Decouplers Fuel transfers through decoupler with crossfeed disabled

[Sea_Kerman]

KSP 0.1.0.0.20892

Windows 10

Ryzen 7 5800H

RTX 3060 Laptop

I expect decouplers to prevent fuel crossfeed but in situations similar to this one, it doesn't.

Some workarounds seem to be to use different landing legs to the medium ones, or to not mount the medium legs on the fuel tank, but the latter does not seem to always work.

No mods

Edited February 26, 2023 by Sea_Kerman

[Sea_Kerman]

it has something to do with your landing legs 

 

[Macenzie]

I do have a lander with legs, however the tank sending the fuel in my design isn't the one with legs attached. This only started happening when I added a fairing further up (to actually cover my lander).

[Macenzie]

39 minutes ago, Sea_Kerman said:

it has something to do with your landing legs 

 

You sir are a genius and a scholar.

So I removed the fareing above he upper stage tank and it stopped feeding through to the lower engines.
I then removed the landing legs from my lander and the lower engine stopped taking fuel from the lander...

This needs fixing ASAP, it makes fareings and landing legs useless!

[csnick]

Having same issue,

It also happens if you have radial decouplers (which I think the clamp counts as a radial decoupler) on the lower stage and medium landing gear. 

 

 

[Macenzie]

Just to add, fairing do this too. If a fairing is touching a tank, it will crossfeed down to the bottom too.

[Euphotic]

When building a rocket that has an upper and lower stage with engines that use methalox fuel, fuel from the upper stage is consumed by the engine of the lower stage across a decoupler, regardless of the fuel crossfeed setting.

From the text representation of a rocket below, if Engine 2 is active, it will consume fuel from Fuel Tank 2 AND Fuel Tank 1, even across the decoupler. I used a 2.5m fuel tank and decoupler with Poodle engines when I encountered this bug. This issue can be worked around by manually transferring fuel back into the upper stage, but this is likely not intended.

 

 

Fuel Tank 1

Poodle Engine 1

2.5m Decoupler

Fuel Tank 2

Poodle Engine 2

[Niko_Dragovich]

Same issue,

• KSP Version :
• 0.1.0.0.20892
• Operating System and version :
• Windows 10 Pro
• CPU and GPU models, any other system information which could be relevant :
• I7 8700K, RTX 3070, 32Go RAM DDR4
• Description of the bug.  
• Fuel of the last stage is used when I decoupling the rocket
  • Expected Behavior
  • Decoupling and don't use the fuel of the last stage
  • Observed Behavior
  • Decoupling and use the fuel of the last stage
• Steps to Replicate
• Use the XS or MD landig leg on the last stage. Launch the rocket and decoupling radial for example.
• Fixes / Workarounds (if known..)
• Don't use XS or MD landing leg
• A list of ALL mods.  If the list is long, please consider using a spoiler window.
• No mods
• Other Notes / Screenshots / Log Files (if possible..)

 

I tryed all landing leg and the XS and MD have the problem.

The bug appears when I use radial decoupler or a decoupler for escape tower for example.

 

Edited February 26, 2023 by Niko_Dragovich

[Sea_Kerman]

very likely same bug as 

 

[Macenzie]

As Sea_Kerman says, it's most likely the landing struts causing the tank to drain.

[omega2003]

This bug appears when the legs are attached directly to a tank.

workaround : Add a (structure) part between tank and your legs.

[Sbeve_Boubemi]

I have fuel flow turned off on a decoupler and fuel still flows through it emptying the tanks. It also doesn't start draining until the first stage is almost out of fuel.

[James Kerman]

1 minute ago, Sbeve_Boubemi said:

I have fuel flow turned off on a decoupler and fuel still flows through it emptying the tanks. It also doesn't start draining until the first stage is almost out of fuel.

Welcome to the forum, I have seen some mentions that certain landing legs are responsible. If your vessel has them could you try removing them and see what happens?

[Periple]

That wasn’t sufficient for me — I had my legs on the command module and the tanks on the same stage still drained. When I added a decoupler between the command module and the tanks, the drain stopped.

Bizarre bug. 

[James Kerman]

Thread moved from KSP 2 Discussion to KSP 2 Bug Reports.

[GamingHen]

I built a Mun rocket that has a first stage, a transfer stage and a lander. Whenever I used the transfer stage, it also drained fuel from the lander's fuel tanks. The stages were separated by a separator and the Fuel Crossfeed was disabled.

[Sea_Kerman]

 

[Nuke]

confirmed. ive seen this. specifically when using surface mounted engines. 

[QuantumG]

Confirmed for me too. 

[QuantumG]

I can confirm the same happens with me. No landing legs were present. 

[Youre_Arent]

Confirmed. I had a transfer stage to the mun that used all the fuel in the lander above it. Note I had crossfeed disabled as per part defaults.

Edited February 27, 2023 by Youre_Arent
Extra information about issue

[MercurialMind]

Confirmed. I played around with it and found that having "medium landing gear" on my lander was causing the issue.

[jfjohnny5]

On 2/25/2023 at 12:03 PM, Razor235 said:

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

At first I had solar panels radially attached, yes. I did remove those as well, but the issue persisted. I then made a brand new rocket following the exact same design.. and this time it was fine. So, it's everyone's favorite type of issue: intermittent.

[Jake1146]

I keep having little to no fuel in my final stage/lander but all of my decouplers have cross feed disabled. So I don’t know why it is using the fuel before I get to that point. 

[Sea_Kerman]

It’s because you have a landing leg attached