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combustion coxial

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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combustion coxial

Unread postAuthor: spudthug » Fri Apr 13, 2007 6:13 am

ok well yester i built my coaxial combustion... i forgot one thing... i used 3" rigid conduit...is this safe for combustions??? ill put pics of the pipe up later today i gtg so school right now....all it says on it is 3" rigid conduit pvc sch40
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4" piston valved cannon-half done..( i spilt my cement...)

Hybrid- 75% done. need to build propane holder and drill/tap sparkplug hole..
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Unread postAuthor: BC Pneumatics » Fri Apr 13, 2007 7:35 am

It will be fine.
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Unread postAuthor: FeLeX » Fri Apr 13, 2007 7:43 am

Sorry for my ignorance (and I did use the quick serach button) but how do they work?
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Unread postAuthor: BC Pneumatics » Fri Apr 13, 2007 7:50 am

Combustions? Coaxials? You obviously didn't search well enough.
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Unread postAuthor: lukemc » Fri Apr 13, 2007 8:28 am

i counted and once i was in the wiki it took me 3 clicks to find this http://www.spudfiles.com/spud_wiki/index.php?title=Coaxial

please look more
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Unread postAuthor: FeLeX » Fri Apr 13, 2007 8:30 am

Guys... I know what a combustion and I know what a coaxial is. Spudthug said Combustion Coaxial. I was wondering how THAT worked.

Edit: I am stupid. Sorrys for all the retardiness. I just went to the wiki and read it again, I get it now. I thought it used pressure just like the penumatic cannon and then you ignited it or something. Sorrys again.
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Last edited by FeLeX on Fri Apr 13, 2007 8:34 am, edited 1 time in total.
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Unread postAuthor: BC Pneumatics » Fri Apr 13, 2007 8:33 am

You know how a combustion works, you know what a coaxial is. A combustion coaxial is a coaxial cannon that works like a combustion.
Just a combustion w/ the barrel inside the chamber.
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Unread postAuthor: lukemc » Fri Apr 13, 2007 8:33 am

well i dont mean to make you sound like a noob but cant you jsut put the two together
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Unread postAuthor: FeLeX » Fri Apr 13, 2007 8:35 am

Yea I figured it, just read my edit. Is it any better in performance or is the easy breech loading part is the only good thing about it?
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Unread postAuthor: willarddaniels » Fri Apr 13, 2007 9:31 am

You don't have to have breech loading with a coaxial, but that is a nice feature. The greatest benefit, in my opinion, is the reduction in overall length of the cannon.
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Unread postAuthor: jimmy101 » Fri Apr 13, 2007 9:49 am

Of course the drawback of a combustion gun that has the barrel extending into the chamber is that it's performance will suck.

1. Too much heat loss from all that surface area in the combustion chamber.
2. Slow burn rate caused by the very small flame front area.

edit: fix bad engrish
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Last edited by jimmy101 on Fri Apr 13, 2007 10:26 am, edited 1 time in total.
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Unread postAuthor: lukemc » Fri Apr 13, 2007 9:55 am

could you make a coaxial hybrid? a regular piston valve but instead of droping the pressure on the pilot side increase the pressure on the chamber side (combustion) or have a simultaneous ignition pilot valve release


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red is the piston
dark bluse is pressureized air
blue with red is combustion
green is a burst disk
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Last edited by lukemc on Fri Apr 13, 2007 10:04 am, edited 2 times in total.
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Unread postAuthor: willarddaniels » Fri Apr 13, 2007 9:59 am

^^^^^^^^^^^^
Solution to 1 : Who cares... it is very minimal at worst. Besides, coaxials aren't built for their increased perforamnace, but ease of use and compactness.
Solution to 2: Use multiple ignition points. This also solves #1.
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Unread postAuthor: jimmy101 » Fri Apr 13, 2007 10:44 am

willarddaniels wrote:^^^^^^^^^^^^
Solution to 1 : Who cares... it is very minimal at worst. Besides, coaxials aren't built for their increased perforamnace, but ease of use and compactness.
Solution to 2: Use multiple ignition points. This also solves #1.


For solution 1: How do you know that? Heat loss in a combustion gun is significant. Indeed spud guns (and real guns) lose perhaps 30% of the energy in the charge as heat transfered to the gun. A coaxial gun is going to lose much more heat (energy) this way. Perhaps twice as much based on having roughly twice the surface area as a comparable non-coaxial design.

For solution 2: Yep multiple ignition points will help but the positioning of the sparks is far from optimal. The best place for the spark is in the middle of the chamber. Pretty hard to do with a coaxial.

A coaxial design really does not save much in the overall length of the gun, if the gun has a reasonable C:B ratio. You shorten the exposed part of the barrel by about the length of the chamber. But, you have to use a longer chamber, or larger ID chamber, to get the chamber volume back to a useable size.

In addition, mixing and venting in a coaxial is difficult. Construction is also a bit harder.

Coaxials are an interesting design with some modest advantages but they really are a step backwards in spud gunning technology.
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Unread postAuthor: willarddaniels » Fri Apr 13, 2007 11:11 am

could you make a coaxial hybrid? a regular piston valve but instead of droping the pressure on the pilot side increase the pressure on the chamber side (combustion) or have a simultaneous ignition pilot valve release

You can, you just need to use a compression spring instead of trying to use a pneumatically pressurized piston. The problem comes in trying to release the pilot at the same moment moment of combustion... get it wrong and you just made a bomb.

For solution 1: How do you know that? Heat loss in a combustion gun is significant. Indeed spud guns (and real guns) lose perhaps 30% of the energy in the charge as heat transfered to the gun. A coaxial gun is going to lose much more heat (energy) this way. Perhaps twice as much based on having roughly twice the surface area as a comparable non-coaxial design.

For solution 2: Yep multiple ignition points will help but the positioning of the sparks is far from optimal. The best place for the spark is in the middle of the chamber. Pretty hard to do with a coaxial.

A coaxial design really does not save much in the overall length of the gun, if the gun has a reasonable C:B ratio. You shorten the exposed part of the barrel by about the length of the chamber. But, you have to use a longer chamber, or larger ID chamber, to get the chamber volume back to a useable size.

In addition, mixing and venting in a coaxial is difficult. Construction is also a bit harder.

Coaxials are an interesting design with some modest advantages but they really are a step backwards in spud gunning technology.

#1 Any additional heat loss is to the interior fo the gun, thus minimizing the effect. Besides, this is a mute point as we have both pointed out that COAXIALS ARE NOT USED FOR THEIR INCREASED PERFORMANCE. Again, who cares? If you want the most efficient gun, don't make a coaxial for this reason- you are only debating yourself.
#2 You are missing the point of MULTIPLE ignition points. Use a spark strip, not just one ignition point in the middle as you suggest if you are so concerned about the efficiency.
Size matters. If you can hide 18" of your barrel in the chamber, you effectively reduce the overall length of the cannon by that same 18". Not a bad idea for those concerned that their gun is too long and want a shorter gun.

The main point: There is more to this recreational activity than building the biggest or most efficient cannon. I am not saying you are wrong, just please be open to the fact that others have different goals, needs or desires than you do. Here, we try to nurture these desires and help everyone get the most enjoyment and success they can out of the projects they choose to do.
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