Falcon 9, grid fin question

[Serenity]

Hello i was watching the latest video of SpaceX, https://www.youtube.com/watch?v=lXgLyCYuYA

and i was wondering, during the landing burn, the grid fins seems to gimble a lot, does it really matter at that point for them to keep working?

Does it have any considerable effect in the landing or just some minor assistance?Or its just there is no point to have a mechanism to lock them again?

 

Edited September 9, 2020 by Boyster

[mikegarrison]

4 minutes ago, Boyster said:

during the landing burn, the grid fins seems to gimble a lot, does it really matter at that point for them to keep working?

Probably not, but maybe any little bit of control authority helps.

[cubinator]

My guess: The grid fins work well while the rocket is moving fast. Only when it's really in the last few seconds of its journey does it slow down enough that they can't really affect the ship's motion. So until it's almost hovering over the pad, the grid fins can help to maneuver the rocket.

Remember how big the rocket is when thinking about how fast it's moving.

Edited September 9, 2020 by cubinator

[RealKerbal3x]

56 minutes ago, Boyster said:

and i was wondering, during the landing burn, the grid fins seems to gimble a lot, does it really matter at that point for them to keep working?

Does it have any considerable effect in the landing or just some minor assistance?Or its just there is no point to have a mechanism to lock them again?

Even though the engines allow quite a bit of control authority due to gimbal, they most likely still use the grid fins during the landing burn as they impart a force further from the CoM (during landing the CoM is quite low due to the mass of the engines and the small amount of propellant left), creating a larger moment arm and allowing greater control. I made a diagram:

Also, gimballing the engines more means less thrust going directly retrograde to slow the stage down, so the first stage's computer probably steers mainly with the grid fins/thrusters to ensure maximum deceleration as well as good control authority.

[mikegarrison]

1 hour ago, RealKerbal3x said:

Even though the engines allow quite a bit of control authority due to gimbal, they most likely still use the grid fins during the landing burn as they impart a force further from the CoM (during landing the CoM is quite low due to the mass of the engines and the small amount of propellant left), creating a larger moment arm and allowing greater control. I made a diagram:

Also, gimballing the engines more means less thrust going directly retrograde to slow the stage down, so the first stage's computer probably steers mainly with the grid fins/thrusters to ensure maximum deceleration as well as good control authority.

Yes, but aerodynamic forces are proportional to V², and grid fins are particularly unsuited to being low-speed control surfaces. So near the very end of the landing, they are doing next to nothing.

[K^2]

Kind of a guess, but if you're setting up a control loop, you'll have something like PID. In practice, probably something a little more complicated, but PID works for sake of discussion. Because the output of PID is desired force from control surface, the deflection angle will depend on velocity. The slow the rocket is moving, the more deflection you are going to need for the same authority, so that will feed into the control loop as well. And there isn't really an obvious lower cutoff point other than hitting the limiters on deflection. Since I'm pretty sure the fins are well balanced, there is probably no reason to explicitly shut this behavior off. So they probably just let it keep running, with gimbal on engine providing all of the authority and fins just going back and forward in futile attempt to help in the last moments of the landing. If it's not hurting anything, figuring out a shut-off timing is just one more potential point of failure.

[mikegarrison]

16 minutes ago, K^2 said:

Kind of a guess, but if you're setting up a control loop, you'll have something like PID. In practice, probably something a little more complicated, but PID works for sake of discussion. Because the output of PID is desired force from control surface, the deflection angle will depend on velocity. The slow the rocket is moving, the more deflection you are going to need for the same authority, so that will feed into the control loop as well. And there isn't really an obvious lower cutoff point other than hitting the limiters on deflection. Since I'm pretty sure the fins are well balanced, there is probably no reason to explicitly shut this behavior off. So they probably just let it keep running, with gimbal on engine providing all of the authority and fins just going back and forward in futile attempt to help in the last moments of the landing. If it's not hurting anything, figuring out a shut-off timing is just one more potential point of failure.

Agreed.

[JoeSchmuckatelli]

Off topic - but I don't think I will ever tire of seeing those things land - nor cease being amazed that we can do it and with such precision. 

[kerbiloid]

Spoiler

They are trembling from despair.

Also on 00:40 they've hit the TV best boy. (Later we can see the reflecting thing he was holding)

 

Edited September 12, 2020 by kerbiloid