- 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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