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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: MrCrowley » Wed Dec 12, 2007 7:31 pm

Not all of it, just skim read over it.

I found that information in my other post for you, so i'm sure you can help yourself out and spend 15minutes having a read of the important stuff, after all you are the one asking for help.

I only know a bit more then you do, yet I still read some of it, so help yourself for once and that link should contain most of your answers as they do an experiment in metal pipe.

Come on don't be lazy just help yourself out until sme more knowledgeable members help you.
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Unread postAuthor: bigbob12345 » Wed Dec 12, 2007 7:35 pm

Okay Ill read it
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Unread postAuthor: Fnord » Wed Dec 12, 2007 7:37 pm

Hey it's only 28 pages. ;)
Not War and Peace or anything.

I read through some of it once, but the original pdf is easier to understand because it has graphs and pictures.

Basically, there's almost no chance of getting DDT in a *normal chamber with propane as fuel and normal mixes.

*Normal meaning ~12" of 2" steel, and mixes under 10x.
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Unread postAuthor: SpudBlaster15 » Wed Dec 12, 2007 7:57 pm

You are not likely to achieve DDT in an average hybrid chamber. At atmospheric pressure, the run up distance in an open pipe with a fairly large diameter, the run-up distance for a stoichiometric propane/air mixture is on the order of 10 meters. I would expect the run up distance to be even greater in a pressurized chamber, as the flame front velocity scales as (IIRC) 1/Pressure<sub>Chamber</sub><sup>0.17</sup>, which means that the flame front velocity will actually decrease with rising pressure.
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Last edited by SpudBlaster15 on Wed Dec 12, 2007 8:02 pm, edited 2 times in total.
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Unread postAuthor: DYI » Wed Dec 12, 2007 7:59 pm

You would really have to be trying to achieve DDT for it to happen. Even then you would likely have a difficult time with it. Without fuels like acetylene or hydrogen, and assuming that you don't have a 3' long 2" ID chamber with 3 running fans, there is extremely little chance of DDT ocurring. It's amazing how people used to think that they were risking DDT in 3" ID chambers only 12" long at 7x.

Without DDT, you have a very small chance of even reaching 2000 psi with C3H8/air combustion. At very high mixes, the combustion actually proceeds more slowly, and energy in the form of heat is dissipated through the walls of the pipe. So you really need not worry about blowing up a steel hybrid.
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Unread postAuthor: bigbob12345 » Wed Dec 12, 2007 8:12 pm

I m done readin now :P
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Unread postAuthor: Novacastrian » Wed Dec 12, 2007 8:14 pm

bigbob12345 wrote:I m done readin now :P


Better go through it again to make sure you didn't miss anything.
Have you taken the Hybrid quiz?
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Unread postAuthor: SpudBlaster15 » Wed Dec 12, 2007 8:21 pm

DYI wrote:At very high mixes, the combustion actually proceeds more slowly, and energy in the form of heat is dissipated through the walls of the pipe.


Eh, the flame front velocity at high mixes is not *that* much slower. Scaling as 1/P<sub>Chamber</sub><sup>0.17</sup>, the flame front velocity in a chamber with an internal pressure of 8 atmospheres is only ~30% slower than it is in a chamber with an internal pressure of 1 atmosphere. Given that this decrease is relatively insignificant, (combustion will still occur within 50ms or so no matter the internal pressure) heat loss is not an issue in a properly sized hybrid chamber. Steel is effective in terms of heat reflection, which is much more important than heat conduction during the small time periods in which combustion occurs.

At 6x mixes, noname was able to achieve internal pressures within (IIRC) 10 - 40PSI of the maximum pressure attainable under adiabatic conditions (no heat loss), which demonstrates the insignificance of heat loss, and the huge significance of the burst disk's failure pressure.
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Unread postAuthor: bigbob12345 » Wed Dec 12, 2007 8:24 pm

I have taken the hybrid quiz and got 9/10
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Unread postAuthor: MrCrowley » Wed Dec 12, 2007 8:28 pm

bigbob12345 wrote:I m done readin now :P


Thanks for reading it, shows that your not some cold hearted, lazy spoonfeedee(?) :lol:




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Unread postAuthor: DYI » Wed Dec 12, 2007 8:41 pm

Eh, the flame front velocity at high mixes is not *that* much slower. Scaling as 1/PChamber0.17, the flame front velocity in a chamber with an internal pressure of 8 atmospheres is only ~30% slower than it is in a chamber with an internal pressure of 1 atmosphere. Given that this decrease is relatively insignificant, (combustion will still occur within 50ms or so no matter the internal pressure) heat loss is not an issue in a properly sized hybrid chamber. Steel is effective in terms of heat reflection, which is much more important than heat conduction during the small time periods in which combustion occurs


You may very well be right, but you seem to disagree with Jimmy. I don't know a lot of the physics behind it, I just kind of learn as I go along. But either way, he's going to need a mix of about 20x or so to rupture the pipe, so he probably doesn't have to worry.
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Unread postAuthor: MrCrowley » Wed Dec 12, 2007 8:47 pm

I think FAE's (fuel air explosives) produce around a 20atm mix, and they blow up buildings and what not :twisted:
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Unread postAuthor: bigbob12345 » Wed Dec 12, 2007 9:03 pm

I only plan to take it up to 4x but would it be safe if I went higher
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Unread postAuthor: MrCrowley » Wed Dec 12, 2007 9:08 pm

Yeah, I would think 8x would be high enough and that's perfectly safe with the metal you're using. Just remember your launcher is as strongas it's weakest component.
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Unread postAuthor: bigbob12345 » Wed Dec 12, 2007 9:18 pm

I think Ill revise my 4x to 6x max
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