[hrnn]

If there is no drag to hold it back, why is there a limit on the speed it can gain?

[techbuttman]

Fuel is the limiter. Tanks of xenon gas are used for fuel.

And if you ever needed to slow down, you'd need to burn even more fuel.

[hnburnsy]

Any idea how much fuel is needed to go a set distance per kg and how it compares to other propellants?