Relighting / staging while the first stage motor is running at "idle" or something like that will be interesting. For a relight, you need some propellant at th

Relighting / staging while the first stage motor is running at "idle" or something like that will be interesting. For a relight, you need some propellant at the feed pipes to restart, though if you have small enough aero forces, you could just vent some stuff to accelerate the rocket forward slightly so you can start ingesting liquid again, instead of the pressurant gas and can then proceed with relight. Then again, since the empty rocket's bottom end is heavy, it could orient itself the right way aerodynamically anyway (acceleration vector pointing the way of the nose) so the liquids go towards the engine end. If it comes like a dart and not tumbling (which presents its own problems), such a big stage has lots of mass per frontal area and it will come in quite fast. Interesting aerodynamic problems. I wonder if we might see some maneuverable mini fins or wings at some point.