Fundamentally, SpaceX chose to go with the cheapest approach that works rather than the "best" approach. Take, for example, the engines. The Falcon rockets ar

Fundamentally, SpaceX chose to go with the cheapest approach that works rather than the "best" approach. Take, for example, the engines. The Falcon rockets are kerolox (kerosene + liquid oxygen) engines. You get substantially more performance from hydrolox (liquid hydrogen + liquid oxygen) engines. The faster you want to go the more difference this makes. However, they recognized that kerolox is far, far easier to work with and that to put a kilogram in orbit on kerolox is cheaper than hydrolox, even though you need a bigger rocket to do so. I can't find any other western rocket that's anything but hydrolox by the time it reaches orbit. But SpaceX decided that standardizing on the same engine (the upper stage of the Falcon rockets uses the same engine, different engine bell) was a better deal. It's all about mass production. They build a *lot* of Merlins so the cost per engine is a lot lower. Starship is using the same philosophy. Metholox (liquid methane + liquid oxygen), a bit harder to handle than kerolox but still far more friendly than hydrolox. Same engines throughout. (Although they might have different bells on some of the engines. A bell meant for vacuum works better in vacuum, but at best worse in atmosphere and at worst will be destroyed. The Shuttle OMS engines had a minimum altitude requirement (actually, maximum pressure) and could not be used to help with a bad landing. Light them too low and they would crumple.)