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Is it aerodynamic drag that flaps the grid fins open? Or is there a controlled mechanism for it? Pictures show the grid fins connect through a small link to something inside the rotating trunnion arm---anyone know what that's for?

enter image description here

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  • $\begingroup$ Note that the picture you provided shows an aluminium grid fin (I believe). New Falcon 9s all use Titanium grid fins which are bigger $\endgroup$ Jun 14, 2020 at 17:09
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    $\begingroup$ Thanks @Speedphoenix, but neither the fin material nor the fin version actually matter here ;-) They all use the linkage circled in red, even if one is aluminum and the other titanium. The picture is of an older fin but it happens to more clearly show the linkage than other pictures I had on hand. $\endgroup$
    – user36480
    Jun 15, 2020 at 5:52

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They are hydraulics which initially used an open loop, using kerosene as the working fluid. Some of the initial landing attempt failures were because they ran out of fluid. Closed loop would keep the fluid in the system, open loop dumps it after it is used.

When they land, the hydraulics also retract the fins so you can see it is under active control.

This is basically the same system steering the fins, just in a different axis.

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    $\begingroup$ Thanks, but geofc, you didn't answer my question. You just presented a collection of facts that almost touch on the question asked but never actually address it. The linkage circled in red in my picture could be hydraulically actuated, but what is that linkage? What does it do, specifically? The type of hydraulic system and the fluid it uses and the landing failures those choices may have caused in the past are interesting but irrelevant here... $\endgroup$
    – user36480
    Jun 14, 2020 at 19:58
  • $\begingroup$ -1 for presenting "a collection of facts that almost touch on the question asked but never actually address it" and for not sourcing said collection! $\endgroup$
    – uhoh
    Jun 15, 2020 at 0:02
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That linkage is connected to a small PFM module inside the rocket. Each fin has its own primary PFM module. Based on telemetry and some very good software programming, sensors on the rocket indicate the orientation and velocity of the returning rocket booster. All that is fed into an onboard computer that directs the PFM modules on how to actuate the grid fins independently. Hope this helps.

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    $\begingroup$ Do you have a source for this information? And what does PFM stand for? $\endgroup$ Nov 10, 2021 at 18:03

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