Fuel cells and crossfeeds

[MR L A]

Hi everyone.

I play KSP with the crossfeed obeys (something) option enabled, It basically makes refuelling and fuel transfer a pain in the behind as fuel cannot be transferred through certain parts. Which I like. However, I currently have a small capsule design which features a fuel tank (oscar b) exclusively for fuel cells. Two fuel cells are directly attached to the tank and this fuel tank cannot transfer fuel to the main tanks which supply the engine.

Annoyingly, the fuel cells, DESPITE CROSSFEED BEING OFF, are somehow able to drain the main tanks and do so BEFORE emptying the tank they are actually attached to. As far as I can tell (though I will test this further) they also ignore the order in which tanks should be consumed.

Whats up with this? Can anybody else replicate this behaviour of fuel cells doing whatever they please?

Thanks!

[Zeiss Ikon]

It's probably going to be critical (for those who'll try to replicate this behavior) to know what version of KSP you're using, and what if any mods.  I wouldn't be surprised if posting a .craft file would be even better -- at the minimum, a VAB image showing the capsule and Ocar-B/fuel cell setup.

Generally, if there's a tank directly attached to a stage, it'll drain evenly with other tanks in the stage (though flow priority can change that).  If the tank is attached via a decoupler, the decoupler's crossfeed setting applies.  If you have a tank that's not draining the way to expect, my experience suggests that the first thing to check is whether the tank has somehow gotten into direct contact with the stage -- I had this problem with strap-on boosters, once, where they'd drain simultaneously with the core instead of using the fuel ducts to keep the core full as the strap-ons drained -- that turned out to be due to the strap-ons clipping through the decouplers to contact the core tanks, making it all one big happy tank family.  They also wouldn't stage away when the decouplers fired, which was the clue to solving the problem.

[fourfa]

I'm afraid fuel cells draw evenly from all tanks, regardless of crossfeed settings.  Similar to how electricity is drawn from all batteries, and monopropellant is drawn from everywhere.

In stock, you can lock off tanks you want left alone and I believe this will stop it (just have to remember to unlock them before you need them!), or let it draw as needed and transfer fuel around when you're ready to stage.

I generally use a fuel pump mod in most saves, and setting up tank priority can totally solve this problem at the cost of some CPU cycles and a little bit of the purity of your soul (I guess mine is in negative territory by now)

[MR L A]

10 hours ago, Zeiss Ikon said:

It's probably going to be critical (for those who'll try to replicate this behavior) to know what version of KSP you're using, and what if any mods.  I wouldn't be surprised if posting a .craft file would be even better -- at the minimum, a VAB image showing the capsule and Ocar-B/fuel cell setup.

Generally, if there's a tank directly attached to a stage, it'll drain evenly with other tanks in the stage (though flow priority can change that).  If the tank is attached via a decoupler, the decoupler's crossfeed setting applies.  If you have a tank that's not draining the way to expect, my experience suggests that the first thing to check is whether the tank has somehow gotten into direct contact with the stage -- I had this problem with strap-on boosters, once, where they'd drain simultaneously with the core instead of using the fuel ducts to keep the core full as the strap-ons drained -- that turned out to be due to the strap-ons clipping through the decouplers to contact the core tanks, making it all one big happy tank family.  They also wouldn't stage away when the decouplers fired, which was the clue to solving the problem.

Latest version and I'll be rebuilding the craft and doing unmodded testing tomorrow 

5 hours ago, fourfa said:

I'm afraid fuel cells draw evenly from all tanks, regardless of crossfeed settings.  Similar to how electricity is drawn from all batteries, and monopropellant is drawn from everywhere.

In stock, you can lock off tanks you want left alone and I believe this will stop it (just have to remember to unlock them before you need them!), or let it draw as needed and transfer fuel around when you're ready to stage.

I generally use a fuel pump mod in most saves, and setting up tank priority can totally solve this problem at the cost of some CPU cycles and a little bit of the purity of your soul (I guess mine is in negative territory by now)

I suspected this. However, I'm annoyed that the fuel cells are actually ignoring the tank they are directly attached to - I'll be doing more test tomorrow and will report back.

Would be nice if squad made them obey the same laws as engines though.