Why isn't pressure used for flight?

Why isn't pressure used as flight?

I've heard that 2L bottles can hold a pressure of up to 90 PSI safely. Since $F = PA$, if the nozzle of a pressure rocket is about 4 inches squared in area, that would be a thrust of 360 pounds!? Is there something wrong with my math, or why don't we just pressurize air and use it for flight?

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How are the bottles supposed to "hold" the pressure while you release it to gain the thrust? –  leftaroundabout Jul 12 '11 at 20:56
A pump could supply more pressure while it's being released out the other side. It would still lose thrust but not as fast –  exosuit Jul 12 '11 at 22:01
"A pump could supply more pressure while it's being released out the other side." Then you have to power the pump. If you are clever you do that by burning fuel in the pressure chamber at which point you have described a turbo-jet or fan-jet engine... –  dmckee Jul 13 '11 at 2:17
As a side note, a compressed air rocket cart does make a good classroom demo. –  dmckee Jul 13 '11 at 2:18

In a way it is. A jet engine will create a high pressure region in the combustion chamber and discharge it through the nozzle. It turns out this is inefficient, and it is better to use the pressure to drive turbine blades that push the cold air around the engine. Remember cold = higher density = higher momentum.

To a lesser degree a propeller will create a high pressure region behind the blades, that results in thrust.

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Nice response. Internal combustion for all its faults is nothing more that expanding gases pushing the cylinders. –  Fortunato Jul 12 '11 at 22:40