Nothing replaces the fuel.
That is the wrong question to ask, the tank becomes an empty room.
The real problem is not the empty space, but the fuel that is left.
In the microgravity of space, the remaining liquid does not sit at the bottom of the tank.
It floats, it breaks apart into a hundred drifting blobs. An engine that sucks in vapor instead of liquid will destroy itself.
So before a main engine fires, the ship must create its own gravity. Small thrusters.
Called ullage motors, fire for a few seconds.
They give the entire ship a gentle push forward – This push forces all the floating blobs of fuel to the back of the tank, settling them over the engine’s intake pipe.
Then the main engine can safely ignite. The problem is not filling the tank, but controlling the fuel.
