Not really. The 2-ring design was designed so the second ring was a "backup", but as you note, this only increases loads that act in the direction of aiding "J

Not really. The 2-ring design was designed so the second ring was a "backup", but as you note, this only increases loads that act in the direction of aiding "Joint Rotation" and hot gas leakage. The third O-Ring was added on the opposite side of the joint from the original O-rings, and the tang was re-designed with an additional clevis socket so that pressure loads are balanced to prevent "Joint Rotation". A comparison of the designs can be seen at https://www.semanticscholar.org/paper/Axisymmetric-shell-analysis-of-the-Space-Shuttle-Nemeth-Anderson/56e940967229d37eaf1256962671e3ba1c5e9ea6/figure/0 https://www.semanticscholar.org/paper/Axisymmetric-shell-ana... . McDonald's book has FEA showing the predicted clevis deflections for both configurations, and the 3 O-ring design significantly reduces the "Joint Rotation." But still, since they were continuing to the same use Viton O-rings, a resistance heater and temperature monitoring was added to address low-temperature performance. (Edit - I think this was McDonald's source for the FEA diagrams in his book - https://web.archive.org/web/20210717050737/https://ntrs.nasa.gov/api/citations/19910005305/downloads/19910005305.pdf https://web.archive.org/web/20210717050737/https://ntrs.nasa...) I've never really liked the description of the failure mechanism as "Joint Rotation" anyway. It's only rotating when you look at it in a 2D cross section. In 3D I'd call it "tang outward deflection." It's increasing in diameter, not rotating. The problem, at it's root cause, was a failure of requirements management. On the night of the launch, no one was able to identify what temperature either the SRBs or the Orbiter had been designed and qualified to. Within the specification for the SRBs, there are several low temperature requirements that conflict with each other, and were misapplied both in design, in testing, and on the night of the launch. Literally no one could say whether it was safe to launch the shuttle that night, because the requirements were not clearly defined. NB- I've personally designed and specified numerous O-ring installations in both static and dynamic aerospace applications, and done all the required DO-160 environmental testing to demonstrate that they perform as required throughout the thermal envelope. I am literally at a supplier/vendor right now doing a salt fog exposure test per DO-160.