My Clumsy Docking Intercepts: Help me clear up the confusion?

[JoeSchmuckatelli]

I'm fairly successful at intercepting and docking - but I'm not 100% sure I understand what is happening and / or doing it right.  My methods work for small, maneuverable craft - but my larger, more complex craft are becoming a problem.  I get that my 'chase' craft needs to have an over-all shorter orbital path to catch up with the target (lower=faster), and I typically 'play with the nodes' until I get a close encounter to try to dock the craft.   So thus far, getting to the point of docking is not the problem... Its the next part that I need help with.  Here's what I do:

• Tweak the nodes until I get an encounter I like.
• Warp to just before the encounter, then switch to ship view
• Watch the target until I like its distance, then 'zero out' the relative velocity.
• Aim at target and goose the engines until I get close (50-75 meters)
• Zero velocity again, then dock using RCS and other fiddly stuff 

The whole time from when I shift from map view to ship view is busy.  This means I really have no idea what the orbits look like, or what is going on globally - I'm so focused on getting the ships together that I just don't know the answer:

Does 'zeroing out' the relative velocity equal 'matching orbits' (i.e. once I'm done - but before docking - do both ships have the same AP and PE?  Or should I be doing something in map view to make sure both ships have the same orbit once I'm at the encounter?

Is there a good way of making one ship switch targets when map view won't allow me to select another craft? (The ship names are all overlaid one upon the other in map view and I cannot get the mouse to let me select a different craft - but they're too far apart in ship view to let me click on individual parts)

How the heck do you dock two huge ships?

 

 

The specific problem is this:

• I've got a large interplanetary ship with crew return module, destination satellite and lander.  It's big and slow turning and I want to refuel it before the launch window
• I've also got a large, clumsy refueling craft with a separable 'fuel tug' that I hope to shuttle between the two  craft
• I get the two big ships near one another, undock the tug from the refueler, undock the payload from the crewed main craft (return module) and start shuttling fuel
• I get the first docking / refueling operation complete
• I notice that my payload and refueler are drifting alarmingly away.
• My fuel tug cannot shift it's target from the return module to the refueler
• I chase the refueler using the Mark One Eyeball, and its a PITA to try to dock without being able to effectively target the craft; even selecting the docking port as the target is not really helping, and it's probably due to something I did in the VAB
• The other two ships continue to drift away, not only from me - but the payload is quite far from the crew return module.
• I get frustrated, realize it's 2 a.m. and I've been at this for hours and I'm fried.  Game off.  Computer off.  Can't sleep - grinding away at the problem.
• Wake up with no real answers.

So... How do I do this?

 

 

Edited December 14, 2020 by JoeSchmuckatelli

[AHHans]

20 minutes ago, JoeSchmuckatelli said:

Does 'zeroing out' the relative velocity equal 'matching orbits' (i.e. once I'm done - but before docking - do both ships have the same AP and PE?

For all practical purposes: yes. If you are close to each other and have the very little relative velocity then you are on near identical orbits. (It is only my nitpicky part, that insists that unless you are exactly at the same location with zero relative velocity then you are technically not on identical orbits.)

24 minutes ago, JoeSchmuckatelli said:

Or should I be doing something in map view to make sure both ships have the same orbit once I'm at the encounter?

No.
(Seriously: I thought about other smart things to say, but they all boiled down to saying "no" with more words.)

26 minutes ago, JoeSchmuckatelli said:

Is there a good way of making one ship switch targets when map view won't allow me to select another craft?

Double click on the ship (or docking port) you want to target in ship view. Or right-click and select "target this" in the PAW.

26 minutes ago, JoeSchmuckatelli said:

How the heck do you dock two huge ships?

Careful.

I use essentially the method described in @Snark's Illustrated guide to docking. Some of my space stations are quite large and with "wiggly" bits sticking out (e.g. a big-ish ship docked with a standard size clamp-o-tron). So when docking I usually don't move the "station" at all(!). (I do switch between ship and station often, to check the alignment of the docking ports, so that I don't need a docking port alignment mod. But when controlling the station I only look at the navball and take care not to give any movement input.) When the ship and the station are close enough to each other, then the orbits are also close enough that I can take my time to maneuver the ship into the correct position for docking. Typical relative velocities when doing that are < 5 m/s. (And for actual docking I usually slow down to 0.1 m/s, so that the prograde marker just vanishes.)

The one thing I do in the VAB is to set up RCS and the reaction wheels so that I control the attitude (yaw/pitch/roll) with only the reaction wheels and use RCS only for translation.

