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New to Cannons

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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Unread postAuthor: cannons-go-boom » Tue Sep 18, 2007 5:23 pm

spudgunnerwryyyyy wrote:For that barrel at the ideal ratio. You would only need a 8inch piece of 4 1/2 inch. I suggest a larger bore.


Whats a bore?

So your saying that I can make it a 8" by 4 1/2" chamber, and a 5 foot 1 1/2" barrel?

Will that shoot Potatoes pretty far?

sorry for all the questions guys...and thanks.
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Unread postAuthor: spudgunnerwryyyyy » Tue Sep 18, 2007 5:25 pm

The bore is how large the inside of the barrel is. Yes this was shoot potatoes pretty far. Don't worry about the q's getting started in this is confusing.
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Unread postAuthor: cannons-go-boom » Tue Sep 18, 2007 5:41 pm

ok, i will make a few barrels eventually but for now i am going to make the 1 1/2" first. is 5 feet to long or to short?
cause thats about as far as i can go because of storage reasons.
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Last edited by cannons-go-boom on Wed Sep 19, 2007 10:54 am, edited 1 time in total.

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Unread postAuthor: Pete Zaria » Tue Sep 18, 2007 5:50 pm

Just to explain the "Ratio thing", in layman's terms:

The c:b ratio refers to the ratio of VOLUME between your chamber and barrel. The "ideal" ratio for a propane-powered gun is .8:1 (the chamber is 80% of the volume of the barrel), 1.1-1.5:1 is better for hairspray guns.

It's easiest to calculate the volume of your barrel and then figure out the appropriate chamber size to match, rather than calc the chamber first.

For instance, let's just take your 5 ft x 1.5" barrel...

First we need to calculate the volume of a cylinder 1.5" wide x 5 ft (60"). Here's a very helpful little tool for that: http://www.online-calculators.co.uk/vol ... volume.php
So to use that calc, we'll put in a radius of .75 and a height of 60, inches, which yields us 106 cubic inches of volume.
So, if we want a .8:1 ratio, we need an ~85ci chamber. For a 1:1 ratio, we'd obviously need a ~106ci chamber, and for a 1.5:1 ratio, we'd need a ~160ci chamber. (I rounded all those numbers up to the nearest cubic inch, btw).
If you wanted to use 4" pipe for the chamber....
For a .8:1 ratio, we'd need a 4" x 6.75" chamber.
For a 1:1 ratio, we'd need a 4" x 8.5" chamber.
For a 1.5:1 ratio, we'd need a 4" x 12.75" chamber.

You can use that calculator tool I gave you a link for to figure out the proper ratios for other sizes of pipe, too.

If you're going to use propane, go for .8:1 - 1:1. If you're going to use aerosol fuels, go for 1:1 - 1.5:1.

Having a chamber that's too big for the barrel, within reasonable limits, won't decrease performance, it will just "waste" the extra power being produced by the chamber (basically, if your chamber is too big, the barrel simply can't harness all of the energy being put out by the chamber, and thus the "extra" power is wasted as noise and heat). So it's better to go for a too-high C:B ratio than too low. I use 1.5:1 for most of my combustion guns.

Hope that helps.

Peace,
Pete Zaria.
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Unread postAuthor: williamfeldmann » Tue Sep 18, 2007 6:11 pm

You remember the area of a circle formula?

A = pi * radius squared

Volume = area of pipe * length

So a 1.5 inch bore that is five feet long = 3.14 * (.75^2) * 60 = 106 cubic inches.

We want a chamber that is .8 of that or about 85 cubic inches.

Now we work it backwards for length of chamber = (85/(3.14*2.25^2)) = 5.3333 inches

The reason for the huge length disparity between parts is the difference in the bore of the two parts.

The area of your barrel is 1.76 sqin, the area of your chamber is 15.89 sqin. If for instance you were using a 3 inch chamber the area is only 7.06. A 4 inch chamber gives 12.56 sqin.

EDIT: Damn you Pete and your fast fingers
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Unread postAuthor: SpudBlaster15 » Tue Sep 18, 2007 7:00 pm

williamfeldmann wrote:I still don't understand the reasoning behind your idea of having a stepped barrel. The dead space between the 4 inch combustion chamber and 1 1/2 actual barrel will decrease the performance you were saying you desired. By eliminating the section of 2 inch barrel you will increase your performance.


There is no 'dead space' in cannon-go-boom's design, as the projectile acts as the valve in a combustion launcher, and thus, any volume behind it will act as the chamber.

However, there is no point (other than possibly for aesthetic purposes) in using 3 different pipe sizes to construct a basic combustion launcher. If you want to maximize the power output for a given length, it is best to use a relatively short, large diameter chamber; and a long barrel. If I were building a 6ft inline combustion launcher, I would use the following dimensions.

4" x 10" chamber
1 1/2" x 62" barrel

Or

4" x 15" chamber
2" x 57" barrel

Both of these designs will have a ratio close to 1:1.
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Unread postAuthor: williamfeldmann » Tue Sep 18, 2007 7:46 pm

That is true about the dead space, I just borrowed the term. The size drop from 4 and a half to 2 inches though with a rear open cannon is going to act like a dead space since it is difficult to get a good mix of propellant even aresol in such a shaped chamber.

Yes the lack of a valve does make the entire space behind the projectile technically combustion chamber, the shape of the space, however makes the 2 inch portion practically useless and greatly reduces the performance of the cannon.
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Unread postAuthor: SpudBlaster15 » Tue Sep 18, 2007 8:43 pm

When an aerosol propellant is sprayed into the chamber, the gases will immediately begin to mix with the atmospheric air within the chamber, and will eventually mix completely, no matter the shape of the container. While it would take longer for the propellant to diffuse and mix with the air inside the 2" portion of the chamber, unless the 2" pipe is exceedingly long (>12" or so) the mixing process would not require a substantial amount of extra time to complete. Should one add a mixing fan, there would be practically no difference in the duration of time it takes for the gases to mix.
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Unread postAuthor: cannons-go-boom » Wed Sep 19, 2007 9:39 am

ok thanks, so I should make it :

4" x 10" chamber
1 1/2" x 62" barrel

Or

4" x 15" chamber
2" x 57" barrel


Which barrel whould you guys choose?
I havent done this before so which one fits potatoes best?
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Unread postAuthor: williamfeldmann » Wed Sep 19, 2007 10:17 am

I would choose 1 and a half first, as most potatoes will fit with no problem. Finding two inch potatos can be more difficult.
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Unread postAuthor: cannons-go-boom » Wed Sep 19, 2007 10:26 am

What if I make both Barrels and have the Chamber be Somewhere Inbetween 10" and 15"?

and what size pvc does a tennis-ball need? anyone ever shoot those?
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Unread postAuthor: williamfeldmann » Wed Sep 19, 2007 10:36 am

You could do that, your cannon would just be a little overpowered for the 1 1/2 barrel no big deal.

Tennis balls fit nice and tight in sch 40 2.5 inch pvc. However, 2.5 inch is tough to get. Your best bet is either a plumbing shop or look for it in the electrical section of your lowes or mendards or home depot. I have found sch 40 2.5 inch electrical conduit in Menards and Lowes. You will want to get your fittings at the same place as most places won't carry fittings for sizes of pipe they don't carry.

If you are an internet monkey and like ordering things you can check out McMaster-Carr here http://www.mcmaster.com/.
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Unread postAuthor: cannons-go-boom » Wed Sep 19, 2007 10:53 am

ok, if they have it i will probably get 1.5" and 2.5" and i need to go get the Chamber cut soon so i can take it with me. (which might be today.)
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