The Falcon 9’s first stage is equipped with hypersonic grid fins, and not planar fins, as shown in the diagram below. Grid fins are used during the re-entry and landing of the first stage of the Falcon 9 rocket. It helps in guiding the first stage return back to the ground location after accomplishing primary mission requirements.
Originally it was made from aluminum, but the latest version of Falcon 9 introduces newly forged titanium grid fins. The specific shape, size, and mount points have changed as well. Check the below image for size comparison.
Elon has often highlighted the high cost of the titanium grid fins. The life and low-maintenance features make them worth their cost.
The grid fins are placed in an X-wing configuration and are stowed on the ascent and deployed during reentry. While the fins are relatively small – they measure just 4 feet by 5 feet – they can roll, pitch, and yaw the 14-story stage up to 20 degrees in order to target a precision landing. (Note that the dimensions are of older design grid fins)
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Why is the base rounded?