Grid Fins of Falcon 9
SpaceX began testing on first stage in 2015. When the rocket re-enters the atmosphere at high speed, it is directed to the landing point with 4 grid fins.
The fins can make the rocket perform up to 20 degrees three-axis movements; roll, pitch and yaw. Although the steering system components are unknown, it is likely that both flight path simulations and actual flight data are used.In the first trials, the Falcon 9 grid fins were made of aluminum. However, the high temperatures generated during re-entry occasionally caused the coatings to burn and the fins to melt.
Then it was switched to titanium. Titanium is a very expensive and costly metal. However, the rocket's reusability benefited because it was resistant to high temperatures. Over time, apart from the material, the size and shape of the fins also changed.The grid fins on the Falcon 9 fold up during lift off. When the rocket re-enters the atmosphere, they open from their hinges.
This was also the case in the Starship design at first, but then the hinges were abandoned due to weight limitations. It was concluded that the grid fins would not distrubt the aerodynamics at lift off. As for material, stainless steel was preferred instead of titanium.