SpaceX is the most successful private rocket launch company in the world, and it's due in large part to the Falcon 9 rocket.
And the journey to the Falcon 9 began with the Falcon 1 in 2006. The first three launches of the Falcon 1 failed, and with only one more shot before the company went bankrupt, they finally got into orbit on the 4th launch.
Plans for a larger Falcon 1e were scrapped, as well as a Falcon 5, so that they could move forward with the Falcon 9 v1.0.
With this first version of the Falcon 9, SpaceX was able to win a contract to service the ISS through NASA's COTS program by proving that the Dragon capsule was capable of carrying out resupply missions.
SpaceX then focused on reusability and developed the Falcon 9 v1.1, which they used to test landings over open water, at the same time testing vertical take off and landing with their grasshopper vehicle.
But it was the next version, the Falcon 9 Full Thrust, that was the first to land, first on a landing pad at Cape Canaveral, and then on a drone ship.
Incremental improvements lead to the Falcon 9 Block 4 and Block 5 that will launch for the first time this April.
Earlier this year, SpaceX launched the Falcon Heavy, which is 90% reusable, making spaceflight even more sustainable, but the ultimate reusable rocket is the upcoming BFR, which is completely reusable.
This is the ultimate implementation of the SpaceX vision.