As to that SRB design...
Didn't I read somewhere that the reason the SRB was designed with these inherently risky joints was to enable the segments to be fabricated elsewhere and then transported to the launch site? In other words, it was a political decision to design it that way, meant to spread the work around.
A one-piece booster could have been fabricated on site, thus eliminating the o-rings and the associated risk of leakage. It would have been too large to build elsewhere and then transport to the launch site.
Is that correct or was it really necessary to design a segmented SRB?