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chamber to barrel ratio for multi-barrels & ignition?

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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chamber to barrel ratio for multi-barrels & ignition?

Unread postAuthor: JDP12 » Sun Apr 01, 2007 4:03 pm

Hello all,

Well first off i just want to say hey, and what a great forum.

But i have a question.

I am building an interchangeable barrel combustion cannon, and want to know how to calculate the correct size of the chamber. I know the ideal chamber to barrel ratio is 1.5:1, but am wondering how to calculate the chamber's volume for several different sized barrels?? The biggest barrel would be a 2.46" Diameter, 4' long tennis ball barrel, and the smallest would be 1"Diameter, 4' long. Should i just average the volumes of all my barrels (there are more than those two) and use that volume or should i just take the volume of the biggest one or which one??


Second question.

Here in Wisconsin, stun guns are outlawed sadly :cry: so I cannot use one for an ignition source. But I still want to have four sparks in my chamber.

Should I just use two BBQ ignitors and have each one fire sparks, and just put the buttons next to each other so I can fire it?? Because I know that one BBQ ignitor can only produce two sparks at most- http://www.members.tripod.com/potatogun ... ltiple.htm
I pretty much know for sure that that would work, just want to see what you guys think.

Thanks for the information,

ilovetoblowthingsup
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Unread postAuthor: benstern » Sun Apr 01, 2007 4:22 pm

I would suggest using a camera flash ignition. It would be impossible to push 2 bbq ignitors in at exactly the same time (give or take a few nanoseconds) to obtain equal ignition. Why do you need four sparks in the first place?
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Unread postAuthor: JDP12 » Sun Apr 01, 2007 4:32 pm

well just to get more equal ignition, but do you know anything about my first question??
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Unread postAuthor: boilingleadbath » Sun Apr 01, 2007 4:38 pm

1) The definition of the ideal C:B ratio varies, but here are some of the more common ones:
a) The barrel length at which all the power generated by the combustion is utilized, about .8:1.
b) The barrel length at which (muzzle energy)/(launcher length) is maximized... which varies with the pipe diameters used.
c) What your uncle recommended - which, naturally, varies widely.

Note that this discusses your BARREL length... because describing the ultimate "chamber length" is silly; there isn't one, ignoring the salt mine paradox. (the salt mine paradox being that there is a point past which, do to a longer period of combustion, the power actually decreases as you increase the size of the chamber. This size is WAY out there, especially with 4 sparks.)

So, what to do?
Make it any size you feel like... but make sure that, with your biggest barrel, the ratio does not go below .8:1, or maybe a bit higher, like 1:1 or something, for aerosol or fan-less launchers... because then the pressure in the bore would actually go below <i>zero psig</i> - which, interestingly, would mean that a shorter barrel would give you higher velocity.
(or, you might download my EVBEC. Search the forums.)

2) As to the "you can only get two sparks out of a BBQ sparker" bit... bullshit. You just have to use shorter spark gaps, because it's the TOTAL length of all your spark gaps added together that determines the voltage required.

If you REALLY want a stun gun, you might see if you can get one as a circuit board. (I am NOT a lawyer, and do not know about the legality of this)
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Unread postAuthor: JDP12 » Sun Apr 01, 2007 4:58 pm

really.. okay didnt know that it was the spark gap that matters. thanks for that, just saved me some money :) So just reduce the distance between the screws?? Kay thanks a lot!

and as for the chamber, just measure my longest barrel, calculate that volume, and use the ratio for that size?? K thanks a lot.

oh and i am including a fan in my chamber, so would it then be okay to exceed the 1:1 ratio??
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Last edited by JDP12 on Sun Apr 01, 2007 5:20 pm, edited 1 time in total.
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Unread postAuthor: lukemc » Sun Apr 01, 2007 5:20 pm

i will now translate the first half or what blb said: make large diameter barrels shorter and small diameter barrels longer to achieve the same volume and something about my uncle... :? :wink:
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Unread postAuthor: JDP12 » Sun Apr 01, 2007 5:21 pm

haha thanks a lot lukemc, i was trying to figure out the same thing, that now makes it a lot clearer.

Thanks
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Unread postAuthor: JDP12 » Mon Apr 02, 2007 7:14 am

okay, problem update....i calculated out my chamber size and everything last night....And found that in order to keep the 1.5:1 ratio, my tennis ball barrel would have to be around 17" long.....now that is not good. I want it to be 3 feet long. And when i calculated out the chamber volume for a 3 foot long tennis ball barrel, i found that my 1.5" barrel would have to be around 8 feet long!!!

