Can you explain this fuel imbalance behaviour?

[Subbota]

I have a fully stock setup with asparagus staging that isn't working as I'd expect it and I'm not sure if it's a bug or something I'm just not understanding. I'm trying to deliver an X200-64 tank into orbit and don't care if it's full when it arrives, so I ran 2 fuel lines down to the first stage asparagus boosters using 2X symmetry. When I run the first stage engines, 4 out of 5 of the first stage -64 tanks stay full as long as they're being fed by the upper tanks, but one of the tanks fed directly from the upper tanks has a small but significant drain as if there's more flow in one fuel line than the other.

Craft file is here. Photos of the vehicle in VAB and during fuel burn on the pad are here.

The goal of connecting the top tanks to the bottom was to prevent a a top-heavy situation when I start my gravity turn. When I finish the initial launch burn I pump fuel from the center -64 back into the upper before separation to let a skipper finish the job.

For now I can work around it by constantly pumping fuel from the top tank to the leaky tank until the top is empty. It works, but it's not ideal and if I can fix this it'll make future launches a lot easier.

Edited July 31, 2013 by Subbota
marking it question answered

[stupid_chris]

The problem is that you created a fuel loop. Basically, you can have multiple fuel lines coming out of a stack, you can have multiple fuel lines coming into a stack, but you can't have one after the other. In other words (or with pictures (kinda)) you can have this: ---<=== you can have this ===>--- but you can't have this ---<===>---. If you take multiple lines out of a stack, you can't get that line to come back into that stack, fuel flow logic really hates it.

You could try TAC fuel balancer instead of manually balancing, that would probably help you a lot. Else, you will need to change your fuel routing if you want the problem to go away.

[Gus]

I'm with stupid. TAC fuel balancer is very handy for moving fuel around and retaining a balanced craft. This and Kethane are the only mods I use.

[sojourner]

just run the final set of asparagus fuel lines to the top tank. Then they should drain in the following order:

Asparagus stages first, top tank next, then middle, and finally the bottom last.

[Klingon Admiral]

just run the final set of asparagus fuel lines to the top tank. Then they should drain in the following order:

Asparagus stages first, top tank next, then middle, and finally the bottom last.

Nope, there is a decoupler between the top tank and the central booster, so the drain order will be: Asparagus set #1, asparagus set #2, central booster, payload tanks.

But the Op wants the upper jumbo tank to drain first to drastically reduce the payload weight.

The easiest solution would be a small correction in the staging, just fire the central lifter engine after the asparagus boosters have burnt out and the craft should work fine.

[Lheim]

I've done similar, wanting to launch a large fuel tank for a fuel depot in orbit and wanting to use the fuel in it during launch.

I did normal asparagus staging, but I had my payload fuel tank with fuel lines feeding to my last two asparagus tanks as well. Really, you could attach them to the first asparagus stage as well.. your first stage would last longer, and after staging it your payload is empty.

Edited July 31, 2013 by Lheim

[mcirish3]

or you could just use these stock orbital fuel tanks found here they were desinged in .20.2 but work just fine in .21 good luck.

[Subbota]

Thanks for all the answers, guys. I'm going to try playing with the design some more to see if I can get it to work better. I'm pretty new to the game so I'm trying to understand the core of the game before I start adding mods.

[sojourner]

Nope, there is a decoupler between the top tank and the central booster,

If only there was some part like a fuel line that could be used to get around this. Oh... wait!