How does a rocket steer? What is gimballing?
Also called: thrust vector control, engine gimballing
Gimballing is how a rocket steers: the engines swivel a few degrees on a hinge, so the push no longer points straight through the middle of the rocket and the whole vehicle turns.
A rocket has no wings to steer with, and for most of its flight there is not enough air for wings to work anyway. What it does have is a very large force coming out of the bottom, and it can aim that force.
Push exactly through the centre of mass and the rocket goes straight. Tilt the engine slightly and the push acts off-centre, which rotates the vehicle — the same way pushing a door near its edge swings it round.
The movement is small. A few degrees is enough, because the engine is a long way from the centre of mass and the leverage is considerable. Hydraulic or electric actuators make constant tiny corrections, many times a second, the whole way up.
On a Falcon 9 the outer engines of the octaweb gimbal and the centre one does the fine work during landing. Once the air is gone and the engines are off, gimballing is unavailable, so small thrusters take over instead.
See it happen
Launch DayCountdown to spaceSources: Falcon 9 Block 5 — specifications · Merlin rocket engine family