I've just watched Scott Manley's video SpaceX's Water Landing Reveals Rocket "Secrets" (or, What We Learned from CRS-16) (again) and there's some rigid body dynamics in there where I'm not sure I am completely understanding exactly what he is saying.
After concluding that this was a single engine burn and so thrust vectoring of the single, central main engine could not address the stuck-grid-fin-induced roll he says:
So I think a lot of the last minute correction comes from the reaction control thrusters. Now another thing to see is that as it gets down, it starts to pitch over. This is what would happen if the engine starts to thrust off center and starts to correct, you would of course get a gyroscopic precession-style effect where the actual rotation would be at 90 degrees to the one the thruster was trying to perform.
When he refers to the single engine correction, is it correction for pitch/yaw, or correction for the roll problem, which he seems to indicate would not work as the single engine is on-axis.
And what is it exactly that is 90 degrees to what else exactly? If there is a governing equation for this effect, it would be great to add it as well.
Did the engine vector 90 degrees to the direction of tilt in order to correct it correctly, or was the correction 90 degrees away from where it was needed because the gyroscopic effect was not properly taken into account?
cued at 05:05