Hot staging for Starship.
It seems that Starship is going further into N1 territory with proposed hot staging. Thus, the second stage will light without the Falcon-9 style pusher piston nor ullage motors.
https://spacenews.com/spacex-changing-starship-stage-separation-ahead-of-next-launch/Open linkView original on lemmy.world
I wonder if this will enable some launch abort scenarios as well.