Unable to decouple after docking two vessels.

[Wedge01]

Hello,

I am experiencing something that I need help to understand (and hopefully correct) involving decoupling. I apologize in advance for the lengthy description.

I've built two ships: a vessel to land and takeoff from Eve, and a "transfer vehicle"  whose purpose is to get the lander to Eve and then bring the crew home. After building the lander I ran a test flight; I used the Debug Toolbar to place it into Eve orbit, and then went through the entire sequence: deorbit, entry, landing, and launch back to orbit. Everything worked well.

I constructed a booster and placed the lander into Kerbin orbit.  I then built the Transfer Vehicle and launched it into Kerbin orbit. After rendezvous and docking the two vessels, I ran another test flight to Eve, placing the "stack" into Eve orbit and again going through the entire sequence. De-orbit, entry and landing went well. But that's when strange things started to crop up. 

The first thing I noticed involved the fuel flow priority. My lander first stage has six X200-32 fuel tanks attached to a central S3-14400 tank using six TT-70 decouplers, The fuel flow priority was set up to have the X200-32 tanks drain first, and then I would jettison them. This worked fine on my first test flight. But this second time after being docked to the Transfer Vehicle the fuel flow priorities were all wrong, and had to be reset (not a big deal, but maybe a harbinger of what was coming).

After launch, the six tanks drained as planned. But when I jettisoned through the staging process, only five of the six tanks decoupled. 

While trying to troubleshoot, it looks like there's a "phantom" TT-70 decoupler preventing the tank from separating. This phantom decoupler isn't on the original lander (the one that flight tested correctly). It only appears to show up after I dock the Transfer Vehicle to the Lander. And I say "phantom" because you can't see it until you go through the jettison process, then you can see that one TT-70 did decouple correctly but there's a second decoupler buried/hidden underneath the first one.

Unable to figure out what was going on, I started from scratch and built a second Lander, tested it around Eve (land and takeoff worked great). But again, after docking with the Transfer Vehicle, the decouple problem with the phantom TT-70 happened again.

So: what's going on here? What is causing a correctly functioning lander to develop this decoupling glitch after docking with a transfer vehicle?

Thank you for your help.

[DennisB]

Fuel flow

I think, it's normal. I ran into this problem on my Jool mission. It happens after docking/undocking, if you have a complicated vessel with many stages and complicated fuel flow. The fuel priority has two components, a base value and a correction value. The base value depends on the staging diagram (changing at decoupling and docking), the correction value you can set manually in the VAB or later during the mission (is constant as long as you don't change it). If your staging order changes during the docking process, or if you use high correction values, it can mess up your fuel flow after docking/undocking.

Decoupler

I have two ideas, but it's only speculation, because it's probably something else.  These are just two cases, which I experienced, when I built my Eve rocket. As I had very high part count in the VAB, and unintentionally detached a larger part of the rocket, which I couldn't attach again correctly, I pressed Ctrl+Z to restore, but it seems, the game couldn't handle this with the high part count, and built in an error. My rocket exploded, and as I built it back to find the error, after I removed a special fuel tank, I've got an error message at saving, so I had to rebuild the entire vessel. The second option is, that you maybe made a mistake in the VAB, when you used symmetry or sub-assemblies, but the chance for this is also low, because you say, that you rebuilt your vessel from scratch, so this could only be, if you did both times exactly the same.

Edited April 21 by DennisB

[Wedge01]

Thanks for your response. I've resorted to doing a re-design of the lander, trying to eliminate as many parts as I can while maintaining functionality. The new lander seems to be less glitchy, but every test seems to generate slightly different results. I plan to just go back to a save point if necessary and repeat the process until I get back into orbit without blowing up (which would be nice).