# Can the grid fins affect the rotation of the Falcon 9 descent?

Can the grid fins control the rotation of the Falcon 9 on descent?

How would they have to be positioned to stop the rotation?

If used to rotate the ship, then how they would they have to be positioned and how fast can they spin the first stage, limited of course by the descent time?

• "how fast they can spin the first stage" is an interesting question, limited, I assume, by the amount of time you have to re-enter. My suspicion is "fast enough to tear itself apart". Commented Nov 2, 2020 at 14:55
• By rotation do you mean roll about the rocket's length axis? Or do you mean also pitch and yaw?
– user39728
Commented Apr 8, 2021 at 16:32
• "How fast" will depend on the steering force exerted on the grid fins, and that must be a function of speed, air density (which varies greatly with altitude), and fin angle of attack. Plus, the steering force will produce acceleration, not velocity, so it's maybe easier to say how fast you can reach some steering rate. Drag would put an upper limit on this number (as it does on a falling skydiver), but there again that limit would depend on velocity and air density which vary greatly as the rocket descends tens of km and sheds maybe 1500 m/s of speed, so no single constant for answer there.
– user39728
Commented Apr 8, 2021 at 16:38

Yes, the grid fins can control the rotation of the stage -- and they're the primary mechanism for that during descent, since the cold-gas thrusters have little force and the engines are shut down for most of the time.

To keep the stage stable, the fins are left horizontal, and adjusted by small amounts to correct for other sources of rotation.

To spin the stage, all the fins are tilted in the same direction relative to their mounting point, so that opposing pairs are producing a horizontal force in opposite directions.

SpaceX accidentally tested your last question on the CRS-16 mission -- all the fins got stuck near the limit stop due to a hydraulic problem. The answer is: quite fast!

EDIT: doodle

• You can supplement this with some dramatic actual footage of CRS-16; youtu.be/EH1nyPIvLjI?t=167 or youtu.be/QyJS1QcPRYM?t=204 or youtu.be/wNpDWSfSd8k?t=258 However there is a small problem with the drawing in the first one
– uhoh
Commented Nov 2, 2020 at 23:05
• Also, during the SAOCOM 1B mission, the booster descended through some cloud layers that rather clearly showed the airflow through the fins: youtu.be/P-gLOsDjE3E?list=PLC474234E124B5213&t=1459 Commented Nov 2, 2020 at 23:25
• @uhoh There's a link to that footage in the original version of my answer. ;-) Commented Nov 3, 2020 at 12:05
• I see that now, great! I've edited a bit to make it clearer and to entice readers to enjoy the original footage. I've also included the self-explanatory title so that if/when the link rots or the video is deleted future readers will know what it is and can look for something similar.
– uhoh
Commented Nov 3, 2020 at 12:34