propane mixes. I wouldn't want to be standing by it when it happens, but it's not likely to be as spectacular and destructive as everyone has made it out to be.

I mean, when ddt happens in a car engine, it will weaken it and eventually ruin the engine unless you fix the problem, but it's not like your engine block explodes.


And just for the record, how thick is an engine block?
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Unread postAuthor: Fnord » Thu May 08, 2008 8:56 am

Depends on the engine block. Yes, it should be stronger than a typical hybrid chamber, but it has to stand up to thousands of explosions every minute at high temperatures.

The pressure spike in a ddt is so short that it just causes fatigue, bulges or cracks, not total annihilation. Noname's results may have demonstrated this.
(Edit: that is, IF you are using a chamber that is not brittle. Otherwise you can expect fragmentation.)


I really need to do some testing this summer...
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Last edited by Fnord on Thu May 08, 2008 9:01 am, edited 1 time in total.
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