# COMMUNITY : FORUMS : OFFBEAT

## Calling all Potato Gun Aficionados and wanna be Combustion Scientists

This is message to anyone who enjoys hearing the sound of forced combustion, or better yet, the loud crack of a potato gun being fired. My co-conspirator and I are in need of some mathematic assistance. Ideally we are looking for a âmagicâ formula concerning barrel length to blast chamber ratio for optimal performance. The ideal fuel would be propane or butane. We have been using a 4â by 33â blast chamber coupled with a 3â by 44â barrel. So far, we have had significant success with this combination, but are curious if there is a better ratio to use. With our current specs, we are able to shoot a 3 â 4 lb. ball of plaster over 250 yards (this is the carry distance, not the total distance traveled) at a 47 degree angle. If anyone out there knows a formula or has advice on Blast Chamber to Barrel ratios we are greatly interested to hear what you have to say.

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Kiteman9 years ago
First point, on a windless day the "ideal" angle is 45 degrees.

Second, why not find out yourselves?

Produce an experimental cannon, specifically for the purposes of calculating the ratio you seek, and devise a way of adding exactly the same amount of fuel every time to fire exactly the same shape and mass projectile every time.

Start with a long barrel (say twice as long as the chamber), and fire ten rounds, recording the range.

Shorten the barrel and try again.

Keep shortening the barrel, recording a table of barrel-length versus range.

Plot the results out on a graph, and voila - the information you need.

Keep a photographic record as well, and you have the makings of both a good Instructable and an eye-catching Science Fair entry.
6 years ago
These things aren't cheap to build, also with still air there is still friction so 45 isn't ideal just on windless days.
6 years ago
(You do know this topic is over three years old, don't you?)

6 years ago
Do now
jj.inc6 years ago
Your gun looks really cool, I saw that you asked about a ignition system and I think you could build one like you wan't really easy, just get a small project box from radio shack and because 9v batteries are smaller just wire 2 or 3 in parralell so you have more room for the coil, and switches, but it will last long enough for good use. You can get cool switches there too.
benihana (author) 9 years ago
Problem solved Volume of Blast Chamber to Barrel Ratio should equal 1.5:1 Fuel volume: Propane is optimal at 4.2% of total Volume for best results. There are lots of handy calculators out there that provide tools for factoring all of this in. The above pictured cannon's ratio is 1.3333:1 Not too bad for a guess. :)
benihana (author) 9 years ago
specs 4 inch diameter by 33 inch chamber 3 inch diameter by 44 inch barrel.
9 years ago
Nice Icon by the way. I own the special dvd.
Lftndbt9 years ago
Check out http://spudfiles.com
The combustion area should give you, all the information you need..
:)