Cross-feed fuel

[DukeJCDC]

I cannot figure out for the life of me how to cross-feed and haven't been able to find any literature explaining how to do it. What I'm wanting to know, is there a way to cross feed fuel via tanks just attached to eachother? Or does it have to have an external fuel line?

[krakerstaker]

I guess theres no literature on the matter because it is so ingrained in the community already that no one thinks it needs explaining.

That said I shall put it into words here.

Basically fuel will flow automatically between tanks that are connected to each other by there top and bottom nodes (the green balls on the ends) i.e. in a stack.

It will flow from top to bottom, or front to back if your making a spaceplane.

If you connect fuel takes together on there sides or through a decoupler, then you are required to use fuel lines. Fuel lines are like struts except they are yellow and they have a specific flow direction.

When placed, fuel will flow through a fuel line from the first point placed to the second point. There is also small arrows on the fuel line which shows the direction of flow.

There is another method to transfer fuel while your flying the vehicle. If you hold down the alt key and click on two different tanks on the same craft, then you are presented with a menu which allows you to tranfer the resources. This works on all containers like RCS, Xenon, and even batteries.

This is how you can refuel a craft by docking with another one that has fuel.

Thats basically it in a nutshell hope this helps. Cheers!

Edited October 26, 2013 by krakerstaker
typoe <--lol

[Kasuha]

There are several inaccuracies in the above description.

1/ engine does not draw fuel just from top to bottom, it can draw also from bottom to top, as long as the engine is radially attached to a fuel tank which has some more fuel tanks below it. If the engine is connected to a middle tank in a stack of tanks, it will draw the fuel from both top and bottom tank at the same time, distributing the load evenly.

2/ fuel pipe does not pump any fuel. If you empty the tank to which it leads, it will not put any fuel to it. Fuel pipe only tells the engine that it should also take fuel from the tank connected by the pipe.

[krakerstaker]

1/ engine does not draw fuel just from top to bottom, it can draw also from bottom to top, as long as the engine is radially attached to a fuel tank which has some more fuel tanks below it. If the engine is connected to a middle tank in a stack of tanks, it will draw the fuel from both top and bottom tank at the same time, distributing the load evenly.

2/ fuel pipe does not pump any fuel. If you empty the tank to which it leads, it will not put any fuel to it. Fuel pipe only tells the engine that it should also take fuel from the tank connected by the pipe.

Number 1 is a fair point. I myself did not know about the radial engines in the middle, or at least didn't observe it, but then again I hardly use the radial engines.

And number 2 seems like good info, but your almost always using an engine to empty tanks. The only way you can empty a tank otherwise is by transferring the the fuel out.

I was merely trying to answer the guys question and give some basic understanding. He's clearly new judging from his join date, so he's still got a lot to learn yet. Giving him a working knowledge of the game and letting him figure out the specifics through trial and error, the heart of KSP as we all know, is a better way to let him enjoy the game.

And if he has another question all he has to do is ask.

There are several inaccuracies in the above description.

I don't think anything i said before was inaccurate.

Also 2 is not several.

[Kasuha]

I don't think anything i said before was inaccurate.

Also 2 is not several.

Sorry if my english is not perfect, it's not my first language. In my language we have a word with the same sense as "several" except it means "low number greater than one". Maybe I should have used "incomplete" instead of "inaccurate", too.

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Just out of curiosity I ran the following contraption to determine how fuel is drawn from stacks of fuel tanks connected by pipes.

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[TechnicalK3rbal]

If you meant cross-feeding fuel between docked craft, you right click on one tank, alt-right click on the other, and you can transfer LF&O between the tanks.

Edited October 28, 2013 by TechnicalK3rbal
There's a button for it in the GUI, and it's instead of alt on mac it's the mod key

[I_Killed_Jeb]

how do you get all the tank info to show up?

[Kasuha]

how do you get all the tank info to show up?

Click on the Resources button in the upper right of the screen (you need to click on it so it stays open), then click on the square next to Liquid Fuel ... or whatever resource do you want displayed.

[Dispatcher]

One may also connect two fuel lines between tanks; one outgoing and one inletting. Sometimes that helps to balance the amount of fuel available to each active engine. Experiments like those done by Kasuha are interesting and show how different results may be obtained by how you set up the tanks and fuel lines.

[krakerstaker]

Just out of curiosity I ran the following contraption to determine how fuel is drawn from stacks of fuel tanks connected by pipes.

Wow excellent work on that. I added to your rep for this.

Looking at your findings there seems to be no intuitive way to figure out which tanks will drain first. I would love to see the actual logic programmed into the fuel flow system. Clearly much more intricate than previously perceived. Maybe I'm easily overwhelmed?

Curious how it drains the the lone tank on the right first. I would have thought that both side would start to drain at the same time, since they are the same distance away (just one fuel pipe).

Your setup is a bit wonky and would never fly, but it does serve to put the fuel flow through an exhaustive test. Maybe now we could do some tests with reverse pipes added like Dispatcher said.

[phunk]

It looks like when a tank has both a fuel pipe and tanks attached above or below, it drains everything it can reach through the pipe before going to the other tanks. That explains his test results entirely.

ETA: I should say, that plus the "drain the farthest tank when they're connected end to end" explains it.

Edited October 28, 2013 by phunk