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Another Docking Thread


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Hello Everyone!

I realise that there are a load of docking threads on here, and have looked at some of the latest ones (including the one currently on the first page of this forum). I have also watched a load of youtube docking tutorials. I'm still getting very frustrated!

My problem seems to be that I get within a few metres of the docking target (see screenie), but on final approach, always seem to overdo it. I end up almost "orbiting" around my target and not lining up properly with the target docking port. It doesn't help that I'm playing this on an underpowered tablet with a lowish frame rate, but don't feel I can entirely blame that.

At the moment, I'm running the "pure" stock game, but would not be against using a (reasonabe) mod to help me. I won't go as far as editing savegame files, though!

Any ideas?

Cheers,

Adam

screenshot0.png

Edited by Pope_Gregory_IX
Answered
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It makes me happy to see that rendezvous is not your problem :D

From what I can see, your RCS thrusters look they could potentially be unbalanced. That could be one source of the problem, as unbalanced thrusters can make it difficult to maneuver your ship. Additionally, if it's taking you a long time to get into position, your position relative to your target is changing as you both orbit around Kerbin. You can tell this is happening if your relative velocity is zeroed out and you head in for docking, but by the time you get there, your target is rotated out of position. There's a few solutions to this problem:

A) Dock faster. The faster you get docking done, the less time your target has to move out of position.

B) Align your docking ports along the normal-antinormal axis of your orbit. This will ensure that your position relative to your target won't change as you orbit. All you have to do is select the target, set "control from here" on the docking port you want, and point at N or S on the navball (assuming you're in an equatorial orbit).

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1) Press "[" or "]" to switch between Near-Craft.

2) Put one of your craft docking port facing "North" or "UP" via the Orbital view (press "V") to switch between view. Then you will not having the Orbiting effect issues.

DR66KUH.png

Edited by Sirine
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Your screenshot doesnt look like you have made the docking port your target.

You will also want to select the docking port on your ship and select "control from here" this will mean the tagets on your nav ball will be the exact targets you need to dock.

I dont think i have ever used the docking mode which uses the rcs thrusters to dock.

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A simple mod to help you would be Navyfish's Docking Alignment Indicator. You don't need to add anything to your craft, and it switches on automatically when you select a docking port to dock with. It doesn't dock your craft for you, that still has to be done manually, but what it gives you is a fairly easy to read indication of when you are lined up with your selected docking port.

Once it comes up, all you need to do is use the W,A,S,D keys to get the orange ball in the middle, use the Q,E keys if you need things lined up in the roll plane, and then its a simple matter of keeping the yellow circle on the point where the green lines cross using the translational keys I,J,K,L until you are docked. Easy.

Docking Alignment Indicator

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Another mod is mechjeb.

NOT for the docking autopilot, but for SMART A.S.S.

You can use it to have each craft target each other (make sure you select "control from here" on the docking ports, and target the other ships docking ports). This way both ships will always be in correct alignment.

Once you have mastered docking doing that, have it target - NML on one craft, and + NML on the other. Slightly more difficult.

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If it seems to you that your target is rotating away from you, then the problem is that orbital mechanics is kicking in - you are not flying in a straight line, you're orbiting something. You need to orient both yourself and the destination docking port in a direction perpendicular to your orbit, which is in most cases (equatorial orbit) directly towards north/south - that's the place where the brown line intersects the horizon line on Navball, or the place directly opposite to that (180°). Then the port will stop rotating away and you'll have plenty of time to line up yourself and dock.

Edited by Kasuha
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Thanks for the prompt replies and all the advice, everyone!

@vetrox: Quite right - I had thought you made a ship, rather than a docking port, the target. I will check this.

I have the target in a N-S position, to attempt to minimise movement due to orbit, but will take all the advice on board and retry.

Thanks Again,

Adam

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^^ You're wrong, because in the second one pairs of engines fight each other's thrust when you do translation (which is in the screenshot).

Too bad for you but I am not.

Yes, in the second set-up there will be 4 nozzles firing instead of 2 but they do so at reduced thrust. The total amount of thrust is exactly the same!

If you don't believe me just install HydroTech. I has a nifty feature that lets you see the total amount of RCS thrust you have in each direction.

Edited by Tex_NL
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Too bad for you but I am not.

Yes, in the second set-up there will be 4 nozzles firing instead of 2 but they do so at reduced thrust. The total amount of thrust is exactly the same!

Yes, each of them will fire at ½√2 times regular thrust (roughly 70%)

So you have TWO thrusters, doing a total thrust of 141% of what one regular thruster does. Power-wise there's no difference but your mono propellant use will be 41% higher.

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^ Exactly. The issue isn't so much the thrust, but efficiency of propellant use. Since they're firing at 45 degree angles relative to the direction of travel, each one needs to fire at slightly more than half-thrust to get the same net effect as they would if they were simply firing in that direction.

In effect, as opposed to an inline arrangement, where the thrust looks like this:

jiCkJGg.png

(Red is the vector for each engine's thrust, black is the final vector of the craft's velocity.)

With an offset arrangement, you end up with something like this:

W1UArOV.png

The black arrow stays the same length, but if you measure the red arrows in each arrangement, you'll find that the ones in the second picture is slightly longer than the first one.

Images are not to scale, but they should be enough to get the point across.

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This ... is bull crap! Both RCS placements ARE and DO exactly the same! Neither is more efficient and/or stronger than the other.

They don't do the same. They are equally powerful in sense of general translation, but if you press just one translation key, the number of activated RCS engines (and therefore monopropellant consumption) is smaller with the arrangement on the left.

If you always press two translation keys at once (e.g. I and J together) then the one on the right is better. But I doubt anyone does that all the time.

Edited by Kasuha
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  • 3 weeks later...

Just wanted to say thanks again. I've been away from KSP for a while, but some of the hints I picked up on the forum finally allowed me to get the hang of docking. The points that were particularly helpful were starting at a high altitude (400k), balancing my craft better (starting smaller) and proper use of targeting. My new space station is a mess (solar panels all over the place, looks odd...), but it's a start and I now have orbital refuelling capabilities!

Cheers,

Adam

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No, it does matter. The second placement isn't aligned with the craft's axis, whereas the first and correct placing aligns perfectly. When you translate left, the thrusters that point parallel to that direction fire, but in example two, the jets are not parallel with the direction of travel, requiring more fuel to be consumed for the same result. Not only is it less efficient, but it also is less responsive.

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