Can High Thrust Rockets Toggle Full Thrust On an Off?

[Spacescifi]

I often see large heavy rockets like what Elon is doing gradually throttle down instead of just shut off.

Is it possible to just shut down a rocket at fulk thrust or do you have to gradually throttle it down?

RCS thrusters are usually not made to be throttled and have static thrust so they are made to be turned on and off at will.

[kerbiloid]

Possible, but needless.
The low thrust replaces the ullage motors, lets separate the next stage more softly, as the empty stage is still controlling its attitude, and lets stop the engine at predictable moment of time, before the fuel tank is actually empty.

[magnemoe]

One issue with shutting down an large engine at max trust might be fuel flow, you have 3 engines and shut down two, pressure into the single engine turbo pump will spike simply because all the fuel and oxidizer in movement.
Yes its ways to compensate but that and other stresses is why you want to throttle down. 
RSC has much lower fuel flow and if engine trust is known its much easier to do an say 5 second burn. 

[DDE]

I suspect it is less advisable to avoid violent vibrations and things like hydraulic ramming...

2 hours ago, magnemoe said:

RSC has much lower fuel flow and if engine trust is known its much easier to do an say 5 second burn. 

Today's RCS tends to operate in small, measured pulses anyway, instead of continuous firing.

[Spacescifi]

Okay then. This definitely should have a bearing on scifi SSTO design then.

Reaction Control System rockets in scifi may be higher thrust especially with high mass vessels like the battlestar galactica or a star destroyer.

I know they don't use or show it onscreen, but given their mass, would not their RCS have to be throttled just like main engines? Due to the sheer mass being pushed (9000 tons or so).

Also I assume with torchdrive rockets, toggling full thrust on and off might be easier.

Why? Less propellant is required for thrust, which means less pressure on the turbopump etc right when full thrust is toggled on and off right?

[jimmymcgoochie]

Throttling a large rocket engine isn’t a trivial process. A small pressure-fed RCS thruster producing tens or hundreds of Newtons can be easily modulated by changing the fuel flow rate and/or firing as discrete pulses, but a turbopumped rocket engine producing MegaNewtons of thrust is a much more complicated beast. Some large engines are throttleable now thanks to modern understandings of things like fluid dynamics and computerised design and modelling, some were throttleable in the past e.g. the NK-15s on the N-1 used differential throttle to steer since they had no gimbals, but the vast majority were either on or off with no middle ground.

Suddenly stopping accelerating isn’t really a problem for humans flying on rockets, let alone for satellites or probes. Ullaging can be managed using solid separation motors between stages, by hot-staging before stage separation, using RCS in space or pushing propellant into the engines under pressure to start them up as for the Starship’s Raptors during landing, no need for fancy throttling on the previous stage in any of those cases.

If you want your magic SSTO engines to throttle down to 0.001% thrust, just make them throttle down to 0.001% thrust with whatever handwavey excuse you like.