The currently controversial theory of “peak oil” holds that the top of the curve of world oil production – a barrels-per-day arc that rises with demand until it begins to fall because of dwindling supplies – either has already been reached or will be soon. Whatever side of the argument you’re on – there are still a few who insist that world oil supplies are for all practical purposes unlimited – it’s apparent that fuel isn’t getting any cheaper. Jet fuel has almost tripled since 2000. The geopolitical consequences of possessing or lacking oil reserves are becoming more important; and alternatives to petroleum-based energy sources have ceased to be the preoccupation of isolated eccentrics.
Nearly all aircraft engines operate by burning a liquid fuel. The resulting release of heat expands a gas, and the ensuing pressure rise does some kind of work, either turning a propeller shaft or driving the heated gases and combustion products out the tailpipe at high speed. Some engines are fussy about their fuels, others aren’t. Rockets are notably omnivorous: Small rocket motors have been made that consume wadded paper, animal fat and even, in the case of motors made of acrylic plastic, themselves. Burt Rutan’s SpaceShipOne flew into space three times, and collected a $10 million prize, on the energy released by burning common rubber in a torrent of laughing gas.
