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If during atmospheric re-entry a capsule (with a shifted center of mass to produce lift) does not produce any rcs thrust, will the capsule naturally roll? and why?

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The capsule will roll into its most stable position (probably minimum lift) and oscillate around that position. It won't rotate and thanks to the bell shape it would keep the heat shield side forward.

It would basically go into what's called a "Ballistic Reentry". Scott Manley has a great video "Ballistic Reentry vs Aerodynamic Reentry" that explains a lot of the concepts.

Unguided, ballistic reentry has happened several times with Soyuz spacecraft over the last 5 decades (nice article about one of those incidents on Universe Today, reason was later determined to be a failure of an explosive bolt that failed and did not cleanly release the service module) they had been in services. G-loads during unguided reentry are significantly higher than with aerodynamic reentry (4-6g instead of 8-10g) which is well within the limits of the human body (especially when lying on the back, firmly strapped in).

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  • $\begingroup$ Why the most stable position of the capsule is that of minimum lift? $\endgroup$ Commented Dec 21, 2022 at 18:57
  • $\begingroup$ @SebastyenLaroche that's a very good question to which I could only answer with assumptions about "minimal energy". It's just what they do when following a ballistic trajectory. I'd suggest you ask this as a new question (I'd be interested in the answer as well). $\endgroup$
    – TrySCE2AUX
    Commented Dec 22, 2022 at 5:59

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