Docking problem: Ships too large, not lined up enough, or something else?

[kasuteru]

So I've tried to dock two ships for the first time, and I guess I got a little carried away with the size of the ships (See screenshot). I've been trying to dock these for about half an hour, and the just don't want to. My question is, what did I do wrong? Are these ships simply way too big for docking with a regular-size pod (The fuel tanks in the "Gas Station" are almost full, thats 9 of the biggest tanks there are). Or do I have to get them perfectly aligned in a line? My understanding was that magnets in the docking ports pull these ships together?

Or did I do something else wrong? I remember that the docking port for the "Gas Station" didn't snap to the middle of the Monopropellant tank it is on, I had to manually align it... might that have something to do with it?

Oh, and since I'm writing this: Is there any mod that keeps pro/retrograde marker on the Navball / somewhere else even when the relative speed is just 0.1? I noticed that in stock KSP, it goes missing sometimes even though the craft is still moving, so I'm never able to bring them to a complete standstill...

Thanks :-)

EDIT: Solved! As the comments say, Docking Indicator Mod (one of them) is essential when docking large crafts! Thanks!

Edited June 25, 2015 by kasuteru
Solved!

[tutike2000]

http://i.imgur.com/9f4TK7g.jpg

Or did I do something else wrong? I remember that the docking port for the "Gas Station" didn't snap to the middle of the Monopropellant tank it is on, I had to manually align it... might that have something to do with it?

Oh, and since I'm writing this: Is there any mod that keeps pro/retrograde marker on the Navball / somewhere else even when the relative speed is just 0.1? I noticed that in stock KSP, it goes missing sometimes even though the craft is still moving, so I'm never able to bring them to a complete standstill...

Thanks :-)

Maybe the docking port is broken because of this. Try using node attachment only (hold down alt when plaving the part).

MechJeb's SmartASS will continue to hold orientation even if you switch to a different craft.

A very easy way to dock is to have MJ on both craft set to Target+, and target each other's docking ports. Don't forget to set 'control from here' on the ports.

[DancesWithSquirrels]

There are magnets in the ports, but they aren't invincible - given ships with the masses you're showing, you're going to need accurate alignment and an approach around 0.01-0.02 mps. Right now your ships look to be 10-12 degrees offset, and that's not going to work.

[ajburges]

That's an awful lot of mass for a standard docking ports. The attractive force won't do much with that mass so you will need to be very precise. I hope you are not trying to tug one ship or the other over just that one connection.

I can't recommend Docking Port Alignment Indicator enough. I give you a toggleable, auxiliary display for bearing, offset, rotation, lateral velocity, and lateral distance. All the info you could want.

[moronwrocket]

I recommend installing Navyfish's Docking Port Alignment Indicator. Makes docking a snap, even in the dark - which inevitably happens every time I need to dock. Use LOCKED camera mode to align your view with your RCS thruster directions. The "Dock Align" window on the right is what DPAI looks like when active. There's an alternate mod that just puts docking indicators on the navball, but DPAI is just too awesome not to use.

I also recommend installing RCS Build Aid. It allows you to place RCS thrusters so you get minimal torque when you try to translate. Looking at your craft on the right, you've got a 3-symmetry vehicle with 2 thrusters on your outriggers and what looks like 1 band of thrusters on the center - that's gotta be a PITA to align. Also, RCS BA allows you to plan around your Center of Mass, or Dry Center of Mass - handy for refueling vessels that need to handle well when near full and near empty.

Re: snapping - Hold down the Alt key, and the docking port attachment node should snap to the tank attachment node. Make sure you have symmetry disabled.

[Sharpy]

I've been docking things that big to the standard port. It IS hard.

First, you need to align the plane so that the ports are flush with each other. There's no chance for them to catch at that angle. Then bring your craft from the front - moving in sideways often doesn't work. Then, once the magnets start pulling, disable all RCS and SAS and don't touch anything. The magnets may need a while to align you right.

Also: Set the port as target, your port as "control from here". Move a good bit back. Align the angle to be flush, then start moving towards the port slowly, and using H,J,K,U (translation controls) chase the "prograde" marker in the navball to be squat in the middle of the "target" marker. Keep it there and let the magnets do the rest.

