Maximum bending moment on that flight occurred at 74.7 sec, just before Max-Q, and was 6.78 meganewton-meters.
Thanks for the correction; you obviously looked up the values while I was trying to remember it. The value I had in my head was for a different parameter of the Saturn v.
Bending moments tend to vary a lot from flight to flight due to differences in high altitude winds. It's why they release so many weather balloons before a launch.
And indeed, as you allude to below, why post-
Challenger launch criteria were amended to add more winds-aloft data.
As I recall, it was upper altitude wind shear that eventually broke the residue and let the plume re-emerge.
You recall correctly. The steering moments generated in response to the wind shear flexed the SRB more than usual and cracked the oxide plug loose. Regardless of how it may have behaved on the
Challenger flight, engineers realized that their design limits for flexure on the field joints were not nearly conservative enough given the emerging knowledge of the joint's dynamic behavior, and this led to more stringent wind-shear requirements for launch. This had the effect of limiting the anticipated steering moments to those within the existing experience base, excluding
Challenger.
Speaking of max-Q, you can really hear the effect in the videos made within shuttle cabins during launch. At liftoff there is a lot of noise and shaking, as you'd expect. That settles down, but as the shuttle accelerates the wind noise grows very loud, reaches maximum at max-Q, and then tapers off again.
This is in part a response to discontinuities in the forward section of the orbiter. The angle between the nosecone and the windscreen "traps" air, and the inset of the windscreen panes certainly doesn't help any with that behavior. There's a lot of dynamic pressure applied to the windscreen, and the inset produces turbulence. That's a huge roar just a meter or so from your face if you're in the commander or pilot seats. Boeing struggled with a similar problem for years; their airliner flight decks were notoriously noisy, for many of the same reasons. Even as late as on the 777, the particular way in which they traditionally formed the forward section of the fuselage included these discontinuities -- the result of structural methods to frame the windscreens. On the 777 tabs were added to the top surface of the nosecone in an attempt to redirect the airflow to the sides of the windscreen. Boeing wasn't responsible for the orbiter cockpit design, but the orbiter designers wrestled with similar problems and design constraints. With the 787 Dreamliner, the entire structural system of the forward section was redesigned from scratch and we were able to entirely eliminate the "Boeing roar" on the flight deck. The 787 windscreen is entirely conformal to the overall fuselage shape.
Also on the orbiter, the flow did not "stick" very well to the top of the flight deck as it came off the windscreen. There was intermittent flow separation right above/behind the windscreen that would have produced a fair amount of turbulence there as well, and that's yet another source of noise. The extremity of this behavior can be seen in
this photo of the stack as it transits the sound barrier.