Dan’s Space on the Web

A couple of weeks ago during my block leave after passing out from BMT, I got somewhat bored so decided to do something I’ve been wanting to do since secondary 3 – build a spud cannon.

Why since then? During that year, we were tasked with building a tennis ball launcher as a physics project. Most teams went with the obvious catapult or trebuchet, but my team decided to try something more exotic like an air pressure based system. That’s when we discovered spud cannons, and eventually didn’t just build a tennis ball launcher, but a tennis ball cannon. It made a loud “boom” each time you fired it, and when placed on the floor launched the ball 40+m. Yea, we totally killed the competition. Unfortunately, our teacher wanted to keep our model to show to future students, and since then we’ve been wanting to make another.

Back then, we used the ball-valve approach. Unfortunately, it’s a pain to find a suitable one – the last time we found it in some obscure warehouse around the bukit timah area, not sure how we actually came across the place. In addition, we were told that such valves would cost about 100 bucks, but back then the man was nice enough to give it to us for free since we were just secondary students.

I was originally going to record our steps here, but due to things not going smoothly I didn’t have time to do so and my block leave ended, and now I have to keep it short. If you’re interested in giving it a shot, just search the web – there are plenty of resources. I’ll mainly only be posting not-so-general info.

This time, we went with the more common combustion design. Went down to one of the stores around Clementi Ave 2 and picked up all our PVC piping needs, then picked up a $1 BBQ ignitor from a convenience store.

Some manual labor later, we had our first prototype. However, initial attempts didn’t work. We’re still not too sure why, because if we were to open the combustion chamber and try igniting, we can clearly see the gas igniting out of the chamber (note: don’t point at anything. or anyone). Most likely it was due to a lack of oxygen. We then changed from using WD40 to Adidas body spray (that’s the best thing we got). Still no luck, but we noticed “more explosive action” when testing with chamber open. After letting the chamber air for awhile and wiping it down (WD40 left a lot of liquid around), we tried again.

I wouldn’t say it was successful after that, because although we got a couple of success shots, it wasn’t reliable and failed most of the time. The success shots, though, were amazing. With a loud boom and some pyrotechnics show, the potatoes really flew fast and far, but we kinda lost track of them so no distance results. Too bad we didn’t get those shots on camera, too.

I tried further modifying it, namely to improve the ignition module, and I even tried drilling in two screws and hooking up the ignitor’s wires to them. It seemed to worked well – when you clicked the ignitor, you can clearly see the electric spark jumping across the screws in the chamber. However this seemed to have even worse results – it never worked once.

Out of time, I put it down and left it for a couple of weeks, until this weekend. However, there’s this thing about leaving a potato in there – it doesn’t just grow buds, there were friggin’ maggots starting to form and crawl around inside. Fortunately it looks like they just started appearing, and weren’t anywhere close to crawling out of the cannon. Still, it’s probably best to just junk this ~$15 attempt.

I suspect the main problem was that our combustion chamber was not big enough. If you sprayed too much, there wasn’t enough oxygen, and if you sprayed too little, probably not saturated enough to reliably ignite.

MK-II shall be built soon, and hopefully this time we figure out what went wrong.