docking alignment problem

[antipro]

hi,

I'm trying to dock a module on a space station.
The problem is that when I try to align the SS with the module, the SS move to a 45° shifted position.
I can't understand why.. it happened when I docked the 2 solar panels modules too.

 

Edited February 22, 2018 by antipro

[Streetwind]

One of the biggest problems in docking actually happens long before you attempt to dock - it happens during spacecraft construction. You want to use your RCS thrusters for "translation" - lateral movement without rotation. However, since there are multiple RCS thrusters firing at the same time, this only works if all of these thrusters are the same distance from the spacecraft's center of mass. If one is further away from the CoM, it gains a longer lever arm, and exerts a greater force on the spacecraft than the other thrusters. And suddenly, your attempt to translate has accidentally induced a rotation in the spacecraft, and you drift out of alignment.

Building perfect RCS alignment is near-impossible in stock KSP, since the editor offers no information on how well aligned the thrusters are. The best you can do is activate the CoM display, and eyeball where to put your RCS. While doing so, you should remember that you need to be balanced for the docking procedure, not for sitting on the launchpad; if your spacecraft expends part of its fuel before docking, then its CoM will change in flight, and your thrusters become unbalanced. In this case, you have to guess how much fuel you will have expended, and empty the tanks accordingly in the editor while placing thrusters. Don't forget to fill them back up afterwards!  

The good news is that SAS compensates small errors just fine, so you can get away without being perfect.

But if you want help, the mod RCS Build Aid will give you visual aides on your thruster alignment. It's highly recommended to have.

[antipro]

ty but I still dont understand why when I select to control the SS form a docking port that isn't aligned with the main ss body, it doesnt work properly

Edited February 22, 2018 by antipro

[linuxgurugamer]

It controls from the selected docking port.  And all control is relative to THAT docking port

[bewing]

It might be a mod screwing up your control algorithm.

I'll take a look in a pure stock game and see if I can reproduce your issue.

 

Edited February 22, 2018 by bewing

[Kerbal101]

Have you clicked on the docking port of the craft you are piloting and selected "Control from here"?

[antipro]

https://www.dropbox.com/s/wn8x4sa8kwfwjed/2018.02.22-13.10.mkv?dl=0

[Clipperride]

Two things to consider.

1. Your space station will rotate around its CoM, so once you start adding extensions out to the sides, it makes the whole thing a lot more complicated to visualise.  I'd be inclined to leave the SS and maneuver the docking vessel into the right position to dock.  

2. As @Streetwind points out, for best results, you need your RCS thrusters equally placed from the CoM.  Although stock KSP doesn't offer any specific tools to help, a simple ladder placed on the CoM and extended works well as a measuring aid when building your docking stage.

[antipro]

I can't understand why you keep continue to talk about rcs, i dont need it.
as I did till now, I align both the ships docking ports, then I push gently(with rcs or main engine) the module toward the SS, and they automatically dock using magnets.
but I cant align the SS when the docking port is perpendicular to the main body axis.
adding another RC-L01 close to the SS docking port axis doesnt help.

 

Edited February 26, 2018 by antipro

[Zhetaan]

7 minutes ago, antipro said:

as I did till now, I align both the ships docking ports, then I push gently(with rcs) the module toward the SS, and they automatically dock using magnets.
but I cant align the SS when the docking port is perpendicular to the main body axis.

For simplicity, let's call the docker the Vessel and the dockee the Station.

Try right-clicking on the Station's docking port and selecting 'Set as target' while you're still in control of the Vessel.  Also try right-clicking on the Vessel's docking port and selecting 'Control from here'.

I suspect that what you are doing (and the images in your first post seem to support this) is targeting the Station and leaving it at that, and the problem is that when you set a target in KSP, KSP defaults to pointing the target indicator towards the target's centre of mass.  The centre of mass on the Station is nowhere near the docking port and it doesn't pay any attention to alignment, so even though you may be pointing the Vessel's docking port straight at it, the target on the navball doesn't make sense.

The ability to right-click the other vessel's docking port and select 'Set as target' was specifically created to solve this problem.

Edited February 22, 2018 by Zhetaan

[antipro]

what you can see from the screenshots is the space station controlled by one of the 2 docking ports T-module.
the target is the LAB-module docking port. and the angle between them is 45°
the lab-module remains perfectly aligned.
 

in the video you can see better

Edited February 22, 2018 by antipro

[Zhetaan]

You've targeted the correct docking port and you're aligned.  There's no reason why you shouldn't be able to dock.  Remember that the target marker on the navball only shows the direction of the target docking port; it can't show the port's orientation.  If you were to wear a GPS tag, for example, I could use a tracker to see where you were in terms of direction and distance, but it could not tell me which way you're facing.  For that, you need either a mod or liberal use of the camera combined with a Mk. I eyeball.

I would suggest that you not bother with trying to target the ship with the station.  It appears to me that your station is fighting its own inertia when it tries to align with your vessel, and that is not good.  I have the best results when I set the station to SAS-normal (that's the  marker) and then dock to it; in other words, make the station as ... er ... stationary as possible, and let the smaller ship do all of the manoeuvring.

Edited February 22, 2018 by Zhetaan

[antipro]

well, i've just tried to dock 2 test vessels using their perpendicular ports, and i have the same behavior, both the ports dont align with eachother.

