Strange fuel line issues (.craft file included)

[Ford_Prefect]

Please take a look at this.

All fuel lines are attached correctly, using 4x-symmetry. It's a staging similar to asparagus, made to shoot this rocket to Dres. Theoretically, the tank at the bottom should become empty, before any other tank even starts draining.

It's hard to describe; put it onto the launchpad and watch it by yourself. Keep an eye on the radially attached tanks.

If everything is normal, and there are no inconsistencies in fuel drain: Congratulations. I am f***ed

Otherwise: How can I fix this? I had the same problem with another craft which basically looked the same, only used smaller parts. Both were made from scratch.

[Johnno]

I took a look out of curiosity, seems to work as intended, the bottom tank drains without the radial tanks starting to drain. However I can't make sense of your staging list, since the bottom tank drains first and is attached with a decoupler, why would you have the untouched radial tanks (and engines) to drop when you ditch the empty tank? On top of that, there are fuel lines from those tanks going upward, there's no reason for that unless you light the upper stage engines as well, which however are jumbled up in the staging and wouldn't light properly unless toggled manually. I just realized I didn't bother looking at action groups though, so maybe that's the answer.

So what exactly do you need help with? I find no inconsistency in the fuel drain, just in your staging.

So I guess I'll ask again, fix what exactly?

Edit: So I had another look with the assumption you intended to light all engines from the start. After sorting the staging I observed it going on the pad, went back to the VAB and redid the fuel lines for a confirm, then testfired again. Likely it's an issue with the fuel line logic of how many tanks it goes through, ie you simply have too many in line. I've seen it mentioned in other cases.

But yeah I can confirm the setup you have, even though logically should work, doesn't (that is assuming all engines are lit from the start).

Edited June 13, 2013 by Johnno

[stupid_chris]

Well I checked it too. And everything was fine. Your staging made no sense at all so I "fixed it" to something that would make sense and everything still worked. But really I hope you don't intend to launch with this cause you really don't have enough TWR. In space this must have an incredible delta V, the job is just to get it there.

[Ford_Prefect]

Apparently the staging is screwed up. The engines should all fire, using the fuel of the bottom-tank. It's an interplanetary stage supposed to go to Dres (and maybe even further). The problem is (at least on my computer): Not only the fuel of the most bottom tanks drains, but the fuel of three out of four radially attached tanks as well. The fourth one stays full. This goes for the upper and the lower 4 tanks; at the same time.

Apparently it's a problem with my computer.

[stupid_chris]

Apparently the staging is screwed up. The engines should all fire, using the fuel of the bottom-tank. It's an interplanetary stage supposed to go to Dres (and maybe even further). The problem is (at least on my computer): Not only the fuel of the most bottom tanks drains, but the fuel of three out of four radially attached tanks as well. The fourth one stays full. This goes for the upper and the lower 4 tanks; at the same time.

Apparently it's a problem with my computer.

I haven't tried firing all the engines at the same time. However I doubt that would work because engines on different stages like these that have to drain fuel through a lot of stage tend to make fuel flow logic break. I'm not sure it would work :l I recommend maybe a complete redesign. Sometimes it's simpler than troubleshooting a whole transfer stage

[Ford_Prefect]

However I doubt that would work because engines on different stages like these that have to drain fuel through a lot of stage tend to make fuel flow logic break. I'm not sure it would work :l

:\

Unfortunately, it is a completely new designed stage... it could be so elegant...

[numerobis]

The main issue everyone has with fuel lines is if you have an engine that has two paths through the fuel lines back to the same tank, then it will not drain in the way you expect.

So for each engine, trace back where it's getting its fuel from. If there's a fork (a tank with two fuel lines going in), follow both branches of the fork. If those two branches ever meet again, you're in trouble.

The other issue is if you place struts and fuel lines that get close to the surface of some object in between, roundoff error can mess up the symmetry, or mess up when you reload or when you hit undo: the fuel line might no longer be going where you want it to go.

[Ford_Prefect]

So for each engine, trace back where it's getting its fuel from. If there's a fork (a tank with two fuel lines going in), follow both branches of the fork. If those two branches ever meet again, you're in trouble.

So this is the reason. But it would be so practical

Ok, with this piece of information I can redesign it. Thank you everyone!

edit: How can I change the Prefix to "Answered"?

edit2: Heres MkIII. It's hell of an interplanetary stage! It's not as mighty as it looks like, though; it's a bit close with the Delta-v (it has reserve tanks, though). I went for a high TWR, because I did not want to overshoot my launch window again

Edited June 14, 2013 by Ford_Prefect