gfunk
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- Joined
- Jul 25, 2017
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Experience is what you get when you didn't get what you wanted. And experience is often the most valuable thing you have to offer. -Randy Pausch
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 shear my switch band from the coupler, plow 3 plastic rivets through almost 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, one still stuck in the liner. These will be a bittersweet consolation prize at next week's elk hunting camp fire.
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.
I assembled this J357 motor under quiet and controlled circumstances in my kitchen. I have a BSME and 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. The threads were not cross threaded. I never removed the nozzle from the liner, though I did pop the foreward closure to drill the delay.
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. Its then surmisable that the plastic rear closure threads heated, softened, and failed.
Remarkably, my DX3s booster section is essentially undamaged. The nosecone shoulder zippered from high speed deployment, but can be repaired with some PE plastic welding. The payload section is banged up but I think I can repair it well enough for another single deploy flight since the entire NC/payload section won't need to separate. The 60" spherachute gore seam stitching show signs of stress from high speed deployment, but I think it's good enough to use again.
I would like to learn anything I can from this experience and welcome any and all opinions and constructive advice. I have submitted a MESS report and assume the vendor will report to CTI.
The motor case fell from a couple hundred feet up. It looks OK at first glance but the there is a tiny flat spot now on 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...
The replacement J357 motor is from 2014, and apparently before the Pro38 forward closure issues. It's tempting to thread this motor into the case with epoxy on the threads and then toss the whole thing... I've had great luck with Pro29s, but 50% success with the 38s has me wondering about trying AT in this size.
Sad ground and on-board video here, pictures below:
[video=youtube;CfkjB5-tN3k]https://www.youtube.com/watch?v=CfkjB5-tN3k[/video]
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 shear my switch band from the coupler, plow 3 plastic rivets through almost 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, one still stuck in the liner. These will be a bittersweet consolation prize at next week's elk hunting camp fire.
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.
I assembled this J357 motor under quiet and controlled circumstances in my kitchen. I have a BSME and 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. The threads were not cross threaded. I never removed the nozzle from the liner, though I did pop the foreward closure to drill the delay.
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. Its then surmisable that the plastic rear closure threads heated, softened, and failed.
Remarkably, my DX3s booster section is essentially undamaged. The nosecone shoulder zippered from high speed deployment, but can be repaired with some PE plastic welding. The payload section is banged up but I think I can repair it well enough for another single deploy flight since the entire NC/payload section won't need to separate. The 60" spherachute gore seam stitching show signs of stress from high speed deployment, but I think it's good enough to use again.
I would like to learn anything I can from this experience and welcome any and all opinions and constructive advice. I have submitted a MESS report and assume the vendor will report to CTI.
The motor case fell from a couple hundred feet up. It looks OK at first glance but the there is a tiny flat spot now on 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...
The replacement J357 motor is from 2014, and apparently before the Pro38 forward closure issues. It's tempting to thread this motor into the case with epoxy on the threads and then toss the whole thing... I've had great luck with Pro29s, but 50% success with the 38s has me wondering about trying AT in this size.
Sad ground and on-board video here, pictures below:
[video=youtube;CfkjB5-tN3k]https://www.youtube.com/watch?v=CfkjB5-tN3k[/video]
