Wicked shimmy due to Docking magnets

[Bogeyman]

Hi everyone,

I am playing KSP in vanilla mode.

I have assembled sort of a "mothership" in Kerbin orbit for longer planetary excursions. It consists of a core with docking nodes and crew habitats, science labs etc. At the end I have docked it with my propulsion unit which consists of a laaaarge pack of fuel tanks and LV-N motors. This thing being quite large, I had decided to connect it to the front of the ship via the large Clamp-O-Tron Sr Docking port.

So far so good.

Now I try to dock my landers to it which I have launched separately. Upon approaching the ship it looks as if the big Docking port gets attracted to my approaching lander due to the magnetism. It bends and tweaks the whole big assembly and it looks as if it is going to break apart. Wicked shimmy, making it impossible to dock.

Is this a bug? Is it possible to deactivate this? Are there modes that correct this behaviour?

Would this not happen had I used the smaller, normal docking nodes?

Best regards

Chris

[klesh]

When you approach, can you quickly switch to the mothership? Does that stop its shimmy? Perhaps a quick timewarp, then back would freeze it in place?

I'm sure its a bug, as I have never experienced something like that.

[Jean Deaux]

If you're using MechJeb, that mod is the root of a LOT of the wobble. On the ship you're docking to, be sure you have the SAS turned on to maintain a lot of stability.

Here's a post I made some time ago very similar to what you're describing, maybe you can find some ideas within.

[Perry Apsis]

Are you using MechJeb or any other tool to point your ships toward their targets? The only time I've seen behavior like what you're describing is when I have Smart A.S.S. pointing my mother ship toward its target and then switch to the target to do the docking. It's especially bad when you have both ships trying to point their docking ports toward each other. I've had a couple of disasters doing that. A better approach would be to use the tool to orient the mother ship correctly, then turn it off and use SAS to lock it into that position, then do the same for the daughter craft when it gets close to the docking port.

[Taki117]

Be sure to disable SAS when the magnets engage, otherwise they will end up fighting each other, especially if one of your ships is coming in at an angle and relying on the magnetism to straighten out.

[klesh]

Ah hah, I've never used MechJeb, so I have never really experienced what everyone is on about.

[Bogeyman]

I have found a workaround, although I don't know why it "solved" the problem.

To explain I need to explain how I built my ship (see Image below). Since that big propulsion unit was not only big but HEAVY , I just added lots of reaction wheels to the front and rears of each cylinder. It functions very well, it turns the whole thing on a dime like nothing.

So, after my problem here I decided to deactivate all of these reaction wheels and it worked. But I still don't know why the propulsion unit tried to crank itself towards my approaching lander...

[jarmund]

I'm thinking something else:

I've noticed that a ship, once it starts calculating its physics, can start wobbling, especially if an enabled onboard SAS is reacting to it. "Crap, nose pointing west, move towards east, crap overshoot, move west (repeat)"

This is especially true for orbital assemblies such as motherships consisting of multiple parts with the large ports connecting the pieces. A workaround if you're willing to use mods is KAS. It comes with a type of strut that can be attached by an EVAing kerbal, and is invaluable for large orbital assemblies (The struts themself is enough of a reason on its own to install KAS. It's a great mod, you'll love it, i promise you)

Sounds to me like the wobble starts once you get within physics range, and then gradually increases to something unmanageable when you get near. Have you tried disabling all reaction wheels on board the mothership to see if it settles?

Related: Just when the ports are about to dock, disable SAS on the docking vessel. Then the ports won't fight each others inclination difference as much, and that should in theory reduce the amount of pull the ports do on each other.

EDIT:

KAS can be found here.

- - - Updated - - -

Would this not happen had I used the smaller, normal docking nodes?

I'm not sure if it's imaginary or not, but i do get the impression that the large docking ports are more wobbly than the medium ones.

Edited March 21, 2015 by jarmund

[Starwaster]

That's imaginary. They impart the same amount of torque and attraction as any other docking port.

The answer is to not come in at an angle. If you're perfectly straight then you don't run the risk of wobbling. There's two kinds of attractive force that comes into play BTW. And they are both configurable in the part with default values of 2. (so if the docking port config doesn't specify, then it's 2)

acquireForce

acquireTorque

It's the second one that is responsible for the wobbling. because it's trying to tug the ports into alignment. Logically if they're both acting at the same time then you might as well be doubling the force involve.

[FlyingPete]

One thing that works to get ships straightened out is to start with SAS turned on. If the docking ports are at too great an angle to connect, turn off SAS on one vehicle. When the magnetism starts to pull them into alignment, turn SAS back on to brake the rotation and prevent overshooting. This stops the pair from oscillating.

[Lindy]

jus' spitballin' here, but an inbalace in the RCS can also induce a bit of a shimmy from fighting against SAS.