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During the take off of a rocket, the exhaust produces some pressure below the rocket, which gives an additional force. How large is this force in comparison to the force produced directly by the exhausted gases because of conservation of momentum?

When the rocket is flying through air, is there any measurable (minimal) effect of the additional thrust produced by pressure because the exhaust hits the outside air?

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Why did you downvote this question? –  martin Nov 12 '11 at 11:26
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Probably because the person misunderstood the question the same way I did at first: the pressure forces inside the rocket are the forces produced by the exhasted gasses because of conservation of momentum. What you are really asking is what is the effect of a barrier placed behind a rocket, which will increase the pressure in the exhaust region, and this can work its way back to the rocket. The effect is probably miniscule, but it isn't ruled out by general principles. –  Ron Maimon Nov 12 '11 at 12:41
    
@'Ron Maimon': Thanks. That's exactly what I mean. Perhaps the question was not clear because of my bad english... feel free to edit it, to make question clearer. –  martin Nov 12 '11 at 13:26
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The opposit will happen. The highest possible exhaust velocity (and thus the highest efficency) is reached in vacuum. In air of 1 bar the exhaust is a bit slower, less acceleration results. –  Georg Nov 12 '11 at 16:39
    
@Georg: At the nozzle throat the choking conditions are formed and father downstream super-sonic velocities are reached, so no external pressure influence is possible. Hence, the exhaust is not "slower". –  Vladimir Kalitvianski Nov 12 '11 at 17:46
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Any pressure difference along the rocket will result in air motion. Normally the rocket jet entrains some air down and makes a negative pressure difference. However, if you enclose the rocket in a container, you can get a positive pressure difference. Watch videos of rocket launching, especially those vapor clouds along the missile, and you will get an idea how important the pressure difference is in reality.

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