This potato cannon has a unique one-way valve system that lets the fuel (propellent) into the combustion chamber and prevents the explosive gases from escaping out through the valve. The cannon can be refuelled and fired in a couple of seconds. We decided to mount the cannon on the bucket (front end loader) of our Kubota BX25 tractor. With the bucket as a shield, we get a big safety advantage, and we also get a nice tank-like experience when manoeuvring the cannon in position to strike the target (see video below).
The cannon is fired with a remote electronic ignitor connected with wires to the spark plug in the cannon combustion chamber. The ignitor is secured to the tractor engine hood with a couple of rare earth magnets. The ignitor is an inexpensive unit available at hardware stores and is normally used to ignite gas barbecues. The wooden cannon cradle is mounted to the bucket with two C-clamps making it quick to install and remove. We mounted a GoPro Hero2 camera on the cannon barrel to record the potato firing action from that perspective.
Step 1: Study what is out there paying attention to the safety aspects of this kind of project
We checked out potato cannons on the web and based the fundamental design on what we learned there. We ended up going with ABS plastic pipes and associated fittings for the cannon. The combustion chamber has an inside diameter of 3 inches and is about 24 inches long. The cannon barrel is 1-1/2 inches by 36 inches. A reduction coupling connects the two pipes together. The other end of the combustion chamber is fitted with a female coupler and a removable (threaded) clean-out plug. Hair spray seems to be the propellent of choice so that's what we experimented with first.
The YouTube video, below, shows the cannon in action and also presents some of the construction details. The Instructable step by step instructions below are quite detailed to help the builder have a greater chance of success.
A second video giving more construction information can be viewed here: http://youtu.be/T7r6I8M4AG4
Be aware that this kind of device can be very dangerous - it would be smart to read and view plenty of background material before construction and testing.
Step 2: Measure and cut the ABS pipe to size
A regular wood cutting blade on a miter saw is an easy way to cut the ABS pipe. A hand saw works too but it is more challenging to make a nice straight cut. After cutting the pipes to size (24 inches for the combustion chamber and 36 inches for the barrel) we removed the burrs from the pipe with sandpaper (a knife or file will also do). Don't forget to wear eye and ear protectors when using the miter saw.
The ABS pipes and fittings are cemented together after the one-way valve and the spark plug are installed in the combustion chamber (see steps below).
Step 3: Make a cutting edge on the barrel to help get the potato (ammunition) in
We found that a pretty sharp edge is necessary on the front end of the cannon barrel to make loading the potato as easy as possible. The combination belt and disc sander, shown here, made short work of this procedure. Best to take your time on the sander to reduce chances of melting the plastic as you shape the end of the pipe. As a final step, we honed the edge of the barrel with a hand file. A woodworking mallet comes in handy to drive the potato into the barrel but a piece of 2x4 should work as well. The sharp edge of the barrel cuts the potato neatly to create a snug fit in the barrel.
Step 4: Drill holes for the spark plug and the one-way valve assembly
Two 1/8 inch diameter holes were drilled for the one-way valve. One hole takes a machine screw that secures the valve flap (two layered pieces of plastic venetian blind) and the other hole, spaced about 3/4 inch from the first, is the fuel injection port. The spark plug requires a hole somewhat larger than 1/2 inch. I first drilled the hole with a 1/2 inch twist drill and then manipulated the pipe, with the drill turning, to increase the hole diameter. The spark plug hole is made about 2-1/4 inches from the pipe end and it is offset from the one-way valve holes by about 1 inch. The offset makes installation and maintenance of the plug and valve easy.
Step 5: Install the one-way valve
The one-way valve flap is made from lengths of plastic venetian blind. I found that two pieces of blind layered together gives a good spring-back action. The flap is held securely in place against the pipe by a round head machine screw, lock washer, and nut.. Long nose pliers aid in the installation of the flap.
Step 6: Install the spark plug
Install the spark plug by first pushing and turning the plug clockwise into the previously made hole. If necessary ream the hole to a larger diameter to get the "tapping" started. I needed to use a socket wrench to get the plug all the way in. As a backup I installed a nut on the spark plug (on the inside of the pipe). I "made" the nut by hacksawing a standard plumbing connector nut in two (see photos ). The nut is probably overkill in terms of securing the spark plug but no harm in being as safe as possible when dealing with a combustion chamber!
Step 7: Give the one-way valve a test
This is a good time to test the one-way valve. This is done by pushing the hair spray feed tube in through the fuel injection port and then pulling the tube out while observing the action of the valve flap. There should be an immediate return of the flap against the inside of the pipe. This is also a good time to check out how the hair spray atomizes in the combustion chamber. (I found that the tube tip should be pushed past the flap to prevent the hair spray from condensing on the flap.)
I replaced the supplied hair spray nozzle with the nozzle and extension tube from a can of WD-40 - works fine.
Step 8: Cement the ABS parts together
This set of photos illustrate the process we went through to assemble the ABS plastic components of the cannon. We cleaned the mating parts with the recommended cleaning agent first. Then, following the instructions on the container, we applied the cement and pushed the parts together as required. You need to work pretty fast to make sure that the parts are properly seated before the cement has a chance to set. The last photo in this series of photos shows the clean-out plug being screwed in place on the back end of the cannon.
With this cannon design the clean-out plug will only have to be removed for cleaning and maintenance - for other designs that we examined, the clean-out input is the fuelling port for the cannon: therefore it is necessary to remove the plug, inject the propellent, and then screw the plug back in place.
Step 9: Make a support braket for the BBQ ignitor
The barbecue electronic ignitor was picked up at a hardware store for about $20. It comes with electrical connectors and wires and it takes one AA battery. We found a metal bracket in the junk box and drilled a hole in the bracket to mount the ignitor. We then screwed the backet to a block of wood to serve as a base for the ignitor. We then attached two rare earth magnets to the bottom of the wooden base so that we can readily postion it in a convenient location on the tractor hood. Once this is done the wires are connected to the spark plug and the cannon is then ready for firing!
Step 10: Make and label a ram rod
We made and clearly marked a wooden ram rod to insure the potato is never pushed too far into the barrel - having the potato lodged in the combustion chamber is not a good idea!
Step 11: Installing the cannon on the cannon cradle and the front end loader
We tried several clamping methods to secure the cannon to the wooden cannon cradle. We settled on a standard worm screw pipe clamp as it is easily loosened and tightened to remove or install the cannon on the cradle for cleaning or maintenance. We also secured the GoPro Hero2 camera to the cannon barrel with a pipe clamp.