A potato cannon (also known as a spud gun) is a pipe-based cannon which uses air pressure (pneumatic), or combustion of a gaseous fuel, to launch projectiles at high speeds. They are built to fire chunks of potato, as a hobby, or to fire other sorts of projectiles, for practical use. The projectile can be dangerous and result in life-threatening injuries, including cranial fractures, if a person is hit. (http://en.wikipedia.org/wiki/Spud_gun)
This instructable shows the steps I took in order to construct my potato launcher.
This project was inspired by the Mythbusters soda can launcher featured on their commercial. Unfortunately theirs was big and bulky I wanted something that closer resembled a rocket launcher, which is how I arrived at my Pneumatic potato cannon.
Disclaimer: Read this instructable at your own risk, under no circumstances should anyone attempt to recreate this extremely dangerous project. I accept no responsibility for any injuries or damage that may occur. DO NOT TRY THIS AT HOME. :)
Step 1: The design
- Combustion launchers generally use the explosion force of a combustive fuel to propel a potato at very high speeds.
- Pneumatic launchers are more complex and use the force of compressed air released into the barrel to shoot a projectile.
I decided to go with a Pneumatic launcher because it seemed like it would be safer and more reliable. It turned out to be very reliable but more dangerous than I had expected... think "vaporized potato at point blank on concrete." I also needed a reason to buy an air compressor but don't tell anyone.
To avoid ending up with just another Pneumatic launcher with a separate tank and barrel with a valve between them, I decided to put the barrel inside of the tank. This would also make the entire launcher self contained and possibly more durable, as expected it also makes this launcher really frustrating to put together. When its finished the tank will be the pressure bearing area between the body and the barrel.
If your looking at the diagram, each piece will show a caption if you hover over it with your mouse.
Step 2: Tools
- Drill press and/or hand drill
- Heat gun
- Saw: can be a compound miter saw, jigsaw or just a hand saw (safer)
- Adjustable wrenches
- 3/4" Fornester bit
- Fly cutter
- 1/8" NPT tap with matching drill bit
- Rubber mallet
- Sand paper and/or sander
Step 3: Parts
I estimate the total is somewhere around $150
- 1 - Rainbird sprinkler valve
- ~3FT - 4" diameter SCH 80 PVC pipe
- ~2FT - 2" diameter SCH 80 PVC pipe
- ~2FT - 1" diameter SCH 80 PVC pipe
- 1 - 4" female SLP to female SLP PVC coupler
- 1 - 2" female SLP to female SLP PVC coupler
- 1 - 4" male SLP to 2" female SLP PVC reducer
- 1 - 2" male SLP to 1" female NPT PVC reducer
- 1 - 4" SLP PVC end cap
- 2 - 1" male NPT to 1" female SLP adapter
- 2 - 1/8" male NPT to 1/8 polypropylene hose right angle adapter
- 1 - 1/4" male NPT to 1/8" female NPT right angle adapter
- 1 - 1/8" female NPT "tee" adapter
- 1 - 1/8" male NPT to 1/8" male NPT adapter
- 1 - 1/8" male NPT to Schrader valve adapter
- ~2.5FT of 1/8" polypropylene hose
- PVC primer and cement good for at least 6" PVC
- 1- 0-60 PSI gauge with 1/8" NPT fitting
- 1 - "air blow off trigger valve"
You will also need some sand, duct tape/masking tape, marker, Teflon tape
Step 4: Cut some PVC
- The larger the PVC the more difficult it is to cut, a straight cut through a 4" PVC pipe is almost impossible without big enough miter or table saw. To compensate, I decided to take a sheet of newspaper and wrap it around the pipe. Then I drew a line at the edge of the paper to make a perfectly straight guide.
- When I started cutting, it seemed like it would be easier using a hand saw to cut about 1/8 of an inch through the PVC all the way around and then go back around and finish the cut.
- I then finished the edges with some fine sandpaper, its optional but it made me feel better about using a hand saw.
- The 4" PVC needs to be cut to about a 32" length
- The 2" barrel needs to be about 18 - 20" long
- I also needed a 6" piece of 1" PVC to be curved (make more than one)
Step 5: Pneumatic sprinkler valve
- First you need to find a sprinkler valve that will fit your needs, the one I decided to purchase was a Rainbird Model #:CP-100. It has 1" threaded female ports on each end.
- Remove the black solenoid with the wires from the top of the valve, you wont need it for this project.
- Disassemble the valve by removing the six screws on top. Pay close attention to how it is put together.
- The holes for the solenoid need to be plugged, I filled the entire solenoid cup with a two part epoxy. Let it cure before continuing, its not easy to clean up.
- The hole in the very center of the top of the valve (where the manual control is) needs to be drilled and tapped with a 1/8" NPT tap.
- You will need one of the right angle 1/8" male NPT to 1/8" polyethylene hose adapters to screw into the hole that has been tapped. (this is the firing control hose) Don't forget the Teflon tape.
- Before re-assembling your valve make sure there are no small shards of plastic to interfere with the operation of the valve.
- Re-assemble your valve exactly how it came apart.
- I added some screen to the intake of the valve to prevent any pieces that may be left inside the launcher from being pulled in.
Step 6: Bent PVC
My original design called for two 45 degree elbows to move the large valve into the center of the body, half way through rough assembly I realized it would bring the valve too far down. My only option was to use a heat gun and bend a piece of PVC. This instructable helped - http://www.instructables.com/id/Bend-PVC-Pipe/
The goal here is to only bent the PVC enough to move the valve about an inch lower, it will take a couple of tries.
- Fill one of the 1" X 6" PVC pipes with sand, pack it down and cap both ends with some sort of tape.
- On a heat resistant surface mark two parallel lines about an inch apart with a marker or tape.
