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The spacecraft doesn't have a fuel gauge ...

Zoom call with 100 Airbus engineers ...

The original software for part of the onboard navigation system was running on a Windows 98 PC that no-one could find the password to and ended up using bolt cutters to extract the hard drives.

Talk about a deep space horror story!

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No spacecraft has a "fuel gauge" in the traditional sense. You don't have gravity to for a float to work like any fuel gauge in a car or airplane. Fuel is driven by surface tension and pressure rather than any buoyancy/gravity forces. These leads to the development of diaphragm type tanks where the pressurized bladder pushes fuel out of the tank or propellant management device (PMD) type tanks [0/1] that use the surface tension to guide pressurant-free fuel to the thrusters.

So determining how much fuel you have left is done by a combination of integrating how much time you had thrusters firing, coupled with what pressure the tanks/lines were at while the thrusters were firing. Errors in these measurements accumulate over time which is why there is a lot of effort in to determining how much fuel is left in a spacecraft. Especially critical for things like big comm birds in GEO where fuel can be limiting in operation and the more fuel you have the longer you can keep station and get revenue from the satellite. But you need to still be conservative enough to have enough fuel to get out of the GEO belt for decommissioning your satellite.

[0] https://en.wikipedia.org/wiki/Propellant_management_device [1] https://www.northropgrumman.com/space/pmd-tanks/

I'm sure we'll get there eventually. But there is a fun idea in there somewhere about a future where the only need for manned spaceflight is so we can get pinged by the computer once every few days and go and hug/wrap a tape measure around the fuel bladder

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.

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?

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.

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

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

Why would anyone need bolt cutters to extract the drives? I have a couple Win98 PCs in the basement, and you just unscrew the drives. The password on it didn't encrypt the drives, either.

As for the password, this is the old days. Could probably google how to crack the password for it. Or just try the classic 123456, querty, password, or letmein.

Why would anyone need bolt cutters to extract the drives?

I've had PC cases that had eyelets for a lock. You'd have to get through that before "unscrew(ing) the drives".

The drives aren't password protected. Why would one need to take the drives apart? Besides, the text was about extracting the drives, i.e. taking them out of the computer box.

Maybe to access the drive, they have to go through a lockable case. If someone lost the keys to that case, they might have had to pop the locks with a bolt cutter

That was exactly what I was driving at, thanks. In order to take the drives "out of the computer box", one needs to get through the lock on that computer box first.

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