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?

  • $\begingroup$ 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

1 Answer 1


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.


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


  • $\begingroup$ 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. $\endgroup$ 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 $\endgroup$ 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 $\endgroup$ Apr 16, 2021 at 0:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.