Monday, April 12, 2010

High Speed Explosions

A comparison of the fuel-air flame speeds in butane/air and hydrogen air mixtures.

The mixtures in each case are near stoichiometry.

Hydocarbons-air mixtures are typically what cars run on, just with octane-air rather than butane air. You can run rockets on liquid hydrocarbons/liquid oxygen, indeed the heavy lift section of the saturn V rocket ran on liquids oxygen (LOX) and kerosene (a moderately long chain hydrocarbon). While per unit volume of gasious combustion this actually produces more energy than the hydrogen-air mixture, the nature of that energy (spread over more molecules, with a lower temperature) means that its not a good a propellant as hydrogen-air.

Hydrogen-air mixture have ferocious combustion speeds, over a vast composition range. As the energy of the reaction goes almost exclusively into the water molecules produces (which are very steam), it makes an excellent rocket propellant. Naturally most of the gas in these bottles is the nitrogen that makes up 4/5 of our atmosphere. If it were simply a H2/O2 mixture the explosion would be about 2-3x as big. Rockets run on liquidized fuel. As a rule of thumb, there is about a 1000 fold increase in density on liquefaction.

This is the mixture the shuttle uses, and it uses some 4000 liters of it per second!
That's the equivalent of burning some 4 million of these bottles per second!

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