Limit to reduction of G-forces during reentry?

Question

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?

Answer 1

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 means a sustained period of high temperatures, and the total heat load needs to be dealt with somehow. The US Space Shuttle had the gentlest g-force profile on reentry of any manned spacecraft, around 1.6g peak, 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.