Why is this fuel line not feeding liquid fuel across a radial decoupler?

[foodini]

Normally, I use fuel lines to set up asparagus stagings. This time, I'm needing to build a ship that lands on a cargo bay so I can drive a rover in and out, so the engines are in nacelles. I want to feed liquid fuel from tanks in the core to the external tanks/engines (and from the ore processors to the central tanks), but a quick burn test tells me that fuel is not transferring from the center to the nacelles. (I have NO IDEA how I'm going to test getting fuel from the processor to the tank without actually doing the launch.)

I _can_ open the tanks, individually, and manually pump fuel around, but that's not going to be convenient when I'm mid-flight. I do have crossfeed enabled on the decouplers.

It's hard to see in the image, but the lines are running from the processors to the center tanks and from the center tanks to the nacelles.

Thanks...

[foodini]

I accidentally allowed the burn to continue until the nacelle tank was empty and discovered that it had started draining the core tank. This doesn't make sense to me, as the fuel line priority is usually the other way around. The tank at the starting point of the fuel line is drained first. For example, in asparagus staging, you run your lines from the first tower to decouple to the second tower, to the third, to the last. The first tower to decouple is drained first, since it's at the source end of the fuel lines. What gives?

[18Watt]

Thread moved to Gameplay Questions from KSP2 Discussion.

[Leganeski]

I don't know enough about VAB mechanics to be confident in my explanation, but my guess is that the priorities of the outer tanks were set when the tanks were first placed on the decouplers, and then never changed after the fuel lines were added, so the central tanks retained the highest priority. If it is in fact an issue with tank priority, you can fix it in the VAB by manually changing the priorities to what you want.

Edit: I hadn't considered the effect of crossfeed on the decouplers. @jimmymcgoochie's answer is probably right.

 

Also, I don't think you need the fuel lines from the converters to the center tanks.

From the KSP wiki:

Quote

Convert-O-Tron units are not subject to crossfeed restrictions in any way whatsoever. A Convert-O-Tron unit can refill tanks on the far side of crossfeed disabled parts even when the difficulty option Resource Transfer Obeys Crossfeed Rules is set. Tanks are filled using the same priority as emptying tanks, tanks with higher priority numbers will be filled first.

This certainly seems true from what I've seen: my current ship has just one converter and no fuel lines at all, and it routinely refuels across both decouplers and docking ports with no issue.

Edited August 28, 2022 by Leganeski

[jimmymcgoochie]

Turn the crossfeed off on the decouplers, then detach and reattach the decouplers (with the tanks and engines still attached to the decouplers) and it should recalculate the fuel priority. Having the crossfeed on completely negates the fuel ducts.

Go to the main menu, click settings and then enable advanced tweakables. Advanced tweakables gives you extra features which are very useful, including the ability to manually set fuel priority on fuel tanks (as well as actually showing the fuel priority in the first place).

Out of interest, why are the engines on decouplers? Why not just radially attach the fuel tanks and/or use a bit of part clipping/rotating to build the nacelles without the added danger of accidentally staging off your engines.