Measuring mass?

[strider3]

How do we measure the mass of something large, like a spacecraft?

[RCgothic]

12 hours ago, strider3 said:

How do we measure the mass of something large, like a spacecraft?

Generally it's weighed with a suitable load cell.

If it's too large for a single load cell, you can use multiple lift points with a load cell on each (or a representative leg if the load is reasonably symmetrical or has load equalisers). Or you can apply a known perpendicular load and measure the angle of the dangle.

If it's extremely big or awkward, you weigh the pieces and the assembled mass is calculated.

Finally, for spacecraft in freefall, you can also calculate the mass from the response to a calibrated input, such as a gyro or thruster.

Edited November 24, 2021 by RCgothic

[Piscator]

Or put another spacecraft in orbit around it and measure the period.

Not practical? How dare you!?

[magnemoe]

Ships are very easy, you simply look at how low it sits in the water. 
Yes this need to be calculated and calibrated beforehand. Its also common to use the double bottom for ballast who is part of your weight. 

[kerbiloid]

In game / in "real" world?

In space / on ground?

[DDE]

There's always the gravitic gradiometer, which measures mass quite directly. Primary use is underwater terrain avoidance without the use of sonar.

[K^2]

3 hours ago, DDE said:

There's always the gravitic gradiometer, which measures mass quite directly. Primary use is underwater terrain avoidance without the use of sonar.

We got to "play" with one back at school. I can confirm that they are sensitive enough to detect a difference between there being a student under a desk and not while the device sits on top of the desk. By moving one of these around the vessel, you can get a pretty good estimate not only for the mass, but mass distribution as well. Though, measuring the mass directly with load cells will still probably be more precise until you get to something the size of a naval vessel.