# How do grid fins actually work? Which aerodynamic mechanism(s) produce the torque on the spacecraft?

I’m trying to model the steering force that grid fins exert on a falcon 9.

I know these things are not just grid-shaped fins but actually grids of fins.

And each of those fins will experience drag and lift forces from which the steering force will ultimately arise.

Do lift and drag both contribute to the steering force on the rocket? Does one dominate over the other or are they both significant?

If you consider the grid fins at a small angle of attack, am I right that drag will act in the direction of the flattened axis (thickness of the grid of fins) and that lift will act perpendicular to it?

• Since your question is very specific and the title was overly general, I refined the title to make sure nobody votes to close as duplicate based on "some matching words in titles" strategy. Please feel free to refine further.
– uhoh
Apr 6, 2021 at 0:01
• Thanks, uhoh! :)
– user39728
Apr 6, 2021 at 2:30
• It's good to include a link to your somewhat different but related and recent question Calculating grid fin steering force
– uhoh
Apr 9, 2021 at 2:30

If you consider the grid fins at a small angle of attack, am I right that drag will act in the direction of the flattened axis (thickness of the grid of fins) and that lift will act perpendicular to it?

For any body in a flow, you can draw a single net aerodynamic force vector. Conventionally, the component of this vector inline with the vehicles longitudinal axis is called drag, and the component inline with the vehicles vertical axis is called lift. For the Falcon 9, the horizontal axis would be the line passing through the engines and the tip; this convention allows drag to be in the opposite direction of velocity.

Do lift and drag both contribute to the steering force on the rocket? Does one dominate over the other or are they both significant?

Yes, both lift and drag contribute to the steering. Consider if only one grid fin was actuated. Net force on the fin is increased which increases both lift and drag. The lift will create a roll moment (and a yaw/pitch moment), and the drag will create a pure yaw/pitch moment.

I would wager that the F9 control scheme has opposite fins deflecting at the same angle more often than not to cancel the moment created by drag. After all, you can get roll, yaw, and pitch from just combinations of lift from the fins.

Worth mentioning that, for considering the surface area of the fin creating the aerodynamic forces, only the grid strips inline with the rotating axel should be considered. The others will remain at zero AoA when the fin is rotated.

• It would be better if you drew from authoritative sources and answered the question more directly; "both lift and drag contribute to the steering" is pretty vague, isn't there some real information about this out there somewhere? Isn't it possible that one is more significant than the other, and depending on speed it might be one or the other?
– uhoh
Apr 6, 2021 at 23:00