Docking

[Kimberly]

I'm trying to dock an RCS tug to a small section of a space station (a single Karmony node). It's my first time docking/using RCS, and it's a bit frustrating. I can rendezvous just fine, to within about 50 meters. Things go wrong when I turn on docking, mode, though... using RCS makes the ship flip every which-way, which in turn makes it impossible to use the RCS controls, and in docking mode I can't seem to realign my ship. What am I doing wrong?

Edited July 17, 2013 by Kimberly

[blizzy78]

Did you balance your RCS thrusters with the center of mass? See this picture:

The thrusters should be placed at the ends of the green line, that is, at the same distance from the center of mass. Just place a ring of 4 quad thrusters at each end, and you're good to go. Also, it helps using the chase cam.

You can also watch my tutorial video (see signature.) Approach starts at 7:22 min, docking procedures at 9:15 min.

Edited July 16, 2013 by blizzy78

[Kimberly]

They weren't quite balanced, no. But if that's absolutely required to use RCS, then what is the use of an RCS tug? I've heard people using them to assemble stations, but they seem useless if they must be balanced, because they'll never be balanced if they have to carry payloads. Also, what's the "chase cam"?

I just ended up docking without using RCS at all, but I'd still like to figure this out for the rest of the station's construction.

[capi3101]

I dock on RCS only regularly and find it easier to just leave the controls in Staging Mode. Only time I use translation is when I'm getting close to docking and my alignment isn't exactly perfect, and even then I'm liable just to use the IJKL keys.

There are folks who suggest that in addition to four on each end, there should be four in the center aligned with the center of mass (I'm one of these folks, incidentally). Those center ones would be used to assist in translation only; one of those instances where the unidirectional block could be employed. Of course, if you use the standard blocks, that's extra thrust for prograde/retrograde maneuvers (and I find that helpful). Here's an example of something built in orbit; there are four distinct pieces here, each one has RCS thrusters fore, aft and in the centerline:

Balanced thrusters aren't required for RCS docking maneuvers, but it does make things go a lot easier. So does having a command pod with good torque (so you're not totally reliant on RCS for rotation; it tends to throw things off a little when you're in close).

Not sure if "chase cam" is an official thing or not (and if it is I'd like to know how to access it). Basically, you just position your camera behind the craft you're controlling. Personally, I like to move it over to the side when I get really close (~3-5 m) for the final docking maneuver.

Edited July 16, 2013 by capi3101

[blizzy78]

Press V to change the camera modes, one of them is the chase camera.

[MonkeyFuzz]

Kimberly, I'd recommend trying out an easier stage in developing your docking skill first. Going for assembly of a space station is a very advanced level to have your first try at.

I think you should try what the Gemini program went with and first try having a mission to simply dock with either another flight or with sections of your launcher that made it to orbit. This can help you hone your skill at designing a well balanced orbital craft, have a better sense as to where center of mass needs to be, and how to finesse yourself through the maneuvers of approach, docking, and then maneuvering with the coupled craft.

It'll be a bit time consuming to work your way up from scratch, but I think it's a mistake to have your first step be the grand finale of station assembly.

[Starwaster]

They weren't quite balanced, no. But if that's absolutely required to use RCS, then what is the use of an RCS tug? I've heard people using them to assemble stations, but they seem useless if they must be balanced, because they'll never be balanced if they have to carry payloads. Also, what's the "chase cam"?

I just ended up docking without using RCS at all, but I'd still like to figure this out for the rest of the station's construction.

Your tug's RCS requiring balancing doesn't somehow invalidate tugs.... If it's not balanced then you're just going to be in for more frustration as you try to dock.

Also, if two ships are RCS balanced for normal flight, RCS will generally function well after docking as well.

Otherwise it's "Garbage in, garbage out."

Anyway balancing isn't that hard. You can either go with the approach that you're going to have two sets of RCS clusters fore and aft of your center of mass or you're going to put one set ON the center of mass. (look at the Apollo craft: ).

Be sure to line it up with pitch/yaw for maximum efficiency. Otherwise, it'll have reduced effectiveness and burn more fuel.

Finally, there's a plugin that helps you balance your RCS in the VAB/SPH. When active, it lets you cycle through various RCS thrust vectors with a green desired vector and a red off balance vector. When the red arrow is gone and the center of balance ball has shrunk as much as you can shrink it then you're at optimal balance.

