My craft yaws/rocks during docking

[Clear Air Turbulence]

I have a miner/lander shuttling between minmus and an orbital refinery. 

Docking is fine when it is fully laden, but every once in a while I have to do a dock with the holding tank empty, and then what happens is that the craft yaws/pitches rocks widely during the final moments.

Translation (IKJLHN) works fine but attitude adjustment  (ADWSQE) produces wild and difficult to control rocking moments.  (If I use Lock View at these times I can actually get seasick )

I assume this is because my craft is balanced for full holding tanks.  I further assume that the rocking is caused by a bad interaction between the SAS and the RCS controls...

But what to do about it?

 

(1) Position the RCS thrusters midway between hold full and hold empty, ensuring it ALWAYS rocks,  but not too badly?

(2)  Make two sets of RCS thrusters, one for each centre of gravity?

(3) Is there  a fiendishly clever mod that causes the RCS thrusters to slide between different positions to make up for shifting CoM

(4) Somehow make up for it with piloting skill?  If so, how?

 

 

 

[Plusck]

I often get mad rocking at the end - but of course it's worse if the docking port isn't aligned with the CoM because the magnetic attraction has a lever effect on the ship, so it misses slightly, corrects, swings the other way, and so on. I normally switch off SAS about 2 seconds before contact because the ship's SAS never seems to make the rocking better, rather the opposite.

The answer may be (3), but not a mod, it's stock: precision control.

Precision control toggles with caps-lock and turns the control sliders on the bottom left cyan rather than red. Only caps-lock simply isn't recognised on my install (1.0.5 on MacOS) so I had to reassign the key (made it keypad multiplication instead). And it appears to apply thrust on a sliding scale that takes your CoM into account. I've just been playing around with it and it does make docking a lot less painful if the RCS isn't really set up to deal with your CoM. I didn't try without SAS on a wildly off-balance ship, but on a reasonably-but-not-perfectly balanced ship it let me leave SAS off altogether and avoid all the constant corrections after each move.

Credit to Snark who pointed this out in another thread recently. I really hadn't explored the possibility before, but it helps greatly.

 

 

Edited December 16, 2015 by Plusck

[Speeding Mullet]

1 hour ago, Clear Air Turbulence said:

But what to do about it?

2 sets of RCS thrusters balanced for COM's and action grouped to toggle would be an easy fix.

Another solution would be to switch to radial tanks and place then around your COM.

You could also try pressing caps lock which takes some of the steam out of the RCS, allowing the SAS module to keep up with the torque and possibly hold the craft steadier for slower, finer control.

It sounds like what you are describing though is when the magnetism is taking effect between the docking ports, in which case one solution would be to use SAS hold instead of target.  If you are using target then the ship can constantly over-correct and go into a wobbling motion much like you are describing.

Couple of pictures of the ship empty and full with COM displayed would be useful to diagnose further, as would clarification of the exact conditions under which you are experiencing jellyship.  Apart from that there is an RCS build aid mod kicking about which might help, and if memory serves another mod called fuel balancer or something, although citation needed on both those and whether they would help.

SM

Edited December 16, 2015 by Speeding Mullet

[RocketBlam]

Regarding #3, MechJeb has something called "SmartRCS" that will adjust RCS thrust if your thrusters are not right on the COM. I don't know how fast it reacts... I usually use it just because either I burned fuel, or I just put the thrusters on wrong, but it can help in those situations, so maybe it will help you. I suspect it works best (or, at all) with two sets of thrusters, above and below the COM.

Edited December 16, 2015 by RocketBlam

[Venusgate]

Assuming by last moments you mean when the docking ports are within each others' influences, I almost always shut off rcs and sas as soon as I see the magic moment when the ports start to attract each other. If I'm anywhere close, the dock is going to happen, and I've had bad experiences with sas and rcs trying to fight the magnets and ripping things off.

 

[Clear Air Turbulence]

I am pretty sure it is not the magnetic forces.  It happens pretty far out from the other craft. It's just that it becomes very noticeable then.  I will experiment with smartRCS...

[Plusck]

Just to add to your option (4) then:

- Speeding Mullet said not to use target hold. I've never used anything but mere stability assist in docking because I find SAS corrections on any other setting are always excessive and wobbly.

- Avoid any WASD corrections whatsoever. Use the prograde movement to bring the target to your crosshairs (i.e. translate until the prograde marker on the navball is on the other side of the target compared to your chevron).

- Exploit the warp effect of killing rotation if ever you really have to change the angle of the approaching ship or the target ship. Switch off SAS, switch off RCS to save monopropellant, turn, then warp for an instant when lined up. Leave SAS off on the target ship anyway.

- Gain mad skillz and do it all without SAS at all. It'll take a while to get used to but if you're using locked camera view, you should quickly learn what WASD corrections you need every time you IJKL...

