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I was wondering whether there is a limit to the amount in which the perceived g-forces during atmospheric reentry can be reduced by trading fuel consumption for smoothed out deceleration peaks.

To define the concept I had in mind; a certain launcher coming from a certain orbit, say the one of the ISS, has a specific reentry corridor (neglecting skipping reentry and straight drops). And the atmosphere has a specific density distribution along it's altitude, so once a certain window of angles, altitudes and velocities for reentry have been selected, I could imagine it is impossible to slow down any slower than that density/atmospheric drag implies, no matter how much fuel you have left to slow the launcher down before the atmospheric drag becomes too dominant (e.g. >2 G's).

(Or as XY-problem, since I could not find a lot of data:) What generally is the minimal perceived (retrograde) deceleration during for the manned spaceflights to the ISS during atmospheric reentry?

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In theory, with sufficient fuel you can come down in a powered reentry as gently as you like to a limit of just above 1g -- this is symmetrical with a theoretical ascent from surface to orbit at just above 1g. The amount of fuel required is prohibitive, however.

With a high-lift reentry vehicle, reentry force can be moderated somewhat. If you had sufficient control of both lift and drag, it would be possible to again achieve any g-force profile desired. In practice, heat is the limiting factor; slow deceleration from orbital speed in atmosphere sufficient to provide lift comes with extremely high temperatures, and the heat needs to be dealt with somehow. The US Space Shuttle had the gentlest g-force profile on reentry of any manned spacecraft, and this required a very advanced thermal system.

Soyuz reentry from the ISS, using body lift to control the trajectory, typically incurs no more than 4.5g force on reentry.

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