Do Falcon 9 grid fins lock up when extended during reentry? Or do they rely on aerodynamic forces to keep them extended once they're deployed? Could the fins flail down during the reentry burn when thrust acceleration is high (>2 g's)?
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If you look at the webcasts you'll see that they are extended while in free fall (before re-entry) and you can kind of see them bounce and lock into place with a jolt. I believe they are hydraulically actuated and do not rely on aerodynamic force to stay extended. I do not know about your second question however.
See the OTV5 / X-37B launch from 2017 just after the boost-back burn (@5:47 in linked video, ~T+3:35 mission clock): OTV5 / X-37B Booster Cam
As you can see they are deployed above 100km altitude so at least initially there are no aerodynamics at play.
You raise an interesting question as to how much help the supporting structure gets from aerodynamic forces while doing the landing burn, anyone else have ideas on that?
answered Apr 20, 2021 at 11:35
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$\begingroup$ Well yes, they use hydraulic actuators to extend, but is there an actual locking mechanism to keep them extended, or do they rely on the sum of the hydraulic actuation force and the aerodynamic force to keep them extended? Using a hydraulic actuator to push the grid fin against a hard stop is not by itself a locking mechanism, as you could still force the grid fin down if you can overcome the hydraulic actuator and aerodynamic forces. A locking mechanism is one that would keep the fin extended even if you removed the hydraulic and aerodynamic forces and tried to force the fin to retract. $\endgroup$– user39728Commented Apr 20, 2021 at 22:58