Quote Originally Posted by OhOh View Post
This is to enable landing the rocket stage and re-use it, yes?

Are there other reasons?

Presumably prior to this landing technology being developed, for say a moon landing module, it was "just" calculating a ballistic trajectory and firing the engines to attain the acceptable landing speed, yes?

I understood the main engines were on gimbals but not utilised as a "steering" tool, weren't side thruster used to alter the rockets orientation and once achieved the main engines just accelerated the rocket onwards and upwards to the stars ?
It was already needed for launches before. But much more critical for landing. On ascent the main engines can be used for steering. But coming down is free fall most of the time. So they need other means for control, like the grid fins.

Roll control was an issue early on, not that easy. Roll being the rocket spins around its long axis. Remember the distinct black&white pattern of Saturn V? It was used so you could see the body spin easily.

You are right that rockets early on used smaller thrusters for steering instead of gimbaling main engines. A gimbal system that can handle the huge thrust of main engines is not trivial. But modern rockets use it. Gimbal is now easier than the plumbing of additional smaller engines called Vernier engines. The very old design of the Soyuz rocket still uses them.

When already in space very small low power thrusters are used to orient the stage in the right direction before the main engine is fired again. Those are called RCS thrusters, reaction control system. It is common to use just cold gas under high pressure, usually nitrogen. They are very low thrust but very simple and reliable. But under main engine power the steering is done by gimbaling or Vernier engines. The small thrusters are also needed to give the stage some acceleration to settle the propellant where the main engine can use it instead of floating around as bubbles.