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A question/theory about DDT

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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Unread postAuthor: SpudBlaster15 » Sat Sep 01, 2007 12:34 am

Should DDT occur in a hybrid fueled with a 4x mix of air and propane, the theoretical detonation pressure is ~1050psi.

As a side note; interestingly, a mixture of oxygen and propane at atmospheric pressure produces only 2x the theoretical pressure of an air/propane mix despite the fact that 4x more fuel is present in the oxygen rich mixture.
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Unread postAuthor: noname » Sat Sep 01, 2007 12:38 am

SpudBlaster15 wrote:Should DDT occur in a hybrid fueled with a 4x mix of air and propane, the theoretical detonation pressure is ~1050psi.


I understood that part. What I didn't understand was that a 4x detonation produces 1050 psi and a 6x detonation produces 840 psi. Get my drift?
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Unread postAuthor: SpudBlaster15 » Sat Sep 01, 2007 12:44 am

What's the average pressure of a 6x propane/compressed air detonation?


Sh*t, for some reason I read that as deflagration.

Anyway, the figure you want is 1,570psi.
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Unread postAuthor: noname » Sat Sep 01, 2007 12:46 am

There's the answer I was looking for.
Apparently the burst pressure of 1/2" Sch 40 galvanized steel pipe is somewhere around 1500-1750 psi, or something close.
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Unread postAuthor: rna_duelers » Sat Sep 01, 2007 1:51 am

I know with metric pipe system you take the maximum running pressure of the pipe and times it by 3 to get its burst pressure.There is no reason why a properly designed chamber couldn't handle the pressure spike of DDT,look at a firearm for instance they produce pressure spikes in the tens of thousands of PSI range.There is no reason why you couldn't make a gun designed to harness the energy of DDT,but I would advise against average plumbing piping,get some custom tubes make up to handle the pressure.
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Unread postAuthor: SpudBlaster15 » Sat Sep 01, 2007 2:09 am

How many times do I have to tell people; it is not the pressure level generated by DDT that is dangerous, it is the pressure spike. Even if the pressure level generated by DDT is less than the rated burst pressure, the pipe is still likely to fail due to the impact of the shockwave with the chamber walls.
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Unread postAuthor: rna_duelers » Sat Sep 01, 2007 2:49 am

So your trying to tell me that when a gun cartridge goes off there isn't a huge pressure spike?Pull the other one.There is no reason why the energy couldn't be harnest.
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Unread postAuthor: From_Hamsterdam » Sat Sep 01, 2007 5:01 am

small pipes have a huge advantage in pressure compared to larger pipes even if the walls are thicker.

gun barrels are designed to withstand the pressure spikes. galv pipes are not. do not confuse cast iron with tempered steel or other high pressure alloys.

you could have 2 burst disks. one to the barrel and one leading back (recoilless rifle style). Have a short chamber and barrel. This would use the shock wave to fire the ammo and might even be recoilless. you could even add a short section behind the back burst disk and fill it with water to enhance its recoillessness (yay new word :D )
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Unread postAuthor: Fnord » Sat Sep 01, 2007 9:18 am

So your trying to tell me that when a gun cartridge goes off there isn't a huge pressure spike?Pull the other one.There is no reason why the energy couldn't be harnest.


Gun powder-based fire arms do not produce a detonation. If steel could handle repeated spikes of detonation, then propellants such as flash powder and high explosives would be used instead of gun powder.
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Unread postAuthor: boilingleadbath » Sat Sep 01, 2007 12:39 pm

Eh, Fnord, there's a reason one doesn't use Unique (tm) in rifle cartriges, and I don't believe it's because it'd detonate...

My understanding is that firearms are designed to reach peak pressure once the projectile is well down the bore - essentially allowing them to act as if they had a larger chamber volume.

Quick burning powders (say, Unique) burn too fast.
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But, yeah, firearms don't detonate their fuels.
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Anyway, it is my expectation that more flame fronts would help prevent DDT.
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And, BTW, telling us the volume of the chamber doesn't really help us much with determining if DDT might be an issue; we need to know the SHAPE.
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Unread postAuthor: Fnord » Sat Sep 01, 2007 12:52 pm

For a rifle with a long barrel, yes, it is more efficient to have a slow-burning powder.
But for small fire arm such as a pistol, a fast burning fuel could probably give greater acceleration in the short time the projectile is in the barrel.
But anyway, that is all beside the main topic here.
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Unread postAuthor: jimmy101 » Sat Sep 01, 2007 1:04 pm

Don't forget that "DDT=death" is not true if DDT occurs late in the combustion process.

If it takes the combustion of 95% of the fuel in the chamber to get to DDT conditions then the detonation of the last 5% of the fuel is really not all that big a deal.
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Unread postAuthor: dongfang » Mon Sep 03, 2007 9:38 am

Hi,

About time that some of us perform some experiments with the purpose of having DDT smash up some pipes.... like, taking a 15 ft 4" DWV pipe with a good mixture and a single point of ignition, and see what happens to it and where, at different pressures....

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Unread postAuthor: Rock » Thu Sep 20, 2007 4:39 pm

How do you know that ddt happens in a chamber. Explosion of the chamber may be a hint that it could have happened, but maybe it was for a completely different reason.
If a chamber is strong enough to handle the pressure spike there won't be a way to find out either the propane burnt fast or detonated.
Maybe the performance of a gun with ddt is better or worse... seems quite complicated.
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