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Calling all Potato Gun Aficionados and wanna be Combustion Scientists Answered

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.

Discussions

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Kiteman

10 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.

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jj.incKiteman

Reply 7 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.

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Kitemanjj.inc

Reply 7 years ago

(You do know this topic is over three years old, don't you?)

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jj.inc

7 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.

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benihana

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. :)

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benihana

10 years ago

specs 4 inch diameter by 33 inch chamber 3 inch diameter by 44 inch barrel.

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bigbadbrad

10 years ago

here are more photos...

gun2.jpgsparkplug_ground_wire_to_co.jpgwad1.jpgwad2.jpg
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bigbadbrad

10 years ago

After a lengthy discussion this weekend with Benihana and some others, We have decided to take another look at the potato gun. While it works well, I am convinced that we can do better. The two main problems, as I see them, are the refire rate, and the ignition system. As it stands it takes about 5 min before we can fire another round. After some discussion with Ben and a few others we think we can resolve this problem by using a cleaner fuel. We are going to go with a Butane/Oxygen fuel instead of the Propane/Oil/Perfume/Oxygen fuel. We think the reason the point on the spark plug will not refire immediately is because of the oil and perfume residue left behind. The blast chamber has to be aired out to let the junk left behind dissipate. We figure we can get a better refire rate if we don't fill the blast chamber with all that extra crap. So that'll be the first step. Getting the thing to light faster and hopefully with a better combustion rate than previously. The other problem that we have is that the blast chamber might be too large but I am hoping that someone can help us with that. Step two and this is where I need some mechanical help is the ignition. The 6v battery to the ignition coil to the spark plug works great. But with the added danger of the pure fuel and a potentially larger combustion, I would like to institute a few changes to the firing mechanism. First I will be building a box to hold the ignition coil so it is farther away from the cannon. And inside the box will be the power source and all the electrical wiring. On the top of the box will be the first of the safety mechanisms. An Ignition Switch Panel Push Button Starter w/Flip Cover. This will allow us to have an extra level of safety when working with the butane. But here in lies the problem. The only way to get multiple sparks to the sparkplug right now is to tap the wire on the battery terminal repeatidly. Since we will be using the push button start, how do you hold down the start button and have it continuously pulse power to the plug? In a car the distributor, run by the timing belt would move over the points so they fired continuously. How do I replicate that? So to sum it up we need to know two things: The optimal length for a 4 inch diameter blast chamber to propel a 3 inch diameter 1.5lb cannon ball about 500 yards using a 4 foot long 3 inch diameter barrel. I do not even know where to begin with that math… any help you could provide would be fantastic. How to get the spark plug to continuously spark while holding down the start button.

gun2.jpgswitch.JPG
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benihana

10 years ago

We found the cannon was more stable at that angle. That is, for the most part, what we did. We were just curious what the Mathletes had to say. You know, searching for that magic number without spending a ton of money on PVC. Trust me, we are all about running the experiments, this thing is a blast to play around with (no pun intended). As for the fuel, we used a count system and an aerosol can. 1 second versus 2 seconds and so on. This varied depending on temperature, and time between firings. The more we fired it, the longer we had to wait in between rounds. As for our projectile. We used the same size and shape to within a 3% difference.