It’s Easter, and in the Fzz family that can only mean one thing: the Easter Challenge. Last year we built an Egg Cannon, but this year the challenge was to build a device to take an aerial photograph of my father’s house. Extra points are awarded for style and the use of mad science.
With those bonus points in mind, my son and I set out to build a hydrogen reconnaissance balloon, complete with a remote control camera and mounting. But we only had a week from idea to challenge, so we had to work fast. Some quick calculations and the kitchen scales revealed we needed to lift around 400 grams, which meant around 500 litres of hydrogen would be needed. Could we generate this much hydrogen in a reasonable amount of time, and could we store it without it leaking away?
Rubber balloons turned out to be a dead loss: hydrogen’s tiny atoms diffuse through the rubber so fast that the balloon deflates in a few minutes. Mylar is better, but where do you get enough mylar to make a balloon that will hold 500 litres? The local hiking shop supplied a couple of mylar space blankets, intended for use when you’re lost in the mountains. Folded over, and taped at the edges, they seemed like they should make pretty good balloon envelopes.
The next problem was how to generate enough hydrogen. You can either go the acid or the alkali chemical route to hydrogen generation, or use electrolysis. Electrolysis is great if you want hydrogen and oxygen mixed as a propellant, but not so good if you want vast quantities of pure hydrogen without the explosion risk. Hydrochloric acid and zinc is pretty simple and effective, but the resulting zinc chloride is somewhat toxic. So that leaves sodium hydroxide (caustic soda) and aluminium.
It took quite a few experiments to get the ratios right, but pretty soon we were producing large quantities of hydrogen. But also large amounts of heat; this is a very exothermic reaction. We weren’t only producing hydrogen, but also very hot water vapour, verging on steam. Not good, especially when we got enough caustic soda foam inside one of the test balloons to dissolve the mylar. So, what we need is a condensor to cool any foam and condense out most of the water vapour from the hydrogen flow. We added a second stainless steel container in the middle of the hydrogen pipe, part filled with water. This container is then immersed in a bowl of water to cool it. The gas bubbles through this water, cooling and condensing out any water/sodium hydroxide froth. In our garage, this seemed to work, and we filled a mylar test balloon successfully the day before the challenge, and lifted a 200 gram screwdriver. It finally looked like this might actually work.
The last component was the camera. I borrowed my wife’s Canon Ixus, which is small and fairly light. Ebay supplied a cheap radio control rig that could drive a couple of servomotors. One of these was to control the camera shuttle release, and the other was to allow the camera to be panned around to point in the desired direction. Some foam polystyrene, duct tape, and tying wire completed the payload. Hopefully this is sufficient to protect the camera from rough landings, but it’s not likely to survive a drop from 200 feet or getting stuck up a tree. Best to avoid those outcomes.
With everything ready but completely untested, we headed for my father’s place near Malvern. Amazingly, the weather was perfect: sunny and very little wind. Everyone teamed up to help, and before long we were happily brewing hydrogen, sorting out a harness for the balloons, and arranging kite strings. After an hour of vigorous bubbling, acrid smells, and occasional topping up with aluminium foil plates and a little more sodium hydroxide, the balloon envelopes were nearly full. Would it fly? We hadn’t even had time to test the remote or the balloon harness; would they work?
Yes!!! It lifts. With hindsight, we should have added even more hydrogen. It lifts quite well, but the wind is stronger than the lift, so it tends to fly with the kite string at a fairly low angle. Keeping it out of the trees was tricky, but we eventually got a fair amount of height, and my son drove the remote control camera. After half an hour and one rather-too-close encounter with a tree (mylar is quite strong stuff it seems), we brought the balloon back down. Would the camera have taken any pictures? A little water had still made it into the balloon, and steadily dripped over the camera. Was it still working? (Next time, don’t put the balloon filler valve directly above the camera).
The pictures worked! The hydrogen reconnaissance balloon was a success.
It turned out the mylar envelopes hold hydrogen very well. We were able to fly the balloon four more times over the next two days without needing to refill it at all. The best conditions were late evening and early morning, though one of my father’s neighbours got a bit of a surprise as the reconnaissance balloon drifted past his bathroom windows when he was in the shower. It’s fortunate he’s got a good sense of humour (and now, a nice set of aerial photographs of his house too).
A selection from the 800 pictures we took are below, plus some pictures of flying the balloon. If you want to see how we built it, and how to make hydrogen, details are over here.