Not exactly right. A part of my point was more that the complexity and finickiness of the STS seem to be largely, though not completely, due to the procurement

Not exactly right. A part of my point was more that the complexity and finickiness of the STS seem to be largely, though not completely, due to the procurement process and some peculiarities of the U.S. political structure. Parts for the STS are made everywhere by subcontractors, in order that politicians can bring jobs to their constituents, and then integrated. I think that's to what you were referring but misunderstood. As to mission-critical parts, there is some leeway in the definition of the term that may explain the confusion. "Mission-critical" means, necessary for the survival of the crew and passengers. The "mission" is, get to the desired location if you can, but abort safely if you can't. For passenger planes, this means that if one engine goes out, the plan can land on the other one (or two or three). If the ailerons go out, the pilot can achieve roll control with rudder and throttle. If the INS goes out, the GPS can get you to a visual on a landing strip. In each of these cases, there is a single failure and nobody dies. Hence, these systems are not "mission-critical" where the "mission" is transporting crew and passengers without loss of life. That the plane goes somewhere specific is a fringe benefit, so to speak. IIRC, the O-ring disaster had to do with the poor design and decision-making regarding temperature limits of the inadequate O-rings. Once hot combustion products get past an O-ring, there will be a failure. On a system where those hot gases can impinge on a fuel tank, there will be a catastrophic failure. On the STS, that makes those O-rings mission-critical. Interestingly, some have said that the choice of Thiokol is the ultimate cause of that disaster, for precisely the reasons I talked about in my original comment. The SRBs reportedly had to be able to be shipped over the road from Thiokol's plant. In order to do so (instead of building a plant near the launch site), the boosters had to be segmented. Segments -> O-rings. That's pure rumor, afaik.