Quick fuel crossfeed question

[Streetwind]

When you stack multiple tanks on top of another and an engine is on the bottom, then the engine will drain from all the tanks, starting at the top. So far, so good.

But what if I have three tanks stacked vertically, and the middle (not the bottom) tank has a fuel line connected to it that leads to a different tank which is drained by an engine? Will all three of my stacked tanks drain through the fuel line, or only the top two? And if all three drain, in which order do they do so?

Edited March 4, 2014 by Streetwind

[Rhomphaia]

Fuel will be drawn from the top and bottom of the stack simultaneously, then the middle one.

[Streetwind]

Cheers, that is a most satisfactory answer! With this, my planned vessel can go ahead.

[Red Iron Crown]

Kasuha wrote up a detailed guide to the fuel flow rules, here. I've found it most helpful in atypical rocket construction.

[Kasuha]

But what if I have three tanks stacked vertically, and the middle (not the bottom) tank has a fuel line connected to it that leads to a different tank which is drained by an engine? Will all three of my stacked tanks drain through the fuel line, or only the top two? And if all three drain, in which order do they do so?

It's always good to do your own experiment - connect your tanks the way you describe in a simple ship deployed on launchpad, mount engines in a way that it won't travel anywhere, start up engines and watch how it drains by right-clicking on tanks. It also helps understanding my description mentioned in the post above which may seem to be very complicated otherwise.

In your layout I'd guess the tank behind the pipe would get drained first, then the stack would be drained, from top tank to the bottom tank.

[Streetwind]

Well, testing is difficult when you come up with a new design while bored at work, with no access to KSP whatsoever

I just wanted a quick confirmation that the fuel line will indeed drain all connected tanks no matter if they are above or below each other, so that I knew whether or not to continue pondering my design idea. The actual building will have to wait until I get home later.