Problems with craft fuel tanks not draining evenly, asparagus staging especially

[smjjames]

Whenever I try to do a complex (as in more than one step) asparagus staging or try having a cluster of engines crossfeed into each other, I keep running into problems with the tanks not draining evenly, can anybody help out here? I have no idea whether it's a bug or I just don't understand whatever new mechanism that the fuel line uses.

Crafts are all stock.

Mjtest ship: This is the asparagus staged one, you can see pretty quickly that they aren't draining evenly, they're also draining in a wierd way. Also, go straight up a few kilometers so that the boosters drop off without colliding into a wing or engine, then do normal gravity turn. KJR is likely required to keep it from breaking apart.

Tankwierdness ship: I tried to crossfeed the orange tanks so that they would drain evenly, but they end up doing the same kind of thing, not draining evenly and in a balanced way.

Fueldraintest: It's upside down for test purposes, just run the engines for a bit and then stop, you'll see that the tanks are draining from the bottom (what would be the bottom if it were pointing up) and they aren't all draining evenly.

Fueldraintest no fuellines: Identical to the previous one, just doesn't have fuel lines. It's to serve as a control and to show what I mean.

Can anybody help with this? It seems like one of those not quite confirmed bugs and one of those things that are just rumored, but it's not rumored for me, I'm having the issue right there.

[SDEngineer]

I'm at work, so can't look at the craft files right now, but there are a few pointers that you might want to check. Your mention of "crossfeeding" and the tanks draining "oddly" suggests that you may have some fuel lines pointed the wrong way. Orientation of the fuel line matters - the side you connect first when placing a fuel line is the "fuel donor", and the side connected last is the "fuel receiver". Making a mistake when doing this will lead to all sorts of problems, particularly if other lines are actually placed correctly. You can see the fuel line orientation if you zoom in close (there are little arrows pointing the direction of the receiver).

Also, generally, you want to keep the fuel line situation simple. KSP is known to do odd things if you try to connect multiple donors directly to multiple receivers. One donor to multiple receivers is usually fine (i.e. a central fuel tank feeding multiple radially attached engines works fine), as well as multiple donors feeding a single receiver (typical asparagus staging for example, where the central tank is typically going to be fed by the two final radially attached engines, or onion staging where the radial tanks all feed the central tank), but if you try something like having each of the asparagus radial stages feed the central tank, and each radial stage ALSO feed the next radial stage in the asparagus sequence, the fuel behavior will likely not behave as expected.

One final caveat - never connect two tanks directly together in both directions (even through a chain). For example, Tank A feeds Tank B, and Tank B feeds Tank A, or A feeds B which feeds C which feeds A. If you wind up with a circular connection the behavior gets very unpredictable.

Edited May 13, 2015 by SDEngineer

[smjjames]

Well, the asparagus staging one is done the standard way of asparagus staging and yes I check the fuel line directions.

[smjjames]

*bump*

Anybody else able to help?

[Red Iron Crown]

I tried "MJtest EveKerbin". It has too many fuel lines, you need exactly one between each pair of neighbor boosters, but that shouldn't affect performance here as they're all in the right direction.. The payload also falls down on the pad, I had to hack gravity to get it off the pad. It appears to drain fuel evenly from opposite sides of the asparagus, top to bottom in each booster pair sequentially.

[smjjames]

I tried "MJtest EveKerbin". It has too many fuel lines, you need exactly one between each pair of neighbor boosters, but that shouldn't affect performance here as they're all in the right direction.. The payload also falls down on the pad, I had to hack gravity to get it off the pad. It appears to drain fuel evenly from opposite sides of the asparagus, top to bottom in each booster pair sequentially.

Lol, yeah, as I said, you're going to need KJR for that one, with it, the thing stays intact.

I'll take another look at the fuel lines. Those extra fuel lines though was an attempt to get it to stay balanced.

Edited May 14, 2015 by smjjames

[tbob]

I built an asparagus last night to loft my mun base (14 Kerbals, Party on the Mun!) and I had a similar problem... looked good up to about 1km then the base kicked sideways as one of the mainsails cut out early.

Turns out one of the central feeders was connected to.... a strut. But I placed fuel lines first! Didn't look that way until it reloaded, then I could see it up close at VAB and the pad - the fuel line was stopping on the wrong side of the upper stage bracing.

Check both ends, and make sure they get all the way to the tank!