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This morning I attempted a L2 certification flight at the Pawnee National Grasslands, which ended with a CTI Pro 38 J357 Blue Streak motor CATO due to rear closure failure (motor date code I believe was November 2016?). The on site motor vendor Moto Joe was awesome and replaced the motor and kept the original packaging. Upon examination of the on board 808 video footage, it is clear the 5 grain case separates from the threaded rear closure while still under boost, pushing out the single deploy main chute on its way to freedom. Because this was a cert flight I had kept things simple (single deploy) so the payload section was empty with only the 11/32” Kevlar harness connecting the nosecone to ebay. At this point the motor impacts the ebay/payload section with enough force to plow 3 plastic rivets through 1” of cardboard air frame and shear the 3 #2-56 nylon shear screws joining the nose cone to the payload tube. Apparently due to the sudden pressure drop, the 5 burning fuel grains promptly self-extinguished, of which 2 partially burned grains were later recovered, one on the range, only one was still stuck in the liner. I had previously assembled exactly (and only) one Pro 38 motor, an I345 3 grain for my L1 cert last month which had a picture perfect flight. https://www.youtube.com/watch?v=Ggnm6cXzij8 I assembled this J357 motor under quiet and controlled circumstances in my kitchen. I have a bachelor’s in mechanical engineering from Iowa State University and I work as a senior staff precision machining engineer where I routinely specify and personally machine threading for a living. I assembled the rear closure until it was snug and just a hair more. Snug, hand tight, but most certainly not over tight. A gap of less than 1/16” was clearly visible between the rear closure and case. It appears to me the seal between the rear closure O-ring and liner failed. The O-ring is visibly eroded, there is a hole in the liner in this area, and there are burn marks on the case. It’s then surmisable that the plastic rear closure threads heated, softened, and failed. Remarkably, my DX3’s booster section is essentially undamaged. The nosecone(!) zippered from high speed deployment, but can be repaired with some PE plastic welding. The payload section is damaged but I think I can repair it well enough for another single deploy The motor case fell from a couple hundred feet up. It looks OK at first glance but the there is a slight dent in the threaded end, it's now a bit out of round. I tried threading in a spent I345 closure and it threads in fine, but this is now one more thing to wonder about... Sad ground and on-board video here: https://youtu.be/CfkjB5-tN3k
