Off-axis docking

[Damien_The_Unbeliever]

I was trying to build my space station earlier and one area I kept going wrong was trying to connect up some final docking pylons that I carried into orbit separately - there are some close in docking ports that (for safety) I disable some solar panels before using - so I was carrying some pylons up that would allow docking to occur further away from the station core.

I was carrying three pylons up and (with some fiddling) I'd managed to fit them to a stack tri-adaptor - but with them all offset by different heights above it. So the plan was that I'd dock the furthest out first, fire off some decouplers, then dock the next highest, etc.

The problem I had was during docking. Normally, when docking I select "control from here" from the port I'd like to use, and then select the port on the target vessel as my target, and so long as I got the navball indicators lined up (my velocity, centre of target), they'd at least hit. But what I was experiencing here was that the ports would slide past each other. I had to do the alignment by eyeball and it took a lot more effort (and didn't seem to match up with what the navball told me).

So - is there some trick to performing off-axis docking I need to learn? Was my previous intuition that setting "control from here" and targeting a port on the other vessel, and keeping my velocity vector and the target aligned should at least cause a collision between those two parts incorrect? Is it in some way related to the COM not being aligned with the part?

(No mods at the time, I've just installed FP)

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Similar situation recreated using a deliberately exaggerated vessel:

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Edited November 21, 2014 by Damien_The_Unbeliever

[DrD]

Well I'm not sure I undertand your issue without a screenshot but I'll give it a stab:

Your navball tells you nothing about the angle of the ports. So the port you're aiming for could be like 90% off and you'll get the exact same steering cues from the navball. There are several solutions:

1. control target vessel and orient its port towards your original spacecraft. The best way to do this is to use a fixed reference that is common to both craft. You can use prograde/retrograde or radial in/out but these will change over time so if you have to re-align you have to start all over. Scott Manley suggests using the normal/antinormal marker as this will NOT change during your orbit. Then go back to original spacecraft and point in the same but opposite direction. Now your ports will be aligned. The tricky bit is you now have to dock using only translation. Even harder is that your target marker and your prograde marker may be out of the visible portion of the navball. also remember to use this you have to change to orbit mode to see your normal/antinormal then back to target mode. You can rotate if you need to to see target marker but then you have to switch back to orbit mode and re-align to normal/anti-normal.

2. use a mod. FDAI gives the most help, in particular it lets you see prograde and target marker even when your orientation causes them to be off the visible nav ball. I use a simpler one called navball docking alignment indicator that just gives you another, red marker on navball that shows you where to point to be at the right angle. Thus if other markers are off the visible navball you can rotate to see them, correct, then rotate back to the red marker. Eventually you want all 3 (target prograde, prograde, and alignment) to match up.

[The_Rocketeer]

I've had this problem before too, I think. Basically eyeballing it was the only answer I could come up with. I don't know why it happens, though your guess about CoM alignment sounds plausible to me.

If eyeballing is the only option, the chase camera can make life a lot easier than trying to do it with the free/orbital cam.

[Laie]

Look here:

http://forum.kerbalspaceprogram.com/threads/86086-docking-woes-navball-parallax-error

[Damien_The_Unbeliever]

Well I'm not sure I undertand your issue without a screenshot but I'll give it a stab:

Your navball tells you nothing about the angle of the ports. So the port you're aiming for could be like 90% off and you'll get the exact same steering cues from the navball. There are several solutions:

I've updated my post with an album, this time I was flying a vessel with a deliberately exaggerated situation

[Damien_The_Unbeliever]

Yes, I think I'm in a similar situation. I've recreated a similar situation and uploaded images for it.

[zekes]

I know this feel. It's eyeball all the way for me. Fortunately i'm really good at it.

[Belphegor]

i find eyeballing a lot easier when you align every axis separately. get close, kill forward velocity, rotate the view sideways and align the vertical rotation of your ship, change to top view and align horizontal rotation. then align the docking ports and kill relative velocity in both views, go forwards till the the docking ports are just out of reach for the magnetic field and check both views again, adjust if necessary and give it a nudge

[Laie]

Again, look at this thread for an over-complete description of why you have the problem you're having, and what you can do about it.

In a nutshell, eyeballing is your only hope, or mods that help with eyeballing (docking cameras), or do the eyeballing for you (alignment indicators).

[Pecan]

Using the NavballDockingAlignmentIndicator be aware that although it accounts for off-axis placement of the target port it does not account for the position of the controlled one. In the OP images it therefore wouldn't help very much. Although it's based on NavyFish's he has updated that since and this may be corrected in the full version.

[Wanderfound]

Yes, they're offline, and it's a nuisance. The best solution is to eyeball it, but it helps a lot if you can get your view into the angle you need.

And that's where Raster Prop Monitor comes in:

All docking ports have an RPM camera by default, aimed right down the barrel of the port. And the RPM displays give you all of the information you need. IVA docking is easy.

[psyper]

I use the Docking Port Alignment Indicator - its a godsend and I never leave home without it!