This weekend I burned all the hair off one of my arms. I've seen people on youtube launching lemonade bottles by setting fire to an explosive mixture of alcohol vapour or propane. I had an empty plastic milk bottle and I own a propane blowtorch so I thought I would have a go at it myself. I am however an idiot. I assumed that it was a type of rocket and that as the exhaust shot out of the bottle conservation of momentum would shoot the bottle in the other direction. If I'd thought of some rough numbers I'd have seen that that couldn't possibly be the case. What does a plastic bottle weigh? 25g? 50g? What does a litre of air/propane weigh? Not much more than 1g. What's the specific impulse of gaseous propane? I don't know but it's going to be tiny. What's the nozzle efficiency of an open plastic bottle? Again I don't know but there is no nozzle. You'd have more chance of a stream of gas moving that bottle if you had an asthmatic child blow at it through a straw.
So what happened is that after a few attempts I got an explosive mixture of gas into the bottle and set light to it. The bottle stayed resolutely still and a brief plume of flame leaped from the open end and ran up my arm. It didn't hurt, the skin is slightly red in one place, but every hair was singed and curled up like a pig's tail.
The actual demonstrations that I had seen are a type of mortar where a "launcher" fits snuggly into the mouth of the bottle.
Richard "Don't try this at home" B
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