SpudFiles

Ok, I built a potato cannon before, but I want to build another and I need your help. I have a few questions about the materials to use and dimensions of the gun. I built my last gun and I had no idea there was a ratio involved with the combustion chamber and barrel. The new gun I want to build, I want a 4" CC and a 1.5" barrel. It will be combustion powered and I would like it to shoot the potato at least 700 feet. With regard to the C:B ratio, what would the ideal lengths of the PVC CC and barrel be? Also with regard to the wiring, which is the best gauge to use with a BBQ igniter? And last but not least, are machine screws better or wood screws for the inside of the combustion chamber? Thanks!

1) The "ideal" ratio varies depending on the fuel you're using. Assuming you go with propane, as most of us do, the "ideal" ratio is .8:1 chamber:barrel. However, to a degree, a higher c:b ratio won't negatively impact performance (out to a certain point anyway), it will just increase noise and waste a bit of gas for a negligible performance. I'd go for 1:1 personally.

2) To lob a spud 700 feet, you need some serious power. That's around 300 yards, and a potato isn't a very aerodynamic projectile. A golf ball is another story.
If you insist on lobbing a spud 700 feet, you're going to need a pretty good size gun. To keep it compact, you'll want to look at an over/under configuration.
A stick of 1.5" pipe 8 feet long yields a volume of 169 cubic inches. For a 1:1 C:B ratio, you'd need around 14" of 4" pipe.
To reach 700 feet with a spud, you'll definitely need a metered propane setup (to ensure a perfect 4% propane to air mixture) and a chamber fan (to mix the propane and air together, accelerate the flame front during combustion, and also vent the chamber between shots).

3) Thick wire is always preferred for any high-voltage system. I use old spark plug wire. Lamp cord will work. Anything well-insulated, really.

4) Don't use screws in the combustion chamber if you don't have to. Any hole you must put in the chamber should be drilled and tapped where a pipe and fitting overlap and there are two layers of pipe.
To attach something to the inside of the chamber, use epoxy instead. Check out my Plans for Advanced Combustion in the How-To section.

Good luck.

Peace,
Pete Zaria.

Maybe Pete misunderstood, but you can use smaller wires for a piezo (BBQ igniter). On my combustion, I purchased a piezo that came with wires and used that. It ran about $10 for the whole kit.

Also, I keep my fan running during the firing process. It helps a little bit. I only have one spark, but it is a smaller combustion. You may want to consider a spark strip configuration.

As far as the interior of the chamber, here's how I set mine up. I cut a piece of pipe about 1/2" thick, then sliced it in the center. I drilled only one hole through the chamber walls to run ALL the wires in (including the fan wires). From there, I epoxied the little strip into the chamber, then added the fan.

Something you may want to look at, as well, is the Burnt Latke Site. They have a lot of helpful information for such a project. Pete's how-to is pretty good, too.

Oh, and good luck with the 700 foot spud toss.

Thanks Pete. I guess maybe shooting for 700 feet was a bit too much. I don't want the gun to be very large. How do you do the math to figure out the ratio? I don't want to sound stupid, but how did you do the math to get the volume of the barrel? Maybe if I knew how to do the math for the ratio, I could design a gun thats smaller, but still pretty powerful.

Mattmp36 wrote:how did you do the math to get the volume of the barrel?

volume of barrel = half width of barrel x half with of barrel x 3.142 x length of barrel

If you use inches for all dimensions, the result will be the volume of the barrel in cubic inches.

Obtain the volume for both, then divide the chamber volume by the barrel volume. Some say .8:1, others say 1:1 (C:B). Use whichever.

To obtain the volume, there are numerous calculators to do so, but it's easier to use water. Fill both up, then measure the water.

Thanks everyone! This all makes sense to me now!

Pete Zaria wrote:3) Thick wire is always preferred for any high-voltage system. I use old spark plug wire. Lamp cord will work. Anything well-insulated, really.

This is not only not true but contradictory. A high voltage low current source works fine with small gauge wire. The resistance and current carrying capability of the wire is basically irrelevant in this application. 12G wire, 20G wire, 30G wire will all work fine. You probably do not want to use spark plug wire, the resistance is a couple orders of magnitude higher than standard copper wire. The only good thing about spark plug wire is that is has insulation that can handle the high voltage. Just use whatever wire you have available and keep the wires well separated from each other (and you). If you use "appliance zip-cord" (lamp cord) you might want to separate the two conductors since the insulation won't handle more than a several hundred volts.

Pete Zaria wrote:4) Don't use screws in the combustion chamber if you don't have to. Any hole you must put in the chamber should be drilled and tapped where a pipe and fitting overlap and there are two layers of pipe.
To attach something to the inside of the chamber, use epoxy instead. Check out my Plans for Advanced Combustion in the How-To section.

A 1x combustion chamber made from pressure rated pipe will have no problems with small (~1/8") holes drilled through a single layer of pipe. Put your spark gaps wherever you think best. I prefer the center of the chamber but the optimal location for the spark gap is somewhat controversial.

Mattmp36: For screws, it is best to use ones with fine threads. Coarse thread wood screws won't have enough thread to pipe contact for a good seal. Fine thread machine screws work well for mounting things like a fan. I prefer fine thread drywall screws for the spark gaps, the sharp tips spark better than do the blunt tips of machine screws.

wait wait, if you want to achieve that range why not build a pneumatic? say burst disk?

Yes a nice big pnuematic with a bust disk will easially acheive that range.
But if you really want to use a combustion I think some 6in diameter chamber and a really long barrel is in order.

I think for the effort put in for a metered propane advanced combustion it would be better to just build a nice pneumatic. Burst disks are probably your only hope of getting a potato that far, because they are still fairly cheep with a wide bore. Or build your own mauler(pison valve with 2 (3?) inch porting)

Skip the spuds and go right for the golf balls.
Pete Z started to talk about GB's, they fly much better spuds.
Almost double the distance, depends on the launcher.
Winter in NE good time to check out the golf courses for ammo.

edit: Start looking for SDR-21 PVC 1 1/2" Or other thin wall PVC

If you're more concerned about the potato going the distance, i suggest ( like elitesniper, bigbob12345 and pizlo mentioned) making a pneumatic cannon. They are somewhat safer (when using pressure rated parts) and have a higher performance rating. Also, the distance/power of your projectile is in your control (not saying it's not with combustions, its just a lot harder). And you could make yourself a huge ass piston valve (2"+ port) for maximum performance (if you wanna stay away from burst disks).

Anyway, the choice is yours, Good Luck!
--sthorne