[bewing]

43 minutes ago, JoeSchmuckatelli said:

How the heck do you dock two huge ships?

AHHans gave a great answer to all your questions. When it comes to docking large, ungainly craft -- there are some little tricks you can do, but generally (as he said), the answer is that you just have to accept the fact that you need to do it slowly and carefully. Which means it's going to take a long time. Which means that if you have more than two ships involved, the other ones are going to drift away. And ships that drift away accelerate away faster and faster. The faster you can learn to dock, the less distance other ships will drift away.

Do you want little tips and tricks for docking large ships? Mostly it's a matter of practice. Zero your velocity at 30 meters, not 50 to 75.

Let other ships drift to most of a km distance, then turn them around and give them 1 m/s toward their targets. That'll give you most of 1000 seconds to dock other craft first. Which is usually enough.

Separate the controls for your rotation from your translation, definitely! 

Move your fuel toward your CoM -- this will help large ships rotate much faster.

I find a .3 to .5 m/s closing rate to work nicely for the final docking. It's slow enough that the magnets get a good grab.

Target the docking port, then spend most of your time keeping your nose right on the target marker on your navball. The magnets will do the rest of the work, provided you are going slow enough and you turn off your SAS when you touch.

Make sure to turn off your SAS when you touch!

 

[AHHans]

[Part 2, answers to the concrete problem:]

47 minutes ago, JoeSchmuckatelli said:

I get the two big ships near one another, undock the tug from the refueler, undock the payload from the crewed main craft (return module) and start shuttling fuel

So you have four different craft: the main ship, the payload (undocked from the main ship), the refueler, and the fuel tug. Keeping them flying in close formation is not going to be easy. I guess the biggest source of your "drifting away" problem is that the docking ports push the craft away from each other upon decoupling, so they don't even start out with zero relative velocity. But even if the did: during the time hour or so that I think your refueling operation lasts they will drift apart (see the "nitpicky" explanation above).

I personally have never tried something like this. When I want several craft to stay close to each other, then I dock them together. Most of my larger craft have more than one docking port so that I can usually find an empty one to stick a temporarily unneeded part. And if I don't have one, then I'm not above using an adapter consisting of only a clamp-o-tron and a Klaw. (Make sure that adapter is always connected to something, otherwise it becomes debris.)

@bewing already gave a tip on how to deal with craft that are drifting apart. My addition (or clarification) would be to regularly switch control to the parts that are drifting away and give them a push back to the main craft. (Which requires them to be individually controllable.

1 hour ago, JoeSchmuckatelli said:

• My fuel tug cannot shift it's target from the return module to the refueler
• I chase the refueler using the Mark One Eyeball, and its a PITA to try to dock without being able to effectively target the craft; even selecting the docking port as the target is not really helping, and it's probably due to something I did in the VAB

I'm a bit confused: when you are controlling the tug and then select a docking port on the refueler as target, then you should have the refueler targeted. Not so? (And no, it shouldn't have anything to do with what you did or didn't do in the VAB.)

[JoeSchmuckatelli]

19 minutes ago, bewing said:

AHHans gave a great answer to all your questions

Agree completely - further, the links he provided are invaluable; even the ancient sublink in Snark's post (turning on fine RCS - which I did not know about) is helpful.  Now I can't wait to fire it up and try again!

 

8 minutes ago, AHHans said:

Most of my larger craft have more than one docking port so that I can usually find an empty one to stick a temporarily unneeded part

I need to start doing this; radial docking ports are not expensive.

 

8 minutes ago, AHHans said:

I'm a bit confused: when you are controlling the tug and then select a docking port on the refueler as target, then you should have the refueler targeted. Not so? (And no, it shouldn't have anything to do with what you did or didn't do in the VAB.)

I think when I flipped the OKTO2 in the VAB, it did something weird.  My navball is effectively backward (I need to get the retrograde marker aligned to have the ships actually close on one another).

The inability to effectively 'shift target' is a big problem for me.  It's a key feature letting me know relative velocity, etc.  Basically - I can only target a part when the ships are Really Really Close.  After they drift - mousing does not work.  Then I try map view - but they're overlaid, and I still cannot select them.

grrr.

 

 

I really need to look into what you describe about keeping the RCS from reacting while trying to use the reaction wheels - that's a neat technique that I was unaware of.

 

Edited December 14, 2020 by JoeSchmuckatelli

[AHHans]

2 minutes ago, JoeSchmuckatelli said:

Agree completely - further, the links he provided are invaluable;

Errr... Thanks, I guess. (IIRC I had only one link...)

