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Combustion chamber shape

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 chamber shape

Unread postAuthor: sacklunch » Sun Oct 09, 2011 7:00 pm

So I'm new to this site, and it kicks a**!!!!! Getting ready to build my first ever spud cannon and I was wondering if shape of the cb matters to combustion force. This is a sad side note....I'm 33 and have never built one.
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Unread postAuthor: Gun Freak » Sun Oct 09, 2011 7:30 pm

Short and fat is better than long and skinny because the gasses don't have as far to travel.
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Unread postAuthor: sacklunch » Sun Oct 09, 2011 8:22 pm

What about end cap and coupler shape? How about a way to internally direct the charge?
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Unread postAuthor: Gun Freak » Sun Oct 09, 2011 8:53 pm

Well the endcap is and endcap, and use elbows if you want an over/under or just make an inline.
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Unread postAuthor: saefroch » Sun Oct 09, 2011 8:56 pm

Spherical chamber with ignition in the center is the ideal. shoot for that. Try to avoid DWV for extra safety should the projectile fail to exit the barrel. You're looking for something that says "NSF-PW" or has a pressure rating stamped on it. If it says "DWV" or "not safe for pressure," I'd personally advise against it, though you're probably okay with just a combustion.

If you stay away from DWV, you're stuck with one shape of cap and coupler shape, so there really isn't a choice there. There isn't really any way to "direct the charge," since you're just generating a pressure rise inside the entire chamber, the quicker you get the maximum pressure rise, the better.
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Unread postAuthor: Technician1002 » Sun Oct 09, 2011 9:30 pm

Gun Freak wrote:Short and fat is better than long and skinny because the gasses don't have as far to travel.


More important is a sphere is the most effecient shape to hold a volume. As such it has the LEAST amount of surface area exposed to the combustion resulting in the least amount of heat loss to the container.

A long skinny chamber has lots of surface area to absorb radiant heat from the combustion.

This is a sad side note....I'm 33 and have never built one.

That is OK. I was over 50 when I built my first combustion.
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Re: Combustion chamber shape

Unread postAuthor: jackssmirkingrevenge » Sun Oct 09, 2011 9:57 pm

sacklunch wrote:I was wondering if shape of the cb matters to combustion force.


It matters, but not as much as properly metered fuel, and ideal chamber:barrel volume ratio and decent ignition - focus on these things if you're aiming for power.

I'm 33 and have never built one.


You don't have to be a teenager to enjoy this hobby ;)
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Re: Combustion chamber shape

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Unread postAuthor: Brian the brain » Mon Oct 10, 2011 12:05 pm

Missed that topic.
36 myself but then again...almost here for 6 years....

A fat shorter chamber is indeed the thing to go for.
A long skinny chamber could induce DDT ( detonation in stead of combustion)

Spare no money on your first.Get the right stuff.
This hobby is addictive.You'll end up spending more to get it right if you don't do it right the first time.
At 33 and with help from this forum you'd be best off going for something with a chamber fan and fuelmeter.
Not too hard to make but so much better!
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Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

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Unread postAuthor: sacklunch » Mon Oct 10, 2011 4:33 pm

Well I'm going to go with a 4" x 24" chamber and a 2" x 83" barrel which gives me a ratio of 1:1.56 for my first cannon.
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Unread postAuthor: mattyzip77 » Mon Oct 10, 2011 4:48 pm

almost 7 feet seems kinda long, I would go 60 inches, but thats me. Good luck, and welcome!! By the way, Im 34!!!
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Re: Combustion chamber shape

Unread postAuthor: D_Hall » Mon Oct 10, 2011 4:56 pm

sacklunch wrote:I'm 33 and have never built one.

No worries. I was probably about your age the first time I built one. By age 41 I'd built what I believe to be the largest, most powerful of them all. Point: A late start means nothing!
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Unread postAuthor: sacklunch » Mon Oct 10, 2011 5:10 pm

So chamber, short and fat over long skinny, got it. I thought that the magic number for cb ratio is 1:1.5? 60 inches seems short for that ratio. Not to change subject.
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Unread postAuthor: sacklunch » Mon Oct 10, 2011 5:21 pm

So has anyone tried to make a chamber that, internally, helps direct the charge or explosion inside the chamber?
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Unread postAuthor: D_Hall » Mon Oct 10, 2011 7:03 pm

sacklunch wrote:So has anyone tried to make a chamber that, internally, helps direct the charge or explosion inside the chamber?


Define "direct the charge." What do you even mean by that?
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Unread postAuthor: Technician1002 » Mon Oct 10, 2011 7:11 pm

I think he is looking in the restriction in the transition between the chamber and barrel. He has a point. Look at Vortex cannons for a chamber shape with a transition cone between the chamber and barrel. With a projectile, don't use a vortex cannon barrel shape. There is no reason the chamber shape would be useless in a spudgun. if the design is to flow near sonic speeds in the transition space between the chamber and barrel.

Is there merit to this chamber shape? It would be more difficult to build from standard plumbing supplies.
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