I have discovered through experimentation that if I place a radially mounted engine onto a radial decoupler which is attached to a barrel of fuel, the radial engine will work without requiring an external fuel duct connection from the engine to the fuel barrel (the fuel apparently travels horizontally through the radial decoupler). However when placing two fuel barrels side by side, and engine mounted to the bottom of one barrel can't access fuel from the other side barrel. Now I'm confused and wondering what exactly are the rules for how fuel flows through components, since it looks like it can travel radially but not if the radial component is another fuel tank, which seems rather arbitrary to me. **Edit** I mentioned that an engine on the bottom of one fuel barrel can't access fuel from a radially mounted barrel, apparently this is only half true, you can still manually transfer fuel between the two barrels, allowing the engine to access fuel from the side barrel by funneling it through the middle one.