(what I mean by "chase": stop your craft for a moment, start moving forward very slowly, then - stop paying attention to the main screen and just observe the navball. Without ever turning your craft, press the translation control keys and see how each your keypress moves the marker around the navball. Note which moves it in what direction. It's actually a very distinct, short move. Using them just push it into the target marker.)

Edited June 25, 2015 by Sharpy

[Kurld]

One thing I managed to do once is use the alignment widget in the VAB to somehow wind up with something that looked right but I actually stacked on top of the docking port. I didn't figure it out until I started taking the craft apart in the VAB to try and figure out what was going on.

[Snark]

I strongly recommend Navball Docking Alignment Indicator (this is a different mod than the one mentioned above):

https://spacedock.info/mod/1098/Navball Docking Alignment Indicator CE-2

I like it because it's minimalist. There's no additional UI anywhere. All it does is add an extra icon to the navball, if you happen to have a docking port targeted. The mod is tiny, reliable, uses practically no memory, doesn't clutter up the UI, automatically gives me exactly what I want when I want it and stays out of my way otherwise. Zero effort on my part required.

With it, docking's a breeze: just set things up so that all three markers (docking alignment, target, target relative velocity) are centered in the crosshairs, then wait until docking happens. It's easy to set this up:

1. Get my ship into approximately the right place pointing in approximately the right direction.

2. Target the docking port. (This causes the mod to add the red "docking alignment" indicator to the navball.)

3. Rotate my ship so that the red docking alignment indicator is precisely centered in the navball crosshairs. 1 down, 2 to go.

4. Thrust forward by 0.2 or 0.3 m/s.

5. Look at the "target" and "target relative velocity" markers on the navball. Goal is to have them both precisely centered on the crosshairs. Without rotating my ship, use RCS to adjust my lateral velocity so that the target marker starts creeping towards the center.

6. Wait until target is perfectly centered. 2 down, 1 to go.

7. Use RCS to thrust laterally to center the target-relative-velocity marker. Bingo.

Then just wait until docking happens.

[kasuteru]

It only took 30 minutes or so, but i finally made it!

Thanks you guys, I'm so happy right now :-) I finally got them to dock, with 0 monopropellant left! The problem was my imbalanced, way to heavy craft. It had to be really precisely lined up for docking, which is impossible to do with the standard Navball. Thanks for everybody who recommended an Docking Alignment Indicator Mod, that was exactly what I needed! With it (and lots and lots of patience) I was able to do it. I haven't tested how stable my setup is, but at least now I can fuel up for Moho

that's gotta be a PITA to align.

Above is very true. Thanks for the RCS BA hint, I'm definitely going to try that out.

I hope you are not trying to tug one ship or the other over just that one connection.

I wasn't planning on doing that (just fueling up for Moho), but since this is KSP, I'm at least going to try it It is currently turning in one piece, albeit very slowly, since I definitely can't use thrusters.

Also, I just figured out that the weird green Smileys in this forum are Kerbals and not, as previously assumed, Zombies.

Edited June 25, 2015 by kasuteru

[Snark]

By the way, one additional hint when you're docking: Turn on "fine control" (default hotkey is caps-lock). You can tell if it's on because the control-status indicators at bottom left will change to cyan instead of their usual red.

Fine-control mode makes all the rotate/translate controls much more "gentle", which is useful in and of itself for docking. But even if you have a surgeon's touch on the controls and scoff at the need for "fine control", this mode gives an extra bonus when you're using RCS thrusters to adjust your velocity:

In the usual default mode, each RCS thruster is either "completely on" or "completely off"; there's no in-between. This means that if your RCS thrusters aren't completely symmetrical around your center of mass, then every time you try to thrust laterally, it's going to induce some rotation (which your ship will then have to compensate for via SAS). This makes handling clunkier and wastes monopropellant.

However, when you turn on "fine control", your RCS thrusters do something special: they turn on proportional thrust so that you get error-free lateral thrust. This makes for stable, rotation-free lateral thrusting, which gives better control and saves monopropellant.

