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Imagine a cylindrical space station module, orbiting at an arbitrary height - the same as the ISS say.

Another identity cylindrical module docks on the end. Real station cross section hasn't changed - the same surface area is pointing in the direction of travel.

How does this effect the total drag caused by the atmosphere? If zero impact, what if we add a third? A fourth? A fifth?

At what point would we materially change the drag experienced?

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  • $\begingroup$ You'll get some skin friction drag from the new section, but at that altitude air density is very small---just 6.017e-12 kg/m3---so even though you're moving at ~8 km/s, and drag is proportional to the square of velocity, you're still looking at drag forces 3 to 4 orders of magnitude smaller than you'd get at sea level (~1 kg/m3) moving at 1 m/s. Plus, friction drag is just one component of the whole drag force, which includes also form drag, and I don't think that would change if your cross-sectional area hasn't changed? Either way, the effects would be measurable only on long time scales. $\endgroup$
    – user39728
    Apr 30, 2021 at 7:13

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Drag is proportional to the cross-sectional area. If you don't change that and keep the other parameters (coefficent, air density, velocity) the same, the drag will not change.

https://www.grc.nasa.gov/www/k-12/airplane/drageq.html

What you will change is the ballistic coefficient, which is proportional to mass.

https://en.m.wikipedia.org/wiki/Ballistic_coefficient

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  • $\begingroup$ In dense air, at least, changing the proportions of a body while leaving its cross-section the same does change its drag coefficient (due to increased skin drag, if nothing else). I don't know if this holds for near-vacuum and v = 7700m/s, though. $\endgroup$
    – Russell Borogove
    Apr 15, 2021 at 16:48
  • $\begingroup$ @RussellBorogove It's at least, a strong function of the windward area "One of the more complex tasks in determining the coefficient of drag is the development of an accurate model for the cross-sectional area of a spacecraft" etda.libraries.psu.edu/files/final_submissions/9285 $\endgroup$
    – Organic Marble
    Apr 15, 2021 at 17:11
  • $\begingroup$ This is interesting. I think skin drag in the molecular flow regime will need its own question. $\endgroup$
    – uhoh
    Apr 15, 2021 at 23:45
  • $\begingroup$ This is interesting. I think skin drag in the molecular flow regime will need its own question. Speed of 1200 K monatomic oxygen molecules at 400 km in the thermosphere is about 1000 m/s so they will be hitting the sides of a long skinny rocket-shaped space station and so will cause some drag, but while we don't yet know how much, we can't say there's no effect. $\endgroup$
    – uhoh
    Apr 15, 2021 at 23:50
  • $\begingroup$ @uhoh I can say there's no significant effect. imgflip.com/s/meme/Change-My-Mind.jpg $\endgroup$
    – Organic Marble
    Apr 16, 2021 at 0:32

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