Potato Gun (Nov 2020)
As with most of my other projects, I was surfing YouTube one day when I saw a slow-motion video of a potato gun being fired. I thought to myself, "Hey, potato guns are pretty simple and I'm sure it would be fun to make a really high-quality one." I also saw an article on the MIT technology review which summed up a study done by the Air Force on the internal ballistics of a potato gun. Why exactly the Air Force studied the ballistics of a potato gun? I do not know. Nevertheless, it's pretty funny.
There are two main types of potato guns: compressed air and combustion-driven. Compressed air guns are, in my opinion, very dangerous and much more expensive than the latter option. The idea behind them is pretty simple: they have a high pressure air reservoir connected to a barrel with a potato in it. The main caveat is the fact that the air inside the reservoir is trying as hard as it can to get out. That means that every piece is a link in a chain, and if one breaks, you're going to get shrapnel. For this reason, I went with a combustion-driven one (granted, when you have combustion in a pipe, you create a pipe bomb if you're not careful). Most of the ones out there use a 4" diameter PVC tube as a combustion chamber with a 2" diameter barrel. You then spray hairspray (or another aerosol) into the chamber, seal it and then ignite it some way. This method isn't very precise, so I tried to make it better.
The plan was to measure out specific amounts of propane from a blow torch and deliver it into the combustion chamber. I also toyed with the idea of adding extra oxygen, but I realized that starts to get into the territory of a potato howitzer. It is, however, still up in the air (pun intended) for future experimentation. I got some brass fittings and got to work.
By knowing the volume of the combustion chamber, adding a specific amount of propane can yield the Goldilocks stoichiometric ratio between fuel and air. To measure that amount of propane, I made the contraption seen in the photo. The clear tube is meant to add some volume to a measuring chamber, which I pressurize to a value I know will deliver the correct amount of propane. I then open the valve and let the propane flow into the combustion chamber, where it can ignite (after turning off and disconnecting the propane tank, of course).
For the ignition system, at first, I used an ignition coil I had that was waiting to be implemented in a project, powered by a 6V battery (left). By rapidly connecting and disconnecting the battery, the secondary coil produces a few kV which can arc across a spark gap (right). Rather than using a relay, I 3D printed a thing (for lack of a better word) on which I can drag an alligator clip or other piece of wire to rapidly connect and disconnect the battery, producing sparks. For my very first test, I brought the launcher onto my patio and aimed at some bricks. I filled the delivery system to the pressure that I had calculated it needed to be, took cover behind a chair and started sparking the coil. Nothing happened, and I figured that was due to the fact that I overlooked the purity of the propane I was using. The next logical step was to "pump more gas 'till she blows," and that's what I did. On the next try, there were pieces of potato all over the place.
I knew that in the future I wouldn't want to be near this thing when it went off, so I made a remote ignition system with two Arduino nanos. That also meant I had to change the ignition system, so I swapped in one of those insanely high voltage boost converters you can get on Amazon. With an ATMega strapped on board the launcher, I figured it was reasonable to try and measure the muzzle velocity of a potato. Thus, I made a muzzle that would aid in this. The muzzle has 4 LEDs; I learned that LEDs can act as photodiodes, so I used two pairs to create two beams. The LEDs are connected to a comparator, so when the beam breaks, a signal is sent to the arduino and a time difference is measured.
For one of my trials, I measured a muzzle velocity of 40m/s (89.5mph), and then again with slightly more propane at 50m/s (111.8mph). The results given by the Air Force study indicate that acetylene blows the other propellants out of the water. Acetylene is much more dangerous to deal with than propane, but I would love to test it out in the future.
A note about accuracy: To make sure that my velocity measurer was accurate, I tested it by dropping a golf ball through a piece of PVC into the muzzle. Knowing the height at which I dropped the ball allowed me to calculate the velocity that the ball should be traveling at that point thanks to kinematic equations. My calculations and measured velocities lined up, so I am confident in the muzzle's accuracy. That being said, I have conducted too few trials to reach a conclusive result, meaning the 50m/s is, at best, an estimate.