- With your heat gun, slowly roll and heat the PVC evenly in the center 3" of the pipe. It will take 7 - 10 minutes to do correctly because the sand also has to warm up. Be careful not to use too much heat or it will burn the PVC.
- When you notice the center starting to twist while rolling back and forth, quickly turn off the heat gun and place it out of the way. With heat resistant gloves grab one end with each hand and slide each to its own line. Hold the un-heated ends parallel with their respective until the center cools.
- Trim just enough of the excess pipe on each end so that the 1" male NPT to 1" female SLP adapters may slide on without being obstructed by the bend.
Step 7: The spacer
This step is probably optional but it may add some durability to the launcher. Once the launcher is assembled the only thing that supports the parts inside the body is the 4" to 2" reducer bushing at the very front of the launcher. If dropped without a spacer the valve may smack against the inside of the body, possibly damaging the launcher.
I cut a spacer out of Lexan plastic, it fits over the 2" male SLP to 1" female SLP reducer and ends up being cemented between it and the 2" female SLP to 2 " female SLP coupler. The drawing is a representation of what my spacer looked like.
- Using a fly cutter, cut a disk that just fits inside of the 4" piece of PVC.
- Next do the same for the center hole, make it just large enough to fit over the 2" male SLP to 1" female SLP reducer.
- I decided to use a 3/4" forester bit to make holes around the edge for air to pass by. The holes don't need to overlap the edge or be very large but they should allow a large volume of air to pass through. (the hose must fit through also)
Step 8: Rough assembly
Without using cement, put the launcher together and check some things.
- Does the bent PVC pipe bring the sprinkler valve to where you want it to be? (See picture)
- Do you understand how the 4" male SLP to 2" female SLP PVC reducer is going to be used and its orientation? (opposite from the way it was intended by the manufacturer)
- Is the interior assembly going to fit inside of the 4" diameter PVC shell or is it too long. If it is cut the 2" diameter barrel to a shorter length?
- Will the spacer easily slide down the interior of the 4" diameter PVC shell or does the spacer need to be sanded down to a smaller size?
- When there is something threaded into the 4" diameter shell, will the spacer be able to slide past it?
- Is there a way for the hose to be installed when things are being put together?
Step 9: Start assembling
- Prime and cement the bent PVC and the two 1" male NPT to 1" female SLP adapters.
- Prime and cement the 2" female SLP to female SLP PVC coupler, the spacer and the 2" male SLP to 1" female NPT PVC reducer.
- Prime and cement the 4" male SLP to 2" female SLP PVC reducer and the 4" female SLP to female SLP PVC coupler(In backwards). Be sure to use a lot of cement, the reducer is not intended to be used this way. You should also do it on the ground on some newspaper, in case you have to stand on it to make it fit.
- Prime and cement the 4" male SLP to 2" female SLP PVC reducer, the 2" diameter barrel and the assembley from step 2.
Step 10: Drilling and tapping
- Mark where the holes will be drilled with a pencil or marker, make sure when parts are installed in these holes they wont obstruct the caps fitting on.
- Using whatever size drill bit is listed on your 1/8" NPT tap, drill straight through the wall of the 4" diameter PVC in moth marked locations.
- Tap the newly made holes with the 1/8" NPT tap, I used a vertical holder that keeps the tap straight as you cut the threads. Alternately you could use a drill press and spin the chuck manually or do it completely by hand.
- Because the PVC and fittings I purchased, I needed to thin thin the wall in order for the fittings to fit together snugly. (see diagram, left side) I used a Fornester bit in a drill press and drilled straight down into the holes that have been tapped to make the wall around the holes thinner. (see picture)
Step 11: Final assembly
- Its probably best to thread the last right angle 1/8" male NPT to 1/8 polypropylene hose adapter through the drilled and tapped hole at the front of the launcher, it must be inserted from the inside of the PVC pipe. Don't forget the Teflon tape.
- Once the right angle 1/8" male NPT to 1/8 hose adapter has been installed, take a more than sufficient length (2.5FT) of polypropylene hose and attach it to the adapter.
- Take the momentary trigger valve and install it onto the 1/4" male NPT to 1/8" female NPT right angle adapter (Teflon tape), then screw the pair onto the 1/8" male NPT to 1/8 hose adapter that is sticking through the PVC body.
- Take the other end of the polypropylene hose, thread it through the spacer if necessary and attach it to the hose adapter on the top of the sprinkler valve.
- The whole assembly from step 9 can now be gently slid into the 4" diameter PVC. It sounds easy but the polypropylene hose will fight you all the way. I had to wrap it around the barrel to make it fit and then spin the entire assembly to unwrap the hose once the assembly was in place.
- The front assembly can now be primed and cemented onto the 4" diameter PVC body. While the cans of cement and primer are open the 4" SLP PVC end cap can also be installed on the other end.
- Last but not least the 1/8" male NPT to Schrader valve adapter, the 1/8" female NPT "tee" adapter, the 0-60 PSI gauge with 1/8" NPT fitting and the 1/8" male NPT to 1/8" male NPT adapter can be put together and threaded into the open hole at the back of the launcher.
Step 12: Additions
- The first thing I added was before I even pressurized my launcher, I was concerned with the pressure inside the tank popping off one of the ends cemented into the body. To make me feel better I drilled and tapped a total of 9 holes for stainless steel screws to reinforce the ends. The cement and fittings are rated to 260 PSI so its really not necessary to do this.
- After a few launches the trigger valve began to spin. To correct this, I cut a small piece of PVC and cemented it under the valve to keep it from moving.
- Paint it, is still on my list of things to do. I tested some paint on a piece of scrap PVC and some paints just wont stick so be careful.
- Sights, right now aiming is more of a guess.