Table of contents:

Introduction
Procedure
Data Collection
Data Analysis
Conclusion
Links

Return to Research Page



Introduction   top

Potato guns use basic laws of physics to shoot vegetable projectiles into the air at high velocities. The body of the gun consists of the combustion chamber, which holds propellant fluid that, when ignited, provides the force that shoots the potato out of the gun, and the barrel, which holds the potato and allows for more accurate shooting. A variety of substances can be used as propellant fluid, the most common of which is a propane-based aerosol.The ignition provides energy which causes the gas in the chamber to expand, pushing the potato out of the cannon.  There are a variety of factors that can affect the precision and the power behind each firing of the gun, some of which are the shape and mass of the projectile, the propellant fluid used, and the body of the gun.

The website “Spud Gun” provides a basic list of the materials needed, as well as step by step instructions for the construction of the gun.It mainly calls for various pieces of ABS pipe, an igniter, hairspray, and other materials used to put the gun together. It also offers basic instructions on the firing of the gun and clean up.A similar site is the Beginner’s Spud Gun page .It contains a similar list of materials and instructions.However, it also gives you the option of using PVC pipe, and also provides a basic diagram of the chamber, igniter, and barrel.The instructions on this page are not quite as thorough as those on “Spud Gun,” but its firing instructions include information on the roles potato shape and amount gas and air in the chamber play on the firing, as well as the basic “only fire in open spaces far away from human life” warning.There are numerous pages like these on the Internet, most of which contain more or less the same instructions.Our specific method of construction will be outlined later.

The Potato Cannon Fun Page is basically a message board where like-minded produce shooters have come together to aide each other in their journey towards the most extreme potato gun experience.Many of the improvements are basic ideas which contribute more towards convenience than to actual performance, such as adding a laser pointer for aiming, or adding a strap so you can carry your gun more easily while riding your bike, or the optimal size “stealth” gun for your nine year old nephew.However, many provide various opinions on the optimum barrel length, propellant use, and firing technique.Overall, it seems that the ideal barrel length is one which provides enough length for the all of the propellant to be used to accelerate the potato, but no longer.Apparently any barrel length beyond this only holds air which the potato must push through, therefore slowing it down.The actual length which does this varies from gun to gun, based on the size of the chamber, propellant used, etc.

Matt’s Potato Gun Page also provides information on the refinement of potato guns, but it focuses on the dimensions, ignition methods, and fuels only, and goes into more detail on each of these topics.He stresses that the ratio of chamber volume to barrel volume is more important than the size of either piece alone, and that ideally the ratio should be between .5 and .85.According to Matt, if the volume of the barrel is more than twice that of the chamber, it is excessively long and will create the drag mentioned above.However, if the ratio is under .85, then the potato does not utilize all of the energy released from the propellant.It also goes into the most efficient ways of ignition, which he states is used with a Barbecue lighter coupled to two sharpened bolts that almost meet inside the chamber.His fuel of choice is butane, which is not too expensive, but is easy to use and gives a good shot.He also tried using propane mixed with oxygen, which, when mixed correctly, creates a very powerful shot, but he advised using extreme caution when using this method.

The Spud Physics web page, like the other web pages, contains information on the general set-up of the gun; however, it also goes in depth on the actual method for calculating acceleration and velocity.The pressure that accelerates the projectile down the barrel can be found if you know the force used to accelerate the potato, as well as the force needed to overcome friction.To be able to the force per unit area it is necessary to divide the total of the two forces by the cross sectional area of the barrel.It then substitutes different equations into the formula to be able to calculate the pressure with the variables known.This page will be very helpful once we have done our experiment and then need to do various calculations to verify or reject the hypothesis.

Procedure   top

There is not one specific way to build a potato gun.  The basic structure consists of a combustion chamber, barrel, and a projectile which is shot out of the gun.The gun can be built from different types of material, or different types of propellant can be used as an igniter.The goal of this experiment is to find the barrel length that will shoot a potato the furthest distance possible.I knew that if a barrel is too short it will not allow full acceleration of the potato, and that if a barrel is too long it will force the potato to push air out of the cannon after the acceleration has ended, slowing it down.An ideal length of barrel would be one which ended as soon all of the power from the explosion had been used up.I hypothesize that the heights reached by the potato would be highest with a barrel that was a moderate length.According to Matt’s web page, this will be when the ratio of the chamber volume to that of the barrel is between 0.5 and 0.85.

