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Every Easter my brother and I have a friendly competition – together with our kids we must build some crazy device to throw an egg as far as possible. The criteria are simple – either build it from whatever is in Dad’s garage (which is full of useful building materials), or it has to fit in the back of the car together with the suitcases, kids strollers, etc. In the past we’ve built a trebuchet and a ballista. But this year my son and I decided to build an egg cannon.
We built the combustion chamber out of PVC drainpipe fittings, and wound it tightly with many layers of duct tape to contain any fragments should the PVC shatter. At the bottom is a copper pipe with a shut-off valve to close the fuel feed. The plan was to use a propane/butane mix from my blowtorch as fuel. Unfortunately plan A didn’t work too well – getting the fuel/air mixture right was difficult, ignition was very intermittent, and the resulting explosion wasn’t terribly impressive. It threw a tennis ball that had been placed in the opening to the combustion chamber only a few feet. Back to the drawing board.
We obviously needed either a much larger combustion chamber, or a better fuel. Then we remembered on old hydrogen-powered rocket kit that had stood unused in the garage for ages because the ignition circuit on it didn’t work most of the time. How about a hydrogen powered cannon?
We removed the electrolysis cell from the kit, and after a little bit of testing discovered that we got a pretty good rate of hydrogen generation from a 12V power supply (much better than the D-cells it’s supposed to operate from). We modified the plumbing on the combustion chamber to fit the top of the electrolysis cell. We replaced the fuse-wire igniter with a spark igniter using an 8KV power supply board I had lying around, and we’re ready to go.
We had no idea how much hydrogen we would need before we could get ignition, so we started carefully. No ammunition, and the top of the combustion chamber sealed with foil. Goggles on! 1 minute: nothing. 2 minutes: nothing. … 5 minutes: nothing. 7 minutes: pop. OK, now we know what the minumum is, lets increase a little more: 10 minutes: big pop. 15 minutes: bang. 20 minutes: ouch! my ears are ringing – shouldn’t do this in the garage late at night – good job we have tolerant neighbours. Next time I’ll use ear defenders.
Anyway, no signs of damage to the combustion chamber, so time to make the barrel. We measured an egg from the fridge, and found some PVC tubing that was just a little larger (later we took a short piece of tubing to Sainsbury’s to test the eggs to find the optimal size – got some strange looks doing that!). We glued two step-down adapters together to reach the right diameter for the barrel. The long part of the barrel is a push fit into the adapter – the plan is to put foil across the bottom of the barrel to contain the hydrogen/oxygen mix, then insert the barrel into the adapter, then rest the egg on top of the foil. The explosion will punch through the foil, and away will go the egg. At least that’s the theory.
Only time for one test run before we leave for Dad’s place. We can’t fire an egg – we don’t have the space, so we put a ball of foil in to test. That won’t carry enough momentum to hurt anyone if it goes further than the end of the driveway. 20 minutes hydrogen generation later: Goggles on! Hydrogen generation off. Fuel valve closed. Stand well back. 5,4,3,2,1, Fire! Bang! What the…..?
Well, we didn’t see where the ball went, but the barrel and adapter flew about 25 feet through the air. The hot glue holding the adapter into the combustion chamber clearly wasn’t up to the job. Oh well, no time to fix it, put it in the car, pack duct tape and glue gun, and off we go.
Dad’s place has a long field so there’s plenty of space to fire without risk of hitting anything or anyone. We’d joked last year that we’d eventually hit the bottom of the field, but everyone thought we were bragging. Re-glued the barrel – four glue-sticks worth this time. OK, time to fire eggs. First firing with an egg, and it’s in front of our rather critical audience. Goggles on! Hydrogen generation off. Fuel valve closed. Stand well back. 5,4,3,2,1, Fire! Bang! Startled cheers from the audience. We’d blown the barrel off again, but this didn’t stop the egg departing on a sub-orbital trajectory. Where did it go? From right behind the cannon, I’d seen it depart, but no-one else had – they’d been watching the barrel take off. We didn’t find that egg in the field, but it was clear it had flown at least a few hundred feet.
Back to the drawing board. Glue isn’t strong enough. We don’t have many choices – can duct tape stand the strain? A quick fix, and we’re generating hydrogen again. Goggles on! Hydrogen off. Valve closed. 5,4,3,2,1, Fire! Ooops. I forgot to weight the cannon with bricks and the recoil knocked it over backwards. But the egg flew, and the barrel stayed on. Three cheers for duct tape! Can we find the egg? Anyone watching must thick we’re mad – three kids and two “grown ups” searching a large field for an egg. But there it is – a bright yellow hard-boiled yolk, and nearby all the pieces of shell. We paced it out back to the cannon – about 600 feet.
One last try, as it’s starting to drizzle. Covered the electronics with plastic wrapping, and started hydrogen generation. Goggles on! Hydrogen off. Valve closed. 5,4,3,2,1, Fire! Wow – that really went! Keep it in sight – just in line with the patch of mistletoe on the trees at the far end of the field. The hunt is on… But we never found it despite seeing the direction it went. The theory is that it flew beyond the trees at the end of the field and into thick bushes. That would make it at least 700 feet, but we’ll never know for sure.
So what next? We could generate hydrogen for longer – 20 minutes probably isn’t nearly enough to fully saturate the combustion chamber with hydrogen and oxygen. But we’ve run out of field. And I don’t feel like testing my luck with combustion chamber pressure – it’s only PVC and duct tape after all. I think we can declare success and return inside for a nice cup of tea and easter eggs. Next year we’ll need a new contest – we’ve done this one now.