Cesaroni J595 6XL 38mm plastic aft closure failure

[billdz]

Tried to launch a Big Nuke 3D today but things did not go quite as planned. Apparently the plastic rear closure on the Cesaroni J595 motor failed, the rest of the motor pulled through the threads and went up inside the rocket until it smacked into and broke the av-bay wooden bulkhead. The motor continued forward and pressed on the sled. The plastic rivets securing the upper airframe to the av-bay were stressed but did not break. It looks like only one propellant grain fired, and the ejection charge did not go off. Fortunately, only the bulkhead was damaged, and the rivet holes need some reinforcement. Video from a distance at: https://youtu.be/ExnD6EfpRpQ

I am aware that Cesaroni did a recall on the 38mm forward closures but this is the first time I've had an issue with the aft closure. I've seen some posts that this closure is easy to cross-thread but I don't think mine was cross-threaded, it screwed in very smoothly.
Today was a beautiful launch day at Labelle, clear skies, no wind, and a lot of big HPR launches, but I had two fails, need to re-group. The other fail was with an Aerotech motor, more about that later.

 

[mikec]

this is the first time I've had an issue with the aft closure.

This is what happens when the Pro38 overpressures. The aft closure itself was probably not at fault, I suspect there was a void in a grain or some similar problem. I had a J420 fail in a very similar way.

 

[tim cubbedge]

Poof!

 

[billdz]

Tim has a slow motion video that disproves my original theory. The motor came out through the back, it did not go up through the rocket. The bulkhead apparently broke when it hit the road, it was not hit by the motor. I'm now thinking retainer failure, see https://www.facebook.com/tim.cubbedge/videos/10211300829188948/

 

[mikec]

I can't tell from the video what happened, but what would cause the retainer to fail at ignition, the thrust is in the other direction? I still think there's a good chance that the motor CATOed, the aft closure blew off, the motor went up and bounced off the bulkhead, and then it went back, tore off the retainer, and came out the back.

My J420 failure was just as confusing but my retainer didn't come off.

Anyway, I'm just speculating; you have the pieces and the full-res video.

 

[Steve Shannon]

I can't tell from the video what happened, but what would cause the retainer to fail at ignition, the thrust is in the other direction? I still think there's a good chance that the motor CATOed, the aft closure blew off, the motor went up and bounced off the bulkhead, and then it went back, tore off the retainer, and came out the back.

My J420 failure was just as confusing but my retainer didn't come off.

Anyway, I'm just speculating; you have the pieces and the full-res video.

The force that blew the aft closure off would be equal and opposite to the force that blew the motor forward to strike the bulkhead. That would be more likely to cause the retainer separation than a bounce.

 

[tim cubbedge]

Anyway, I'm just speculating; you have the pieces and the full-res video.

here is you tube link. you should be able to view it in 1080HD. I have the original uncut in 4k if you want to see it.

https://youtu.be/Gu-23AiY_zE

 

[billdz]

Thanks for the HD video, Tim. Before seeing Tim's video and pics, I felt sure the motor had gone up through the airframe and struck the bulkhead. But it seems clear from the vid that the casing, grains, and aft closure came out the back, in pieces, and they came out right at ignition (video at 23 seconds), before the av-bay even was completely separated from the lower airframe. The video does not appear to support Mike's theory that "the motor went up and bounced off the bulkhead, and then it went back, tore off the retainer, and came out the back." It happened too quickly, and it seems unlikely the motor could have found its way back into the motor mount on the way down. There's no sign of any impact on the shock cord, the drogue chute, or its nomex protector, which the motor would have had to pass if it came forward and struck the bulkhead. On the other hand, there is a dent on the sled inside the av-bay, and the plastic rivets are stressed, so it sure seems some blunt upward force was applied to the bottom of the av-bay. And it looks like the rocket was not high enough off the pad for the 6XL casing to come out the back freely.

It all happened in a split second. In both frames below, the timer shows 23 seconds. I don't really understand exactly what happened.

 

[Dwatkins]

Watch the slow mo again. Liner and grains come out the back, but in the bottom of the recovery gear it appears the case is coming out the front of the booster.

 

[billdz]

You may be right. I've looked at the video thirty times and don't see a point of exit for the casing, kind of looks like it is coming out the side. It looks like the casing came out first, followed by the grains, which suggests the casing came the top. But the casing is rising, which suggests it bounced up off the pad. I don't see a frame where the av-bay is out of the rocket when the casing is visible. At 29.992 seconds, the av-bay is firmly in place and nothing seems wrong, while at 23.072 seconds (less than 1/10 of a second later) the casing is out and next to a fin. So I'm confused.

 

[Dwatkins]

Stop the above video at 24 seconds. Look at the bottom of the wad of recovery gear. Is that not the case. What I think you are seeing coming out the aft end is the liner and not the case.

 

[tfish]

this?

Tony

 

[billdz]

Good eyes, thanks much for solving the mystery. It was the liner I was seeing.

 

[goodeyes]

This is what happens when the Pro38 overpressures. The aft closure itself was probably not at fault, I suspect there was a void in a grain or some similar problem. I had a J420 fail in a very similar way.

My J285 blew apart at about 20 ft after liftoff; some of the grains separated. The explosion blasted off the Aeropack retainer from inside and the rest of the motor and casing ejected. Going with metal aft retainers from now on.