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The best description of the Soyuz docking system I could find is in the proceedings of the 6th Aerospace Mechanisms Symposium (scroll to PDF page 139, labeled page 143). The alignment process is described as follows:

As the spacecrafts are pulled together, their alinement is accomplished by an electric drive, which retracts the large screw. Pitch and yaw alinement is controlled by the linkage assembly, while roll alinement is achieved by pulling up the latches inside narrowing socket slots. To reduce friction during alinement, the arms are fitted with rollers. Once the docking rings have made contact, the force required for capture is produced by the safety clutch of the drive and maintained by a locking clutch.

Does this mean that the shape of the slots in the docking cone interfacing with the ball on the probe lead to roll alignment? Can I find a description of those slots somewhere?

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    $\begingroup$ That document's pretty old, the system looks a bit different now. I didn't find enough to write an answer, but there's some info in the Soyuz Crew Operations Manual infosondas.com/2017/04/… on pdf page 59. It calls the slots "trapezoidal" - I'd interpret it as saying that the latches on the tip of the probe go into the slots, which narrow down and constrain the vehicle in roll. This says to me that you couldn't be wildly off at docking in roll, which actually makes sense. $\endgroup$ Commented Oct 12, 2021 at 23:31
  • $\begingroup$ Given that the only slots visible are inside the end of the drogue where the ball of the probe sits and latches, Organic Marble is probably correct. There is roll seen when the probe screw retracts, and this is guided by the alignment levers (for yaw and pitch, not roll), but its always when the probe cone is already inside the drogue and you never see it. By the time the (8 or 12) latches on the ring lock, the guide pins are in their socket and everything is already lined up before hard-dock is applied. $\endgroup$ Commented Sep 30, 2023 at 9:17

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Does this mean that the shape of the slots in the docking cone interfacing with the ball on the probe lead to roll alignment?

Yes. Bar minor changes, this aspect of the docking design has remained unchanged since it was first implemented in the 1960s.

As Organic Marble says in his comments (i cannot see the PDF):

It calls the slots "trapezoidal" - I'd interpret it as saying that the latches on the tip of the probe go into the slots, which narrow down and constrain the vehicle in roll.

Correct, you can see the trapezoidal slot - outlined in green - below:

enter image description here

when the probe head enters the drogue, the (4) latches extend from the head and engage within the (4) sockets at the end. When the probe head retracts (screws), the latches are dragged along the edge of the narrowing socket edge until they become fully engaged and cannot move from this position (to do so, the latches would be retracted, for removal of the hatch and for undocking). This is where the final, minute (within 10 degrees of error), roll alignment is carried out.

Small video to show its operation:

latches on the probe head seem part-extended in this view, alignment levers (with wheels attached) can be seen retracted inbetween the 'cone legs'.

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Larger video culled from ESA that shows the graphical representation of the docking procedure, with the probe head contacting the drogue cone, entering the socket when the latches catch and then the probe head retracts, simultaneously the alignment levers have extended forward into the drogue to alleviate friction as final pitch and yaw alignment is carried out, and as the latches on the head are dragged along the edge of their socket the final roll alignment is performed, bringing the alignment pins and sockets and interfaces into alignment on the docking rings, and final retraction beings the hooks of the 8 (or 12 on APAS type) latches together for hard-docking sealing.

enter image description here

trapezoidal sockets, allow for a very mechanical way of roll alignment.

From OP's PDF link:

When the probe head has been caught by the drogue socket and relative spacecraft motions have been attenuated, the docking mechanism’s drive is actuated to pull and align the spacecraft. The alinement is accomplished by the narrowing socket slots and through expanding the linkage assembly (for pitch and yaw alignment) by a cam rotating synchronously with the screw movement.

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Early development sketch, showing socket design:

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Text, from left to right: Docking mechanism, Receiving cone socket, Profiled groove

Russian text:

на пассивном — приемный конус с ответным гнездом под головку штыря. Профилированием паза обеспечивают выравнивание по крену в процессе стягивания.

On the passive - a receiving cone with a counter socket under the pin head. By profiling the groove, roll alignment is ensured during the tightening process.

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    $\begingroup$ @OrganicMarble - removed all the previous maneuvering bits and just focused on the probe and drogue aspect, thanks! $\endgroup$ Commented Sep 30, 2023 at 15:08

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