We got the second rocket prepped at this point, flying on another N5800 just like we did in August and November 2019. This motor used the warrantied casing to replace the one that bulged the 6G-XL casing from the November flight. It came with dinged threads which we straightened out with a mallet, the liner had a small chip at the top edge, and we had to omit the bottom-most grain spacer O-ring to get the nozzle to fit flush inside the liner. None of these would have prepared me to witness what came next. The Bigger One weighed 128 pounds and sat ready on the pad. When the igniter burned, the motor lit for about a half second before the rocket got blow apart, ejecting parts of the motor, including a flaming propellant grain onto the grass. After 4 years of high-power rocketry, I was responsible for my first CATO, and it had to happen with a $1300 reload on the 7th launch with our customer. I didn’t really feel that bad personally; I was more baffled by what could have possibly gone wrong to cause such a failure. Unfortunately when we collected the pieces, the image became even less clear. There were 2 unburned propellant grains on the ground. All of the motor hardware was intact. The forward closure and threaded retaining ring were blown clear, but both sets of threads contained almost no observable damage. It’s almost as though the grain which caused the overpressurization did so by unscrewing  the retaining ring on its own.