[Uncle Fud]

Has there ever been a throttleable engine on any operational spacecraft or booster other than the lunar module and the space shuttle? Can't have been many. All the Titans, Saturns etc. I am sure were on/off just like a solid.

I don't see anything inherently unsafe about riding a single SRB to space. Remember it was not the O-ring failure on the SRB that directly doomed the Challenger, rather it was the ensuing explosion of the main (liquid) fuel tank.

[RightWhale]

The liquid fuel engines on the Shuttle are throttleable to a degree. Upper stage engines on most large boosters can also be shut down and restarted.

[Boundless]

> All the Titans, Saturns etc. I am sure were on/off
> just like a solid.

Dunno about the throttling, but the SRBs are not "on/off".
They are "on until fuel exhausted". They cannot be shut
down prior to that, and jettisoning them while still
burning at full thrust is apparently not an option.

> Remember it was not the O-ring failure on the SRB
> that directly doomed the Challenger, rather it was
> the ensuing explosion of the main (liquid) fuel tank.

Had the ET burn-thru not ocurred, the asymetric thrust
would probably have shortly caused stack loss by
exceeding the directional control provided by engine
gymballing and aero-surface deflection.