One very clever part of the Starship Super Heavy booster is that the outer ring of Raptor engines are needed only for ascent: the inner, steerable engines are sufficient for the boost-back and landing burns of the nearly-empty stage. Consequently, there is no need for the resources needed to start the outer engines to be on-board. The gases to spin up their turbopumps are supplied by ground support equipment and delivered through the launch mount, then disconnected at liftoff. This saves weight and reduces complexity compared to carrying the requirements for engine start on the booster.