Fuel Crossfeed does not work in crossfeed enabled docking ports connected after launch (but does work if connected in VAB/SPH)--PLEASE HELP!!!!!

[StevenLawyer]

Hello, all.  I've spent weeks building a beautiful reusable space plane capable of sea level landings on Eve and Laythe and descent to 0 meters (sea level) on Jool.  After a flight, the spaceplane is refitted with jettisonable fuel tanks and jet engines/electric fan blade motors depending upon the planetary body.  The refitting is done using clamp-o-tron docking ports--one on the spaceplane and one on the fuel tank or engine to be attached.  I've meticulously built a mothership with robotics to fit the parts on the spaceplane.  Now I am finding that, even though crossfeed is enabled on both docking ports, the engines won't draw fuel from tanks across the docking ports.  At first I thought I had done something wrong, but I have built two very simple and seemingly identical ships that demonstrate the problem.

Take a look at the pictures in the links below:  The ships are identical (other than the paint scheme), right?  Notice that they both have an upper and lower fuel tank connected by docking ports.  You can see that on each ship both of the docking ports have crossfeed enabled.  And yet, while the orange ship performs as you would expect (fuel drawing evenly from both tanks), the white ship will only draw fuel from the bottom tank.  What's the difference?  The yellow ship was built with the docking ports connected in the VAB--normal top to bottom construction.  The white ship was built with the top and bottom docking ports not connected in the VAB, but rather placed immediately adjacent to each other so that they would dock as soon as the ship loaded on the pad.  Once on the pad, the ships are identical in apparent construction, and yet no fuel will flow across the docking ports that are docked after launch.  Without that capability, the only option for using drop tanks attached in flight is to do manual transfers--which aren't always a viable option if fuel draw is happening quickly, such as in a return to orbit.

https://drive.google.com/file/d/1iAdn7XVQgtV1Dy15jj2Ub4PekmsNjCxS/view?usp=sharing

https://drive.google.com/file/d/1CYP1SKITTRaJCUTRWQUBkrnz8nYc3IUH/view?usp=sharing

HEY KSP GUYS--I know you're very busy with KSP 2, but what about a little love for us long time KSP users (I go way back) who want to keep playing while we wait for KSP 2?

If anyone wants to see this problem for themselves, the two ships in these pictures can be downloaded here:

https://drive.google.com/file/d/1Fp7Juix5nLlfCrv_GrNRKPMHFCJKx71_/view?usp=sharing

https://drive.google.com/file/d/11M-pqvgITFbKp--IQeZPdD5CfEmMMZSS/view?usp=sharing

[Hotel26]

Just a hunch but check your fuel priorities.  Engines will draw first from tanks marked highest priority.  (Hmm, although you do say the 'orange ship' works OK.  If all your relevant fuel priorities are identical in this case, try setting a priority order.  Back in KSP pre-1.0, there used to be a very disappointing aspect of the fuel draw algorithm based on loops in the connections (two paths to a tank) in which the algorithm selected the first it found in the paths.  There was a detailed article in the KSP bug-tracker, of all places.)  Just guessing at this point...

Edited January 12, 2020 by Hotel26

[new02DAY]

Gotta point out that they're not technically identical. I think the way that the white rocket is built (bottom half rooted to the radial decoupler) is messing with vessel-vessel interaction. I think the game is picking them up as two separate rockets (after stage 1) that are being suddenly docked. I found that after stage 1 on the white test rocket, selecting the top docking port and making it the "Primary Docking Node" solves the fuel transfer issue.

Edit: Removed double post.

Edited January 12, 2020 by new02DAY

[StevenLawyer]

2 hours ago, new02DAY said:

Gotta point out that they're not technically identical. I think the way that the white rocket is built (bottom half rooted to the radial decoupler) is messing with vessel-vessel interaction. I think the game is picking them up as two separate rockets (after stage 1) that are being suddenly docked. I found that after stage 1 on the white test rocket, selecting the top docking port and making it the "Primary Docking Node" solves the fuel transfer issue.

New02Day--Thanks for the response.  Yes, they're not identical.  The orange rocket has the docking ports connected as nodes, whereas the white rocket they are docked.  But once docked, they should function identically.  I'll try the primary docking node on my spaceplane and report back.

 

3 hours ago, Hotel26 said:

Just a hunch but check your fuel priorities.  Engines will draw first from tanks marked highest priority.  (Hmm, although you do say the 'orange ship' works OK.  If all your relevant fuel priorities are identical in this case, try setting a priority order.  Back in KSP pre-1.0, there used to be a very disappointing aspect of the fuel draw algorithm based on loops in the connections (two paths to a tank) in which the algorithm selected the first it found in the paths.  There was a detailed article in the KSP bug-tracker, of all places.)  Just guessing at this point.

Hotel26--Fuel priorities on the tanks in the sample rockets are the same level.  But, with my spaceplane, even when the docked tanks have a larger priority number (and therefore should drain first), they don't.  In fact, they never draw--the engines flame out for lack of fuel with the docked tanks full.

[new02DAY]

9 hours ago, StevenLawyer said:

New02Day--Thanks for the response.  Yes, they're not identical.  The orange rocket has the docking ports connected as nodes, whereas the white rocket they are docked.  But once docked, they should function identically.  I'll try the primary docking node on my spaceplane and report back.

Yeah, it's weird. They should function properly, but I think the game has trouble with vessels docking too quickly, or from a too close position. I've had this issue when loading a game just before docking and completing the docking. Not sure why the game has trouble with it though.

[StevenLawyer]

2 hours ago, new02DAY said:

Yeah, it's weird. They should function properly, but I think the game has trouble with vessels docking too quickly, or from a too close position. I've had this issue when loading a game just before docking and completing the docking. Not sure why the game has trouble with it though.

This issue just about docking on load up.  With my space plane, it returns from a mission, docks with the mothership, and then robotics attach the new modules.  Of course, once the plane docks with the mothership, it is all one vessel from KSP's perspective.  Then the robotics rotate the new modules in place and the docking ports take hold.  Then the connection to the robotics is jettisoned using couplings.  Lastly, the spaceplane undocks.  What I have found is that I have to go to each docking port on the spaceplane after it undocks and make it the primary docking node (thanks for that tip!).  Once I do so, fuel flow works again.  It seems that the docking ports leading into the vessel containing the "control from here" point need to be primary for things to work.