In article ,
says...

Jeff Findley wrote on Fri, 7 Dec 2018
07:17:04 -0500:

In article ,
says...

Niklas Holsti wrote on Thu, 6 Dec 2018
19:01:58 +0200:

On 18-12-06 13:06 , Jeff Findley wrote:

Also, the stage managed to land very well on the ocean (as can be seen
in a video posted by an observer on social media). SpaceX also released
the on board camera footage from the stage which showed that once the
landing burn started, the engines were able to negate the roll caused by
the stuck grid fin.

I thought the landing uses only one engine, therefore probably the
center engine -- then how can the engine control roll? I don't understand.


The same way any other single engine booster controls roll, I would
think. The engine gimbals.

That would be a neat trick.


Or not.


On Falcon 1, that version of Merlin had a gimbal on the turbo-pump
exhaust for roll control. As far as I know, they deleted that feature
on Merlins used for Falcon 9 since the extra complexity wasn't needed
anymore.


Wiki indicates that the ability to gimbal was removed as of the Merlin
1C engine. Everything else I find, including the Payload User's
Guide, indicates that the Merlin 1D engines on a Falcon 9 do indeed
still have the ability to gimbal and gimballing is used to control
pitch, yaw, and roll for the first stage.

I agree that the center engine can gimbal. But during the final phase
of landing, only the center engine is firing. So how can you generate a
torque in the direction of the vertical axis of the rocket when the
center engine is located on that same vertical axis?

Jeff
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