Starship is a pretty radical redesign with the main goal being reusability. Everything about it is with reusability and and thus scale in mind. A new engine wa

Starship is a pretty radical redesign with the main goal being reusability. Everything about it is with reusability and and thus scale in mind. A new engine was needed because while the Merlin is a great engine, it could never be reused the thousands of times Raptor is designed to be reused. You can think of engine design as the simultaneous, interdependent optimization across 10+ dimensions, but as an example of one of dimensions - Raptor uses Methane (rather than Kerosene) because it burns cleaner (does not leave residue) and can be manufactured on Mars more easily. Here's an in-depth video about Raptor: https://www.youtube.com/watch?v=LbH1ZDImaI8 https://www.youtube.com/watch?v=LbH1ZDImaI8 The rest of the rocket is similar, in that it is done with manufacturability, reusability and scale in mind. The Falcon design could never stretch to these demands.