3 minutes ago, JoeSchmuckatelli said:

even the ancient sublink in Snark's post (turning on fine RCS - which I did not know about) is helpful.

I don't actually use that too often: when you separate the controls for rotation and translation, then rotation won't affect translation (because reaction wheels can't do that) and translation won't affect rotation as long you have SAS on (because SAS will use the RWs to counteract any rotation from the RCS thrusters).

6 minutes ago, JoeSchmuckatelli said:

Now I can't wait to fire it up and try again!

Great! Practice makes perfect! (As annoying as it may be in the meantime.)

[JoeSchmuckatelli]

2 minutes ago, AHHans said:

Errr... Thanks, I guess. (IIRC I had only one link...)

Your link + Snark's.  ;D

[AHHans]

8 minutes ago, JoeSchmuckatelli said:

I think when I flipped the OKTO2 in the VAB, it did something weird.  My navball is effectively backward (I need to get the retrograde marker aligned to have the ships actually close on one another).

Select "Control from here" on the docking ports that you want to use! (On both "ship" and "station".) That way the orientation (or lack thereof) of the probe core doesn't matter, and the "target" marker will point in the direction from one docking port to the other.

10 minutes ago, JoeSchmuckatelli said:

The inability to effectively 'shift target' is a big problem for me.

Hmmm... Double click (with the left mouse button) doesn't work? Double click on empty space should deselect the target and double click on a craft should select that craft as a target. (Or are you playing on console?)

13 minutes ago, JoeSchmuckatelli said:

I really need to look into what you describe about keeping the RCS from reacting while trying to use the reaction wheels

Open the PAWs and de-select the rotation controls on all the RCS thrusters. (You may need to enable "advanced tweakables if you haven't done that already.)

[Snark]

2 hours ago, JoeSchmuckatelli said:

So... How do I do this?

So... depends on how attached you are to the "what" versus the "how".

Everyone has their own playstyle, of course, and that's fine.  My own take on this would be that you're making life far too difficult for yourself by trying to fly multiple ships in formation, and doing far more docking operations than you need to.

It boils down to this.  My observation has been that,

• Docking two-- and only two-- ships together, is easy.  For any ships, no matter how big they are.  (Yes, even if they're a thousand tons each.  If this is your concern, there's a separate conversation we can have about that.) 
• Flying in formation (i.e. station-keeping near each other, without being docked) is really hard.  (Yes, there are ways to do it, and if you're insistent that you have to, we can talk about how best to do that.  But my own advice would be, simply, "don't".)

I haven't seen what your ships look like, exactly, but it seems to me that you'd have this solved in a trice if you'd just limit things to two ships (or, at least, two ships at a time).

For example:  I get that you have a big interplanetary mission ready to launch, with separate pieces.  Why all the shuttling back and forth?  If it needs a bunch of fuel, why not just have a single big fuel hauler that holds enough to fuel the whole thing, and just dock the hauler to the mission, transfer fuel across in one go, and you're done?  You'd have the whole problem wrapped up in five minutes with no fussing about, and only one docking required.

 

In my own games, I generally design stuff with this philosophy in mind.  I dock craft A to craft B, and that's it; no shuttling back and forth between multiple craft that need to stay parked right next to each other.

On those occasions when I do need multiple things... then that's when I use a "space station" to tie things together.  My space stations are typically not grand affairs-- basically just something with a decent sized fuel tank and perhaps a bit of crew accommodations, but mainly what they offer is a bunch of docking ports in various sizes, widely spaced on projections that make it easy for the structure to serve as a "parking lot" to which I can dock as many ships as I need to.  In a scenario like that, what you could do is this:  Don't try to park all these multiple ships in formation and then shuttle back and forth between them.  Instead, dock them all to a space-station hub, one at a time.  While they're all docked, you can easily transfer resources back and forth among them-- no shuttling back and forth needed.

 

So, those would be my own recommendations to you.  Idea 1 (the simplest), just make this into a two-ship scenario, where your fueler docks to your interplanetary mission and that's it.  Idea 2 is to use some sort of space-station hub to "glue" everything together, so that if you must have multiple ships involved, you can just dock them all to the station.

Would one of those ideas work?  Or is there some reason why you're especially attached to your current design (or some reason why you dislike one or both of those ideas)?

 

[JoeSchmuckatelli]

 

 

So - I finally did it.  And thanks to all of you for your help.

2 hours ago, Snark said:

• Docking two-- and only two-- ships together, is easy.  For any ships, no matter how big they are.  (Yes, even if they're a thousand tons each.  If this is your concern, there's a separate conversation we can have about that.) 
• Flying in formation (i.e. station-keeping near each other, without being docked) is really hard. 

