Can a rocket engine be throttled?

Many rocket engines cannot do this at all — they light, run flat out, and stop. Being able to vary the thrust is a genuine engineering achievement, because the pumps, the chamber pressure and the propellant mixture all have to stay stable across the whole range.

It earns its keep twice on every Falcon 9 flight. First about 70 seconds up, when the engines ease off through Max Q so the airframe does not have to be built stronger than it needs to be. Then again on the way home.

The landing is the harder case. By then the booster is nearly empty and very light, and even one engine at its lowest setting still pushes harder than the empty stage weighs. The rocket physically cannot hover — so the burn has to be timed and throttled so that speed and altitude reach zero at the same instant.

The upper-stage engine throttles too, down to 360 kN, which lets it place a satellite precisely rather than simply shutting off and hoping.