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2nd Potato Gun - A Few Quick Questions for You Pros :)

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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2nd Potato Gun - A Few Quick Questions for You Pros :)

Unread postAuthor: cameronfield » Tue Dec 04, 2007 11:43 am

So some friends and I want to build an awesome potato gun. I have one that a friend and I built years ago, which works pretty well. Just your basic 3" chamber, 2" barrel, etc. Normal BBQ ignition, no chamber fans or anything.

We have about $50-$75 to sink into this project. I've been looking around, and I'm trying to figure out the best measurements to get all the ratios right. So here are my questions:

1) What's the best chamber diameter to use? 3"? Is it practical to use 4" chamber?

2) What's the best location for the sparker? I've read in the middle or in the end cap, but I'm not sure which is right.

3) If I want the gun to be 5ft long maximum, what should my chamber/barrel length be? 2ft chamber 3 ft barrel? Or 1.5ft chamber 3.5ft barrel?

I'm using aerosol for fuel. I'm planning on adding a chamber fan. Any tips/advice on that?

By the way I'm going to do interchangeable barrels so that I can take it apart to transport it (and so I can have different barrel sizes :o).

Here are the plans I've been looking at so far:

http://potatocannon.nodice.org/cannonV2.html

What do you guys think? You'll know 100x more about this than me so tell me exactly what to do to make that spud fly as far as possible :). Thanks in advance!!
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Unread postAuthor: octane89 » Tue Dec 04, 2007 11:56 am

1. You will want a ratio (chamber to barrel) to be between .8:1 (most efficent least power) to 1.5:1 (most power, least efficent).
2. Towards the back to allow your flame to burn forward.
3. Depends on the ratio.

To get a ratio:
V= 3.14 x r(squared) x length (in inches)

You will do that for the barrel and chamber, and divide the two. You should come out with a number between .8-1.5
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Unread postAuthor: JDP12 » Tue Dec 04, 2007 11:57 am

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1) Doesn't matter, use 4" if you want a shorter chamber
2) Put the spark in the middle or towards the front of the chamber
3) Depends on what barrel diameter you will be using mostly. Calculate the C:B ratios of different barrels and different chamber sizes. .8:1 by the way
4)Chamber fan- click here http://burntlatke.com/fan.html
5) Concerning the plans, meh they're pretty good i guess.
6) If you can go with metered propane
7) Search some more

welcome to spudfiles
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Unread postAuthor: paaiyan » Tue Dec 04, 2007 11:58 am

1) Diameter isn't all that important, volume is.

2) Spark location in an aerosol-powered cannon isn't really going to make much difference. Probably middle though.

3) The length of the barrel and chamber are entirely dependent on the diameter of each. The volume of the chamber and barrel should both be about the same. On my aerosol cannon, the chamber volume is .8 that of the barrel, and I wouldn't change it.

Different barrel diameters will need to be different lengths in order to maintain the proper ratio. Smaller diameter = longer. Also, use SCH 40 pressure rated PVC. Prime and solvent weld all joints, except where you're goign to switch out the barrels. Make sure you can lock the barrels in somehow so they don't fly off.

Chamber fan, well, just mount it in there with epoxy or something. wherever you drill to hole for the wires, make sure you drill through a joint, that way it's double thick and stronger. Seal the hole with epoxy.
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Unread postAuthor: psycix » Tue Dec 04, 2007 11:58 am

1) Smaller diameter pipes are more pressure resistant but since you are using pressure rated pipe this is no problem.
Some people may say the reverse, but I think combustion goes faster in a big diameter pipe rather then a small one (wich has to be longer), because the flamefront has to travel less distance to burn all fuel.

2) I think its a little back of the middle.
A spark at the back creates one flamefront that moves forward.
A spark in the mid creates two flamefronts that move both directions. - faster combustion.
Now it is said that when the spark is at the back it might create a shockwave to the front burning up faster. Some people use to place their sparks in the back because of that theory but I think its the best to place it near the middle of the chamber.

3) Your barrel should be as long as possible for maximum power, and your chamber size should grow along with that.
Its all about C:B ratios. Dont go much higher then a 1:1 and dont drop under 0,5:1
The higher the ratio, the louder the bang. You make your choice and calculate the needed chamber lenght with: V = (pi)*R*R*L
Where V stands for volume and R for radius of the discussed cillinder shape and L for the length.

