# Why do Falcon 9 grid fins open in piecemeal fashion?

The grid fins on Falcon 9 seem to open in piecemeal fashion: extending a few degrees, pausing, extending a few degrees, pausing, etc.

Any reason why this happens? The rocket stage is around 80 km high, where drag is negligible, so that shouldn't be a factor.

The engines aren't firing, so thrust acceleration shouldn't be a factor either.

So it all seems to come down to the fin-extend actuators, which are said to be hydraulic.

But why wouldn't they open the grid fins in one continuous shot? Is this by design?

video cued at T+ 00:02:55 into the launch, just before the grid fins begin to deploy and do so at an irregular rate:

• If i were to guess, it maybe because at this phase there are cold thrusters for attitute control and if they deploy them at one shot the rocket might wobble a little bit, another reason might be that they are continiously checking deployment status and going ahead, if at any point they notice grid fin deployment failure they may have a failure mode where they can land despite one grid fin failed ! – Prakhar Apr 8 at 2:50
• Another possibility might be that the hydraulic system is optimized for small and fast movements, not large continuous movements. After all, this event happens exactly once per flight, and this axis is normally not used for control, so it is only exercised once. Why spend engineering effort and (probably) weight on something that is completely irrelevant? It doesn't matter how smooth or fast the deployment is, as long as the fins are fully deployed before any significant aerodynamic control authority is established. – Jörg W Mittag Apr 8 at 6:40