Flying in formation / station keeping is really hard.  Harder than it had to be... but I ran this mission because I was really bad at docking.

I did not realize that I was really bad at docking until today! 

I figured that my inability to dock two large ships wasn't fixable - but it turns out, that through this process I figured out what I was doing wrong, and how to fix it.

3 hours ago, AHHans said:

Hmmm... Double click (with the left mouse button) doesn't work? Double click on empty space should deselect the target and double click on a craft should select that craft as a target. (Or are you playing on console?)

This was the life-saving feature of my overly-complex, challenging and ultimately fun refueling efforts.  Being able to double-click in ship view was literally game-changing for me.  I did not realize this feature existed -so thanks for the info!

3 hours ago, bewing said:

Make sure to turn off your SAS when you touch!

This is another thing that also made my life easier; shutting down SAS in a timely manner really kept me from bouncing and breaking stuff.

The thing I discovered is that I've been using RCS wrong all along.  Now that I've figured RCS better - I think I can attempt larger ship docking in the future.  Because - keeping this flotilla together was a beast.  Here's how it went:

• Got the two large ships at zero relative velocity 80m apart.
  • Undocked the lander and satellite combo mission package and drifted it off to the side and safely out of the way
  • Undocked the fuel tug and maneuvered it over to the return ship,
  • Docked and sent fuel the big engines' tanks
• The initial refueling complete, I notice the mission package and fuel ship have drifted far away
  • Moved the refueler back to the vicinity of the return ship
  • Moved the mission package back to the return ship
    • (Okay - I really did not complete the moves; I just kept the mission package generally moving towards its mothership and moved the fuel tanker back to w/in 150 and 0m/s)
• Undocked and moved the fuel tug back to refueler
  • Docked and transferred fuel
  • Adjusted the relative velocity of the mission package to keep it in sight of the return/mothership
  • Moved the refueler and tug back into position of the return/mothership as one vessel
• Undocked and moved the fuel tug back to the return/mothership
  • Docked and transferred fuel
  • Adjusted the relative velocity of the mission package to keep it in sight of the return/mothership
  • Moved the refueler back to the return ship

• Undocked and moved the fuel tug back to refueler
  • Docked and transferred fuel
  • Adjusted the relative velocity of the mission package to keep it in sight of the return/mothership
  • Moved the refueler and tug back into position of the return/mothership as one vessel
• Undocked and moved the fuel tug back to the return/mothership
  • Docked and transferred fuel
  • Adjusted the relative velocity of the mission package to keep it in sight of the return/mothership
  • Moved the refueler back to the return ship
• Undocked and moved the fuel tug back to refueler
  • Docked and transferred fuel.  This emptied the fuel tankers big tanks (it's 3 stages at this point because I overbuilt).
  • Separated the empty stage from the refueler and let the debris drift.
  • Flew the mission package - which was quite far from the return/motheship back and docked
• I'm now back down to two ships - a smaller refueling craft and the interplanetary mission craft.  Ready for mission!

So yes, very very complicated - but through all that docking I learned how to use RCS correctly - even though I still had RCS puffing when trying to orient with the reaction wheels - which was annoying, so I kept turning RCS on and off - and also the main engines of the various craft as I've fat-fingered the shift key before!  As @Snark writes: way too complicated.  But a good learning exercise!

FWIW - it was about half-way through all of this that I realized I could move the prograde marker juuusst a weee little bit using RCS... which was the thing I did not know how to do well before.

Again - thanks all!

 

Edited December 14, 2020 by JoeSchmuckatelli

[AHHans]

18 minutes ago, JoeSchmuckatelli said:

I figured that my inability to dock two large ships wasn't fixable - but it turns out, that through this process I figured out what I was doing wrong, and how to fix it.

Well, docking two large ships isn't really much different than docking two small ships. You mostly have to take more time and take it slow, because you can't throw large ships around like smaller ships. But don't expect to learn in half a day what took me a few months to learn.

But I'm glad I was able to help.

 

[JoeSchmuckatelli]

Grin - I'm not saying I can do it with panache - but I think I now have a conceptually correct understanding of how to dock two ships that is lightyears ahead of what I used to do.  Whether I can build and fly a large craft capable of docking remains yet to be seen - but I now feel competent to try!

[Snark]

4 hours ago, AHHans said:

The one thing I do in the VAB is to set up RCS and the reaction wheels so that I control the attitude (yaw/pitch/roll) with only the reaction wheels and use RCS only for translation.

