pneumatic potato gun i made

[vivek16]

hey guys. ive been working on a pneumatic potato gun all summer and i tested it for the first time today and it didnt work

i was hoping you could give me some tips on how to make it better.

well i used the kind of minimatic cylinders that were in the kit this year.

i pumped up one of them to 90 psi and attached it through a valve to the end of a 2 foot long peice of 2" diameter PVC pipe. i cut a potato to a bit smaller than the diameter and put it in the pipe. but when i release the valve i hear air coming out but the potato just sits there.

im not sure what i can do besides get more air tanks and put them on a manifold. i am going to try to get a part so i can put a second tank on there.

but does anyone have any tips on how to make it work better?

thanks, vivek

[Pat McCarthy]

I built an air powered potato gun last fall.
From experience, I've found the best way to fit a potato to the barrel.

• Cut a potato in half down the center, into two bullet shaped halfs.
• Place one half, cut side down, on the ground and force the PVC pipe over the half to cut the potato to the perfect fit to your barrel.
• If you need to, take a stick and push out the shaped half, and reinsert it into the barrel so that the slightly rounded end is pointing at what you want to shoot at. (This makes for a more stable shot)

Try this, if this doesn't work, some pictures of your rig would be helpful for more advice.

[vivek16]

thanks, ill try that and get back to you with some pictures.

-vivek

[DonRotolo]

Exactly: A tight seal is a requirement.

Also, how much air are you pushing? It needs to be about 4 times the volume (at least) of the PVC tube.

Lastly, with 90 PSI, consider that the shock loads may cause the pipe to shatter. Two ways to make it safe, first encase the bottom 2/3 of the pipe in a larger pipe (no matter the size as long as it fits over the inner pipe), so the larger pipe will catch any shrapnel, OR second wrap pliable tape (like electrical, duct, or fiberglass packing tape) in a 1/4" to 3/8" layer around the lower 1/2 of the pipe, that'll slow down the shrapnel to nonlethal speeds.

[vivek16]

Originally Posted by Don Rotolo

Exactly: A tight seal is a requirement.

Also, how much air are you pushing? It needs to be about 4 times the volume (at least) of the PVC tube.

Lastly, with 90 PSI, consider that the shock loads may cause the pipe to shatter. Two ways to make it safe, first encase the bottom 2/3 of the pipe in a larger pipe (no matter the size as long as it fits over the inner pipe), so the larger pipe will catch any shrapnel, OR second wrap pliable tape (like electrical, duct, or fiberglass packing tape) in a 1/4" to 3/8" layer around the lower 1/2 of the pipe, that'll slow down the shrapnel to nonlethal speeds.

well the volume of the barrel is about 80 in" squared. and there is definetely no chance of exploding pvc. no pvc is ever pressurized in my setup.

here is a sketch.


sorry about the size but there are 2 metal tanks that go through a valve and then to the barrel.


single tank with pressure gauge

the whole setup as it it now but that lone peice of pvc near the bottom is the tank inside its pvc casing through the valve to the barrel.

the inside of the barrel and the bulkhead where the air comes out of (yes the barrel screws off makes it so much easier to carry around and put in more potato pellets)

-vivek

[DonRotolo]

OK, so you're using a piston to push air from the big PVC pipe into the smaller PVC pipe. I agree there is little safety to be concerned about with this scheme, but think about what is pushing the potato.

One clippard tank isn't enough, you don't have enough volume to move the piston fast enough. Try using as many as you can get, but plumbed in parallel, not in series, such that the greatest volume of air can move. Here pressure is secondary to volume.

If this isn't clear, I can explain further.

Don

[vivek16]

Originally Posted by Don Rotolo

OK, so you're using a piston to push air from the big PVC pipe into the smaller PVC pipe. I agree there is little safety to be concerned about with this scheme, but think about what is pushing the potato.

One clippard tank isn't enough, you don't have enough volume to move the piston fast enough. Try using as many as you can get, but plumbed in parallel, not in series, such that the greatest volume of air can move. Here pressure is secondary to volume.

