Stage Decoupler Crossfeed not working properly

[Basto]

I am having issues where my first stage ignores the fact that crossfeed has been disabled on the decouplers and it just uses all of the fuel of every tank in all stages evenly... also the fact that we cannot set the fuel tank priority order makes it difficult to keep your rocket stabile when launching (if you can set this I have not been able to determine how).

[Basto]

I think I have figured out part of what is causing this.  AS long as I do not use the launch clamps it does not happen...  still wish we could set the priority on the tanks.

[Ambious]

This has been driving me nuts!
Thanks for the workaround suggestion!

[Sequence]

18 hours ago, Basto said:

I think I have figured out part of what is causing this.  AS long as I do not use the launch clamps it does not happen...  still wish we could set the priority on the tanks.

Just tested this. Didn't fix it for me. Bit thanks for the suggestion. 

[TwitchTVAlba]

1 hour ago, Sequence said:

Just tested this. Didn't fix it for me. Bit thanks for the suggestion. 

It was landing legs causing this for me. When i removed my landing legs from my lander it no longer used up the fuel in my lander on the initial accent

[Periple]

Same. Also, putting the landing legs on the command module and putting a decoupler between it and the tanks + engine stopped it from triggering for me. I.e. it would always drain if I had landing legs on a stage with fuel + an engine.

This doesn't fix it for everybody however. It's a bit of a mystery and I hope there's a fix soon.

[TwitchTVAlba]

1 hour ago, Periple said:

Same. Also, putting the landing legs on the command module and putting a decoupler between it and the tanks + engine stopped it from triggering for me. I.e. it would always drain if I had landing legs on a stage with fuel + an engine.

This doesn't fix it for everybody however. It's a bit of a mystery and I hope there's a fix soon.

ah nice workaround, ill give it a shot.

[LameLefty]

2 hours ago, TwitchTVAlba said:

It was landing legs causing this for me. When i removed my landing legs from my lander it no longer used up the fuel in my lander on the initial accent

I was burned by this in the first test of a Mun lander. Dropped my crasher stage and went to land using the descent engine and my lander tanks were empty.  After reading another thread about this, I mounted small cubic strut segments clipped into my lander body and attached the lander legs to those. That stopped the "leg-induced crossfeed."

What a weird bug.

[whatsEJstandfor]

I was having the same issue with the stock Kerbal-K2. Like some others in this thread, I tracked it back to adding landing legs. If you open the vessel in the VAB and launch it without touching anything, it's fine. If you add a landing leg, the fuel flow breaks and has this same behavior, where fuel from all tanks drain evenly to the core stage.

 

[Kleos]

1 hour ago, Periple said:

Same. Also, putting the landing legs on the command module and putting a decoupler between it and the tanks + engine stopped it from triggering for me. I.e. it would always drain if I had landing legs on a stage with fuel + an engine.

This doesn't fix it for everybody however. It's a bit of a mystery and I hope there's a fix soon.

THANK YOU! This worked for me, and only required the decouplers to be used. I didn't have to move the position of my landing legs to the command module.

 

I will add a few more details from my own debug in case it is helpful to anyone else:

- This only happened with the medium landing legs. If I put the small landing legs in the same spot, I would not get the fuel crossfeed bug.

- This only occurred when I had boosters in the first stage that I dropped off. Steps would be 1.) Ignite boosters + medium engine at launch. 2.) Radially detach boosters with medium engine still burning. Once this happened the total fuel for my medium engine would jump from 7.7 tons to 8.5 tons (because at this point the crossfeed of my decoupler in the next stage failed and it combined this fuel tank with my first stage. If I launched the rocket without these boosters, then the crossfeed would hold even with the medium landing legs still on my lander.

- I still had the same bug when I put the landing legs on a small strut connection instead of directly on the fuel tank.

- The only thing that fixed it for me without drastically changing the purpose of my rocket was to attach the landing legs to some radial decouplers, and then only activate those decouplers in the stage after I was done using that engine (effectively the stage that I am dumping the lander engine anyways). My guess is that the bug only occurs when the medium landing legs are a part of the same stage as the engine in question that will have all of its fuel stolen with this bug.

Edited February 25, 2023 by Kleos
Minor clarification