How do I find out the ambient pressure for designing a vaccum rocket engine? I am using the propellant combustion charts from the braeunig website. But I cannot figure out the ambient pressure for the 200-500 km LEO OR SSO. Since the pressure in space is never completely zero I will need a value for the ambient pressure for moving on with the design. Please help. Thanks.
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$\begingroup$ Have a look at the (currently unanswered) question Why does Earth's atmospheric density have a big “knee” around 100 km? Is there a good analytical approximation? as well as grc.nasa.gov/www/k-12/airplane/atmosmet.html and also ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539.pdf if you enjoy reading further. $\endgroup$– uhohApr 21, 2019 at 10:04
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$\begingroup$ Here's one more! This and this answer link to braeunig.us/space/atmos.htm which seems to be gone. However it is archived here, yay! $\endgroup$– uhohApr 21, 2019 at 10:16
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$\begingroup$ @uhoh do you know anything about how to choose the 'low' or 'high' or 'mean' solar activities columns given in the tables in the website while designing a rocket engine? $\endgroup$– user167195Apr 21, 2019 at 12:42
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$\begingroup$ A nozzle expanding to vacuum pressure is impossible to build, its length would be infinite. But a nozzle expanding to ambient pressure for the 200-500 km LEO would be too long too. $\endgroup$– UweApr 21, 2019 at 12:50
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1$\begingroup$ Use zero, cut off the nozzle based on weight or packaging limits. $\endgroup$– Organic MarbleApr 22, 2019 at 18:05
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Since the pressure in space is never completely zero i will need a value for the ambient pressure for moving on with the design.
Looks like you only consider the Isp to get the area, and your model always gives bigger Isp with bigger area. As soon as you're interested in something else - mass of the nozzle, for example - or the model of the nozzle is different - for example, too cold a gas can condense and then it stops adding to the thrust - you can find your optimum.
In practice, gas does condense in vacuum nozzles, there are some pictures of that with, IIRC, Arian vacuum stage. Added thrust is smaller and smaller for each extra unit of nozzle exit area, while mass of the nozzle is bigger and bigger for the same unit of area, so if you consider full Tsiolkovsky equation there will be some optimal area.
Look at pictures of vacuum nozzles to get an idea. Everybody has his own limitations - staging issues (avoid collision with previous stage), or rocket length (will need a longer interstage), or material of the nozzle (light carbon composites can allow wider and longer nozzles), or mechanical side forces for thrust vectoring (you'll have to consider nozzle inertia when swiveling the chamber) or manufacturing capabilities etc. so numbers can differ.