Now i would like all barrels to be around 3-4 feet long, but still have maximum power...Does anyone have a way to remedy this??? :?
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Unread postAuthor: cerberus » Mon Apr 02, 2007 8:22 am

To put it simply, it can't be done. You're going to have to compromise one way or another. Build the chamber to match the barrel you think you're going to use the most often and then make other barrels as close as you can fiesably do so to their ideal length.
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Unread postAuthor: JDP12 » Mon Apr 02, 2007 10:01 am

so i'll mostly be using my golf ball or just plain 1.5" sch 40 barrel..so i should just measure my chamber for that then??? And then would it still be okay to make my tennis ball barrel.....say 3.5' long..and still get a tennis ball to go super far??How much power would be lost as a result approximately?? My guess is not much, but i just want to check.
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Unread postAuthor: aturner » Mon Apr 02, 2007 10:22 am

You may want to consider making a launcher with an optional chamber extension. Sorta like this...
Image

You could pick a chamber size that works with several of your barrels that have a smaller volume, and then for larger barrels add a chamber extension.

Here's an example of this in use:
http://loose-cannon.home.mindspring.com/blackhawk/

And an example of a completely modular chamber:
http://loose-cannon.home.mindspring.com/modular/
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Unread postAuthor: SpudBlaster15 » Mon Apr 02, 2007 10:50 am

A 1.5:1 ratio, to put it simply...sucks.

(Now I am going to assume that you are using an aerosol fuel, and a fairly accurate fuel mixture)

Aim for a ratio of around 1:1. This ratio, given a fixed chamber size, will perform far better than a 1.5:1 ratio.

A compromise must me made somewhere. It is idiotic to assume that a fixed ratio can be achieved using a chamber of a set volume, and barrels of set lengths, but variable diameters. A tennis ball barrel of a 3.5 foot length will require a chamber with a volume of 200 cubic inches to perform well with an aerosol fuel. With your theoretical setup, the tennis ball is likely to leave the muzzle at a velocity no greater than what could be achieved by throwing it, and plop down about 100 feet from the muzzle (unless you are using metered propane, of course). With a chamber this size, a 1.5" barrel with a length of 4 feet would give you a ratio of ~2.2:1, which would be extremely loud, and terribly inefficient.

I suggest building multiple launches, each with a barrel of your intended length, and a chamber built around it to achieve an ideal ratio. A detachable chamber extension would work, but would have to be large enough to nearly double your chamber volume.
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Unread postAuthor: aturner » Mon Apr 02, 2007 3:58 pm

Right now I wish I had a link to some of Jimmy's website, or his posts on the old spudtech forums. I'll do my best impersonation....

Nothing "sucks" about a high ratio. I happen to like 1.5:1 ratios because the additional noise and muzzle flash are part of the whole enterainment package. Just a preference.

Folks get in a big tizzy about ratios, when in reality, 99% of the time, what really matters for the typical spudder is barrel length. We usually have plenty of additional power in the chambers we build. There is no need to aim for any magic ratio unless somone is worried about noise or is trying to win an efficiency contest. The key to ratio is to fall below a certain ratio. A bit of additional chamber volume won't hurt, but a lack of chamber volume will literally suck. Therefore, focus on a design with the longest barrel that is still practical.

For me, I say a 6 foot barrel is just about ideal in most typical spudgun situations. Much longer, and it's a PITA to transport/setup/support/aim. Much shorter and the performance drops considerably.

Your mileage may vary.
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Unread postAuthor: SpudBlaster15 » Mon Apr 02, 2007 4:38 pm

Alright, so I should have worded that differently. A 1.5:1 ratio will suck... performance wise.

Sure, if you are looking for the noise output and muzzle flash aspects, then by all means stick with it. However, if one desires maximum performance, then size the barrel to the chamber, or vice versa.
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Unread postAuthor: boilingleadbath » Mon Apr 02, 2007 5:48 pm

lukemc misunderstood my statements.

Here are some actual numbers generated by EVBEC (using 100g projectile for all, for the sake of comparison)
<table><tr><td>Chamber volume</td><td>barrel volume</td><td>velocity</td></tr><tr><td>2060 ml</td><td>1325 ml</td><td>260 fps</td></tr><tr><td>4100 ml</td><td>1325 ml</td><td>300 fps</td></tr><tr><td>2060 ml</td><td>1950 ml</td><td>280 fps</td></tr><tr><td>2470 ml</td><td>2800 ml</td><td>310 fps</td></tr></table>

What are these launchers?
The first one is your launcher with the 17" tennis ball barrel
The second is like the first, but the chamber is twice as large. (barrel is 17")
The third is like the first, but the barrel is long enough to make it (roughly) a 1:1 ratio.
The fourth is the the 3' tennis ball barrel paired with a chamber that gives it a .88:1 C:B ratio.

As we can clearly see:
1) a longer barrel, as in launcher 3, increases muzzle velocity.
2) a bigger chamber, as in launcher 2, increases muzzle velocity.
3) there is no real requirement to stick to a specific ratio; launcher 4 preformes just fine.

...honestly, just download EVBEC and play with it a bit.
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