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?

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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 $\endgroup$ – Speedphoenix Jun 14 '20 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 '20 at 5:52

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 '20 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 '20 at 0:02

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