Best Way to Create a multiple docking port setup?

[jkool702]

I did a quick search and game across some threads that said multiple docking port configurations were possible (ex: a senior port in the middle surrounded by a few normal ports), but these threads were all really old and indicated that it was almost not worth doing due to how difficult it was and how it messed with the games physics engine. So now with KSP 1.02 are multiple docking port configurations better / easier to (successfully) make? And if so whats the best way to set one up? I gave it a try and it didnt work....not even a little bit....but I'd like to give it another shot.

[Snark]

Short answer: No, it's still not worth it. (Or at least, it's not worth it any more than it used to be.)

Nothing changed about docking port physics in 1.0.2, so anything old you read is still relevant.

The easiest setup for multiple docking ports is to use one of the multi-couplers (the structural components that go from a single node on one side to 2, 3, or 4 on the other side), so that you can dock several ports at once.

In my experience, it's not really worth the bother, but YMMV. If I want a stiffer connection, I just use a Clamp-O-Tron Sr. instead of the plain-vanilla variety, and I find that that's generally good enough for what I need to do.

[jkool702]

Alright, thanks. I use the Clamp-O-Tron Sr. too, and find that while it works decently well it doesnt completely remove wobble from larger ships, but I guess I'll just live with it.

[Geschosskopf]

Alright, thanks. I use the Clamp-O-Tron Sr. too, and find that while it works decently well it doesnt completely remove wobble from larger ships, but I guess I'll just live with it.

Wobbles from docking ports mostly happens when the joined objects are considerably wider than the docking port between them. Then you have a stress concentration with a big moment of inertial on each side of it. So if you limit the size of your modules, you'll usually be OK. However, a cross-shaped station is always going to wobble because the arms are heavy and way wider than any port.

[jkool702]

My main motivation for this is connecting a lander to a main engine block on an interplanetary ship. While the main engine block is about the size of the docking port (a little wider since i use a Mk3 fuel tank), the lander is considerably wider (it has to be wide to be stable when landing). Initially struts keep it stable during launch, but after the first disconnect those struts are gone for good (at least on stock KSP they are).

[Laie]

My standard example for a three-port connection: This vessel is just as rigid as if it was built in one piece. A good deal of that is due to the three colums strutted against each other, though.

Building multi-port connections has become a lot easier with the rotate and offset gizmos.

Using them is as difficult as ever: you have to get the rotation just right. I wholeheartedly recommend Mechjeb's Smart A.S.S. for the purpose.

You don't have to use several senior ports as I did in that picture. However, the amount of thrust a port can take is limited; if you want to put more than a Skipper behind it, there should be a senior port in the center. The surrounding ports may be small or even tiny.

For stability purposes, it's not important to have many ports. Besides, you have to unbotton all ports when you detach the vessels. What's more important is the distance between them: the further apart the better. That comes easily if your craft happens to have outriggers anyway -- if it doesn't, I'm not sure if it's really worthwhile to add them.

Also keep in mind that only one of the docking port connections will allow fuel flow, and there's no telling which one it will be. My experiments with a pilot connection that sticks out a little farther have worked more often than not, but it's still a gamble. This may give you the oddest fuel flow, leading to imbalance, leading to your craft going in circles.

Edited June 19, 2015 by Laie

[FleshJeb]

Also keep in mind that only one of the docking port connections will allow fuel flow, and there's no telling which one it will be. My experiments with a pilot connection that sticks out a little farther have worked more often than not, but it's still a gamble. This may give you the oddest fuel flow, leading to imbalance, leading to your craft going in circles.

Great post. Thanks Laie.

Just came in to point out that TAC Fuel Balancer makes it possible to mitigate this problem.

[ajburges]

The docking port sr has all the connection strength most craft need. The issue comes when you have forces that result in torque at the more elastic connection docking ports give.

If possible, tug your lander while it has a lower fuel load. Also, unless your drive vessel lacks sufficient reaction wheel torque, disable reaction wheels on the payload. Having two reaction wheels on opposite sides of a flexible connection leads to positive feedback control loop if they are sufficiently more powerful than the connection.

I once had a LKO pod retrial mission. After capture with the Klaw (on a sub tonne orbiter) I disabled pivot lock to align target CoM with the thrust vector. I forgot to disable the freshly revived pod's reaction wheel first. I thought I was going to shake myself to light speed!

Edit: derp! Forgot to answer OP question.

Simply make a sub-assembly with some form of symmetry for the multi ports. Then just use said sub-assembly for both craft. Any ports not placed at the same point (center vs others) will need distance adjusted, but alignment will always be correct.

Related: how do standard docking ports handle shear forces. I have this crazy idea to make a Mk2 tug design that can radially dock with sister/cousin ships for increased payload capabilities. Such a system would dock with a pair of inline Mk2 docking ports separated by a fuel tank. Would that idea be practical in game? I'm worried about the shear forces either breaking the connection or allowing engine play and steering losses.

Edited June 19, 2015 by ajburges