Edited February 22, 2018 by antipro

[HvP]

@antipro I would strongly recommend that you try installing one mod that will go a long way to helping you with docking.

https://forum.kerbalspaceprogram.com/index.php?/topic/40423-131-docking-port-alignment-indicator-version-670-updated-053117/

Docking Port Alignment Indicator gives you an extra window with orientation guides to help you point in the right direction for docking. Be sure to watch the video in that link as it will give you detailed instructions on how to use it (make sure Youtube's annotations are turned on to see the video text.)

Edit: I see from your pictures that you actually mean the station SAS isn't rotating properly to automatically face the target. I see no reason for that. I still recommend the above linked mod, but it is used for manual docking control and won't fix your SAS problem, sorry. I need to have a think about this.

Edited February 22, 2018 by HvP

[HvP]

@antipro Let's try and work the problem from the ground up.

Do you have sufficient control authority to turn? That includes reaction wheels and electric charge to run them or sufficient monoprop remaining.

Also, do you have the MechJeb mod installed? It has its own version of SAS that if selected will override the stock SAS directions.

Edited February 22, 2018 by HvP

[HvP]

35 minutes ago, antipro said:

well, i've just tried to dock 2 test vessels using their perpendicular ports, and i have the same behavior, both the ports dont align with eachother.

Can we see a picture of these test craft, too?

[GrouchyDevotee]

If I'm reading the problem correctly...

Set the station to hold a set position. (it is always easier to move the smaller craft)

move the space craft that is trying to dock close to the station

then switch the RCS from linear to rotation

realign the space craft with the docking port on the station

switch the spacecraft RCS back to linear and get a bit closer

keep switching back and forth until you have the rotation of the space craft in alignment

then use the linear controls to advance into a docking berth.

.

The relative positions of the station and the spacecraft will always continue to drift a bit due to the fact they are orbiting a planet.

Edited February 22, 2018 by GrouchyDevotee

[HvP]

9 minutes ago, GrouchyDevotee said:

If I'm reading the problem correctly...

I think the problem is shown in the second screenshot. The station's SAS is set to hold target, but you can see from the navball that the SAS is not aligned with the target marker at all. The pitch/yaw indicators in the lower left also show that the station isn't even trying to move to align with the target. For some reason SAS control isn't functioning correctly, and I'm not sure why.

Edited February 22, 2018 by HvP

[GrouchyDevotee]

1 minute ago, HvP said:

The station's SAS is set to hold target, but you can see from the navball that the SAS is not aligned with the target marker at all.

Yes. I'm suggesting that the station should be simply be holding position while the incoming module does the final manoeuvring.

OP has said that the procedure being attempted hasn't worked twice before, and I've not heard of this working before.

I'm on a PS4, so I don't have any of the MODS, and I've never attempted to use "hold target" with a space station.

I wasn't sure if my suggestion was the solution because I have not tried to get the station to change orientation for docking ever since I figured out how to use the Rotation and Linear functions of the RCS.

In this case, I still suggest the station holds its position.

The module needs to rotate so it's not pointing at the docking port, but is instead simply squared up with the station, then use the linear RCS to move in front of the dock.

[HvP]

I see what you're saying @GrouchyDevotee and it's true that what you're describing is the preferred docking method. But the original poster says that he can't get other test vehicles to aim correctly either so I think it would be worth it to try and track down what's actually causing the problem.

Just to be clear @antipro it is normal operation that the two ports won't necessarily be aligned square to each other, but they should at least point at each other. Do the station or your test vehicles ever move or do they just sit still? And if they do move, do they ever point at the target marker on the navball?

[antipro]

https://www.dropbox.com/s/y17scd1uzg8irtk/2018.02.23-00.19.mkv?dl=0
 

and in the video i've posted above,
https://www.dropbox.com/s/wn8x4sa8kwfwjed/2018.02.22-13.10.mkv?dl=0

at 0:36 when the station moves, it's me with the keyboard to show you how is it moving back to the original position.

and also when I said that the angle between ports was 45°, I was wrong.
the angle change continuosly according to their distance.

and also when I control the station from a perpendicular port, the Q and E keys dont work.
the station non longer rotate.

Edited February 22, 2018 by antipro

[HvP]

20 minutes ago, antipro said:

https://www.dropbox.com/s/y17scd1uzg8irtk/2018.02.23-00.19.mkv?dl=0
 

and in the video i've posted above,
https://www.dropbox.com/s/wn8x4sa8kwfwjed/2018.02.22-13.10.mkv?dl=0

at 0:36 when the station moves, it's me with the keyboard to show you how is it moving back.

Excellent videos showing the problem. Are you running an older version of KSP by any chance? I seem to remember a bug like this being addressed in a changelog somewhere.

In the meantime, I'm going to do some testing on my setup and get back to you.

[antipro]

v1.3.1.1891

[HvP]

23 minutes ago, antipro said:

v1.3.1.1891

Confirmed, I'm getting the same behavior on my system using a recreation of your test vehicles.

This appears to be a stock bug that I was unaware of. Off-axis control points do not correctly aim at target when under SAS control. It seems to be a problem with the stock target tracking and not the docking ports themselves because MechJeb WILL target correctly under the same conditions using it's "Smart A.S.S." targeting controls.

It appears to be a continuation of this bug in the bug tracker. I've added my own report to the bug tracker now and would suggest you do as well if you'd like.

For now it looks like the only work around is to do the docking without SAS target tracking assistance or use the MechJeb mod's "Smart A.S.S." controls to do the orientation instead of the stock SAS.

Edited February 23, 2018 by HvP

[Clipperride]

18 hours ago, antipro said:

 

Edited February 23, 2018 by Clipperride