Fuel going from one tank to another for no apperent reason.

[Chronin]

So I'm currently making my Eve Super-Mission (the Extravagant Exploding Eve Expedition) and I'm trying to get the interplanetary stage into orbit. This stage is made up of seven orange tanks, with a total of 13 nuclear engines and 3 skipper engines. The NERVAS are going to be doing most of the work, but the Skippers are there for a little extra power. Anyway, I was sending it into an 80km orbit to dock with the other parts of the ship. I told mechjeb to do it, since it's much more efficient and precise than me. The first few stages went with no problems, but when it activated the Skippers and NERVAS one of the skipper tanks was apparently taking fuel from one of the nuclear engine tanks, which is strange, considering there are no fuel lines on the whole ship (the first few stages are asparagus staged, but they're gone at this point). Is there anybody who knows what this is or how to fix it? Thanks

[technicalfool]

Have you got a screenshot or .craft file that people can play with? There can be some parts which will allow fuel to transfer through them, and if you're sticking two tanks together with such a part, well, you might have a problem.

[Othuyeg]

This is something that happens to me very often, and if it is caused by the same thing, then you want to go check that your fuel lines are all connected correcly. For me they sometimes disconnect or connect to something else, such as a structural part.

[allmhuran]

A pic or craft would indeed help, but in the meantime, I'd check your asparagus stages and make sure that all of your 7 final tanks are actually full at the point where you drop the final asparagus columns. It's possible that one of the asparagus lines got messed up, but the craft was still balanced enough for mechjeb to be able to compensate. Once all the asparagus stages are dropped and the final stage ignites the imbalance would become more pronounced, creating whatever issues you're experiencing.

[James_Eh]

Make sure you disable crossfeed on any docking ports in the upper stage to prevent unintended fuel flow.

[Kasuha]

You can try to find the problem using information in this thread. My bet is it might be an instance of this problem.

Or post a screenshot or a .craft file (but I can only help you with .craft file if you don't use any mods).

[Red Iron Crown]

They really should turn off fuel crossfeed on radial decouplers, it's counterintuitive and rarely the desired behavior.

[Kasuha]

They really should turn off fuel crossfeed on radial decouplers, it's counterintuitive and rarely the desired behavior.

I agree. The only case where I can imagine it as useful is radial engines mounted on decouplers, allowing you to drop engines to reduce mass and TWR when your fuel tank is getting empty - an early predecessor to asparagus staging, perhaps from times when there were no fuel lines in the game. But who knows if anyone has ever used them that way.

[Claw]

They really should turn off fuel crossfeed on radial decouplers, it's counterintuitive and rarely the desired behavior.

I agree. The only case where I can imagine it as useful is radial engines mounted on decouplers, allowing you to drop engines to reduce mass and TWR when your fuel tank is getting empty - an early predecessor to asparagus staging, perhaps from times when there were no fuel lines in the game. But who knows if anyone has ever used them that way.

Has this been formally suggested? I know we talk about it, but I don't know if it's in the bug tracker.

[Red Iron Crown]

A cursory search of the Suggestion forum and bugtracker doesn't reveal anything. I'll start a thread, I don't think it's really a bug per se.

Edit: Suggestion thread here.

Edited May 21, 2014 by Red Iron Crown

[Chronin]

Sorry about not giving you guys pictures, my mistake. This is the ship after the first stages are gone, where the problem is.

http://imgur.com/a/9BRLc#0

[CaptRobau]

I agree. The only case where I can imagine it as useful is radial engines mounted on decouplers, allowing you to drop engines to reduce mass and TWR when your fuel tank is getting empty - an early predecessor to asparagus staging, perhaps from times when there were no fuel lines in the game. But who knows if anyone has ever used them that way.

I would've, had I known. But I always thought they weren't crossfed. But most of the time you'll be using it for a situation where you don't want crossfeed.

[Kasuha]

Sorry about not giving you guys pictures, my mistake. This is the ship after the first stages are gone, where the problem is.

http://imgur.com/a/9BRLc#0

I'm afraid I need to see how exactly is that skipper part attached to the rest. There seem to be some decouplers involved, but a radial decoupler between a tank and another tank does not transfer any fuel because of Rule 5. So for that to work this way, you'd need to have the decoupler between the battery and the central column, or have some forgotten fuel lines in there.

Edit: on further investigation of your images, are you sure you don't have some mod transferring fuel between tanks? Such as TAC fuel balancer?

Edited May 21, 2014 by Kasuha

[Whovian]

I don't see any indication of a mod like TAC fuel balancer, so probably not.

Looking at the screenshots, it looks like there isn't a perfect symmetry between the tanks; the strut structure, for instance, is a little asymmetrical. Were they put on freelance, in two chunks of 3x radial symmetry, or ... how does the symmetry on this ship work?

[Chronin]

The symmetry is 2 chunks of 3x symmetry, so i didn't think that would be a problem. I can't give you the .craft file because I'm trying to rebuild it, so I updated it. Really haven't thought this true.

Also, I only have Mechjeb and Hyper-edit as mods. I'm trying to keep it mostly stock.

Edited May 21, 2014 by Chronin

[Kasuha]

The symmetry is 2 chunks of 3x symmetry, so i didn't think that would be a problem. I can't give you the .craft file because I'm trying to rebuild it, so I updated it. Really haven't thought this true.

Also, I only have Mechjeb and Hyper-edit as mods. I'm trying to keep it mostly stock.

Your craft file does not exactly correspond to your screenshots, there are all kinds of parts missing (most importantly struts, it behaves like jellyfish even with hacked gravity). And it works correctly, no fuel is transferred anywhere, each engine takes it from its tank.

Beside the fact that the staging is not set up correspondingly to intended asparagus (which may be only problem in this craft file, not the final ship) the only thing wrong I noticed on it is that you tried to set up tanks on three radial decouplers. It looks right but it does not work, the tank is only attached through the middle decoupler and the other two are there just for looks and do absolutely nothing - they're not connected to the tank and since ship's parts cannot collide with each other, they even don't prevent the tank to wobble.