For example, suppose you have a long skinny craft, and one ring of 4 thrusters up at the front end, and one ring of 4 thrusters at the back end. Also suppose that your CoM isn't in the middle, but is actually towards the back of the craft.

If you try thrusting sideways without fine-control on, then all the thrusters (front and back) fire sideways at full power. But since the back end of the craft is heavier than the front end, that means that the ship is going to torque; the RCS thrusters are not just pushing the ship sideways, they're also trying to rotate it in the direction you're thrusting. Then SAS kicks in to correct (which will use RCS since it's turned on, which in turn wastes monopropellant).

On the other hand, if you thrust sideways while fine-control is on, then the RCS control system automatically calculates the needed thrust based on the positioning of your thrusters around your center of mass. In this case, it will fire the front-end RCS ports at a lower power than the back-end ports (since the front end of your ship is lighter). Result: the ship moves perfectly sideways, without any torquing. Better control, and no wasted monopropellant.

[ajburges]

Fine control RCS drives me crazy! It is so busy balancing itself, no thruster fires fully.

Just balance RCS to no more than ⅓ your torque from reaction wheels at all fuel levels and it will behave reasonably well. Also, crunch the numbers on translation acceleration. I hate ships that can't manage .05 m/s² translation or better while full!

[kasuteru]

...

On the other hand, if you thrust sideways while fine-control is on, then the RCS control system automatically calculates the needed thrust based on the positioning of your thrusters around your center of mass. In this case, it will fire the front-end RCS ports at a lower power than the back-end ports (since the front end of your ship is lighter). Result: the ship moves perfectly sideways, without any torquing. Better control, and no wasted monopropellant.

This is a great hint, thanks! I knew about fine-control, but I wasn't aware that it changes how the thrusters behave. I had the impression that sometimes my craft starts spinning when translating and sometimes it doesn't, but I couldn't understand why. This is probably the reason. (I switched between fine and raw control a lot)

Oh, and I was able to get from a 100km to and 1,500km orbit with some very careful burns with my craft. I had to turn SAS off though because by trying to stabilise the craft, it always broke it apart.

[cephalo]

Another thing that can be incredibly frustrating is when your target ship is rotating slightly without you knowing. Anytime you bump a craft with a near docking miss it can start to rotate just a tiny bit, then the variables you must mind and adjust go from 4 to 8. Make sure SAS is on in the target craft and that it's stable.

[Kurld]

This.

I find it helps me zero out relative motion if I orient the target vessel so that it is pointed at normal or anti-normal on its orbital navball.

[moronwrocket]

NO WAI? That's stock? That's totally news to me. Where is that documented? I'll have to use fine alignment more often I suppose, even with RCS BA it's sometimes impossible to get 0 torque during translation, especially with nuke engines on the bottom of long tanks.

- - - Updated - - -

@Kurld,

Yeah, good point, learned that one the hard way when I kept wondering why my target vessel kept rotating. Now I always set my targets vertical as I dock. (most of my ports now are on the side anyways)

[Laie]

If you have a skilled pilot or high-tech probe core, having the vessels target each other can ease the job *a lot*. Smaller vessels can be docked without any RCS at all, with big ones it will eventually start overreacting as you close the gap. Still, it helps a bundle.

In a pinch, setting SAS on both vessels to hold (anti-)normal orientation will also ensure a plain and level alignment.

[RW-1]

Another thing that can be incredibly frustrating is when your target ship is rotating slightly without you knowing. Anytime you bump a craft with a near docking miss it can start to rotate just a tiny bit, then the variables you must mind and adjust go from 4 to 8. Make sure SAS is on in the target craft and that it's stable.

I'm sure we all have had that occur ... I typically at that point retreat back slightly and null, then switch crafts to then engage or reorient the ship that I'm docking to then switch back, unless you like the challenge, makes it kinda fun

[Snark]

(regarding using "fine control" mode when using RCS)

I knew about fine-control, but I wasn't aware that it changes how the thrusters behave. I had the impression that sometimes my craft starts spinning when translating and sometimes it doesn't, but I couldn't understand why. This is probably the reason.

One thing to bear in mind-- even though fine control mode certainly helps, there's no substitute for having a properly-designed ship that makes at least a half-way reasonable distribution of RCS thrusters around the CoM. If things are too lopsided, fine-control mode may not be able to help you.

