Combustion Chamber Potato Cannon

A few days ago I saw an article in the local Popular Mechanics magazine that featured a potato cannon. This inspired me to build my own. I started collecting all the necessary parts today. I have quite a list, and was not able to source all of the items. The PVC that our local hardware supplier has is not pressure-rated, and I am afraid that the combustion will cause it to shatter. I will start looking for stronger fittings and PVC piping later this week.

I will be using Brut deodorant as a fuel, as it contains both propane and butane (a simple hydrocarbon that holds a fair amount of energy). I will be spraying the deodorant into the chamber for 1-2sec (I will need to experiment), close the endcap, and ignite the gas using a spark. The spark will be created via a piezoelectric switch. Piezoelectricity (for those that do not want to read the wikipedia entry) is basically energy generated by a certain type of crystal. Mechanical deformation of the crystal (via the button) creates a potential difference (voltage), which I will use to create a spark over two wires.

While I will be testing the cannon with potatoes, I am more keen on what else I can do with it. I am already coming up with ideas to launch model airplanes with it. This will require a special ammunition piece, lowered combustion power, and an extra long barrel. I am also trying to think what I can achieve with small rockets.

Another idea of mine (more of a challenge) is to try break the sound barrier. With the plans I am using, the piece of potato should be exiting the barrel at around 500km/h (138.8m/s). I think the sound barrier challenge is achievable using a hybrid cannon. I’m thinking of filling a chamber with compressed air, and remotely pumping a propane/butane mixture in. The valve release/ignition of gases would have to be very well timed, so as to happen at exactly the same time. I would need a more reliable igniter. The one I set up this afternoon has about a 70% success rate in producing a decent spark.

The project is getting me quite excited. My interests lie in both mathematics and physics, and here I am able to apply both, even some chemistry. Now I just need a way to integrate some electronics, and it will be the perfect hobby.

Let me know if anyone wants to see the cannon’s calculations and plan. Using the current plan, there should be an exit speed of about 500km/h, which will give me a height (if aimed vertically upwards) of over 900m, and air time of around 28sec.

See above for an example cannon. Only difference is that my chamber to barrel ratio will be 1.5:1 and the one in the picture is 1:1. The 1.5:1 ratio gives a slightly more powerful cannon, reason being that the potato will still be accelerating the moment it leaves the barrel, as all the gas in the chamber will not yet have escaped. Using a 1:1 ratio however, the potato will leave the barrel when all the gas in the chamber has escaped, slowing it down slightly near the end of the barrel. Oh, and I will also be using long wires running to the igniter’s switch. I am quite careful when it comes to my safety, so I will be keeping a fair distance away from the cannon.

Let me know if anyone else finds this interesting.