[WalterBright]

The Apollo capsule didn't have a fuel gauge, either, because nobody could figure out how to make a fuel gauge work in weightless conditions. Various schemes failed. The solution turned out to be to have a reserve tank with enough propellant in it to do reentry. Then when the main tank ran dry, you knew just what you could do with what was left.

[barelyauser]

Since you know how much fuel you start with, can't you integrate something like the time spent firing the engine and use that as gauge?

[dr_orpheus]

Yes, and that is what most satellites due to estimate fuel. But any errors in that measurement can accumulate. See my other response on this thread for more details.

[WalterBright]

I'm sure they thought of that, so there must be some reason why it is inaccurate.

[tekla]

No, since engines aren't perfect and wastes propellant. Also fuel boils off in space.