The same, its just a much higher proportion of gas rather then liquid. Basically on the pad its mostly full with liquid, as it launches, it pumps back part of the gas created in the engine back into the tank. That called 'Autogenous pressurization'. So they don't need an extra gas like helium, as for example Falcon 9 needs.
The big issue during landing is that you need to make sure that the engine doesn't suck in gas. That causes bubbles and can destroy the engine. This was actually the failure that caused some of the earlier SN flights to explode or not produce enough power from the engine.
You need to either have header tanks, like the booster. Or some kind of method to push the liquids into the right place.
If you want to deep dive into the whole problem, 'CSI Starbase' on youtube has a brilliant series on all the engineering problems with all of this. Its a very complex problem.
The big issue during landing is that you need to make sure that the engine doesn't suck in gas. That causes bubbles and can destroy the engine. This was actually the failure that caused some of the earlier SN flights to explode or not produce enough power from the engine.
You need to either have header tanks, like the booster. Or some kind of method to push the liquids into the right place.
If you want to deep dive into the whole problem, 'CSI Starbase' on youtube has a brilliant series on all the engineering problems with all of this. Its a very complex problem.