Jon S. Berndt wrote:
Has an STS SRB ever failed (in flight or in test) "catastrophically"?
For the STS SRB, human rating _was_ designed in.

There have been three failures of large segmented
solid rocket boosters since 1986, inclusive. They
we

1/28/1986 STS-51L (We know all about that one)

4/18/1986 Titan 34D-9, Vandenberg SLC-4E, SRM case
insulation and case burn through. Catastrophic
explosion over pad 8.5 seconds after liftoff.
Believed to have been caused by insulation
debonding from the internal surface of the motor
case.

8/2/1993 Titan 403-K11, Vandenberg SLC-4E, Vehicle
exploded 101 sec. after liftoff. The cause was a
case burn through of solid rocket motor 1, case
segment 3. The failure was traced to a faulty
repair of a "restrictor" that involved cuts into
the propellant grain that were improperly potted.

Two of these three were instantaneous solid booster
case failures. In addition, no one noticed the
51-L joint burn-through until examining tracking
film after the incident. The vehicle disintegration
itself was all but instantaneous.

NASA upgraded its nondestructive test methods after
the 51-L and 34D-9 failures, but no test can
absolutely preclude the possibility of an
instantaneous case failure, even on shuttle SRBs.
The K-11 failure is instructive in this regard,
because all of the post-34D-9 NDT methods were in
use during its prelaunch checkout.

- Ed Kyle