Why does one of my couplers break off when disconnecting a bi-coupler?

[Dunbaratu]

Here is a screenshot of it about 15 seconds after it happened so you can see:

I was attempting to use the construction technique I'd heard of where you can force docking couplers to couple rotated the way you like by using two of them next to each other, so your RCS thrusters on your components are aligned and not off-angle from each other.

But the problem is that I can't decouple them without one of the bits breaking off as shown in the picture. I rightclick on one port in the pair and say "disconnect", then quickly click the other one and do the same, and the two ship modules drift apart as expected, but one of the couplers breaks off its module when I do so. This keeps happening each time. I scrap the mission and try again, and after three tries I'm getting frustrated.

What am I doing wrong?

I thought that disconnecting both at the exact same time might help so I put them into a common control group to be activated with one keypress, but when I press that key.... nothing happens. The only decoupling I can do is manually with the rightclicks.

Edited May 8, 2013 by Steven Mading
IMG URL not working

[HoY]

Take a look at this video. It will show you how to attach those parts properly in the VAB

In the video I used the tri coupler but its the same concept as the other one

[Noticeably FAT]

It has to do with the way KSP connects parts, they can only be connected to one other part at a time. So, as you are building you have a bi-coupler, two docking ports connected to that, two more docking ports connected to those, then another bi-coupler that is only connected to one docking port. The one that is falling off was never connected to the second bi-coupler, only the first docking port.

I've never tried building one of those in the VAB (mine are always on separate ships and docked in orbit), so I don't know if this will work. Place your first bi-coupler, then the docking ports. Then a single docking port, and then your second bi-coupler. Then, attach the fourth docking port to the second bi-coupler, not the first docking port. You may have to temporarily put something between the first docking port and the third, then remove it after the fourth is placed. The second and fourth ports won't be connected, but they should dock on the pad.

The action group decoupling does work, but only if you select the right option. There are two available, undock and decouple, I think you need decouple.

[HoY]

I always add Undock AND Decouple to the action group for the ports, not one or the other. That way the key works later into the mission as well

[Dunbaratu]

So am I supposed to connect it like this, essentially?

Do I understand it right?

So, by the way, what on earth is the difference between the meaning of "undock" and "decouple", then? They sound like they mean the same exact thing to me.

Edited May 8, 2013 by Steven Mading
URL for image didn't work first time.

[Eric S]

Yes, you understand it correctly. I'm not sure on the difference, I think it's that if you place a docking port on another docking port in the VAB, they are coupled rather than docked. If you put anything else on the docking port instead of another docking port, you can still decouple it.

EDIT: the "not really connected" parts will connect as soon as physics kicks in before you have the chance to launch, so it will be effective for the launch.

Also note that you might have problems getting fuel to flow through two multi-couplers facing each other. You'll be able to transfer fuel from above to below them, but engines on one side won't draw from tanks on the other. I'm not sure this happens in all cases, but I've seen it happen fairly regularly lately.

Edited May 8, 2013 by Eric S

[Noticeably FAT]

EDIT: the "not really connected" parts will connect as soon as physics kicks in before you have the chance to launch, so it will be effective for the launch.

A small correction: this only applies to docking ports facing each other as they will immediately dock with each other. As the OP found, connection nodes not connected in the VAB are still disconnected at launch.

Also note that you might have problems getting fuel to flow through two multi-couplers facing each other. You'll be able to transfer fuel from above to below them, but engines on one side won't draw from tanks on the other. I'm not sure this happens in all cases, but I've seen it happen fairly regularly lately.

I trick I learned from someone else is to attach fuel lines from the tanks to the docking ports. If you do that when you dock the fuel will automatically feed through the docking ports with no messing around with manually transferring fuel. It should only work in one direction, but in a large craft I have set up to feed from direction into a lander, I found that the interplanetary engines were feeding from the other direction and draining my lander tanks. I may have just installed things bas-akwards, I'll have to check later.

Edited May 8, 2013 by Noticeably FAT

[Eric S]

A small correction: this only applies to docking ports facing each other as they will immediately dock with each other. As the OP found, connection nodes not connected in the VAB are still disconnected at launch.

Correct, I wasn't specific enough, thank you for clarifying.

I trick I learned from someone else is to attach fuel lines from the tanks to the docking ports. If you do that when you dock the fuel will automatically feed through the docking ports with no messing around with manually transferring fuel.

Correct. It's the multi-couplers that have the issue, it's not actually the multiport docking that causes it, so if you use fuel lines to skip over the multi-couplers, you'll be fine. In fact, you only need to skip one of the multi-coupler.

[Noticeably FAT]

Correct. It's the multi-couplers that have the issue, it's not actually the multiport docking that causes it, so if you use fuel lines to skip over the multi-couplers, you'll be fine. In fact, you only need to skip one of the multi-coupler.

Are you sure? I've had problems with fuel feeding right going from tank>port>port>tank>engine. The ports are labeled as fuel crossfeed capable, but they don't seem to actually function that way. It does seem to work if you put a fuel line from the tanks to the ports, even if there isn't anything between the tank and port. What seems really weird to me is that doing that seems to work in both directions, even though the fuel lines generally only work in one direction. My suspicion is that you aren't supposed to need fuel lines, but placing them tricks KSP into making the ports work properly.

[Eric S]

Am I sure that multi-couplers cause fuel flow problems in the absense of docking ports? Yes. As a test when I first heard about this problem, I built a small test rocket that was "probe module/tank/bi-coupler/inverted-bi-coupler/tank/engine" and despite the total lack of docking ports, the engine could exhaust the bottom fuel tank but wouldn't touch the top one. I further simplified it to "probe/tank/bi-coupler/tank/engine" and the engine drained both tanks. "probe/tank/inverted-bi-coupler/tank/engine" again demonstrated the problem, with the top tank remaining untouched.

The problem is that the single side of the bi-coupler can't draw fuel from either of the nodes on the other side of the bi-coupler. Either of the two nodes on the double side can draw fuel from the single side, however. The two nodes on the double side cannot draw from each other, however. So it appears to me that the only fuel flow allowed through a bi-coupler is from the single node to either/both of the double nodes.

I also tried variations of each case where the engine could drain the tank, and the presence of docking ports did not affect the results.

Am I sure that docking ports do not cause fuel flow problems in the absense of multi-couplers? No, but I've never been able to create a case where this happens, so at the very least, I feel that docking ports are at least unlikely to cause this kind of problem. I've tried replacing both bi-couplers in the non-working designs with docking ports and it has always worked fine for me at that point. If you could recreate the config you had problems with, I'd like to see it. I'm not going to claim that my scientific method is perfect (my anything, for that matter), so I'm quite open to scrutiny.

Am I sure that you can get away with bypassing only one of the bi-couplers with a fuel line? I think so, but I'm not sure as I haven't tried it. Since bi-couplers let fuel flow one way but not the other, then only one bi-coupler is the problem, so bypassing the problem bi-coupler should resolve the problem.