system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How long can a fighter jet fly inverted or straight up vertical before fuel supply stops? For example; drive a car upside down, the engine would stop getting fuel because the fuel is at the wrong side of the tank, does a jet fuel tank work differently _because_ it flies upsidedown?
gpt
There are ways of making fuel tanks work upside down. For example you can have a long fuel pickup tube with a float at the end so it will always bob up to the surface with the end sticking into the liquid no matter where that surface may be. You can also have multiple fuel pickups and lift pumps, two is usually enough for most maneuvers. It is also possible to have a flexible membrane in the fuel tank so it will always be full and the volume of the tank changes as it drains, etc.

Where it gets tricky is the fact that a vehicle does not have to have only one fuel tank. An airplane typically have multiple fuel tanks both in its wings and its main fuselage which is used as ballast to make the center of mass optimal. Military airplanes also have external drop tanks for extended range when needed. But even cars may have more then one fuel tank. Installing systems to handle extreme g forces like negative in all tanks may be too heavy and complex. So you typically do not have full fuel capacity while flying inverted.

A common thing to do is to install a small feeder tank for each engine in order to supply the engine with fuel at a constant pressure and flow rate no matter the external forces. This is sort of a bigger version of the carburetor fuel bowl. By making sure this tank can pick up fuel while inverted you do have good protection against flaming out during heavy maneuvers. And you can do things like switch between different pickups or different tanks depending on the forces without a gap in fuel supply. Even some cars have this type of tank installed if they are expected to see extreme cornering forces, inclines or freefall.