Edited July 16, 2013 by Starwaster

[regex]

Look in my sig for the RCS balancing mod. Remember that fuel usage will eventually unbalance your carefully placed RCS nodes; with experience you can alleviate that to some extent. The tendency for the ASAS to overcompensate unbalanced craft when RCS is enabled (wasting all that fuel!) should go away in 0.21. Until then, you'll have to learn to toggle ASAS on and off as needed.

Also, I used to use docking mode heavily but I recently gave the IJKLHN keys a try and, I have to say, it is a world of difference. No more hitting space to change to rotation or translation mode, it's almost like having two sticks for movement. I highly recommend you give them a try.

[Fractal_UK]

Proper RCS balancing is totally unneccessary for docking, you can do it well enough to dock by eye and you can dock some horrendously unbalanced ships with a bit of patience.

The key piece of advice that I can give you is not to use docking mode, stay in staging mode and make full use of the navball - the navball is absolutely critical to docking. Once you get close, make sure the yellow prograde to target marker is lined up perfectly with the purple direction to target marker. If they aren't, use the i,j,k,l keys to translate your ship to push them into alignment. If they drift out of alignment, do some more corrections and keep them aligned.

If you can get within 50m, you're already pretty much there. Reduce your relative speed to about 0.3m/s and make sure you're lined up - as above.

Also, press the caps lock key if your ship is proving hard to control, that will toggle between normal and precision control. Precision control makes it much easier to make very tiny maneouvres to keep yourself perfectly aligned with the target.

[Kimberly]

But if I dock a payload to the RCS tug, won't it always be unbalanced? Unless I make the tug so that it will be balanced for one specific payload, which is pretty impractical.

I'll try to make use of those suggestions regarding the controls, though, maybe they'll make the second docking easier.

[blizzy78]

The key piece of advice that I can give you is not to use docking mode, stay in staging mode and make full use of the navball - the navball is absolutely critical to docking.

I find that the navball can be totally ignored when approach is done and things transition over to docking. I usually dock by visual judgment only. The chase camera is extremely helpful here.

[regex]

But if I dock a payload to the RCS tug, won't it always be unbalanced?

Very true. In those cases my best advice is to use ASAS sparingly and for stabilization only, and take advantage of time-warp's ability to stop all rotation. If you feel that time-warp rotation stabilization is "cheating" then ASAS can help with major stabilization, but you're going to have to handle the fine work.

[Starwaster]

Proper RCS balancing is totally unneccessary for docking, you can do it well enough to dock by eye and you can dock some horrendously unbalanced ships with a bit of patience.

Then you've never approached a docking port with an RCS system so horribly unbalanced that translations result in yaws. Attempting to correct by yawing back into alignment results in pitching wildly in the wrong direction.

This is what 'unbalanced RCS' means.

[allmappedout]

But if I dock a payload to the RCS tug, won't it always be unbalanced? Unless I make the tug so that it will be balanced for one specific payload, which is pretty impractical.

I'll try to make use of those suggestions regarding the controls, though, maybe they'll make the second docking easier.

If you dock with something with your tug, you're only unbalancing the centre of mass towards the attached object. Therefore, for example if it is attached behind your tug as expected, forward/backward translation, and roll are not affected by the change of CoM. Depending on the size of the object, and, importantly, if it has its own RCS thrusters on it, will determine how much impact you will see when you subsequently attempt to move with it on.

A lot of the issues with RCS can be partially alleviated by small adjustments followed by a correction using the pod's torque. It's more frustrating, yes, but as others have said, as soon as your RCS fuel starts burning, you're unbalancing your craft, so it's a battle!

[capi3101]

Anyway balancing isn't that hard. You can either go with the approach that you're going to have two sets of RCS clusters fore and aft of your center of mass or you're going to put one set ON the center of mass. Be sure to line it up with pitch/yaw for maximum efficiency. Otherwise, it'll have reduced effectiveness and burn more fuel.

Make sure if you go this route that you're using a manned control pod (i.e. one with a lot of torque available). I've tried the single RCS quad along the center of mass with an unmanned craft before; handled like Elsie the Cow...

[Kimberly]

I've been having great success so far with constructing my space station without the use of RCS at all...sticking to staging mode and using tiny engine burns, plus switching to the target to realign docking ports if necessary, seems to be doing the job. In any case, thanks for all of your suggestions, guys!

[Vanamonde]

RCS balance is helpful, but not at all necessary. It does mean that you have to manually control attitude as well as translation without the help of ASAS, but I often use subships that need to be asymmetrical for mission-specific reasons, and routinely dock them to my carrier ships. Sometimes it's quite a fight, but it certainly can be done.