MK-2 Spaceplane falls apart when undocking

[JayKay]

I'm having a problem with MK-2 spaceplane parts. I docked a plane to a space station using the retracting docking port, and when I wanted to undock, the plane broke in two instead of undocking:

I'm running MechJeb, Kethane, Kerbal Alarm Clock, Precise Node and SCANsat, all the latest versions for .25. Only a MechJeb module is involved in the planes other than stock parts. The large cargo bay is connected to the docking module, and I installed a standard docking port on the wall of the cargo bay connected to the docking module. I had launched a load out of the bay before docking (a double-ended docking extension using a structural fuselage with docking ports on either end.) I brought another plane up to the station and it does the same thing.

[Master Tao]

Could you provide a save file where the plane is still docked? Also, a log file and the KSP and mod versions information like the examples in the How to Get Support (READ FIRST) sticky? You also may want to subscribe to the mods' threads, because KAC is outdated.

Those purple icons in Blizzy's Toolbar make me think something's installed incorrectly.

[JayKay]

I forgot about Toolbar. Yeah, I know the toolbar is not installed right. I'm not sure how that could affect this, but you never know. I removed it and every other mod except MechJeb from my install and it still does the same thing. Where can I upload my save so that it can be linked?

[Master Tao]

Anywhere that lets you publically link files, like Dropbox or Google Drive.

[JayKay]

Ok, this is the link to the save file:

https://www.dropbox.com/sh/3gm6hbylydgq28j/AADE6SmZn-FzNSKO0pfSTgn5a?dl=0

The MechJeb version is 2.3.1.0

The KSP version is 0-25-0

All other mods have been removed. You can get to the right place just by hitting F9. Just select the docking port on one of the planes, and back of the plane will separate instead of the two ports undocking. The plane's port will remain locked after that, still attached to the station.

There was a short patch this morning which may have changed the version, but the planes behaved the same way anyway.

[Master Tao]

I suspect you're running into a variation on these docking bugs, but I haven't been able to reproduce it with similar crafts.

Are these the steps you took?

1. You launched an MKX-3 spaceplane
   
2. On approach to Unity Station, you removed the docking extender from the cargo bay and docked it to the Mk. 2 Docking Port.
   
3. You then docked with Unity Station using the docking extender.
   
4. You tried to undock, but that option wasn't available, so you clicked "Decouple Node" instead.
   
5. Instead of decoupling the Mk. 2 Docking Port from the station, the game decoupled the back end of cargo bay port, breaking off part of the ship.
   

I suspect that the actual bug happened somewhere in step 2, so the game started treating your ship like it had been assembled in the SPH rather than in space.

How did your build process for the MKX-3 go? It looks like you placed these parts first:

1. Cockpit
   
2. Mk. 2 port
   
3. Cargo docking port
   
4. Docking extender assembly
   
5. Cargo bay
   

What happens if you rebuild the craft, placing the cargo bay before the cargo docking port? You can now Alt+click to disable radial attachment, making it easier to place things inside the cargo bays.

[JayKay]

That is exactly the process for docking and the subsequent problem while undocking. I'm not sure why "decouple node" would even be an option for something like this; it's not like you are likely to try to take off with something protruding from the open docking port. In any event, it should only decouple something attached to the docking port, not elsewhere on the part.

The assembly of the ship went much as you described, though there was some partial disassembly and reassembly before I was done. The interior docking port was a bit of a problem, and thanks for the tip on the Alt+click. I have been planning to try building the plane another way, but I thought I would try bringing up a docking extender on another ship to see how it goes when that part is not part of the plane out of the hangar. That mission is in progress right now.

[JayKay]

You know, I had a thought. At one point while I was trying to get the internal docking port to fit where I wanted, I turned the plane up on its nose to get a better view. I wonder if somehow the program decided that the back of the part was the top at that point. Something to try, I guess.

[JayKay]

I installed a new extension delivered by a rocket. However, I decided to make some modifications to the spaceplane including changing out the main engine for a rapier and adding solar panels and lights to the docking extension in its bay and sent it up to repeat the original mission. It did not go smoothly. I buggered up the ascent which wasted some fuel and then ran out of power trying to wait out a rendezvous. Long story short, I had to rescue the plane twice because it kept running out of, well, everything. I tried deploying and docking to the extension so I could use its solar panels, but ran out of monopro before I could finish, and it drifted off and had to be retrieved by another ship, and, well, it was a fiasco.

Anyway, by the time the plane and its extension arrived at the station they had been separated and docked with other ships a couple times. The plane docked using the extension with the station and then undocked from the extension properly this time. The mystery is not exactly solved, but everything is working as it should now.