Problem ISS attitude control problem

[DanM]

So I was trying to dock the soyuz with the ISS last night, but it was difficult because it kept "bouncing" while I was on final approach. Is this normal?
Does the real ISS do this, I haven't noticed it in videos.

 

[tblaxland]

Can you elaborate what you mean by "bouncing"? Was it a sudden shift in either the relative rotation or position of the two spacecraft (very strange and unusual), or was it a slow drift of the relative rotation (quite normal, and in real life the spacecraft correct for it with their attitude control systems).

Also, what are the versions of the addons are you using in the docking process (Soyuz, ISS, and any addon MFDs)?

EDIT: Do you see the same behaviour between the Delta-Glider and the ISS included in a clean Orbiter install?

 

[garyw]

In real life the ISS goes from US CMG control to free drift. This allows an approaching vessel to dock without the risk of the ISS using CMG's to maintain orientation.

It sounds to me like you have something active which is firing a thruster on the ISS and changing it's orientation. When I dock I always make sure I've used the "Kill Rot" autopilot beforehand as this makes sure the ISS is stable for my approach. This is my version of "free drfit"!

 

[tblaxland]

In real life the ISS goes from US CMG control to free drift. This allows an approaching vessel to dock without the risk of the ISS using CMG's to maintain orientation.

Someone correct me if I'm wrong, but my understanding is that during approach the ISS's attitude is still controlled by ISS's attitude control system (normally the CMGs). This keeps the ISS in a stable (but rotating) +XVV ZLV* attitude. The attitude control systems of both the ISS and visiting vehicle are only switched to free drift after soft-dock capture to allow relative motion to damp out prior to hard-dock, after which ISS attitude control is reactivated.

* Means +X axis aligned with the velocity vector and Z axis aligned with local vertical, both of which are rotating wrt an inertial frame.

Your method of using killrot is not like real life, but is much easier

 

[DanM]

Can you elaborate what you mean by "bouncing"? Was it a sudden shift in either the relative rotation or position of the two spacecraft (very strange and unusual), or was it a slow drift of the relative rotation (quite normal, and in real life the spacecraft correct for it with their attitude control systems).

The best way I can explain it is that it pitched up a few degrees then pitched back down for a fraction of a second.

 

[tblaxland]

The best way I can explain it is that it pitched up a few degrees then pitched back down for a fraction of a second.

Were any autopilots running at the time? Eg, prograde, retrograde, Attitude MFD, etc? It sounds like a conflict either between autopilots or between an autopilot and manual RCS input.

Referring back to my earlier question - can you replicate the problem with the Delta-Glider and the ISS included in a clean Orbiter install?

 

[Izack]

Are you using Thorton's ISS? If so, its CMG autopilot is a little 'bumpy' and behaves a lot like you describe. If so, you can temporarily disable it from within, and reengage it when Soyuz is docked.