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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
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"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

2 Answers 2

The principal disadvantage is the indirect use of energy. Energy is used to compress air, which in turn provides the energy to run the rocket. Any conversion of energy between forms results in loss. Hence it's more economically efficient to burn fuel rather than compress gas and then release it. This idea has been used in Compressed Air Cars much before. Also look at Compressed Air Energy.

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What is "indirect" use of energy? Did You ever calculate the energy of such a 2 ltr-bottle at 90 psi? –  Georg Jul 12 '11 at 21:25
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@Georg the point is there is an energy cost to compress the air inside the bottle to 90psi which would be greater than or equal to energy of such a bottle at 90psi. –  David Jul 12 '11 at 21:30
    
so you're saying it does generate a lot of thrust, but it's more efficient to just burn it rather than exert energy to compress it? –  exosuit Jul 12 '11 at 21:55
    
@phycker I pretty much think this is not the reason. The energy per unit mass of fuel is a much more important factor for aircraft, and long range aircraft in particular. I think a compressed air system would be too heavy for it to get off the ground for most practical flights. –  Alan Rominger Jul 12 '11 at 21:57
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How meany cubic m of air at say 3000 psi do you need to flay a 747 for 5 hr. Now there's the question. I guess it wont fit in the plane –  Fortunato Jul 13 '11 at 1:54

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

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