bi-directional fuel feed and tank equalization

[barfing_skull]

I have a rocket first stage that seems to work pretty well....except when I try and turn at altitude. I'm pretty sure I know what the problem is, just not how to fix it.

The rocket is a main cluster of tanks and engines (7 in all, the central plus 6 surrounding). Outside of that there's another ring of 6 tanks and engines, which are asparagus staged. There are some SRB's too, but they're not the problem. The rocket works fine until I've dropped the SRB's and asparagus stages, and I'm down to the inner cluster of 7 tanks and engines. The central cluster has bi-directional fuel lines - one each direction - between the outer 6 tanks, and unidirectional lines all pointed to the central tank. This works great, as the asparagus stages expire as planned, and when down to the central 7, all the 6 outside engines have the same amount of fuel available to each. However, that fuel doesn't seem to be weighted equally among the 6 radial central tanks (some tanks show full while others empty at that stage of flight), and this, I believe, is what's causing the wild directional behavior when the rocket tries to turn. I don't see any imbalance of thrust.

Since it's asparagus staged, the last asparagus tanks empty into 2 of the 6 central radial tanks. I'm assuming this is what causes the imbalance, and that it's not redistributing that fuel among the inner tanks evenly. What I'd really like to have happen is for the fuel/oxidizer levels to remain equal among all the inner tanks. Essentially, I want it to function as one big tank, with all tanks being symmetrically filled/drained. Is there some way I can do this?

Cheers,

-BS

[cicatrix]

TAC Fuel balancer might help, but if you don't want to use mods there's not much you can do except trying to manually balance the fuel levels.

[Wanderfound]

If you're burning evenly, they should be draining evenly. Most likely, you have a misplaced fuel line somewhere.

[katateochi]

unless something changed in 0.25, you can run fuel lines in both directions and the tanks will drain evenly.

[magnemoe]

If you're burning evenly, they should be draining evenly. Most likely, you have a misplaced fuel line somewhere.

Yes my best advice is to do an static burn on pad and look at the flow.

Most probably an pipe is missing or the wrong direction, this will work until one engine burn out while the other is burning and you get uneven trust.

However the staging looks a bit weird, it looks more like an onion staging, 3 layers+ srb and 4 stages before horizontal.

[Laie]

unless something changed in 0.25, you can run fuel lines in both directions and the tanks will drain evenly.

Yes and no.

When the central tank/engine has six lines connected to it. One of them is checked first. Then comes the line that connects that tank to the next, and the next, until the circle is complete. Only after one set of fuel lines has been followed through will the next line even be considered. This leads to a tank we've already had, and another we've already had, and so on. So these are all discarded. Same for the other four lines connecting the center tank.

So in effect, of the six lines connecting to the central tank, only the one that is checked first will count, because the other five only lead to tanks that have already been considered at that point.

It might be possible to make it do what you want, if you put down the fuel lines in just the right order. But that's mind-boggling. It will be much easier to rewire the rocket so you keep staging in pairs.

Edited November 14, 2014 by Laie
removed copypasta

[GoSlash27]

What Laie said.

Having multiple fuel lines come into a single tank will work fine, but having multiple fuel lines coming out of it will not. The script will only ever follow one of those paths and the others will be ignored.

Best,

-Slashy

[Red Iron Crown]

In addition to what Laie mentions, it's important to note that fuel lines never move fuel between tanks. Fuel lines determine which tank an engine should draw fuel from, once the path is set the engine draws directly from that tank.

Also, no discussion of fuel flow is complete without mentioning Kasuha's Fuel Flow Rules thread, it is an excellent resource for understanding fuel flow and diagnosing problems.

[Superfluous J]

I don't see why you're doing all that complicated fuel line doohickery. I'd just run fuel lines from the interior asparagus stage (stage 12 I believe) on each side to the 3 onion-staged tanks closest to it, and then from each of those 3 tanks to the interior tank.

Better yet I'd asparagus those 6 tanks as well

[barfing_skull]

Thanks all for the replies and ideas!

My impetus for keeping the central 7 tanks as one is that if I asparagus the inner tanks and engines, I'll be left with too little thrust for the large amount of stuff that's on top of it. One S3 engine will leave the rest of the stack plummeting back to Kerbin. So I think I'd like to keep that inner circle together. Besides, they like each other.

Onion staging is a possibility, but I'd have to look at how much delta-v I'd lose in that case. My first impulse is to remove the bi-directional links and then try some other unidirectional links from there. I think my main thought crime was that the bi-directional links would act as some sort of equalizer. I'll keep in mind the depth-first search for the tank algorithm, too.

Again, good ideas for places to start.

Cheers,

-BS

[Kryxal]

How about this: it looks like the second ring is mounted a bit higher than the first ring, so run fuel lines from the outer ring to the center tank, then out again to the 6 inner ring tanks. Your center rocket will run out of fuel, of course, but you'll still be thrusting on 6 ... or you could lose the center rocket entirely, whatever works.

On second thought, that would run into the multiple fuel lines in AND out issue, maybe just accept that you'll need to do things differently ... maybe cut the center tank out of things entirely, dial the center engine down to 50% or use a lighter engine, and just make it work that way.

Edited November 15, 2014 by Kryxal