Hope this helped.
Good luck :)

EDIT:
For a chamber fan I used to drill bolts through the PVC like you I did for the spark gap electrodes. Connect fan wires to the bolts inside, and power source on the outside.
Thats my non-leakable way of bringing in the power for the fan.

EDIT 2:
Corrected the formula.
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Last edited by psycix on Tue Dec 04, 2007 1:22 pm, edited 1 time in total.
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Unread postAuthor: cameronfield » Tue Dec 04, 2007 12:11 pm

Wow such quick responses. So 4" chamber sounds good cause I want a shorter chamber. I'm confused on how to do the calculations. Can you'll help me out? 4" chamber 5ft long max (whole thing). My "default" barrel size will be 2". I'll use 2.5" for tennis balls sometimes.

I read somewhere else that a longer barrel isn't always better because energy is lost. Is this true??

Also using interchangeable barrels can you breach load or will it mess the threads up?

I gotta go to class but I'll be back soon. Thanks again guys!
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Unread postAuthor: JDP12 » Tue Dec 04, 2007 12:15 pm

Longer barrels only lose energy if they are too long to optimize the proper C:B ratio. Take a look here for examples of that. http://burntlatke.com/15cb-testday.html

the barrel was too long, so the combustion energy stopped, and turned back in on itself, creating a vacuum
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Unread postAuthor: octane89 » Tue Dec 04, 2007 12:18 pm

4"ID chamber that is 12in long and 1.5"ID barrel thats 72" long= 1:1 ratio
Same but with a 2" barrel that is 44" long equals 1:1 ratio.

This will help with an idea perhaps too-
http://www.burntlatke.com/ratcalc.html

You wont mess the threads up either, they will eventually just get dirty.
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Unread postAuthor: paaiyan » Tue Dec 04, 2007 12:22 pm

Hey psycix, you gave the formula for area bro.

If you're using 4 inch for the chamber and 2 inch for the barrel, a 1 foot chamber and 4 foot barrel will give you a 1-1 ratio. A 10 inch chamber and 50 inch barrel gives a .8-1 ratio, which is what mine uses. Though I'm not saying mine is perfect. Somewhere around there would be good.

EDIT: Octane, he wanted a 5 foot gun.
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Unread postAuthor: octane89 » Tue Dec 04, 2007 12:24 pm

For some reason I blanked out on that and thought 50inches. Its been a long day.
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Unread postAuthor: paaiyan » Tue Dec 04, 2007 12:28 pm

Cameron, I meant to mention. With a .8-1 ratio, my gun makes a dull "whump" sort of sound when it fires. That tells me that I'm about as efficient as I can be. Louder guns are loud because there is not enough barrel for the combustion to completely take place inside, therefore some happens outside and it's louder. This is less efficient, though some will argue much more fun.
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Unread postAuthor: octane89 » Tue Dec 04, 2007 12:32 pm

I have one thats 1.3-1.4: 1 and it's more of a crowd pleaser. But I can add a longer barrel if need be.
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Unread postAuthor: paaiyan » Tue Dec 04, 2007 12:35 pm

octane89 wrote:I have one thats 1.3-1.4: 1 and it's more of a crowd pleaser. But I can add a longer barrel if need be.


Definitely a crowd pleaser. Nice big bang yea? My friend and I made my aerosol cannon. Loud? No. But it's pretty darn strong for a spray&pray.
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Unread postAuthor: octane89 » Tue Dec 04, 2007 12:38 pm

Yea it is a aeresol cannon. One that I made a while ago, just something to do in the winter. It kicks hard, big muzzle flash, and shoots pretty damn far. I would say around1 150-250yds
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Unread postAuthor: psycix » Tue Dec 04, 2007 1:23 pm

paaiyan wrote:Hey psycix, you gave the formula for area bro.

If you're using 4 inch for the chamber and 2 inch for the barrel, a 1 foot chamber and 4 foot barrel will give you a 1-1 ratio. A 10 inch chamber and 50 inch barrel gives a .8-1 ratio, which is what mine uses. Though I'm not saying mine is perfect. Somewhere around there would be good.

EDIT: Octane, he wanted a 5 foot gun.


Yup youre right.
Small mistake :D
I corrected it now.
Thanks for mentioning :)
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