Fuel Line Behavior

[Flying_Brick]

Hey everyone. So I just started playing KSP pretty recently now and I am having a great time with it. Quite excellent and entertaining (thank you developers!). Anyway, I've started working on some more complex designs using fuel lines and came across something kind of interesting (to me anyway). So I have a situation like this:

My naive first thought was that it would drain the single tank on top, then the two tanks connected with fuel lines simultaneously, then down to the rest of the main body. What I assume actually happens is the game looks up the chain of tanks and lines and marks the final tank it can reach as active, then follows any alternate fuel paths as far as they will go marking the last tanks in those paths active as well. In my case however since the very top tank was marked active first, the single remaining alternate path stops a tank below it, since the next one in line is already marked as active, and activates the one it stopped on.

So for one thing, I might suggest that if a chain of fuel tanks and lines ends with a tank that is already marked as active then it should simply do nothing instead of activating the next tank down. That suggestion of course is based on quite a few assumptions about how the game is coded on my part and there could be negative consequences on game mechanics that change could have that I am not considering, so feel free to ignore it if I am off on any of those points.

My main question though is, are there any suggestions on getting something like this to drain the way I would like it to as the game currently works?

[amore555]

A nice feature when creating ships would be to be able to number your fuel tanks, so as to choose in what order they drain.

[Awaras]

There is a number of known bugs in the fuel system that cause a lot of weird behavior. Many of these should be fixed in 0.17.

[nhnifong]

There is a number of known bugs in the fuel system that cause a lot of weird behavior. Many of these should be fixed in 0.17.

AFAIK, it's just that if a part can draw fuel from N other parts, it will take 1/N units of fuel from each, regardless of whether any of them are already empty.

[Vanamonde]

Forgive me if you already know this, but the fuel lines only pump one way, which is the direction of the little gray triangles on the tube. I can't tell from your picture if they're all flowing in the directions you intend, but might that be the problem?

Oh, and you might not need that third, lowermost set at all. Those 3 bottom tanks are already connected because they're in contact with each other, but the outer two draw from the center, so the only reason you might to want lines there is to override that and have the middle engine be the last to flame out.

Edited September 13, 2012 by Vanamonde

[softweir]

Flying_Brick, that is, so far as I understand it, a pretty good guess as to how the fuel-line logic works. If you do a search for posts by HarvesteR using the key word "fuel" then you will find a few mentions of it - none are complete, but they give you the idea.

Fuel-tank drainage is being adjusted (yet again!) for release 0.17, so with luck this "feature" will change.

[Bluejayek]

Other obvious comment: A second ASAS on a ship does absolutely nothing, you can remove it and save weight.

[Flying_Brick]

I really only designed this rocket to test the fuel drain logic so it wasn't really meant to be functional. I actually noticed things acting funny in a space plane design where I wanted to route fuel from a tank in front of the cockpit to two tanks below it to one tank behind it. I just used the two ASAS units to block the fuel from feeding through that section, the arrows are definitely going the correct direction, and the fuel lines at the bottom were pretty much just tests to see how I could affect the fuel flow. Anyway, thanks for all the comments. I very much appreciate the responses. I'm sure I can just design out the issue or work around it but I was curious enough about the cause to ask about it here. At least until .17 (possibly) changes how things work that is