The first thing I had to do was build the gun.To do this we went to our local hardware and bought the necessary parts, which were:

2” ABS Pipe (barrel)

3” ABS Pipe (chamber)

Pipe fittings (3” to 2” reducer, screw on lid)

ABS glue

Barbecue lighter

Butane

After getting all of our supplies, I built the gun.I made the barrel 1.56 meters(5 ft.).Then I made the chamber 34.5 cm long (again, originally in feet), and connected the two with the reducer, and glued on the screw-on ending.Once I had the body of the gun built I used a thin nail to drill two small holes in opposite sides of the chamber; these would be where I stuck the wire from the barbecue lighter into make a spark.Once I had the wire in there, I taped over the holes with duct tape to keep the wire in place and keep the combustion chamber as airtight as possible.Now that I had it built; it was time to begin the experiment.

I decided to shoot the gun off at a local school, where there is a lot of open space and not too many people (especially kids) around.I planned to shoot the gun straight up and time how long it took to get down; this way I could find the initial velocity, height reached, etc.I made sure that the lighter was sparking, filled the chamber with butane, screwed on the lid, pushed down the potato, and got the timer set up.It was the moment of truth.I braced myself in case anything dangerous happened when it went off, and then counted to three. Click.Nothing.I clicked the lighter spark again and again, and nothing happened.I spent all afternoon adjusting the wires in the chamber, bringing them closer to each other, further apart, at different angles, but to no avail.They would make a spark, but nothing would explode.I tried different amounts of butane, but nothing seemed to work.The beginner’s Spud Gun web page had suggested putting nails connected to the wires in the cannon to create a bigger spark, but this didn’t work either.Finally I called a fellow classmate.

Matt is experienced in the art of potato cannons.He kindly explained to me the various things that might be causing my problem, such as moisture in the chamber or a too small spark, and gave me various methods to fix these, such as a towel to remove any moisture, alcohol to induce ignition, and the ideal space between each wire to create a big enough spark.I tried these, but still nothing worked.I called him back, asking him if there was anything else he could think of.He asked if my wires from the barbecue lighter went in a full circle back to the clicker, which they didn’t, because I didn’t know this was necessary.He pointed out that this was the problem; the wire should go from the lighter into the gun, at which point the electron (or spark) would jump to the other wire and go back around to a little copper fixture on the lighter.He said to do this and it would create a much bigger spark.With this newfound knowledge I set back to the field filled with hope and excitement.

However, once again, I found myself clicking the lighter over and over, but the potato stayed lodged in the barrel of the gun.My classmate had mentioned off hand that his grandfather, who introduced him to potato guns, had one set up with a small hole in the side of the chamber.He would put butane into the chamber, and then hold a flame up to the hole, which would ignite the butane leaking out of the hole and eventually shoot the potato.To me, this sounded a little dangerous and even worse, like something that wouldn’t result in maximum firing potential, due to explosive power exiting through the hole and not the barrel.But that potato was not going to come out on its own, and I decided to go for it.At first it didn’t work, so I made the hole progressively bigger until it was almost 3 centimeters in diameter.My gun caught on fire.I was desperate. Nothing seemed to be working.Then I noticed that the long part of the lighter was just thick enough to fit through the hole.Common sense told me that just sticking a flame into a pipe of flammable gas wasn’t very safe, but not only was I desperate; I was jaded.I had resigned myself to the fact that the gun would never fire, but I gave it a try anyway.I pulled the trigger on the lighter, and instead of the pitiful click I was so accustomed to, a powerful boom echoed through the valley.It had finally worked.

Data Collection     top

Once I had the potato gun shooting, it was time to get to work.I shot the gun straight in the air, timing how long it took for the potato to come back down.I did this five times with the gun at its full length, and repeated the process for different lengths, each time sawing off 16 cm.Here follows a table of my findings.One may note that the times in each grouping vary a good bit.I attribute this to the fact that butane is picky when it comes to how much oxygen it combines with to make and explosion, and so sometimes we would put too much in and have to air it out, and sometimes I put too little in, hoping to not have to air it out.However, this happened with every grouping, so it can be assumed that the discrepancies in each group are about even and cancel each other out.
 

