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9x mix and beyond Pneumatic cilinder

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: DYI » Thu Jul 26, 2007 4:56 pm

Steel doesn't have enough impact resistance to withstand DDT. It may survive the first one, but even if it does, it won't be useable afterwards. Steel is strong, but it has undesirable failure characteristics. It won't "shrapnel" like PVC does, but DDT could cause it to shatter in my opinion. 'Normal' steel fittings definitely will not cut it, and even with 6000psi forged steel fittings, I wouldn't want to be anywhere near it when it went off at 20x.
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Unread postAuthor: paaiyan » Thu Jul 26, 2007 4:56 pm

DYI wrote:The chamber should be OK, but the fittings may be where you start to run into trouble. Steel fittings would likely shatter, and fittings made of other materials wouldn't be strong enough. Good luck getting C2H2 to 20 bar. If you try that, the gruesome scenario that I described in my above post may well become reality, especially if said C2H2 is mixed with air, or, god forbid, pure Oxygen.


Look, don't be that guy alright? As long as the amunt injected will not surpass the burst pressure upon ignition, he'll be fine. He just needs to do a bit of math and find out how much that is.
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Unread postAuthor: DYI » Thu Jul 26, 2007 5:09 pm

Well yes, it stands to reason that a material will fail if taken above its burst pressure. There is no way he will be generating over 50 000 psi, the problem is that of detonation. I'm not trying to rain on anybody's parade, sorry if I sounded negative. I haven't been attempting to undermine anyone's intelligence, just pointing out that DDT can cause materials to fail far below their burst pressure due to the sudden spike, and it is difficult to predict at what point it will occur, and to design something that will withstand it repeatedly, which jumps out to me as being the real problem: it doesn't need to simply survive a detonation and remain intact, it needs to survive a detonation shot after shot, which could make it even more brittle. I don't profess to know if a 20x mix in this chamber will cause a detonation, but if it does, it is one more thing to incorporate into the design, and it complicates things immensely
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Unread postAuthor: Jumpin Jehosaphat » Thu Jul 26, 2007 5:22 pm

Does anyone know the exact conditions for DDT of propane or acetylene?
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Unread postAuthor: paaiyan » Thu Jul 26, 2007 5:44 pm

DYI wrote:Well yes, it stands to reason that a material will fail if taken above its burst pressure. There is no way he will be generating over 50 000 psi, the problem is that of detonation. I'm not trying to rain on anybody's parade, sorry if I sounded negative. I haven't been attempting to undermine anyone's intelligence, just pointing out that DDT can cause materials to fail far below their burst pressure due to the sudden spike, and it is difficult to predict at what point it will occur, and to design something that will withstand it repeatedly, which jumps out to me as being the real problem: it doesn't need to simply survive a detonation and remain intact, it needs to survive a detonation shot after shot, which could make it even more brittle. I don't profess to know if a 20x mix in this chamber will cause a detonation, but if it does, it is one more thing to incorporate into the design, and it complicates things immensely


Well, A sudden spike shouldn't matter as long as it's under the burst pressure. Especially if it's 50k PSI under. The only thing to worry about would be if it detonated whilst fueling. Then the pressure and gasses would most likely attempt to go out the barrel, as well as through the fueling apparatus, which is bad.
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Unread postAuthor: DYI » Thu Jul 26, 2007 6:06 pm

Au contraire. A DDT created by a propane/air mix has been measured to produce about 50bar. Noname has proven with a test, that this is sufficient to do significant damage to Sch80 1/2" steel pipe, rated for well over 100bar (probably over 200bar). The pipe did not shatter due to the detonation, but there was a noticeable bulge on the side of the pipe. It probably wouldn't have survived a subsequent detonation. Although I believe this cylinder would survive a detonation at 20x, I question how many it could survive before the multiple shock loads made it brittle and weak to the point that it could not withstand any more. The cylinder should be fine for a while though. It is the problem with any attached steel fittings that could maybe withstand 2 detonations max, without rupturing.
And yes, the mix autoigniting during fueling would be very, very bad. In this case, I think that not only remote ignition, but remote fueling, would also be a necessity.
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Unread postAuthor: Velocity » Thu Jul 26, 2007 8:47 pm

Can someone please provide a link to noname's test? It has been referenced several times before, yet I know nothing of it...
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Unread postAuthor: Fnord » Thu Jul 26, 2007 9:06 pm

Here's a few things I have to mention:
First, propane and air should not liquify at 20x. I'm fairly sure propane has to be in high concentrations to liquify.
Acetylene is fairly unstable and tends to combust when you pressurise it.
As for propane and air combusting on its own, just watch the temperature once it gets past 10x. Hopefully the metal chamber should keep the mix cool.