[Snark]

All you need to do is to turn on precision control by hitting caps lock.  It will make all your problems magically go away.

Here's how/why it works.

 

[WuphonsReach]

Note that if you do use MechJeb's Smart A.S.S. feature to point yourself at the target (docking port), turn it off when you are within 5-10m.  Use DPAI and guide it yourself the last 5m (if it's a correctly aligned docking attempt, there's not much to do during the last 5m anyway).

[Aethon]

I've noticed craft seem more wobbly in this version.  I trust KER when it says I have zero thrust torque.  I'm not sure what it's from.

[Clear Air Turbulence]

On 12/16/2015, 8:15:52, Speeding Mullet said:

 

oops, how do i delete posts?

Sorry, I just wanted to say to Speeding Mullet - thanks, yes, I have RCS Buildaid.  it is useful. I don't find caps lock makes much of a difference.   for now I will just have to brush up my skills... .

[Gojira1000]

This may be a silly question, but are you running an ISRU unit while in flight? That, for me, results in some manic wobble and veering under acceleration. More than the ore-fuel mass shift would account for.

[Plusck]

23 hours ago, Snark said:

All you need to do is to turn on precision control by hitting caps lock.  It will make all your problems magically go away.

Here's how/why it works.

 

You may have to reassign another key to precision control, though, if my experience (the game simply does not recognise caps lock, even if I try to redefine it in the settings) is anything to go by.

[Snark]

55 minutes ago, Plusck said:

You may have to reassign another key to precision control, though, if my experience (the game simply does not recognise caps lock, even if I try to redefine it in the settings) is anything to go by.

Interesting.  I've never had any problem with it myself, but I have the feeling that you're not the first person I've seen complain about something like this.  I have a vague memory that it may have something to do with the platform-- by any chance are you running on a Mac?

[Aethon]

1 hour ago, Plusck said:

You may have to reassign another key to precision control, though, if my experience (the game simply does not recognise caps lock, even if I try to redefine it in the settings) is anything to go by.

I've noticed caps lock being a little wonky.  If you go to a craft with caps locks on, sometimes you have to turn it off to get precision control to come on.  Just yaw or pitch to find out whether the game thinks precise control is on.  If it moves a lot, hit caps lock and see if your control input is damped down.

[Plusck]

4 hours ago, Snark said:

Interesting.  I've never had any problem with it myself, but I have the feeling that you're not the first person I've seen complain about something like this.  I have a vague memory that it may have something to do with the platform-- by any chance are you running on a Mac?

Yes, and the oldest possible (while still being reasonable) OS too (10.6.8). One day I'll surely have to upgrade, but there's a bunch of software that will no longer work :/

Still, I have no idea why the game simply refuses to recognise that there is a caps lock key. Or recognise a different keyboard (the keyboard is French so all keys have to be reassigned. Switching input keyboard in the OS, to pretend that it's an English layout,  doesn't work, oddly....). Ho hum.

Edited December 17, 2015 by Plusck

[Snark]

7 minutes ago, Plusck said:

Yes, and the oldest possible (while still being reasonable) OS too (10.6.8). One day I'll surely have to upgrade, but there's a bunch of software that will no longer work :/

Yeah, in that case I think that's the issue.  Dunno what to tell ya (I'm a PC gamer myself), other than maybe asking in the relevant support forum about it.  Something screwy about Macs and caps lock.

[MaverickSawyer]

I've always put my docking ports at the cardinal points of the Navball (Pro/Retrograde, Normal +/-, Radial +/-) and use the Smart A.S.S. to lock in a particular orientation. There've been a few notable exceptions, which required me to use the Surface mode and dial in a particular pitch, yaw, and roll setting that had been determined in the design of the station, but it worked splendidly: The ship stays rock solid as I move in with the RCS, and I'm not slewing about wildly trying to align with the target.

 

Notable exception to this rule: Debris removal. Spent stages are almost NEVER aligned right, so you need to eyeball it and use the Target mode. And yeah, my ship goes real wild as I get in close, so I tend to do my big changes further out, then smoothly come inwards and Klaw the part I'm trying to deorbit.

 

Back on topic, though: The reason you're going nuts at the end while the tanks are empty is just that: The craft is lighter, and the CoM is located at a different location, so the RCS is working doubletime to try and dampen down the oscillations, and that only makes it worse. There's a reason I rarely use the RCS for attitude control...

[RocketBlam]

On 12/16/2015, 8:28:43, Snark said:

All you need to do is to turn on precision control by hitting caps lock.  It will make all your problems magically go away.

Here's how/why it works.

 

I didn't know about this, thanks. It sounds like that may be what MechJeb is using for "SmartRCS".