A comment below Is Starship planned to fly directly to the ISS without first stage? Is it even possible? says:

to my best knowledge, the Starship will never be near the ISS. it is too large to dock even if you fit the correct docking adapter on it. The control thrusters of such a large and heavy vehicle would be a possible danger for the ISS. (emphasis added)

For background, this answer to How hard was it for the Shuttle to perform attitude control for the entire ISS? Did they have to calibrate? indicates that (unsurprisingly) the Shuttle was able to perform attitude control for the Shuttle + ISS docked superstructure without modifications.

Question: But can a spacecraft be "too large to dock" to the ISS? Please consider both issues of thruster spatial configurations and exhaust plumes, and their strengths and torques to the ISS, as well as any other potential issues.

To me it seems that Starship could easily be configured so that it would be safe to dock with anything either much larger or smaller than itself, or of a similar size; but I'm no rocket scientist.

That the ISS could have its attitude controlled by torques applied from the docked Shuttle also shows that the ISS is designed to have this done to it as long as the torques are below some established limits. Larger docked craft would have larger thrusters but also have a larger moments of inertia, so the ISS wouldn't experience larger torques just because it's bigger.

However larger thrusters means bigger exhaust plumes that might be directed towards the ISS, and there are a lot of ISS intrinsic systems and addded-on experimental packages that might be compromised.

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    $\begingroup$ It may be possible if a too large mass is docked to the ISS, the attitude control algorithms of the ISS become unstable. An unstable attitude control would waste any amount of thrusters fuel up to zero. $\endgroup$
    – Uwe
    Jul 23, 2021 at 23:44
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    $\begingroup$ If the station's control software couldn't be updated, it could easily be put into free drift mode, allowing the Starship to control the attitude...IIRC, this was the typical mode of operation with the Shuttle docked, though I'm having a hard time finding specific mentions of this. It especially seems likely the Shuttle was in full control during reboost maneuvers. $\endgroup$ Jul 24, 2021 at 1:05
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    $\begingroup$ @ChristopherJamesHuff: I believe there is an answer by Organic Marble somewhere on the site with a quote of all the detailed steps from the Operations Manual, and from what I remember from reading that answer, indeed, one of the steps is to put the station into free drift and have the Orbiter control the combined stack. $\endgroup$ Jul 24, 2021 at 8:58
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    $\begingroup$ @JörgWMittag: This answer about ISS reboost talks about free drift mode, but from my reading is is only applied during the reboost activity. This answer mentions free boost during the docking phase, but not once docking is completed. $\endgroup$
    – Jan Fabry
    Jul 24, 2021 at 9:34
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    $\begingroup$ I haven't seen a study for something the size of starship to dock with the ISS, but I did see something about having 2 Orbiters docked at the same time (During the time it was Freedom). This was soon shelved due to the structural integrity not being up to it. Also saw something about different ways to dock and their effects - something along the lines of 'bumping and grabbing' (also Freedom period, and then docking was dumped entirely for 3 years). And there was a video about the plume effects from Orbiters manoeuvring thrusters - but this may have even been during the Dual Keel Freedom days.. $\endgroup$ Jul 24, 2021 at 20:31


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