How to force fuel tanks to drain simultaneously?

[Ninzoris]

I'm working on a SSTO and after many, many failures from a mysterious pulling of the plane to the right starting at 30km I finally discovered what was at fault. One of the two fuel tanks hanging under the wings (right one) was draining before the other tank was. Even though I had symmetrical fuel transfer lines running from each tank to the engines. I thought it could be a bug from the order in which the plane was assembled, so I unclipped the tanks and clipped them back in using symmetry and put in new fuel transfer lines. Now the problem persists but this time with the left tank draining first.

Any help would be appreciated.

[frozenbacon]

If the tanks are connected to a part capable of fuel crossfeed, they will ignore the fuel lines. You will need to put something that does not have fuel crossfeed between the fuel tank and the wing. if you want it to follow the fuel lines. I think there is a hardpoing that could used for this purpose.

[RoboRay]

You either have some asymmetry in your fuel line connections, or you have created a loop in the system that's confusing the fuel-flow logic (never create a situation where fuel can leave a tank and flow back into the same tank, where a tank can follow more than one path to reach an engine, or where the flow splits from one tank into more than one path then merges back together again later).

[Ninzoris]

Thank you!

It was a fuel line loop.

[Payload]

Download the TAC fuel balancer. Leave out the fuel lines all together. That is what I do. Small planes are easy because you can balance them around one central tank or something. Huge cargo ships almost require the fuel balancer mod as a rule.

[M5000]

never create a situation where fuel can leave a tank and flow back into the same tank

Not always true. I actually do this a lot on spaceplanes and such. You run a line from the tank and a line to the tank, and, assuming you've linked one stack to one stack, you've basically just made one "big" tank that will drain equally depending on what resources are being used.

Hear that, OP?

If you want to have multiple "stacks" (fuel tanks all connected via connection nodes and NOT radially attached) drain as one large "tank", say, if you have several radial tanks on a central stack tank, connect them all with a fuel line going to the center stack, and a fuel line coming from the center stack to the outside. This will cause them to drain equally, regardless of how fast each engine consumes fuel. So, if you have, say, a center stack with a Mainsail, and 6 side-stacks with LV-Ns, the fuel meter for all 7 engines will be exactly the same length and will all drain at the same rate, which I believe would be some average based on the mean average of all the fuel consumption rates of involved engines..

Edited May 5, 2013 by M5000

[Sovek]

Yeah, poor advice RoboRay. Sometimes you need the extra thrust and ÃŽâ€V that a couple more engines and fuel would give you. My most recent design had two 3.75m segments, By themselves they wernt enough for my needs (that being free return trajectory around Mun), so I attached 2 boosters on the side, this had enough. However... The boosters either drained too fast (fuel lines to the center 3.75m tanks) and went from over 2 T:W down to an anemic 1.3 T:W, If I forgo the lines, the center engine cuts out before the side boosters finish. By having them drain semi-evenly, I was able to keep a decent T:W and not jettison the first stage until 20km, where the second stage was enough to get me into orbit and around Mun.

Sometimes you just need to do it that way.

[RoboRay]

Yeah, poor advice RoboRay. Sometimes you need the extra thrust and ÃŽâ€V that a couple more engines and fuel would give you.

"Adding more engines and fuel" has absolutely nothing whatsoever to do with avoiding loops in the fuel-flow system. Loops and lines are not the same thing. Perhaps that's what's confusing you.

Not always true. I actually do this a lot on spaceplanes and such. You run a line from the tank and a line to the tank, and, assuming you've linked one stack to one stack, you've basically just made one "big" tank that will drain equally depending on what resources are being used.

You are fortunate and the the last of the 25 (I think) cycles the fuel flow logic runs through before terminating with a silent error is stopping on a point in the infinite loop you've created that results in balanced fuel flow for your implementation. If you added one more less step to the flow, it would result in an imbalance. This is based on what the Devs have said, not merely my own experiences. We experimented carefully with the flow logic a while back to determine exactly how it works.

EDIT: Unless they've changed something in the last couple of patches in regards to fuel-flow, and I don't recall seeing that in the patch notes.

Edited May 5, 2013 by RoboRay

[M5000]

Yeah, poor advice RoboRay. Sometimes you need the extra thrust and ÃŽâ€V that a couple more engines and fuel would give you. My most recent design had two 3.75m segments, By themselves they wernt enough for my needs (that being free return trajectory around Mun), so I attached 2 boosters on the side, this had enough. However... The boosters either drained too fast (fuel lines to the center 3.75m tanks) and went from over 2 T:W down to an anemic 1.3 T:W, If I forgo the lines, the center engine cuts out before the side boosters finish. By having them drain semi-evenly, I was able to keep a decent T:W and not jettison the first stage until 20km, where the second stage was enough to get me into orbit and around Mun.

Sometimes you just need to do it that way.

I wouldn't exactly say poor advice, as I'm sure there's a logical way the fuel flows and maybe I've just been lucky... BUT....

"Adding more engines and fuel" has absolutely nothing whatsoever to do with avoiding loops in the fuel-flow system. Loops and lines are not the same thing. Perhaps that's what's confusing you.

You are fortunate and the the last of the 25 (I think) cycles the fuel flow logic runs through before terminating with a silent error is stopping on a point in the infinite loop you've created that results in balanced fuel flow for your implementation. If you added one more less step to the flow, it would result in an imbalance. This is based on what the Devs have said, not merely my own experiences. We experimented carefully with the flow logic a while back to determine exactly how it works.

EDIT: Unless they've changed something in the last couple of patches in regards to fuel-flow, and I don't recall seeing that in the patch notes.

I don't know, it's always seemed to work flawlessly for me. I might try adding some more tanks with one-way connections on there and see if they drain to/from the main "psuedotank" or if it causes a fuel clusterfark.

[foamyesque]

Download the TAC fuel balancer. Leave out the fuel lines all together. That is what I do. Small planes are easy because you can balance them around one central tank or something. Huge cargo ships almost require the fuel balancer mod as a rule.

Who says? What're you defining as huge, here?