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 “magic” formula concerning barrel length to blast chamber ratio for optimal performance. The ideal fuel would be propane or butane. We have been using a 4” by 33” blast chamber coupled with a 3” by 44” 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 – 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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Kiteman10 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.
jj.inc Kiteman6 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.
Kiteman jj.inc6 years ago
(You do know this topic is over three years old, don't you?)

jj.inc Kiteman6 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) 10 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) 10 years ago
specs 4 inch diameter by 33 inch chamber 3 inch diameter by 44 inch barrel.
Nice Icon by the way. I own the special dvd.
Lftndbt10 years ago
Check out http://spudfiles.com
The combustion area should give you, all the information you need..
:)
bigbadbrad10 years ago
here are more photos...
gun2.jpgsparkplug_ground_wire_to_co.jpgwad1.jpgwad2.jpg
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