For our project, we wanted to build a potato gun.Until that time, we had only heard of potato guns that were fueled by hairspray or a similar combustible substance.Mr. Murray advised us to build a pneumatic (air-powered) gun because the combustion-powered guns were less reliable and not conducive to experiments.

We downloaded several potato gun plans from the Internet and compared them to determine which model would be best for our experiment.Our criteria consisted of:

Along with building a potato gun, we needed to design an experiment to test some aspect of the gun.We decided to test the effect of pressure on the distance a potato is launched.Some of the questions we had were “Does doubling the pressure double the distance?” or “How does the distance vary if you increase air pressure in 100 kPa increments?” or “Is there a formula we can find to determine distance based on the amount of pressure in the cannon?”We decided to attempt to answer these questions by launching potatoes with different amounts of compressed air and measuring the length of their flight, then analyzing the data we gathered.We determined that, in order to keep the pressure as the dependent variable, we needed to keep both the mass of the potato and the angle of trajectory constant.

Theory and Construction (Return to T of C)

The basic pneumatic air cannon consists of a body and barrel connected by a bell reducer, a female adapter connected to the body, and a diaphragm held in place between the adapter and a cleanout plug through which air is pumped using an air compressor, bicycle pump, etc. Some models are equipped with a valve attached to the cleanout plug that seals off the air prior to the launch. When compressed air is pumped into the cannon, the pressure forces the corners of the diaphragm around the edge of the barrel, sealing it off and directing the air flow into the outer chamber of the body. When the nozzle of the air source is removed, the diaphragm releases the seal over the barrel, allowing air pressure from the outer chamber to flow into the barrel and build up behind the potato, thus initiating the launch. If the air cannon is equipped with a valve switch, the switch is flipped before removing the air source, thus sealing the outer chamber, and then released to break the seal of the diaphragm on the barrel.

Building the Pneumatic Cannon (Return to T of C)

Our first step in construction was to cut the PVC pipes that made up the body of the air cannon. We cut a 36" length of 3" PVC pipe for the outer chamber and a 72" length of PVC pipe 1.5" in diameter for the inner chamber and barrel using a hand saw. We then cut a 3 3/16" diaphragm from a plastic bin lid with an Exacto knife. After drilling a hole in the cleanout plug, we glued the bell reducer and the female adapter to each end of the 3" pipe. We placed the diaphragm and the female adapter and screwed in the cleanout plug. We then pushed the barrel through the reducer until it sat firmly against the diaphragm and glued it in place.

Testing the Cannon (Return to T of C)

We began our testing procedure by prepping our ammunition. We selected ten potatoes of comparable size and weighed each potato to determine the smallest mass. Using a piece of PVC pipe, we cut each potato to ensure a snug fit inside the barrel of the air cannon. We then re-weighed each potato and trimmed each to equal the mass of the smallest potato, 0.6 kg.

Our next step was to prepare the launch site. We propped the air cannon on a sawhorse and calculated the angle of trajectory using trigonometry. We kept the angle of trajectory constant to isolate the pressure variable in determining the distance.

We then marked off 25 ft. intervals down the flight path to facilitate distance measurement. We used the air compressor from the trunk of the car to fill the cannon and measured the pressure in kPa using the pressure gauge. We placed a potato 2.5 inches down the barrel of the air cannon and filled the outer chamber with 200 kPa of compressed air, then flipped the valve controlling air flow to the inner chamber, thus closing off air flow to the barrel. We removed the air compressor hose from the nozzle of the air gun and released the valve, thus allowing air to rush into the inner chamber and through the barrel of the air cannon, launching the potato. We tracked the flight of the potato and measured its flight distance to be 9.7 m. We repeated the process for 300 kPa and 400 kPa, launching the potato 14.6 m. and 16.8 m., respectively.

While setting up another trial, we accidentally pushed the potato too far into the barrel, past the 2.5" mark. Because it would be impossible to retrieve the potato, we decided to move the potato 6.5 inches down the barrel and launch it using 400 kPa of compressed air. The potato flew 39.6 m., over two times the length it flew when only 2.5" down the barrel. Our examination of this unexpected increase in distance led us to form an interesting theory.

The force propelling a potato through the barrel of an air cannon is the compressed air building up from behind the potato. The farther the potato is from the compressed air, the farther the air has to travel to reach the potato and push it through the tube. Because it has to travel some distance, the compressed air decreases in pressure due to the increased volume in which it can expand. Therefore, by the time the compressed air reaches the potato, the force it exerts on the potato is lessened. However, if the potato is pushed farther down into the tube, the compressed air has less room to both travel and expand; therefore, it retains more of its air pressure and can exert a greater force on the potato to propel it through the barrel. Also, when the potato reaches the end of the barrel, the air seal is broken and the compressed air is released into the atmosphere, no longer using force to give the potato momentum. If the potato was situated closer to the opening of the barrel, the compressed air would have little time to exert force on the potato before it was released into the atmosphere. However, if the potato was farther down into the barrel, the compressed air would have an increased amount of time to exert a force on the potato, thus accelerating it to the maximum velocity it could reach using the available pressure before the potato reached the end of the barrel and broke the air seal. because the potato was positioned farther down the tube, it would have more time to accelerate and more time for the potential energy in the compressed air to be converted to potato inertia, thus providing greater launch propulsion.

With this in mind, we increased the distance of the potato to 12.5" down the barrel and continued our launch. Our first trial with 300 kPa of compressed air yielded a 41.12 m. launch, with 400 kPa a 58.5 m. launch, and with 500 kPa a 83.2 m. launch. We repeated our 500 kPa launch, yielding 81.08 m., which demonstrated the relative accuracy of our air cannon.

Evaluation and Synopsis (Return to T of C)

We were able to create formulas to project distance based on pressure by analyzing the data we had gathered. At first, we entered the data into a program to find a linear equation. After re-entering the data to check the equation, we found that the data didn't quite match up to the values produced by our equation. At first, we assumed that the equation was an estimate, but as we graphed the data points, we realized that the graphs were in quadratic form and not linear. We re-entered the data in a quadratic program and found two equations: for the 2.5" graph, y = (-1.38165 E -4)x ^ 2 + (.1188155)x - 8.5798 and for the 12.5" graph, y = (3.659 E -4)x ^ 2 + 32.94.

Our accidental discovery about potato position in the barrel of the air cannon provided an insight into the process a cannon undergoes in launching a potato. Our mistake really helped us to understand the principles of air pressure and pneumatic-powered mechanisms, more so than would explanations or diagrams. Other possible experiments could compare distances based on potato position while keeping pressure constant, which, if not just interesting, could aid in potato gun design reconfiguration for a greater launch distance.

1. Pneumatic Spud Shooter: www2.csn.net/~bsimon/pngun.htm
This website was the most informative and had the plans we ended up using to build our gun. The directions are fairly simple and all of the parts are easily procured.

2. Ultimate Potato Gun Page: angelcities.com/members/spudgun/spudgun.html
This page contains information on both pneumatic and combustion potato guns, as well as ideas and tips on firing and designing cannons.

3. The Spudgun Technology Center: www.goldmann.com/spudgun.html
This website has incredibly high-tech potato gun plans- if you are really into mechanical and engineering stuff, this one's for you.

5. Tony's Spudgun Page: www.tommasi.org/spud/index.html
This site, like the Spudgun Technology Center, features incredibly complicated designs with high-tech parts. Not for the amateur builder!