Air cannons are an iconic DIY project. You can find a lot of great examples online. There are cannons that are powered by combustion and cannons that are powered by compressed air. They can be used to fire anything from golf balls, to water bottles, to candy.
But in this project, we are doing something a little different. We are going to make a baking soda and vinegar cannon. This is very similar to the baking soda and vinegar volcanoes that we made as kids. I have just scaled up the reaction so that it can be used to send a potato flying through the air.
Step 1: Safety Warning
This should really go without saying, but POTATO CANNONS ARE NOT SAFE! If you do not handle them properly, they can seriously injure or kill you. They can also cause great damage to property. Mixing chemicals in a closed container can create high pressures that can cause the container to explode. Build this project at your own risk. Please be safe. Have responsible adults nearby at all times. Don't do anything stupid.
Also, it may not be legal to fire a potato cannon in your area. Be aware of your local laws.
Step 2: Materials
Here are the materials and tools that you will need for this project.
4 inch diameter PVC pipe
2 inch diameter PVC pipe
Two 2 inch to 4 inch PVC pipe adapter
Two 2 inch PVC pipe ball valve (solvent weld)
Step 3: The Baking Soda and Vinegar Reaction
Vinegar is a weak solution of acetic acid (usually 5%). Baking soda is powdered sodium bicarbonate. When one molecule of acetic acid reacts with one molecule of sodium bicarbonate it produces one molecule of sodium acetate, one molecule of water and one molecule of carbon dioxide. The carbon dioxide is what produces the characteristic foaming and bubbling. In a closed container, the carbon dioxide that is produced builds up pressure inside the container. This pressure can be used to send a potato flying through the air.
The ideal ratio of vinegar to baking soda is 14 milliliters of 5% vinegar for every 1 gram of baking soda. This can be approximated to about 1 cup of vinegar for every tablespoon of baking soda.
All these chemicals are generally considered to be non-toxic and safe (in small quantities). To clean up, everything can be safely poured down the sink. Although it is always a good idea to dilute any chemical that you are disposing of.
Step 4: The Basic Design
The cannon consists of three main parts: the reaction chamber, the fittings and the barrel. The reaction chamber is where the baking soda and the vinegar will mix. A pressure gauge is mounted on the side of the reaction chamber to monitor the pressure inside. On each side of the reaction chamber there is a reducer that lets you connect to 2 inch diameter pipe. Then a small piece of 2 inch diameter pipe is added on each side so that you can connect to two ball valves. The ball valves make it easy to add the reactants and then quickly seal the chamber so that pressure can build up. They also make it easier to clean out the cannon after each use. Lastly on one end there is a long section of 2 inch diameter pipe that acts as the barrel.
Step 5: Cut the Pipe
The reaction chamber is made from a two foot long piece of 4 inch diameter pipe. The Barrel is made from a two foot long piece of 2 inch diameter pipe. There are also two small pieces of 2 inch diameter pipe that connect the reducers to the ball valves that are each about three inches long. These dimensions do not have to be exact. Feel free to modify them. Larger reaction chambers and barrels will give you more power but they take up more space and can make the cannon difficult to handle.
I used a hack saw to cut the pipe to the appropriate lengths. I recommend clamping the pipe to a table to help keep it from moving while you are cutting. So hardware stores may cut the pipe for you when you buy it.
Step 6: Dry Fit the Pieces Together
Before gluing anything, you want to fit all the pieces together to make sure that the spacing works. This will also let get an idea of where you want to place the pressure gauge. If any pieces get stuck together and don't want to come apart, you can use a rubber mallet to gently separate them.
Step 7: Glue the Pieces Together
Pieces of PVC pipe are fixed together in a process called solvent welding. The two pieces of PVC are essentially chemically melted and cemented together. This process should be done outside or in a very well ventilated area.
First make sure that all the surfaces are clean and dry. Then apply a coat of purple PVC primer to all the surfaces that will be connected. Follow the instructions for the primer that you are using. You may need to apply a second coat of primary to some areas.
Then apply a coat of PVC pipe cement to all surfaces that will be connected. Again, follow the instructions for your PVC cement. You may need to apply a second coat to some areas. After applying the cement quickly stick the two parts together. Once the pieces are inserted all the way turn them 1/4 turn. Hold the two pieces together for 30 seconds. After assembling all the pieces, let it sit until it is fully cured.
Step 8: Connect the Pressure Gauge
The pressure gauge can be mounted to any point on the reaction chamber. I recommend mounting it at one end near the ball valve.
Start by drilling a small hole through the side of the PVC pipe. Then re-drill the hole with larger bits until the hole is just smaller than the fitting on the pressure gauge. You want the pressure gauge to be fit tightly in place so that it doesn't blow out when the tank is pressurized. You may need to smooth out the sides of the hole with a file. Then apply plumber's tape all around the threads of the pressure gauge. Using a wrench screw the pressure gauge into the hole until the threads are just sticking out of the top.
Step 9: Load the Potato
Open up both valves. Then push the potato into the barrel. You want it to make a good seal. Using a pole, push the potato to the bottom of the barrel. To help judge the distance, you can hold the pole up to the side of the barrel so that you know how deep the potato needs to be pushed in.
Step 10: Pour Vinegar into the Reaction Chamber
Close the ball valve that is connected to the barrel and open the ball valve at the back of the reaction chamber. Pour in four cups of 5% vinegar. Then close the ball valve to seal the chamber.
Step 11: Load in the Baking Soda
As soon as the baking soda comes in contact with the vinegar it will start to react. So you want to add the baking soda as quickly as possible. The best way that I have found to do this is to make a small packet of baking soda by wrapping it in a thin napkin.
Take 1/4 cup of baking soda and place it in the center of a napkin. Then lift up the sides and gently twist them so that the baking is wrapped into a small packet that is about 1 inch in diameter. Tear off any excess napkin on the end.
Open the ball valve on the back of the reaction chamber. Then take this baking soda packet and insert the whole thing into the tank. As soon as the baking soda is inside, quickly close the ball valve to seal the chamber.
Step 12: Gently Shake the Chamber to Help the Reactants Mix
The reaction will happen most slowly if all the baking soda stays in one big chunk. So gently rock the chamber back and forth to help break it up and mix better. As you do this keep an eye on the pressure gauge. Once the pressure stops going up, the reaction should be mostly complete. It should get up to about 20 PSI.
Never let the pressure get too high. I recommend keeping the pressure below 1/2 of the capacity of the lowest rated component. In this case, the weakest component was rated for 80 PSI. So I always kept the pressure below 40 PSI.
Step 13: Fire the Cannon
Clear the firing range. Aim the cannon at your target. Grip the cannon tightly. Then quickly open the front ball valve to fire the cannon. If everything performed properly, the potato should fly over 100 feet. The final range will depend on the pressure in your tank, the size of the reaction chamber, the size of the barrel, the size of the potato and the wind.
The potato will also be accompanied by a large blast of liquid, smoke and foam.
Step 14: Clean the Cannon
After each use, you need to clean out the cannon. You don't want any reactants left in the barrel or the reaction chamber. These can cause problems for the shot. Also it is not good to let vinegar sit in the cannon. This can lead to the parts wearing out prematurely.
I dump the remaining contents of the reaction chamber into a large bucket. Then I rinsed out the whole thing with a garden hose. After cleaning out the cannon you should be ready to load it and fire it again.
Step 15: Bonus Test Footage
Here is some extra test footage of the cannon being fired.