Colossal Cannon: Building a Behemoth Piston-valved Pneumatic

So you have a decent sized vacant lot of land (be it farm country, woods, tundra, ice shelf) and you think to yourself, "Hmm...I wonder just how far I can hurl stuff, maybe even into the next county." Or maybe you just like the idea of kinetic energies ten times that of a 50 cal bullet shot out of a sniper rifle. Or maybe you want to get into Punkin' Chunkin'. Well, my friends, you just might want a piston-valve pneumatic cannon.

It'll sure beat the pants out of your neighbor's ball valve pneumatic or hairspray combustion cannon.

Update 2014 - We were young and stupid when my friend and I first built this cannon. We made a lot of mistakes and a lot of safety oversights. I highly suggest reading up on modern potato cannon theory and design before delving into a project of this magnitude. Use your head!

Videos of the recent 2in valve:

Three test shots at 40psi, two with paper towel, the last one with a whiffle ball bat. We were gonna move on to damage shots, but it got dark. I should have damage vids up today or tomorrow.

Videos of the old Mk IV 3in valve:

Preparing for launch, I couldn't find the rocket. So I put in a can of lemonade instead. "There's the rocket!" refers to my sudden realization that the rocket was in the barrel the whole time. Rocket, lemonade, and wadding all shot out over 120 m.

My cannon has a record distance of 1900 feet (using Google Earth along with terrain mapping - it happened to land right between two distinct knolls that I could see from GE). That was a 70 psi shot with a 2.5in x 20ft barrel that was so long it needed it's own suspension system.

I have a video of that shot somewhere but I have it find it I'll post it as soon as I do. That was also the shot that broke the cleanout cap.

During the testing phases, the cannon was barely charged halfway, so I could get away with plugging my ears. I make sure to grab hearing protection for anything 65+ psi.

For these two shots we had an audience.

They were thoroughly impressed:

This shot, something caused the piston not to actuate, and we had a dud. They weren't as impressed :-P:

So over the past two summers my friend and I have been working on my brainchild: a 3" porting piston cannon. Porting is the spudding term for the size at the greatest restriction between the pressure chamber and the barrel. The air can only move as much through the barrel as it does through the porting (usually the valve) so if you want more air to hit the projectile, you need a bigger porting.

Piston valves work on a principle similar to a plug in a drain. Pressure on one side of the piston, causes the other side, or sealing surface, to make a seal against the barrel, the porting. This keeps the rest of the air in the chamber from escaping. The region behind the piston, called the pilot, is dumped to ambient pressure, and the pressure difference from the valve causes the piston to get shot back quite forcefully. The porting is now wide open, and the air from the pressure chamber rushes into the barrel, launching the projectile.

Sounds fairly straightforward, right? We thought so. Many people had already build 2" porting valves with little trouble, but I wanted to go bigger. One would think that you could just change the size of the parts from 2" to 3" and voila, you have a bigger valve. Well, yes and no. Moving up to 3" increases the size of the sealing surface by 50% or more, and achieving the right seal can prove frustrating, difficult, and expensive.

So, to save you from some of my mistakes, I present you with two, count 'em, two different designs. One is the 2" porting, which still had quite a kick, and the 3" porting valve I used on my cannon, for those of you with higher levels of patience.

Mechanics:
1. Under Load: The valve pressurizes from the pilot end, pushing the piston against the sealing surface. The pressure equalizes between the pilot volume and the chamber. Because the surface area on the back side of the piston is larger than the surface area on the opposing size, the force pushing the valve forward will always be greater.

2. The pilot valve is opened. The pressure behind the piston drops quickly, not allowing the chamber side time to equalize. The force pushing the piston forward drops, while the force pushing the piston back remains nearly constant. The two force equal and then...

3. Actuation. The force pushing the piston back exceeds the force pushing it forward, and the valve opens, allowing the air from the chamber to flow into the barrel.

This part of the instructable is called covering my own ass. :-P I take absolutely no responsibility for whatever happens to whoever uses this instructable. None. Zip. Zilch. Nada.

***IMPORTANT NOTE***
I haven't worked on this project, or spudguns in general, for a while now. I am not 100% positive this design is optimized. I highly recommend that you read up on valve design and theory and really understand what you are doing before building a piston valve.
Check out http://www.spudfiles.com for all the information you need.

This project involves large volumes of air at high pressure using PVC pipe. PVC was never meant for compressed air technically, though under most circumstances it does fine. This project also involves PVC under load taking large impact stresses, something it isn't terrifically good at to begin with.

I know some of you reading this have been making spudguns for a while and have been mostly lackadaisical about construction materials, I'm sure you've read the warnings "Don't use XXX pipe!" that it could frag and grenade, and have done it anyway with no consequence. I've made non-pressure rated cannons too, and they've worked just fine. However, this is the big league. This stuff can most definitely injure you. If the instructions say to use a particular type of pipe, the only acceptable substitute would be something even stronger.

My cannon worked fine for a while with no problems, but when I decided to improve the seal, the valve worked too well in essence, opened too fast, and smashed the valve. The first time it just broke the fill system, that was an easy fix. The second time snapped the valve in half. This is serious power, people.

Update 2014 - I've come to the conclusion that there really is no 'safe' way to build a PVC cannon. Even using pressure-rated pipe, PVC is designed to handle mostly static pressure, with some water hammer. The forces and shocks involved in any sort of potato cannon, let alone a piston valve, is above and beyond the safe operating limits of PVC pipe and well into the danger zone. I haven't played with cannons in a long time, but I've adopted the philosophy that PVC cannons made from pipe (DWV or otherwise) that experience shock loads ought to be operated in a manner such that if there is a fragmentation, the operator or bystanders won't be injured. That means remotely fired, behind a sturdy wall, dirt berm, etc. I was young and stupid when my friend and I built the first large cannon. Had that valve fragmented at 100 psi and we were standing in our usual spot, we could have easily been seriously injured.

Solvent Welding - PVC is not glued (even though it's referred to as "cement", it's solvent welded, which is where the parts are dissolved and meld together into one solid piece. A good weld should be stronger than the pipe itself, see my failure pictures. If you don't know how to solvent weld PVC properly, I highly suggest taking a course at Home Depot or other hardware store, they're usually free, it will save you a lot of hassle.

**A quick tip on solvent welding: After covering both sides of the joint with cement, push the parts together while simultaneously turning 90 degrees. This ensures the strongest, most airtight joint.**

I don't care what the can of PVC cement says, I think it's a general rule of thumb to give 24 hours cure time before putting joints under pressure. 2-3 hours is plenty if you just want the part workable, for drilling, cutting, and sanding you can start almost immediately.

I tried to give as close to step by step instructions, but since a large portion of these are after the fact, there are going to be minor details left out. This project is a pretty big undertaking, and I'm assuming you have enough sense to try and bridge minor engineering gaps. You will probably have to interpolate a little bit with sizes, just figure out what works for you.