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Does anyone know what material the booster rocket joints were made out of? Thanks smart people! I know its steel, but what type?
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There's a drawing here:
Shuttle Joint. The joint consists of many materials. The steel in the cases is D6AC.
The Rogers' Report is worth reading if you are interested in the details of the disaster.
Feynman's Appendix is especially worthwhile.
It is a gross oversimplification to say "the o-rings failed." The people in the system also failed, miserably. Feynman put it this way: "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled."