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Step 2Planning
General Theory: For those of you who remember shooting spitballs out a straw, this pneumatic cannon operates on essentially the same principles. High pressure air in a tube will force any obstructions out the end at high velocities. The larger diameter pipe, here I've used 3" PVC, is used as a pressure chamber, holding compressed air. The valve holds this air in until manually or electrically opened. The pressure gauge is used to ensure you don't exceed the safe pressure limit of 100 psi. The tire valve stem is used to fill the pressure chamber. All details will be explained in more detail further down this page.
As is customary for my projects, I've included a diagram of the project. Hopefully it helps you to understand what I've done. Here, I've included both the 1.5" and 0.5" barrels. They are interchangeable, by unscrewing them from the valve, and attaching another barrel. It is important to note that while I have placed specific couplings and bushings in the diagram, the important thing is to find what works for you. If you can find an easier/cheaper way to convert a 1.5" pipe to 1" male threads, by all means go for it. I built with what I had, or could get from the local home improvement store. Some home improvement stores won't carry all the pieces you need, so you might have to search elsewhere. I personally have had good experiences with http://www.plumbingsupply.com for finding exotic parts.
Safety Alert!!! An astute reader ( bigbob12345 ) pointed out that most bell-reducer couplers are NOT PRESSURE RATED. Please go read this page on the spudfile wiki. It is suggested that instead of using the bell reducer, that you simply use a series of regular pressure-rated bushings to connect the 3" pipe to the 1" pipe.
A note about length of pipes: The lengths of the main pressure chamber and barrels are up to you. The first cannon I ever built I used 4' pipes for both, which worked quite well, but was a bit heavy and a bit long. That cannon was broken years ago, so when I rebuilt this new cannon (that I am documenting here), I've chosen to use shorter pipes. I used a 2' pressure chamber, and cut the barrels to be about 4" longer than the end of the pressure chamber, when both are connected to the valve.
About the Valve: The brand of in-ground sprinkler system at my local hardware store was Orbit. The specific valve I used is an Orbit WaterMaster 1" Automatic Anti-Siphon Valve, with standard 1" threaded fittings, 24 Volt solenoid, and 125 psi maximum pressure. Product Webpage The anti-siphon aspect doesn't matter to me, but I happen to like the U-shape of the valve, which allows for a compact, easy to carry/hold/shoot cannon.
About the tire valve stem: I picked up a two-pack of replacement tire valve stems from the local auto parts store. Be sure to get generic valve stems, not anything specific to a particular brand of tire. You want to be able to drill a hole in the endcap, slide the valve stem through, and tighten it down airtight. The particular tire valve stems I purchased had rubber gaskets on the inside, which really help make a good seal. Another advantage to the standard tire valve stem, is that nearly every air compressor has an attachment to fill tires, so you shouldn't need any additional hardware for your compressor.
About the pipe: The pipe should be regular Schedule 40. My 3" pipe is rated for 260 PSI, the 1.5" pipe is rated for 330 PSI, and the 0.5" is rated for 600 PSI. The valve can only handle up to 125 PSI, and since I want at least a small safety margin, I'm only going to run this cannon at up to 100 PSI. This gives me a comfortable safety rating on all the PVC involved, and I have never had one of these valves fail on me, regardless of pressure.
The first image is my plan for the cannon. I hope you can understand what i'm trying to indicate. The drawings took me way too much time.
The second image is a picture of the sprinkler valve's box.
The third image is a blown-apart view of the actual fittings for the pressure chamber side of the cannon.
The fourth image is a blown-apart view of the adapters for the 1.5" barrel.
As is customary for my projects, I've included a diagram of the project. Hopefully it helps you to understand what I've done. Here, I've included both the 1.5" and 0.5" barrels. They are interchangeable, by unscrewing them from the valve, and attaching another barrel. It is important to note that while I have placed specific couplings and bushings in the diagram, the important thing is to find what works for you. If you can find an easier/cheaper way to convert a 1.5" pipe to 1" male threads, by all means go for it. I built with what I had, or could get from the local home improvement store. Some home improvement stores won't carry all the pieces you need, so you might have to search elsewhere. I personally have had good experiences with http://www.plumbingsupply.com for finding exotic parts.
Safety Alert!!! An astute reader ( bigbob12345 ) pointed out that most bell-reducer couplers are NOT PRESSURE RATED. Please go read this page on the spudfile wiki. It is suggested that instead of using the bell reducer, that you simply use a series of regular pressure-rated bushings to connect the 3" pipe to the 1" pipe.
A note about length of pipes: The lengths of the main pressure chamber and barrels are up to you. The first cannon I ever built I used 4' pipes for both, which worked quite well, but was a bit heavy and a bit long. That cannon was broken years ago, so when I rebuilt this new cannon (that I am documenting here), I've chosen to use shorter pipes. I used a 2' pressure chamber, and cut the barrels to be about 4" longer than the end of the pressure chamber, when both are connected to the valve.
About the Valve: The brand of in-ground sprinkler system at my local hardware store was Orbit. The specific valve I used is an Orbit WaterMaster 1" Automatic Anti-Siphon Valve, with standard 1" threaded fittings, 24 Volt solenoid, and 125 psi maximum pressure. Product Webpage The anti-siphon aspect doesn't matter to me, but I happen to like the U-shape of the valve, which allows for a compact, easy to carry/hold/shoot cannon.
About the tire valve stem: I picked up a two-pack of replacement tire valve stems from the local auto parts store. Be sure to get generic valve stems, not anything specific to a particular brand of tire. You want to be able to drill a hole in the endcap, slide the valve stem through, and tighten it down airtight. The particular tire valve stems I purchased had rubber gaskets on the inside, which really help make a good seal. Another advantage to the standard tire valve stem, is that nearly every air compressor has an attachment to fill tires, so you shouldn't need any additional hardware for your compressor.
About the pipe: The pipe should be regular Schedule 40. My 3" pipe is rated for 260 PSI, the 1.5" pipe is rated for 330 PSI, and the 0.5" is rated for 600 PSI. The valve can only handle up to 125 PSI, and since I want at least a small safety margin, I'm only going to run this cannon at up to 100 PSI. This gives me a comfortable safety rating on all the PVC involved, and I have never had one of these valves fail on me, regardless of pressure.
The first image is my plan for the cannon. I hope you can understand what i'm trying to indicate. The drawings took me way too much time.
The second image is a picture of the sprinkler valve's box.
The third image is a blown-apart view of the actual fittings for the pressure chamber side of the cannon.
The fourth image is a blown-apart view of the adapters for the 1.5" barrel.
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Nov 5, 2008. 10:03 AMapothocry of hypocrysy
says:
sick knife in the corner of the second picture step 1... I have the same one.
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Jun 9, 2006. 5:50 PMbunky11
says:
Hey, This is EXTREMELY COOL. I have made lots of bizzare stuff in the past. A Spudzooka type project is what I am currently interested in. Seeing your cannon has got me seriously interested. Now, which is more powerful your design with air or a combustion conventional type of spud gun if both are of about the same physical size? How far would you expect a 100 psi charge to fire say a sock projectile? Thanks, Bunky.
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6
Nov 7, 2006. 11:11 PM
matthewbeckler (author)
says:
matthewbeckler (author)
says:
Hey, In town, where my house is located, in a subdivision type thing, I usually only fill up to 40-60 psi. Using the rolled up and zip-tied wet sock projectile, I'd estimate that at 100 psi, it'd go a pretty long ways, probably at least a standard city block. As to the never-ending combustion vs. compression debate, I personally prefer pneumatics for the following reasons: 1. Safety: I know exactly how much pressure is in the system, and I don't have to worry about timing sprays of fuel. All I have to look at is the nice pressure gauge. 2. Consistency: Again, for a given pressure, I get pretty much the same range and target. 3. Power: Related to consistency, I have friends who swear they can get more range out of their combustion cannons, but it seems to take a lot of trials, and requires a lot of luck as to the amount of fuel as well as the mixing of the fuel and air in the combustion chamber. Basically, if the maximum power of a combustion cannon on a good day, with lots of luck, is X, then I'm more than happy to consistently get 90% of X every time.
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Mar 18, 2006. 11:21 PMMoogle
says:
It might be a better idea to put the fill valve on the side of the endcap where it overlaps, so you're going through a double wall. It should be a lot stronger there.
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