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Oxy/Acetylene mini cannon questions.

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Re: Oxy/Acetylene mini cannon questions.

Unread postAuthor: Tango71 » Fri Feb 20, 2015 8:24 pm

Somebody please chime in on my previous post. I'll be going to buy materials tomorrow and don't want to screw this up.

Also, what kind of calculation do you guys do for metering chamber losses. I imagine that as soon as I open the valve on the metering chamber to fill the combustion chamber some pressure and gas remains inside the metering chamber after I close the valve. So I dont get 100% of what was in the metering chamber in to the combustion chamber.
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Re: Oxy/Acetylene mini cannon questions.

Unread postAuthor: jackssmirkingrevenge » Sat Feb 21, 2015 6:24 am

Tango71 wrote:My biggest thing now is figuring out fueling. I have already determined the ideal combustion chamber size using HGDT and from what I'm understanding it looks like I need fuel at 4% chamber volume give or take a bit. Plus a 1:4 ratio of fuel and oxygen, someone mentioned 1:5 but 20% and 80% is 1:4 is it not? So (and correct me if I'm wrong) For every 1cc of fuel I need 4cc of oxygen.


You will need to purge the chamber with oxygen - as in oxygen going into the chamber with oxygen going out, in order to replace the air. After that, you meter enough fuel to give you 20% of fuel total in both your meter and your chamber.

My new combustion chamber volume (without going coaxial like the original plan) is 1235cc so 4% of that is 49.42cc so roughly 50cc. This means that at a ratio of 1:4 I need 200cc of oxygen?


You misunderstood the idea here, atmospheric air has 20% of oxygen, so the ideal ratio of fuel is 4%. If you want a 5x mixture at atmospheric pressure, then you need to replace all the air in the chamber with oxygen, and then inject 20% of fuel into that chamber.
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Re: Oxy/Acetylene mini cannon questions.

Unread postAuthor: Tango71 » Sat Feb 21, 2015 9:33 am

Ah yes got it! Sorry, I somehow missed that part. A 5X mix should be doable with a small 12v compressor I think and still gives me the performance I'm looking for. I crunched more numbers yesterday and realized how expensive it will get running oxygen. So as I initially thought, I have no choice but to make this a hybrid.

I designed an interesting breech lock system that uses a sliding gate behind a sealing plug. The plug contains o-rings and seals in all the air pressure and the gate holds the plug back during the detonation. Everything is 1/2" wall pressure rated steel, a bit overkill for under 500psi chamber pressures but you gotta realize that I'm sitting next to this.

I will have design pics of this up later this week in the hybrid forum.
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Re: Oxy/Acetylene mini cannon questions.

Unread postAuthor: DYI » Tue Feb 24, 2015 10:56 am

Well, this thread turned into a total cluster**** while I was gone...

Just about everything said since my previous post is in some way wrong or ambiguous, including JSR's advice on the ideal mix ratio (using the mix ratio he proposed will lead to low performance and high risk of detonation, and his estimate of the pressure generated is off by a factor of two, consistent with having failed to use GasEq :roll: ). Without attempting to correct all the factual errors (which would take several hours, based on a quick skim), I do agree with the decision to move to an air/fuel hybrid, given the design requirements.

I am somewhat confused as to how you plan to seal the rupture diaphragm in your design. Could you expand further on that?

Also, based on said factual errors, don't expect this thing to perform even remotely close to your "modeling" results :lol:
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Re: Oxy/Acetylene mini cannon questions.

Unread postAuthor: Tango71 » Tue Feb 24, 2015 10:00 pm

Here are the pics I promised. The combustion chamber contains a piston with o-rings around the perimeter. It is backed by a spring assembly which allows the piston to travel back but keeps the unit as one piece. This makes the unit easily removable to reload. The backing plate on the other side of the spring extends slightly past the combustion chamber so when the rear gate is slid closed it compresses the spring which presses the piston in to the barrel.

Image
Inside combustion chamber showing piston and projectile position

Image
Breech gate open

Image
Breech gate closed

Obviously there is a bit more to this than what is shown. The breech gate will have a lock on it to lock it in place. No room for it to slide from the top down so it must go from the bottom up. Let me know what you guys think.
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