The auto-thrust-adjusting feature can only really help you if you happen to have thrusters on both sides of your CoM (so that it has something to balance). If your thrusters are hopelessly placed, it can't help. For example: Imagine you have a long, skinny ship, and a ring of RCS thrusters at one end, and no RCS thrusters at the other end. Your CoM is way offset from your RCS thruster ring, with no balancing RCS thruster ring on the other side. In that scenario, fine-control's auto-balance feature is helpless.

Moral of the story: place your RCS thrusters well.

If you have a reasonably compact shape (i.e. isn't long and skinny), like a lander, then aim for a single group of 4 thrusters in a ring, which is centered as closely as possible on the craft's CoM.

If you have a long, skinny shape, put a ring of RCS thrusters at each end. Ideally with the CoM halfway in between them. If the CoM is significantly off from halfway, then fine-control mode will help you with that, but you can make its job easier by shifting thrusters around to be more symmetrical around CoM.

[Coga19000]

Well, I'm about to be the weirdo that actually knew that Fine Control provides RCS thrust balancing, but is still super happy to read that thread because he finally found out WHICH IS THE GODDAMN FINE CONTROL BUTTON! YAYYYYY

OK, I'll just calm down now for a moment.

Oh, and if you have slight rotation on the target craft, there is a (bit cheaty) way to solve this ;just turn on (non-physical) timewarpfor a split second, and it will be gone! This ability is a double -edged sword, though, because the reason it happens is the same reason you cannot fire Ion Engines in non -physical timewarp.

[Wemb]

Docking Alignment is very useful- I've also found that, if you can work comfortably without the exterior view, the docking MFD display in RasterPropMonitor gives even more useful data - displaying, numerically, both the lateral and rotational error and currently velocity - this is very, very useful and can really help fine tune a heavy close approach to a docking port.

I generally use the exterior view to get on the right side and get my apporach sorted out - my target will have MJ on and so will be rotating or doing something stupid, I retract any at-risk solar panels, and I get within 30-50ms, and switch to the IVA mode, and finish the docking on RCS using the above mentioned MFD panel - I don't need to 'see' where I'm going, and flying with those numbers, if I'm in a good starting position and you're not too heavy on the RCS is normally very easy, even if it feels hairy not 'seeing' what's going on.

But, as others have said, i) Docking Alignment Mod, ii) Fine Control iii) if you're using MJ's Smart-ASS to keep your craft pointed at the target, I'd suggest turning it off when you're very, very close and about to connect - I've had MJ's SASS end up 'fighting' the magnetic clamps.

Wemb

[ygarl]

http://i.imgur.com/9f4TK7g.jpg

Or did I do something else wrong? I remember that the docking port for the "Gas Station" didn't snap to the middle of the Monopropellant tank it is on, I had to manually align it... might that have something to do with it?

I feel this is by far the most likely explanation. I'd guess it was visually in line with the part, but hadn't gone "green".

Alt-mouse (or R-shift if you're a Linux bod like me) is utterly crucial.

If I had a penny...

[CmaN]

I had an issue with my standard sized docking ports and learning docking.  Even though I had everything lined up (was using docking port alignment mod), the magnets were not kicking in, and after hours of trying, could not make the connection.  I had ensured power to both ships, and correct docking port alignment (neither part was backwards).

I found an old bug report from 2013 about a problem that can happen to the state of the docking port, as recorded in the save file.  I made a backup of my save file, and searched for 'state = acquired', found an entry and found that the port corresponded to one of the vessels I was attempting to dock.  I changed the entry to 'state = ready', but still had the same issue with being unable to dock.  I looked at the file again, and on the port, there was an entry for 'dockUId' right below the 'state =' line I had changed.  I set dockUId to = 0 (kept the 'state = ready' change, rather then the 'state = acquired' that it was previously set to), loaded the save, and they docked almost immediately. I'm guessing dockUId records what part the port is docked to.

I had been making quicksaves during the docking process, when I had everything lined up and was ready to close distance, and after reading the bug report, and a few other posts, concluded that the saving during the closing seconds of docking may have caused an error in the save.