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In Musk is testing x-wing style fins, spaceport drone ship there is mention of two Musk tweets:

Testing operation of hypersonic grid fins (x-wing config) going on next flight.

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Grid fins are stowed on ascent and then deploy on reentry for 'x-wing' style control. Each fin moves independently for pitch/yaw/roll.

What is the function of these fins?

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Actually, they are deliberately draggy.
During descent, they would produce significant drag given the waffle pattern producing vortices in the air around them (the hairs on a bee's wings have the same effect). In addition to attitude control, they could serve to reduce retrorocket burn and save a bit of fuel. They would definitely serve to keep the top of the rocket from heeling over when the thrusters ignite, acting like small drogue chutes.

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    $\begingroup$ Do you have a citation for the dragginess? $\endgroup$
    – Nathan Tuggy
    Jan 14, 2015 at 4:36
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Grid fins can be lighter, lower drag, and improve maneuverability at supersonic speeds relative to conventional planar fins. Russian designers seem to like them.

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  • $\begingroup$ In this video, are they being used for landing? I can't imagine how they should prove to be useful at that low speed. $\endgroup$
    – Daniel F
    Dec 9, 2014 at 21:25
  • $\begingroup$ In that video, they don't need to impart a lot of control authority because the maneuvering is very gentle. The main thing they're doing in that scenario is adding drag, moving the center of pressure/center of drag toward the top of the rocket, helping keep the rocket stable. $\endgroup$
    – Russell Borogove
    Dec 9, 2014 at 23:17
  • $\begingroup$ Didn't you mean to write that they increase drag (as they can have bigger surface area and remain stable compared to planar fins at supersonic speeds)? $\endgroup$
    – TildalWave
    Dec 19, 2014 at 20:13
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    $\begingroup$ According to the wikipedia article, they have less drag in the supersonic regime, but F9 is using them at lower speeds for drag as well as control. $\endgroup$
    – Russell Borogove
    Dec 19, 2014 at 21:58
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They are aerodynamic control surfaces.

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Also since the rocket is only using one engine it has no control over rotation about the long axis of the rocket the fins provide that.

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  • $\begingroup$ During powered descent, when the grid fins are used, only one of the Merlins is running - the stage is very light on fuel at that point, and they want it to come down rather than to keep going up. :) $\endgroup$
    – Russell Borogove
    Dec 19, 2014 at 21:59
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I am not an aerospace engineer, but it seems to me that because each grid fin can be independently controlled, then each can be rotated at extremely high speeds to create small vortexes which would allow discrete control of aerodynamic forces on the overall moment of the inherently unstable vertical proportion of the Falcon booster. A really eureka solution to creating means to achieve a soft landing in atmosphere...gyroscopically controlled rockets might require more fuel and weight than these fins. Very very impressed with what SpaceX and Musk are accomplishing...just hope that they partner up with Sierra Nevada and Dream Chaser to provide a more complete U.S. space capacity...after reading another article it seems the grid fins do not rotate at high speeds...rather they operate to assist in stabilizing and steering the booster as it descends at hypersonic then transonic velocities...sorry for the confusion but am not a rocket scientist...

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    $\begingroup$ Welcome to Space Exploration SE! Please note that good answers are characterized by references to primary sources. Would be nice if you could substantiate your claim to "rotation at extremely high speeds" as the mode of operation of the fins. $\endgroup$
    – Deer Hunter
    Jan 13, 2015 at 15:03
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    $\begingroup$ Possibilus, with your last edit you're kinda disqualifying your own answer. Could you please edit it again to include a link to the article that you mentioned, and perhaps extract from it some useful information that's relevant to the questions and hasn't yet been discussed? Assuming its a credible source, of course. Thanks! $\endgroup$
    – TildalWave
    Jan 14, 2015 at 3:33

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