Once I had collected my data I decided I should calculate the uncertainty. The main thing that would cause uncertainty would be inaccuracy in the timing, such as starting or ending the timer too soon or too early.I found the uncertainty for each barrel length by calculating the average time that the potato was in the air and then subtracting that from the time which was the furthest away from the average.Then I averaged all of those uncertainties to get the overall uncertainty in the timing, which I found to be 0.72 seconds.

However, midway through my research, I realized an interesting prospect.When I loaded the gun, I would put the potato in the end, leave it there, fill the chamber with butane, and then push the potato down.The Potato Cannon Fun Page suggested this because it increases the pressure in the chamber and creates a more powerful shot.However, I realized that this could seriously influence our results; with a shorter barrel length, the pressure built up by pressing down the potato would be less.This gave rise to question over what had the most influence over the power output of the gun: the increase in pressure or the longer barrel length?Most logically it is a combination of the two, but if the pressure varies greatly and plays a major role, my results are not nearly as valid as I hoped for.If I had realized this before, I would have pushed the potato all the way down the barrel before spraying in the butane in order to keep the pressure constant.However, it was too late for that, as most of the barrel had already been cut off.

To try and figure out how much of an effect the pressure had on each shot, I decided to look at the acceleration.If the acceleration stayed fairly constant throughout the different lengths, then it would indicate that the power energy released by each explosion was fairly equal, meaning that the pressure did not play as big of a role.On the other hand, if there was a significant decrease in the acceleration as the barrel lengths got shorter, this would indicate that the pressure directly influenced my data.To find the acceleration, I first had to find the velocity of the potato as it exited to gun.

To do this I used the formula v = u + at.I knew that the times I had were all twice the time it took for the potato to reach its peak, where the velocity (v) is zero.I also knew that it underwent a constant acceleration of –9.8.Knowing this, my final formula was u = (-9.8)(t/2).Once I had the initial velocity of the potato, I could use the formula v^2 = u^2 + 2as.The initial velocity from the previous formula is the final velocity after accelerating through the barrel, and the initial velocity in the barrel is 0.This turns into the formula a = (v^2)/(2s), where s was the length of the barrel.Here is a table of my calculations:

Heres a graph, of the results:

The potato reaches its maximum acceleration at 124 cm, which seemed odd because it reached its highest initial velocity at a length of 140 cm.However, this starts to make sense when one thinks of how a combination of both the length of the barrel and the pressure affect the velocity.At 140 cm, the extra built up pressure is enough to overcome the resistance of the extra barrel length, in which the potato begins to slow.For this reason, the potato goes far even if the acceleration was not as large.At 124 cm the acceleration is very high for two main reasons; the barrel is still long enough to create a lot of pressure in the chamber, creating a high acceleration, but it is short enough that the potato exits before it starts to slow down.The reason this did not create the highest velocity is because the potato exited before it used up all of the power from the explosion.After this point the accelerations start to shrink because the pressure is not as great.This supports my hypothesis that both play a role in creating the longest shots.Also, by realizing that at 140 cm the barrel is too long and the potato starts to slow and that at 124 cm the barrel is too short and the potato does not reach its full potential, one can see that the best length (for a gun with a chamber of my size) would be somewhere between those two lengths.

Finding this ideal length would be one of the things that I would do to continue this investigation.By having a gun with a barrel length which only varied from 124 to 140 cm, I could narrow down our results and pinpoint an ideal ratio.Another thing I would do would be to investigate the effects of pressure and barrel lengths separately.To do this, I would use one gun where the barrel length would remain constant, but the pressure would vary by having the potato at different levels before I added the butane and pushed it back.I would also have a gun in which I always put the potato at the same level (very close to the chamber) before adding the butane, and cut the barrel to different lengths.Also, to make my results more reliable, I would try to find a system where the right combination of oxygen and butane was always in the chamber.This would most likely only be done after a lot of practice shooting, so I figured out the right amount to spray in and consistently did so.Another thing that would help improve the efficiency is to have an ignition system that was more reliable; even after I began to use the lighter, sometimes it would fail to light, causing me to believe that there was a problem with the amount of gas in the chamber, even though it was just the lighter.An additional thing that I probably should have taken into account was the mass of the potatoes we used.Although they were generally the same size and cut to fit the circumference of the barrel, slight variances may have affected our data.This would be another thing that could be further investigated by using a setup where the only variable was the size of the potato, which would show if it had any large affects.