Spud Cannon

Disclaimer: Proper operation of a potato gun requires that no persons be targeted and that the user(s) be free of the effects of alcohol and/or other drugs.

As writers for the Hawkeye Engineer, we wanted to write an article that allows us to apply our engineering knowledge and skills while still having fun in the process. As engineering students, we must deal with our fair share of equations and boring derivations, so we tried to stay as far away from them as possible. In the past, this has included brewing beer and making a lava lamp. This semester we decided to construct a potato gun. We hope that you find this a little more interesting than the physics that describes the reason for the change in barrel diameter that we used.

A1*V1 = A2*V2.

Prior to beginning this project, we called the Johnson County Sheriff's office. The receptionist checked with the proper authorities and assured us that it was not illegal for us to build a potato gun. There are two types of potato guns, combustion or pneumatic. Both are similar in cost, but their methods of propulsion differ. Combustion guns use an explosion to produce a change in pressure that forces the potato out of the gun. Pneumatic guns use compressed air to launch the potato. Pneumatic guns also have an additional chamber where compressed air is stored. When a valve is opened, the compressed air rushes out of the chamber and propels the potato from the gun.

Which to make, combustion or pneumatic?

We chose to make a pneumatic potato gun for a variety of reasons. First and foremost, we thought it would be safer to deal with air versus explosive fuel. Another consideration was that the use of a pneumatic system instead of an ignition system eliminated any fumes or chemical use, making our potato gun environmentally friendly. Upon firing, we also found that there was no cleanup required with our device as opposed to an ignition-based system. A combustion potato gun leaves residue behind from the fuel.

Supplies we used:

5 foot piece of 2' PVC
3 inch piece of 2' PVC
2' street elbow
5 foot piece of 1.5' PVC
3 inch piece of 1.5' PVC
1.5' ball valve
2' to 1.5' reducing bushing
2' end cap
Tubeless tire valve
3 foot dowel rod
Can of PVC primer
Can of PVC glue
Some potatoes
Tools we used:

Drill with 9/16 drill bit
Saw
Knife
The pieces go together exactly like you see in the picture. The 3 inch pieces of PVC pipe were cut for us by a Menard's employee. Prior to construction, we drilled a hole using a 9/16ths drill bit in the end cap. This was done to make room for the tire valve. The hardest part of putting the potato gun together was waiting for the cement to set before we could test it. The directions for using the primer and cement are found on their containers. Following these directions was fairly easy to do. When gluing the ball valve, caution must be used to make sure that you do not glue it "open" or "closed". After all the pieces have been assembled, we recommend letting the cement cure overnight. However, we only waited 5 hours.

10-9-8 Launch Sequence A Go

Once we had completed our design, we needed to perform a test launch. We allowed the glue to set for several hours to insure that the joints were properly sealed. We chose a test site that was free of structures and pedestrians for safety reasons. We used an air compressor that was connected to the power adapter in a '93 Buick Regal. The air compressor could provide air pressure up to 140 psi. The maximum pressure rating for the PVC pipe used was 280 psi, but we were not prepared to trust the PVC cement or the ball valve at anywhere near that high of a pressure. A literature review prior to our building the device suggested we not use a pressure over 100 psi for safety reasons.

We started off cautiously by filling the pressure tank of our potato gun to 30 psi. This is similar in magnitude to the pressure commonly found in car tires. There were no audible leaks and the tank sustained 30 psi for several minutes, so we declared our first pressure test a success and moved on to 50 psi. We achieved 52 psi in the second test. We noted that there were still no audible leaks and the pressure stayed constant. We trimmed a potato down to size to fit into the launch tube and loaded it about 2 feet deep in the tube using a 3 foot dowel rod. We noted that this first potato did not exhibit a high degree of friction with the tube. We opened the ball valve and unleashed 52 psi on the unwitting potato. We estimate that this first launch was at an angle of 30 degrees above horizontal and achieved a flight distance of approximately 150 feet. All distance calculations were paced off using an average stride length of 3 feet.

We were slightly disappointed with this launch so we tweaked some of the launch parameters. We increased the pressure from 52 psi to 90 psi, which was still within our preset pressure limit of 100 psi. A slight leak could be heard through the ball valve at this point. We also increased the angle of trajectory to 45 degrees above the horizontal. Finally, we were more careful in preparing the potato for launch. We made sure that the cross section of the potato was as close to circular as possible. This made for a better seal around the potato when the pressure was released and theoretically this would result in more energy being transferred to the potato and its flight. The impressive result of the second test was that the potato flew an estimated 300 feet.