How do the F9 grid fins extend and rotate?

When the vehicle launches they are flat against the surface of the vehicle.

Old and new grid fins with mount points

As the first stage begins reentry, the fins extend and start 'flying' the vehicle.

What is the mechanism that extends and controls them? Hydraulics? Electric?


1 Answer 1


The earliest use of the grid fins failed to land successfully because it ran out of hydraulic fluid. They use the RP1 from the main tank to run the hydraulics and they were initially open loop (the hydraulics). This meant they would use the fluid, and throw it away as they used it. Thus they ran out.

They later switched to a closed loop system so they do not run out of hydraulic fluid during landing.

We see the fins start against the side of the vehicle. They pop out during landing, and then after the vehicle lands, they safe themselves and retract back to the closed position.

  • $\begingroup$ @uhoh Yes and edited to make it clearer. $\endgroup$
    – geoffc
    Jul 17, 2017 at 19:26
  • $\begingroup$ Nice collection of facts about grid fins, but nowhere in here is an answer to the actual question: what causes the grid fins to extend and retract? Aerodynamic drag? Hydraulic or pneumatic actuators? Pushers, maybe? That was the original question ;-) $\endgroup$
    – user36480
    Jun 14, 2020 at 6:17

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