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Quick Questions

A place to ask general spud cannon related questions.
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Unread postAuthor: xpiC » Wed Jun 18, 2008 11:29 pm

Hello im building my first cannon
I have uploaded a picture of what i hope to build it like.
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MEASUREMENT'S
A) 100mm
B) 50mm

I am pretty sure they are the size's
But i have not cut any of it up i have about 1 meter of both A and B
How big should i make A and B?

Please help i dont want to reck it

Thank you
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Unread postAuthor: DonTheLegend » Wed Jun 18, 2008 11:36 pm

It depends on exactly how big you want it. I would keep B at about 1 meter and cut down A accordingly. A good reference is in the wiki:
http://www.spudfiles.com/spud_wiki/inde ... rrel_ratio
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Unread postAuthor: xpiC » Wed Jun 18, 2008 11:46 pm

Thanks for the fast reply
ill keep B at 1m but im not sure how long A should be?
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Unread postAuthor: Hubb » Wed Jun 18, 2008 11:50 pm

Search for chamber to barrel ratio.
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Unread postAuthor: xpiC » Wed Jun 18, 2008 11:53 pm

I tryed but im still not sure should i do it like half a meter?
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Unread postAuthor: Launch-Tek » Wed Jun 18, 2008 11:54 pm

I put the spark igniter in the cap. It is easy to change the flint and just feels better to me. And when I store it I put the cap in backwards and push the igniter in all the way so the launcher can stand on end. But that is just my opinion. :)
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Unread postAuthor: xpiC » Wed Jun 18, 2008 11:55 pm

yeah can anyone help me how long should A be?
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Unread postAuthor: Hubb » Wed Jun 18, 2008 11:59 pm

I lost my spoon. Sorry.
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Unread postAuthor: xpiC » Thu Jun 19, 2008 12:06 am

haha fair enough i just want to know lol
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Unread postAuthor: DonTheLegend » Thu Jun 19, 2008 12:06 am

Did you look at that page I linked to in the wiki? It tells you how to calculate the C:B ratio and there are graphs that show that an optimal C:B ratio is around .7 So calc the volume of the barrel, then multiply by .7 to get the volume of the chamber. Plug in your pipe diameter and use the equation for volume backwards and that will tell you how long everything should be. Sorry, don't feel like finding a calculator to figure it all out.
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Unread postAuthor: Eddbot » Thu Jun 19, 2008 12:08 am

hubb017 wrote:I lost my spoon. Sorry.


awww, that's kinda mean, but i guess he deserves it...

and putting the ignitor in the cap can make it structurally unsound, put it through a double layer of pipe, like at the couplers, and if you use a bell reducer make sure that it is pressure rated and not DWV
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Unread postAuthor: psycix » Thu Jun 19, 2008 5:11 am

When B is 1 meter, and 50mm, then the volume is:
pi*2,5*2,5*100=1963 cubic cm

The volume of A is: pi*5*5*h
For a 1:1 ratio, h = 1963/(pi*5*5) = 25 cm
For a 0,8:1 ratio, h = (1963*0.8 ) / (pi*25) = 20cm

Using Ecxel, you can easily dish out all numbers for any C:B ratio you want:
Code: Select all
Length-C:B ratio
12,5   0,5
15,0   0,6
17,5   0,7
20,0   0,8
22,5   0,9
25,0   1
27,5   1,1
30,0   1,2
32,5   1,3
35,0   1,4
37,5   1,5
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Unread postAuthor: jimmy101 » Thu Jun 19, 2008 12:25 pm

Eddbot wrote:and putting the ignitor in the cap can make it structurally unsound, put it through a double layer of pipe, like at the couplers, and if you use a bell reducer make sure that it is pressure rated and not DWV

Besides weakening the cleanout plug (which isn't pressure rated to begin with) if you set the gun down on its butt while loaded it may fire.

Put the sparker through one of the fittings (probably the cleanout fitting).

A BBQ piezo is a much better way to go. More expensive (~$14 versus ~$3) but it is much easier to build the BBQ piezo into a handle. The problem with the flint sparker is it takes all of one hand to operate. That leaves just one hand to hold the gun. This gun'll have a significant recoil, best to have a two handed grip on it when fired.

The bell reducer is fine for a standard combustion gun. The cleanout plug isn't pressure rated and is much more likely to fail than the bell reducer. A bushing reducer would be preffered but isn't required for safety.
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Unread postAuthor: DonTheLegend » Thu Jun 19, 2008 4:45 pm

For a basic combustion, I'm more partial to the lantern sparker. Works every time no questions asked. Also, for a basic combustion, the cleanout cap is probably fine. I had one on a propane gun with no problems. Best of luck.
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