The solid rocket motors on the space shuttle accounted for the majority of the launch mass and launch thrust. They're the most powerful rocket thrusters ever flown, at least until the 5 segment versions take flight with SLS in the next year or so.
I've often described solid rocket motors as being relatively simple compared to the complex plumbing, pumps and turbines of liquid rocket motors. However there's still a huge amount of critical engineering and science that goes into these boosters.
The design of the boosters were also partly responsible for the accident that destroyed Challenger during launch.