 

 Me too... but I can never remember to do that, and also it's a pain to go 'round and do if I've got a lot of RCS thrusters present.  So I just made some ModuleManager config that tweaks all the RCS thrusters to have yaw/pitch/roll disabled by default.  You can find the config here:

https://github.com/KSPSnark/SnarkTweaks/blob/master/stock/NoRCSRotation.cfg

...just grab that file and drop it anywhere in your GameData folder.  As long as you have ModuleManager installed, it'll set them all to default yaw/pitch/roll control authority turned off.  (You can always turn it on, manually, if desired.)

4 hours ago, JoeSchmuckatelli said:

I really need to look into what you describe about keeping the RCS from reacting while trying to use the reaction wheels - that's a neat technique that I was unaware of.

Yep, it's really handy.    See the above link for a handy config file that will make them default to that state, if you don't want to have to remember to do it manually every time.

 

[radonek]

A few observations to already good answers:

• Undocked crafts slowly drifting away is a Good Thing to see. You can't make them perfectly stationary, and I don't think  you want them closing in and smashing into something.  Well, in theory you can make them stay put on exactly same orbit, meaning directly prograde or retrograde. Dunno if it's actually possible to do. In my experience, good place to park stuff is on planar axis (under or above). If you zero velocity there, you should have very similar orbit in all but inclination - stuff will drift closer or farther out but not move around. (This is why prolonged docking maneuvers are sometimes done in this axis.)
• With circular orbit being, well, round, things drifting away tend to be coming  back after half-orbit. If you fiddle around for long time, keep eye on stuff floating around and re-zero relative velocity where necessary. In theory, this means one can't really loose anything, but in practice anything that gets outside physics range can get on slightly different orbit.
• RCS build aid mod is a great help when creating big ships.
• Switching between rotation and translation should not be a problem in a first place. Get some mod that will show you target docking port axis. (I use "Navball Docking Alignment Indicator" as it only adds a marker on a navball.) If you align axes first, you don't need to touch rotation controls afterwards. Just get roughly  into position, turn towards marker, turn on RCS, translate to dock. With a bit of practice, you can do it by navball alone, which is immensely useful when docking in darkness.
• If you have a fat fingers, disable engines in docked vessels. Yes, it does not play well with stagging, but neither does docking anyway.
• "Closing in" phase of docking is mostly unnecessary. Here is what you do:  burn for rendezvous as usual. Right after MECO, align your ship along cardinal axes, switch to map view and right click on encounter marker to keep distance visible. Now switch on RCS and just tap any translation key and you will see encounter distance  change in realtime. I won't go into details, just play  around with translation keys and see what it does to distance. With responsive RCS, I can usualy get within 100m of target right on. In fact, one downside of this method is that there is a real chance of crashing into target, so don't over do it if your vessel lacks TWR for fast velocity matching burn.

[vv3k70r]

14 hours ago, JoeSchmuckatelli said:

I think when I flipped the OKTO2 in the VAB, it did something weird.  My navball is effectively backward (I need to get the retrograde marker aligned to have the ships actually close on one another).

Check what is Your control point and what craft is in prograde/retrogade direction to each other.

 

[Spricigo]

11 hours ago, JoeSchmuckatelli said:

I think I now have a conceptually correct understanding of how to dock two ships that is lightyears ahead of what I used to do.  Whether I can build and fly a large craft capable of docking remains yet to be seen - but I now feel competent to try!

The thing with docking (as well orbital mechanics in general) is that it is counter-intuitive. We're  so used to how objects move there in the ground, restricted by all sorts of drag, that we expect a similar behavior when I space.  With some effort and a few good explanations don't take that long to understand that what is happening is normal  for things moving in space but to it feel normal, that is another issue altogether. 

So, while I understand your desire for some why to learn it quickly, lots of practice seems like the way to eventually get it. 

[radonek]

On 12/15/2020 at 12:04 PM, Spricigo said:

So, while I understand your desire for some why to learn it quickly, lots of practice seems like the way to eventually get it. 

So true.

I learned a lot about subject from tutorials and good people around here. But I did not really understand it until I parked a lab at low Mun orbit and started gathering science from all biomes.

[Zhetaan]

On 12/14/2020 at 11:12 AM, JoeSchmuckatelli said:

So... How do I do this?

I don't have much to add, except to say that one thing that I did not notice (and if someone did say it above, then please accept my apologies) is that you get less drift when you are in a higher orbit.  The orbits are larger, which means that the angular sweep over the same period of time (meaning the size of the angle in the orbit that you travel through during the operation, or, to be more technically accurate, the change in true anomaly) is less.  Also, the orbital speed is lower, which reduces the sweep even more.