Now about ddt:
At 20x a detonation is probably certain to happen.
I'm suprised no one has thought of this before, but how about making a ddt bumper? That is, a chamber lining made of something with a little give to it. Something kinda like foam rubber, only heat resistant and about 100x times stronger.

Such a lining would help absorb the initial shock and reduce the stress on the chamber.

I don't really think a detonation will destroy your chamber though, I mean, ddt occurs in engines doesn't it?
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Unread postAuthor: DYI » Fri Jul 27, 2007 1:06 pm

I don't think that DDT regularly occurs in engines, they probably wouldn't survive too long if it did.
As I said, the partial pressure argument has been going on for a long time, and as far as I know it has never been proven one way or the other, and I don't really know enough about the gas laws to have a strong opinion on it. The idea of a bumper when using a less shock resistant chamber is a good idea. Something like aluminum, lead, brass, etc... could work because they are relatively soft, meaning they could absorb the impact better. ABS could work if it were thick enough, but something like rubber would probably be too soft to provide enough protection unless it was in unreasonably large quantities.
If you had enough knowledge in the area , you could probably calculate at what point the kinetic energy of the particles would surpass the ignition energy of the mixture. If using very high mixes, I think that slow pressurisation would be preferable so that the gases don't become too heated and autoignite.
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Unread postAuthor: SpudBlaster15 » Fri Jul 27, 2007 1:58 pm

DDT can occur within gasoline engines, should the compression ratio be set too high, or if engine temperature exceeds a certain threshold. If it does occur, it can severely damage the engine, and it's force is known for destroying pistons.

Preignition is certainly possible, as this is similar to the concept of a diesel engine's operation. At a ~15x mix, the air within the cylinder is so hot, that the fuel, when injected, ignites instantly. However, this process is aided by engine heat, and diesel engines require glow plugs for initial cold startup.

A propane/air mixture should liquify at the same pressure as pure propane, although more slowly. The propane molecules will be farther apart due to dispersion in the air, but as they move around, they will circulate past one another, thus forming "clumps" that will puddle at the bottom and sides of the container as liquid propane.

A shock absorbing material lining the chamber would help lessen the shock of DDT, but how long would this lining last before said shock weakens it beyond usefulness is the question.
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Unread postAuthor: CannonBall » Wed Aug 22, 2007 3:30 pm

well.. if a standard moped cilinder has a wall thickness of 0.8 centimeter a ik know about ddt in moped... the cilinders survives only the piston gets destroyd.. if my cilinder is 6.5 centimeters thick...
I dont think it blows...
the army fire's shell with cordite.. in a stainless steel chamber of aprox 12 centimeters thick..
Then my cannon probably wont blow only problem is how i get the right feul amount in this chamber im still designing ... (carburater)
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Unread postAuthor: noname » Wed Aug 22, 2007 4:06 pm

Just looked at this topic, and Rmich, I never really did any "test." I attempted DDT and tried to get steel pipe to fail, and managed to get it to bulge near the union with a high mix. That was it.
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Unread postAuthor: CpTn_lAw » Wed Aug 22, 2007 5:18 pm

DDT occurs constantly in turbojet engines (turbo reactors). EGV is approx 1400 km/h in a little model one, and higher in normal sized aircrafts.
no need to precise that the EGT exceeds 700°c, in my little one, it exceeded 1000°C with butane and managed to melt the brass nozzle. (became liquid)
So, DDT provides instant power, but, instant shockwave. I doubt your chamber will break, 5000 bars of working pressure is extremely high. I suggest you make your barrel big enough though...
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