SRBs weren't reusable in any real sense, what was left after splashdown had to be completely disassembled, separately refurbished, and then reassembled. Using the SRBs also made operations much more expensive, since any prep work done after they were mated to the Shuttle was extremely hazardous.

The Shuttle's main engines also had to be disassembled, refurbished and reassembled, rocket engines are complicated

I've read that's in part because they were pushing the edge of the state of the art at the time they were designed, sports cars were used as an analogy.

Of course, it remains to be seen if SpaceX's scheme for engine reuse will pan out.

Which is another way in which the Shuttle platform wasn't that much reusable. Falcon 9 and future SpaceX rockets are intended to be just refueled and relaunched, without having to disassemble the engines after every landing.

"Had to be..." is not quite true. How the SSME/RS-25 was designed to operate and could be operated is not the same as how NASA chose to operate it. Particularly after Challenger.

That's a solid point, but the Ariane 5 does not do manned flight either. SRBs are less safe than liquid fuel engines too, no way to shut SRBs down on ignition ... sweet.

Welcome to rocket science. Not all of this is 100% safe

(see past discussion of a Shuttle mission auto-shutdown, where the Shuttle main engines were shut down seconds before lift-off)

> Welcome to rocket science. Not all of this is 100% safe

Yes, but the Shuttle was far worse than most in that regard. It's abort modes were limited at best and crazy/non-existent at worst.

Yet, from 270 SRB's flights one of them failed. STS-135 included parts flown on 59 previous missions, including STS-1.

The main problem was actually the weight load as the shuttle was hanging off the side.

Its also that when they fail they create dangerous falling debris that could threaten the pad or an escaping crew compartment.