bames53

Thanks, Kasuha, that's exactly the sort of thing I'm hoping checks out. It seems like the rules you show will depend a lot on exactly how the depth first traversal works. Already it's clear that, if your rules are accurate, the traversal doesn't necessary visit all the connected parts. If it did then a centrally connected engine could take fuel from radially connected tanks. Anyway I'll have to take a closer look and test out these rules. Thanks again.

The third image in my original post shows a part pulling fuel from a point other than it's default attachment point. So far, it's true, I haven't been able to get fuel to flow 'backwards' through a quad-coupler, but that seems to be part specific as fuel can flow backwards through the other parts I've tried E.g. flowing up instead of down through a structural fuselage.

Here's my test for fuel flowing 'up' a vehicle's hierarchy: And here's an example of fuel flowing through a probe core.

The KSP wiki doesn't seem to include any comprehensive article on fuel crossfeed. Can anyone explain the rules, if in fact there is a consistent set of rules? I've been experimenting and I'm starting to wonder. For example these configurations allow all engines to take from the central fuel tank first: But for some reason this doesn't, and instead uses only the radial tanks: Also I can't explain why this doesn't work: And I had a hypothesis as to why this doesn't work: but it was contradicted by some of the above tests. And this doesn't work: despite the fact that the engine and fuel tank have the same relative configuration as one of the earlier, working tests. In fact this lead me to a test that seems to prove that not only the connections between components matter, but that the positioning of components within the vehicle's heirarchy also matters. These two images are of two separate vehicles, where the only difference is the order in which the parts were added. Despite the identical connections between components the vehicles function differently. My initial hypothesis was that engines only take fuel from components higher up in the hierarchy, but that was disproven when I did manage to built a vehicle with an engine taking fuel from a sibling component. So my questions are: is there some way of understanding fuel crossfeed that reveals a coherent and consistent system, or does fuel crossfeed involve a bunch of bugs and part specific idiosyncrasies that eliminate all hope for such consistency? If the latter, has experience with previous releases given any indication that this issue has received work in the past and is likely to improve with future releases?