Falcon 9 v1.1 was the first SpaceX rocket variant to use grid fins for atmospheric guidance of its returning first stage.
The upgraded rocket first flew in September 2013 and was substantially larger and more powerful than the original Falcon 9 v1.0. Its first stage used nine Merlin 1D engines, while its predecessor used Merlin 1C engines.
Grid fins are deployable aerodynamic control surfaces. During descent, they help a booster steer through the atmosphere, working with engine burns and other control systems. They became a defining feature of SpaceX’s reusable-launch program.
The first Falcon 9 v1.1 stages were not recovered successfully. SpaceX later refined landing legs, grid fins, navigation software, and landing-burn procedures before achieving a successful booster landing in December 2015.