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How to stop spinning in circles while docking?


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I play the game stock. I can rendezvous and dock no problem, but often times both the passive and active vehicles will begin spinning as they approach each other, kind of like ice skaters holding hands and rotating on the rink. Is there a way to stop this? I try multiple times to bring velocity to 0, then point the vehicles towards each other using small reaction wheel movements, and then resume approach, but it still happens.

This is not a vital issue, just a quality of operations type thing :)

I do have tried both keeping SAS on and off on the passive vehicle during approach. It will still start moving away laterally. I do not use SAS for the active vehicle at all, not when aiming towards the target nor during approach.

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  On 11/17/2021 at 2:23 AM, SunlitZelkova said:

I can rendezvous and dock no problem, but often times both the passive and active vehicles will begin spinning as they approach each other, kind of like ice skaters holding hands and rotating on the rink. Is there a way to stop this? I try multiple times to bring velocity to 0, then point the vehicles towards each other using small reaction wheel movements, and then resume approach, but it still happens.

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@HebaruSan is correct, so I'll give a bit more context and explanation.

You can't stop it.  This is an emergent property that arises because you're in a rotating frame of reference.  Because your orbit is a curve, the direction of forward motion (what you and I know as prograde) changes with respect to any absolute frame of reference.  Anything that moves to intercept is necessarily on a different curve, and so the different directions of prograde will result in relative drift.  This is true even if you match orbits perfectly first, because moving to intercept necessarily changes the shape of your orbit.  This is also the reason that all of the counter-intuitive manoeuvring (i.e., decrease altitude to increase speed, and so forth) works in orbital mechanics:  you have to account for the rotating frame of reference.  That's true on the surface, as well, but we can usually approximate things with straight lines and flat surfaces (though there are notable exceptions in navigation:  following a great-circle course, with a few edge case exceptions, means constantly changing your heading).

This rotation affects everything in any combination of prograde and radial directions.  You may note, for example, that radial in at the periapsis points in the opposite direction of radial in at the apoapsis.  Over the course of the orbit, the compass rose of prograde, retrograde, radial in, and radial out rotates around one time.  This rotation is parallel to the axis of the orbit and perpendicular to the plane of rotation, i.e., about the normal direction.  What this means is that normal does not change over the course of the orbit, and you can use this to your advantage when docking.

Provided that you do not change inclination much, you can approach from normal or anti-normal and get a dock without the target vessel rotating away from you.  This only works for small relative inclination differences because matching orbits in every way but inclination means setting yourself up for a collision later--either controlled docking, or the more explosive version.  Other alternatives include docking in a high orbit (with its slow rotation) or simply docking quickly (this could mean rendezvousing, matching at a close distance, and waiting for a potion of the orbit for the target to point the docking face that you want towards you).

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I am guessing you are docking from quite far away? By the time I point noses towards each other I am within 3-4 ship lengths, and no frame of reference effects are noticeable to me in the short time until dock. I would be interested in seeing your docking style :)

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  On 11/17/2021 at 2:23 AM, SunlitZelkova said:

I do have tried both keeping SAS on and off on the passive vehicle during approach. It will still start moving away laterally. I do not use SAS for the active vehicle at all, not when aiming towards the target nor during approach.

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If at least one ship has SAS with "target tracking" available (from a pilot or from an advanced probe core), consider using it.

Turn on target tracking for your active ship (#1) to aim at ship #2.  Press "]" to take control of #2, the now passive #1 will continue to track as #2 moves. Note that if you press "]" again to go back to #1 then #1's target tracking will turn off, so you may have to re-select target tracking every time you change active ships.

In other words, when a ship becomes passive it keeps its SAS mode, when a ship becomes active its SAS mode resets to stability assist.

What does all that mean? If your active ship "spins in circles" during approach, you just need to fix that one ship's approach. A passive ship with target tracking will use its SAS reaction wheels to automatically spin itself to aim wherever the active ship ends up.

(I think this is stock behavior. I have a lot of mods but I don't think any of them change SAS.)

Edited by DeadJohn
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  On 11/17/2021 at 5:38 PM, splashboom said:

I am guessing you are docking from quite far away? By the time I point noses towards each other I am within 3-4 ship lengths, and no frame of reference effects are noticeable to me in the short time until dock. I would be interested in seeing your docking style :)

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I don't, but it is possible that the sloppy nature of my docking style might affect it, now that I think about. The distance that I begin docking is similar, but I usually come screaming in at something like 40-50 m/s towards the target before hard braking about 20 or so meters off, then dropping to 0 m/s, then turning, then proceeding to dock :D

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