Aligning correctly multiple docking ports

[MrUberGr]

I'm trying to build a Scapeship that will have a small landing ship that will meet up with a larger part to travel. Can't explain it really good, I hope you can see what I mean from the screenshot. My problem is that I can't align the damn docking ports... I've been trying for over an hour but to no result. Any suggestions?

[Pecan]

Navyfish's docking alignment mod will give you an exact roll figure, as will RPM's docking mode (I believe). Alternatively you can always let MJ do it for you.

[MrUberGr]

Nope, that's not the problem. The problem is that the distance is not equal to each other. If one docks, the other won't be able to because it will need more space.

[NoClass]

Here's a technique I used, though it's from an old craft I no longer have (so no pics , words must suffice.)

Basically, pick one of the craft and build a spacer part that fits snugly onto both docking ports in the assembly building. This part doesn't have to be fancy (and it will never fly), it's just a few structural components and docking ports that line up with the craft.

If you save this part to a subassembly, you can then mount it to your other craft, and it will show you exactly where to put your ports on that other craft.

FYI, it's really hard to get these things perfectly. Allow for a little bit of flex in your craft, and expect to be slightly off balance in flight.

[LordFerret]

I wonder... will dual docking ports as such fail?... much like trying to use triple docking ports on the Mk1 Stack Tri-Coupler? I tried that and learned the hard way - http://forum.kerbalspaceprogram.com/threads/85580-Docking-Undocking-Issue-Why?p=1259303#post1259303

[Pecan]

It depends how it's built LordFerret. Yours was a matter of them not being constructed in the right sequence in the VAB/SPH - if they are attached to their parent craft properly in the first place then they will work in practice and make for stronger connections.

For the problem in hand - that they don't align properly anyway - there's nothing that can be done in flight. NoClass's advice to use a template-piece is about the best. Otherwise built subsidiary craft on the original in the VAB/SPH then save them as sub-assemblies. For the immediate issue I'd suggest that taking two landers using one docking port each may work. If you connect just one then you may get balance issues but if it works then that will be lighter. If none of those options is viable then it's time to scrub the mission and de-orbit those ships.

[Laie]

Also, if you want to fight wobble, you need at least three connections. Two ports will still allow the attached piece to flap like a door on its hinge.

Docking ports make for a rather strong connection in terms of force needed to pull them apart. But they have about as much wiggle room as the upper lip of a docking port is thick. The small ports, giving and taking one inch at either side, can easily bend by ten degrees. The large ones bend much less simply by virtue of having a wider ring. If you want to reduce wobble even more, you don't need many connections, but you want to space them as far apart as possible.

Besides, keep in mind that you have to unbutton all ports if you ever want to un-dock.