One of my tanks drains too fast.

[Fenris]

So this took me awhile to figure out since the fuel indicator on the left of the screen shows all engines having the same amount of fuel. But part way through my launch the rocket tips into an unrecoverable dive. I finally clicked on all the tanks as this was happening and saw that one of the tanks was nearly empty.

The setup has a center tank and 1 tank north, south, east, and west (for a total of 5 tanks, with the middle one being the largest). It looks like an "X" from a top down view. I ran one fuel line from each tank into the middle one. and one fuel line from the middle one to each of the outer tanks thinking this would balance out the fuel as it drained. I have checked countless times and all fuel lines are going in the proper direction and are attached to the tanks, but the one in the "back" (the one that faces away from you as you enter the launch screen) drains faster creating a balance problem.

So ... what am I doing wrong?

Thanks,

[Wayfare]

KSPs fuel logic is easily confused by loops and/or a mix of converging and diverging flow routes. In order for fuel to flow properly, you should plan the route to be either converging (many to few) or diverging (few to many) with no loops.

In your setup, having the outer tanks feed into the inner one could work, or having the inner tank feed the outer tanks, but not both as that creates a loop.

[JoCRaM]

KSPs fuel logic is easily confused by loops and/or a mix of converging and diverging flow routes. In order for fuel to flow properly, you should plan the route to be either converging (many to few) or diverging (few to many) with no loops.

In your setup, having the outer tanks feed into the inner one could work, or having the inner tank feed the outer tanks, but not both as that creates a loop.

a fuel line from the outer tanks to the inner one, and then a line from the inner tank to the engines (or something uptree from the engines - but not the tanks) will probably do what you want.

Alternately use the Fuel Balancer mod.

[Kasuha]

Fuel lines do not pump any fuel, they are just traversed by engine feeding algorithm determining from which tanks and in what order should it drain fuel. This means that how the fuel is drained does not depend on just how fuel tanks and pipes are connected but also on how engines are connected to it.

My guess is, you have an engine below each of the tanks. If you don't use any fuel lines, the side tanks drain too fast. If you use fuel lines from the middle tank to the side tanks, the middle tank drains too fast. And you maybe want them to drain fuel from all tanks so they all run off fuel at the same time.

If that's the case, notice that you don't have to run the fuel line just between tanks. You can also run the line from a tank to an engine. So you can run a fuel line from the central tank to each of the side tanks, then run a fuel line from each side tank to the central engine. This will do that.

[Wayfare]

If that's the case, notice that you don't have to run the fuel line just between tanks. You can also run the line from a tank to an engine. So you can run a fuel line from the central tank to each of the side tanks, then run a fuel line from each side tank to the central engine. This will do that.

Won't that just drain the central tank first?

[Sirine]

I have the same experience many times.

and my solution....rebuild the whole things.

Remember to save often when adding parts. After launch, try save and load again to make sure all the struct and fuel line are properly in place. (Connected to where they design to be.)

[Kasuha]

Won't that just drain the central tank first?

It will, does it matter? All engines will have fuel anyway.

The question is what you're trying to achieve.

[Skyrunner27]

Try putting on the small hardpoint. That is in the structural tab. It has cross fuel capability so all the tanks should have a even amount of fuel. Which should solve what you are trying to do.

[Wayfare]

It will, does it matter? All engines will have fuel anyway.

The question is what you're trying to achieve.

True, but I understood your post as a suggested way to make sure all five tanks drain evenly. The only way to do that would be to have each tank feed into the engine independently, with no fuel lines between the tanks.

[Taki117]

Might I suggest asparagus staging? It wont solve your problem, but it will help get big things into orbit.

[Kasuha]

True, but I understood your post as a suggested way to make sure all five tanks drain evenly. The only way to do that would be to have each tank feed into the engine independently, with no fuel lines between the tanks.

There is no general way to make all tanks of different sizes to be used evenly. If the big tank is twice the size of small tanks, you may achieve even usage by attaching two engines to it instead of just one, for instance. But usually the task is to not lose balance and eventually to make sure stages to be dropped are empty first.