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The Great Physics Projectile Motion Lab Project
Learning with a "Potato Cannon"

Project Links
* Project Overview

* Lab Day Pictures

* Lab Worksheet
        (MS Word File)
        (PDF)

* Example Lab Worksheet

*
Design Calculations

* Construction Details

Project Objective:
The object was to create a projectile motion lab rich with calculation verifiable data as well as induce a little excitement into class by creating a "significant emotional experience", which aids in the learning process. The projectile motion lab included with the school's curriculum was less than exciting, to say the least, as well as deficient in verifiable data calculations. Additionally, the preciseness of measurement has a greater impact on small-scale flight distances. So, you either have the option of spending money on more precise equipment or make the measurements greater. Because of this and the previously noted memory aiding "wow" factor, I decided to lean toward the bigger is better concept.

Idea Stimulator:
One of my physics students said they had spent some time over the weekend firing a "potato cannon" (otherwise known as a "spud gun") and thought it would be a great idea for the class to build/fire one. Being a typical male who likes loud noises and high velocities (which translates into model planes, motorcycles, rockets, etc), I was intrigued with the idea and told him I would look into the possibility.

Feasibility Research Information:
This stage of research fell into three categories: legality, practicality and options. In terms of legality, I found that at a national level as long as the muzzle velocity is less than 500 feet per second, the feds (Bureau of Alcohol, Tobacco & Firearms) do not consider it a weapon and thusly do not subject it to their regulation. The thought crossed my mind that had they consulted a physicist, the limit would have probably been in terms of kinetic energy and not just velocity. States and local agencies may have more restrictive requirements. One web site made an off-handed remark that they are okay anywhere in the US except New York and California. Iím in California and have scoured the penal codes, but have not found these devices specifically mentioned. After building one for the school, a student from another class indicated that he heard that possessing one could result in a citation. The more conservative inquisitors should probably contact local authorities before embarking on constructing one. I have decided that as a result of the facts that I have de-tuned mine from the normal "potato cannon", it is being used for teaching purposes and is being called a "tennis ball launcher" I am probably not at great risk of being cited by the local sheriff.

Under the category of practicality, I was mostly concerned about being able to safely launch the projectiles and keep them in the confines of available open space on the campus. Secondly, I needed consistency in terms of the projectile mass and the chamber pressure. Lastly, I wanted it to be easy to lug the equipment to the field and back to the lab.

With respect to options I'm referring to the choices such as building materials, projectiles, the propulsion thereof and equipping for measurement. Some of it is related to the practicality issues noted above.

Decisions as a result of Research:
Early in my investigation, I decided that consistency was needed. The pneumatic gun would be the most consistent, but would require more bulk in terms of equipment (compressor, etc.). Further research revealed that by using metered propane as fuel I should be able to obtain consistent combustion chamber pressure. I also wanted a consistent projectile mass, which could not be had using potatoes, so I decided to design the launcher to shoot tennis balls.

My first observation was that most of the potato cannons described on the web were capable of shooting the projectile more than 200 yards. Since my objective was to make the flight distance this side of 100 yards, I had to find a way to scale down the typical cannon. I first calculated the needed V0 (muzzle velocity) to obtain the desired flight distance. With that in mind, my next inclination was to try to use the chemical potential energy per given volume of propane to determine the amount of propane and thus the chamber volume needed to propel the tennis ball the desired distance. However, I quickly discovered that there would be too many variables to determine a chamber volume this way. I then found a web site with performance data (www.burntlake.com) which provided enough information to allow me to "guestimate" a chamber volume. My "guestimated" size turned out to be valid as the first shot of the first launcher traveled about 110 yards. Since I wanted to use the chamber combustion pressure to compute the muzzle velocity by kinetic energy, the barrel length would have to be sufficiently short to allow the ball to exit before the combustion was complete. According to one site (www.spudtech.com), the ideal chamber to barrel ratio is 1:5 to 1 for optimization of combustion pressure. However, the test data on the previously mentioned site seemed to indicate that the other publicized optimum ratio of 0.8 to 1 is best. I settled on a ratio of about 1:1. Since I reduced the combustion pressure to well below that of any available rated pipe (20psi to 30psi, schedule 40 pipe is rated for more than 160psi), materials used would depend on availability and price. It turned out that I used a combination of materials as I found a cement that would work with both PVC and ABS and even join them together. Below is a table of the construction parameters of our tennis ball launcher.

Projectile Launcher Parameters
Propulsion Type Combustion
Fuel Metered Propane
Construction Materials PVC & ABS Pipe
Ignition Flint (Lantern Lighter)
Chamber Volume 154 in3
Barrel Length/ID 32 in x 2.5 in (Volume = 157 in3)
Chamber/Barrel
Volume Ratio
0.98:1
Projectiles Tennis Balls (57 grams, 2.5" OD)
Enhancements Ventilation/Mixing Fan, Pressure Gauge (retains high reading until reset)

Missteps, Stumbles and Falls
On my first launcher, I decided to not make a removable back since I was thinking that it was only needed for adding a "hair spray" type of fuel and I was using directly injected propane. The result? Great first launch, but next to impossible to get enough fresh air into the chamber to obtain any further launches. I also had a pressure gauge that did not hold the high reading, which turned out to make it difficult to obtain a high-pressure reading. Lastly, I used a Bar-B-Q electronic igniter, which quit working after less than 25 ignitions. I read on a couple of web sites that some users had the same experience, while others had more success. I next tried a car coil to create the spark. While it did create a spark, I was not happy with its intensity. I finally settled on a flint igniter since its drawbacks would not apply in my situation (i.e. coating when hair spray fuel is used - we're using propane and difficulty holding cannon and twisting to fire - ours will be mounted in a stand). The most recent launcher also includes a fuel distribution manifold, mixing fan and rear ventilation port. It has been dependable, only requiring the ventilation port to be open for a few seconds while the mixing fan is running to assure an ignition following an ignition failure. Lastly, by reversing the quick-connect coupler components I was able to remove one valve from the metering pipe and minimize waste of propane. However, my propane density is now a little rich, as the short small pipe between the valve on the propane bottle and the quick connect is now part of the metering pipe.

Anticipated Improvements
I have found some web sites where people have designed carburetors to go between the metering pipe valve and the combustion chamber to automatically mix the fuel and air to the right proportions as the propane enters the chamber. This would effectively eliminate the need for the mixing fan. I will likely construct and add this to the launcher. Lastly, I will probably eventually obtain a stun gun to power a multiple spark ignition system to replace the flint igniter.

Though not really an enhancement to the launcher, building a ballistic pendulum would enhance the lab by giving us yet another way to determine the muzzle velocity. Thus far our readings with the radar gun have not been consistent enough to provide usable data for comparison purposes.


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