Re: Blowout Ports
You can kill the thrust with a solid motor, but you can't stop the burn.
Controllable liquids are the answer (and they give better isp, too)
Even with blowout ports the Challenger accident would have been unstoppable once started.
(If noticed early enough a total abort and dump of the boosters and External Tank would have saved the crew, but led to a very exciting abort-to-base flyback trajectory for the shuttle)