Decouplers Fuel transfers through decoupler with crossfeed disabled

[FuelX]

Hi I have trouble with fuel draining from my upper stage. Fuel crossfeed is off. Also at stage separation remaining methane and/or oxidizer is not calculed properly. Is there a way to make sure my fuel will not drain into the first stage?

Here first stage is almost empty :

 

Looking at delta v, we already see there's something wrong and the upper stage leaked.

Then stage separation, upper stage is only half full or half empty :

My setup in the VAB:

 

 

If you have any tips or workaround for this bug, it would be appreciated  

Thanks!

 

Edit: To troubleshoot crossfeed, I made this testbed. Faster iterations and less kerbals stranded around the mun :

Edited February 24, 2023 by FuelX
reformat

[Dman979]

Moved to KSP2 Gameplay Questions

[Sixstrings]

I've just had the same problem. Having the crossfeed switched on or off didn't change anything, it looks like it's always on. Will try again after dinner.

[TCAlex01]

I also have the same problem. Very good explained by FuelX.

[FuelX]

My finding s so far:

• The upper stage usualy leak once the lower stage is empty so it's possible to work around this simply by disconnecting stage just before the lower stage run out of fuel.
  • This is tricky because there is no display of the remaining fuel in every tank. 
• Both small and big rockets can leak.
• Sometimes, the whole upper stage is drained, sometimes only half.
• When the first stage is a single tank or almost empty, sometimes it does'nt leak.
• Maneuver nodes can help predict when the current stage is empty, it will display a "no fuel" warning.
• The stock kerbak-k1 prebuilt rocket did not leak at all.
• Displaying the staging sequence details (with the small arrow next to Launch) can help understand the problem.

For example, staging sequence for my rocket vs stock k1:

Edited February 24, 2023 by FuelX

[Schaufel]

Same question here.

Edited February 24, 2023 by Schaufel

[FuelX]

I restarted the game and rebuilt my rocket and the bug stopped.

Edit: it's quite random, sometime stage separation works, sometimes it does'nt.

Edited February 24, 2023 by FuelX

[Kbu]

My rocket (based on the Kerbal X) show the correct deltaV for the stages, but still emptying the upper stages tanks The last stage of the aspargus emptying at the same rate than the main thrusters and once they are empty, the next stage tank is empty too :(

[FuelX]

Kbu, you're right, my previous observation was wrong. Upper stage leaking seams a bit random. 

[Sixstrings]

I'm back from my second session. Restarting the game and building a fresh new rocket "fixed" the crossfeed bug. If as you guys said it's random, which seems so,  that could be a sensitive issue when building more complex designs. Hope they fix it soon !

[attosecond]

Same just happened to me... critical bug that makes the game unplayable when it happens. All that time spent in VAB and the only solution seems to be to start fresh with a new vehicle. Ugh.

[Periple]

Same here. I've been trying to investigate it, no luck so far. It seems random -- for example, I test-fired a rocket in launch clamps without SRBs firing, no drain. Built the fairing, changed the staging so launch clamps detach and the SRBs fire, drain. Another time I got in the air without drain, but when I staged my sustainer, drain.

I'll try rebuilding the rocket from scratch from the top down to see if that helps.

Edit: it didn't. Weird

Edited February 25, 2023 by Periple

[Audionut]

As someone entirely new to KSP, glad I'm not the only one experiencing this.

Tried a 3 stage to start, drained the 3rd stage at an equal rate to the 1st stage.

Then with a 2 stage, the 2nd stage drained equal to the 1st.

Will see if a restart on a completely new save fixes things.

[Saphi]

I had a similar issue with my mun lander.
I had to put the lander Legs on radial decouplers in order to prevent fuel from leaking.

[TCAlex01]

1 hour ago, Saphi said:

I had a similar issue with my mun lander.
I had to put the lander Legs on radial decouplers in order to prevent fuel from leaking.

I removed the LT-2 "Wombat" landing legs from my upper stage and this also worked for me.

Edited February 25, 2023 by TCAlex01
text correction

[TCAlex01]

43 minutes ago, TCAlex01 said:

I removed the LT-2 "Wombat" landing legs from my upper stage and this also worked for me.

Made a test now,  changed to the LT-1 "Wallaroo" landing legs and everything works fine, changed back to the LT-2 "Wombat" got the fuel issue again. Can anyone confirm?

1 hour ago, Saphi said:

I had a similar issue with my mun lander.
I had to put the lander Legs on radial decouplers in order to prevent fuel from leaking.

Confirmed!

[FuelX]

Thanks, I will test that too. My testbed didnt have landing legs and was leaking, but the rocket from my screenshots had wombats i'll change it's legs and give an update.

Another finding on my part, it's also related to the staging sequence. Bug start after moving in the staging sequence. By removing side boosters or a first stage for example.

Also just adding or removing a fuel tank from the main stage can change staging sequence enought to get rid of the bug.

And there's a link to a similar topic:

 

[FuelX]

42 minutes ago, TCAlex01 said:

Made a test now,  changed to the LT-1 "Wallaroo" landing legs and everything works fine, changed back to the LT-2 "Wombat" got the fuel issue again. Can anyone confirm?

Yes, it works! I removed the wombats from my rocket and it's not leaky anymore.

It may not be a fix, but it's a fine workaround. I want to try Saphi's trick too.

[Periple]

I can also confirm. "Wombat" and "Bandicoot" cause the problem.

I was also able to work around the problem by placing a decoupler between my command module and its engine, with the legs on the command module. 

It appears that the bug triggers if the legs are on a stage that contains fuel. If they're on a fuel-less stage, no drain.

[Motokid600]

So THIS is why my Mun lander failed last night. It had 1800Dv, but when it came time to land it didn't have enough. 

[jfjohnny5]

2 hours ago, Periple said:

I can also confirm. "Wombat" and "Bandicoot" cause the problem.

I was also able to work around the problem by placing a decoupler between my command module and its engine, with the legs on the command module. 

It appears that the bug triggers if the legs are on a stage that contains fuel. If they're on a fuel-less stage, no drain.

This doesn't appear to be a consistent fix. I currently have no landing legs at all, and it's still draining the upper stage.

[Periple]

16 minutes ago, jfjohnny5 said:

This doesn't appear to be a consistent fix. I currently have no landing legs at all, and it's still draining the upper stage.

Blimey. Any idea what could be triggering it?

[Sea_Kerman]

[Razor235]

44 minutes ago, jfjohnny5 said:

This doesn't appear to be a consistent fix. I currently have no landing legs at all, and it's still draining the upper stage.

Are there other radial parts on the upper stage at all?  Hazarding a guess it's related to those rather than just landing legs.

[Macenzie]

I'm getting super frustrated by the game mechanics being so buggy it appears to be random what will screw up my mission next.

Right now I have my damn rocket feeding upper stage fuel to my lower stage engines through a decoupler that has crossfeed off. What is the point if you can't trust the BASIC staging features to work every time...