L2 Failure, CTI Pro38 J357 CATO

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gfunk

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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. 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 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]

0781c4f9-85dc-45b1-a4ed-8e18a07fcb64-o.jpg

preflight_happiness.jpg

mach_diamonds.jpg

DSC02068 - Copy.jpg

vlcsnap-2017-10-08-15h04m26s079.jpg

yardsale_3grains_leaving_booster.jpg

eroded_Oring.jpg

melted_liner.jpg

IMG_2409.jpg
 
Did you check the seal before you put it together? Would recommend that check out the motor before use. Consider greasing the oring while your at it.
 
Sorry to see that. While I have not flown that motor, I have flown several Pro-38 motors. I've also seen MANY Pro-38 motors flown and I have never seen that failure. Your assembly of the motor mirrors mine. I have never greased anything except a very light grease of the inside of the case. That is only to facilitate easy removal of the post flight waste. There is no mention of grease of any kind in the Pro-38 instructions. Even though the motor was dated as 2014, I would contact your vendor. Also, please fill out a MESS report. I hope you are able to get satisfaction from your motor vendor. I also hope you get to use CTI motors again. I use both CTI and Aerotech. You'll get the cert. next time. It's a drag to lose it when it wasn't your fault.
 
threading plastic into metal is just a bad design. seen it many times, some day multiple times in a day. There's a reason they moved to a metal closure when they made the 29's. No idea why they didn't fix the 38's back then too.

Loki and AT make nice 38's.
 
Joe is a terrific guy and a very upstanding vendor. I’m not at all surprised that he took care of you.
I’ve flow or seen flown hundreds of Pro38 motors. I have seen this happen once or twice. It’s a fluke and you should not worry about it happening again. I would continue following CTI’s instructions without adding any additional steps.
 
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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. 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 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]

Sure looks like the o ring failed. Do you think maybe the o-ring was pinched or cut? Did you remove nozzle from liner before installing into the case? Somewhere some how o ring failed and caused the burn through. CTI is a very good product and this could happen with any motor brand. I would get a new case and try again.
 
threading plastic into metal is just a bad design. seen it many times, some day multiple times in a day. There's a reason they moved to a metal closure when they made the 29's. No idea why they didn't fix the 38's back then too.

+1 on this. I wish CTI would redo the 38's with a metal rear closure, but then they'd have to recertify all their loads, too. I have at least one reallly nice 38 6G load that I can't fly because the stupid plastic nozzle got cross threaded... it's easy to do, and I always lubricate the case and the threads.
 
+1 on this. I wish CTI would redo the 38's with a metal rear closure, but then they'd have to recertify all their loads, too. I have at least one reallly nice 38 6G load that I can't fly because the stupid plastic nozzle got cross threaded... it's easy to do, and I always lubricate the case and the threads.

the number of cases and loads they've had to warranty out has to be more than what'd take to recertify
 
+1 on this. I wish CTI would redo the 38's with a metal rear closure, but then they'd have to recertify all their loads, too. I have at least one reallly nice 38 6G load that I can't fly because the stupid plastic nozzle got cross threaded... it's easy to do, and I always lubricate the case and the threads.

I am sure you can get a new nozzle from CTI or one of its dealers. I know guys who have reused the once fired CTI nozzles.
 
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+1 on this. I wish CTI would redo the 38's with a metal rear closure, but then they'd have to recertify all their loads, too. I have at least one reallly nice 38 6G load that I can't fly because the stupid plastic nozzle got cross threaded... it's easy to do, and I always lubricate the case and the threads.

I can’t speak for CAR’s MCC, but I don’t think they would have to recert all of their loads. As long as they could show that the replacement nozzles and aluminum aft closures were functionally equivalents to the existing plastic aft closure and nozzle assembly, it should not be much different than certifying any kind of hardware change.
Also, you should be able to request a replacement aft closure for your cross-threaded one. Ask any one of the larger CTI dealers to help you.


Steve Shannon
 
George - bummer about the failure. I was out on the Pawnee Saturday but heard Sunday was better. I certified many moons ago on the H125 and then J285 combination.

Joe will definitely take care of you. I had a similar failure on a 3 grain about a decade ago. Rear closure came out, and ruined the case. I've seen hundreds of CTI motors flown and very few failures.

As others have suggested, file a MESS report.

As for switching to another brand, they all have their pros and cons. Typically it comes down to what is available at your local launches. I personally like the CTI motors for their ease of assembly. The 38mm motors take no tools.

See you out on the range!

Edward
 
Thanks, all!

Yep, I submitted a MESS report.

No, I didn't inspect the failed rear O-ring before flight... I never removed the nozzle/rear closure from the liner.

CATO damage repairs are going well, glad I have a plastic welder for the nosecone shoulder zipper! Hoping to try this all again at the November NCR launch if conditions allow.
 
threading plastic into metal is just a bad design. seen it many times, some day multiple times in a day. There's a reason they moved to a metal closure when they made the 29's. No idea why they didn't fix the 38's back then too.

