That is not a breakthrough.
The only other player in this space is Blue Origin, and they are still working on their commercially-viable engines. Everybody else throws away their rocket after the launch.
Are you saying only SpaceX is capable of doing this? Seriously?
- Building an engine capable of multiple restarts while facing into a supersonic headwind, which are necessary for any kind of successful propulsive landing.
- Designing a mass-thrifty first stage simultaneously lightweight enough to be able to accommodate the added weight of landing equipment, but strong enough to handle the tensile stress and buffeting of atmospheric reentry.
- Developing ultralight landing legs to enable stable landings at low mass cost.
But most of the innovation has been in the realm of integrating existing technology in new ways. They brought together cold gas thrusters, grid fins, and thrust vectoring - none of which were new - into a complete package capable of precisely controlling the flight of a 15-storey first stage from the upper atmosphere to the ground. It's the act of getting technologies to work together that is often the source of the greatest gains.
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The breakthrough is actually doing it. A lot of space engineers were saying before Falcon 9 that what SoaceX were trying to do was impossible.
Hindsight is 20/20. It's easy to see now what steps were required to pull this off and that clearly those steps were possible. But if it's so clear, why wasn't it done before? The ottoman line is only Must and SoaceX were prepared to get the job done. Nobody else was even trying.