Solid Rocket Booster's were one of the design features of the space shuttle that was supposed to reduce the costs of going into space. An SRB is basically like a bottle rocket. Solid propellant is formed and placed inside the rocket. A star point hole is drilled through the center of the solid propellant and the interior of the rocket is designed to ensure that all sections of the propellant are ignited appropriately.
And like a bottle rocket once you ignite it, there is no turning back. There is no engine shutdown. ...You've just lit rocket fuel, good luck if you've screwed anything up, because there is no way to stop it until it's burned every last inch. That was Werner Vaun Braun's problem with SRB's, so he stuck with liquid rockets with lots of turbines and moving parts in early days of NASA. With liquid rockets you can shut down the engine. It was the legendary argument in Engineering favouring control over cost. Quite frankly based on my experience in small piddly engineering projects I'm more apt to favour control as well so I can't blame the grand master Vaun Braun himself.
Though SRB's are simple. No moving parts, and they should be relatively easy to build. Like a child building bottle rockets it should be easy. Somehow the costs doubled from the Apollo years.
But this is the final nail in the coffin for SRB's:
The Space Shuttle's solid rocket motors are periodically tested to ensure they can withstand environments similar to those generated by an actual motor during flight. For example, the motor's insulation materials must withstand chamber gas temperatures that reach 5,652 degrees Fahrenheit during a Shuttle launch.
If it worked once, twice, thrice, and a hundred times, why do they need to test it periodically? Maybe somewhere out there someone has an awnser to this, but to me it makes no sense. All of this should have been done during the design phase of the space shuttle. For them to do continously makes me wonder.
If I designed a process that required a central component to be tested periodically I'm sure the Engineers around me would conclude that the system I designed wasn't very robust. I would probably agree, and hope I wouldn't get fired.