Loki and AT make nice 38's.

The Pro38 is the way it is because they're already manufacturing this for another application (likely military). Do you think the military would allow them to do this if it wasn't extensively tested and proven? Not a bad design. Engineering isn't about overkill. It's about meeting requirements with minimum cost (time, materials, labor).
-Ken
 
The Pro38 is the way it is because they're already manufacturing this for another application (likely military). Do you think the military would allow them to do this if it wasn't extensively tested and proven? Not a bad design. Engineering isn't about overkill. It's about meeting requirements with minimum cost (time, materials, labor).
-Ken

I'd be willing to bet that the military wasn't reusing the casings, either... they probably treated it as a on-time use motor. Given their likely use, they probably didn't expect to get it back in one piece.
 
I'd be willing to bet that the military wasn't reusing the casings, either... they probably treated it as a on-time use motor. Given their likely use, they probably didn't expect to get it back in one piece.

The 6061 aluminum casings aren't the weak part of the system. We all use similar casings multiple times. From the pictures shown, it looks like the o-rings was chopped/nicked and didn't seal, that let hot gas escape, heated the thermoplastic nozzle assembly and then the threads failed and the nozzle assembly let loose.

Edward
 
Thanks, all!

Yep, I submitted a MESS report.

No, I didn't inspect the failed rear O-ring before flight... I never removed the nozzle/rear closure from the liner.

CATO damage repairs are going well, glad I have a plastic welder for the nosecone shoulder zipper! Hoping to try this all again at the November NCR launch if conditions allow.

Don't risk it.

You should send all your Pro-38 loads to HARA for "Proper Disposal". Better safe than sorry :cool:
 
I'd be willing to bet that the military wasn't reusing the casings, either... they probably treated it as a on-time use motor. Given their likely use, they probably didn't expect to get it back in one piece.

Years ago we sold a very large number of chutes to the Navy for training purpose. The were using AT 29mm reloadable motors. The Navy would fire the rockets off the ship deck and the guys would practice tracking it all the way down into the water. The whole rocket, chute, and motor were not recovered.
 
Years ago we sold a very large number of chutes to the Navy for training purpose. The were using AT 29mm reloadable motors. The Navy would fire the rockets off the ship deck and the guys would practice tracking it all the way down into the water. The whole rocket, chute, and motor were not recovered.

Holy cow. Somewhere there's a patch of ocean with bunch of corroded 29mm cases in it.......
 
Sorry to hear about your woes. I started out with CTI 29mm motors and enjoyed them. When I "upgraded" to the 38's I always wondered why the design seemed less robust than the smaller 29's. While I have not seen your particular failure before, I have seem plenty CTI aft closure failures. I stopped flying them the day I cross threaded one on and I am about as careful and deliberate as they come. It's a shame too because I genially liked certain loads in certain formulas, just not worth the risk. I also had one load where the little paper seal holing the ejection charge was peeled back and emptied. I ended up pouring more powder in and taping a new seal but I had the stuff on hand, would have sucked if I didn't. I now fly with cases that require me to build the motor from start to finish so I know it was put together as good as I could and not rely on some guy I will never meet.
 
One thing I do with CTI 38mm loads when assembling them is to make sure the entire load is in the case fully before staring to thread the aft closure/nozzle on by pushing the casing in with a spacer. Then lube the o-ring on the nozzle and push it in place. That way you're only trying to push in the nozzle and not the entire reload. Especially on the looong ones...
 
I now fly with cases that require me to build the motor from start to finish so I know it was put together as good as I could and not rely on some guy I will never meet.

If you fly CTI 38 then this is what you do. Take it apart and inspect it, that way you aren't relying on some guy you never met.
 
The Pro38 is the way it is because they're already manufacturing this for another application (likely military). Do you think the military would allow them to do this if it wasn't extensively tested and proven? Not a bad design. Engineering isn't about overkill. It's about meeting requirements with minimum cost (time, materials, labor).
-Ken

I find it hilarious when people say something is "military grade" or the "military uses it" like thats some kind of proof it's a good idea. Haven't spent much time working for the gov't, have you?
 
Oh, not only those. There are Pro 150 casings all over the Persian Gulf. (and likely elsewhere) That's what takes a some cruise missiles off ships that deploy them.

Really? Which type of cruise missiles? I believe Tomahawk boosters are substantially larger than 6"
 
Really? Which type of cruise missiles? I believe Tomahawk boosters are substantially larger than 6"

These bad boys? They look like 20" in diameter.

WMUS_Tomahawk_IV_rear_pic.jpg
 
Pretty sure he meant UAV, not cruise missile. In theory though, I think those got reused.
 
Didn't Myth Busters have an episode where they used Tomahawk booster rocket motors to test stopping a run away trailer. If I remember right, it was a huge failure.

It might have been the Rocket Battles(?) that used a pair of Tomahawk booster to launch Mini Coopers up a ramp and tried to drop them on a target. That didn't work so well either.
 
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