If this isn't clear, I can explain further.

Don

no, i am NOT using a piston. i hooked up the tank to a valve which sends all the air to the barrel through a push in connector.

ok, i will try to get some more parts to get the second tank to be in parallel with the first one. i think i know how i can do this.

[DonRotolo]

OK. Part of the problem is I can't see the sketch too well.

The air that enters the barrel is pressurized. That is the pressure I was writing about with safety and shock loads. As you continue, please consider what I wrote. PVC pipe might be rated for 150 PSI, but that's not a shock load - it might only withstand 1/2 that.....

To launch a projectile, you need to get a lot of pressurized air behind it. If the barrel is 80 cubic inches, and you have 8 cubic inches of air (about what a clippard tank holds) at 90 PSI, releasing it into the barrel changes the pressure to 9 PSI. Do you see how that works? (The calculations are actually more complex, really resulting in less PSI, but the principle is the same).

Now, if you release 80 cubic inches of 90 PSI air into the barrel, you end up with a total of 160 cubic inches of 45 PSI air. Start with 800 cubic inches of 90 PSI air and you get 880 cubic inches of about 80 PSI air... See the way this is going?

OK, now the other half of this is getting the air into the barrrel quickly. If you use a 1/8" pipe to move the air into the barrel, it'll take 3 or 4 seconds to let the air in, and that's much too slow. Use 3/4" copper plumbing pipe, and the difference is dramatic.

OK, that's enough for you to go on, let us know how far your half of a potato goes...

Don

[vivek16]

the pvc i have is rated for 280 psi so i think ill be fine there. im using 1/4" tubing. i think ill cut the barrel down to 2/3rd size

-vivek

[Jay H 237]

PVC isn't rated for air though. Due to it's properties it can shatter into sharpnel and the reason OSHA won't allow it for air lines in shops. If the potato gets stuck or something else happens the pipe can explode like a hand grenade. Just something we're trying to make you aware of.


1/4" ID tubing will really restrict the volume, and more volume is what Don was hinting at for you. If you don't have anything bigger than 1/4" (I imagine you are using the same SMC polyurethane tubing we got in the KOP) and you don't want to buy anything bigger then try adding several feeds of the 1/4" tubing to your gun. This will help the tanks discharge faster which equals greater volume.

[artdutra04]

After I was bored at a summer robotics meeting last year, I tried to make a pneumatic cannon out of only KoP items from this and past years. Even with 3/8" tubing, the volume of air rushing through the hose was barely enough to launch the plastic lid from a spray paint can about 20 feet. You need at least 3/4" ID pipe/tubing to make it work well.

[vivek16]

1) i am not using pvc to hold the pressurized air, i am using the type of air tanks that we got in the kit this year.

and i think that i might perhaps cut the "barrel" part of the gun down because one tank full of air causes the potato to go 2/3rds of the way through the barrel so i think i will add the second tank and maybe it will work then.

also, the tip on how to make an air tight seal really helped, thanks.

-vivek

[vivek16]

well i made them into a paralell connection but the brass fittings in between them are rather small so when the air comes out it sort of flows slowly. I thing that i might try to launch a potato but i was too tired yesterday.

-vivek

[Schnabel]

One precaution I will give you is that we have been building many air cannons over the years and every time we used pvc because of the cost for supplies and every time we got screwed over. Basically you have to treat it like glass because it will crack slowly with the pressure built up inside and will being put down to the point that it can't be used anymore. You may want to consider using something else.

Originally Posted by Tytus Gerrish

90psi is nothing for a hand held gun i typically use co2 at about 850Psi. even with compressed air i go up around 250psi.

Just remember, the TechnoKats built a cannon this year and we can shoot into the crowd ~70+ ft. away at 50 psi.

[vivek16]

ok, I tried hot gluing a straw to the other end of the valve and i can shoot mini pellets like 50 feet now.

the pellets are about the size of mechanical pencil erasers. I think i will get a smaller tube ~ .5 in. in diameter and launch baby carrots