Could someone please explain this CATO damage?

[billdz]

Had a CATO on Saturday. The rocket was an EZI-65 and the motor was a Cesaroni J420. At ignition, the plastic aft closure apparently popped off and 4 grains fell to the ground. The rocket lifted up a few feet, then fell back to the ground, the fin section touched ground first, then the rocket fell on its side (see attached video). The rocket remained in one piece, the ejection charge did not fire and stayed in place, on top of it was the dog barf, then the chute and shock cord. The payload section did not separate from the fin section of the airframe.

The only damage to the rocket was 1) a hole on the side of the fin section airframe, 2) the payload section bulkhead broke at the eye bolt (where the shock cord connects), and 3) the coupler buckled just above the bulkhead (see attached photos). I'm trying to figure out what caused this damage. I assume it must be either upward force from the motor at the moment of the CATO, or impact with the ground after the CATO. From the video, it seems the rocket was already bent as it ascended, and it does not seem the rocket hit anything that caused the damage. Any ideas as to the cause of the damage?

Yes, I have filed a MESS report.

Thanks,
Bill

 

[mikec]

The motor overpressured, the aft closure blew off and the grains were ejected. The rest of the motor, now unrestrained from going up through the rocket, smashed into the bulkhead, bounced, and was retained by something. I'm guessing, but this is a pretty common failure mode with Pro38.

 

[billdz]

The motor overpressured, the aft closure blew off and the grains were ejected. The rest of the motor, now unrestrained from going up through the rocket, smashed into the bulkhead, bounced, and was retained by something. I'm guessing, but this is a pretty common failure mode with Pro38.

Thanks for the reply. What you describe has happened to me before, but that is not what happened this time. The rest of the motor did not move, it was in exactly the same place where it was mounted, and the dog barf, chute, and shock cord were undisturbed.

 

[mikec]

If the motor didn't move, what kept it from going forward? All I can say is that I've had this exact same failure, and the sequence I described is the only way I could think of for it to happen.

 

[billdz]

If the motor didn't move, what kept it from going forward? All I can say is that I've had this exact same failure, and the sequence I described is the only way I could think of for it to happen.

Guess the motor was too tight in the mount to move up in the rocket. I also can't think of any other way it could have happened, but the motor definitely did not move up.

 

[tsmith1315]

It looks like the payload section was pushed forward by the time it was just off the rail.

I don’t see any tape on the coupler, was it a loose fit?

 

[jmasterj]

It looks like the payload section was pushed forward by the time it was just off the rail.

I don’t see any tape on the coupler, was it a loose fit?

Yeah, the rocket already looks bent before it comes off the rail. I don't have any guesses to add, unfortunately.

 

[billdz]

It looks like the payload section was pushed forward by the time it was just off the rail.

I don’t see any tape on the coupler, was it a loose fit?

It was a good snug fit, not loose at all.

 

[tsmith1315]

It was a good snug fit, not loose at all.

I can’t see the video very well on my phone, but it looks like the coupler may be exposed pretty far down on the rail.
Where is the hole in the airframe? Maybe some exhaust was able to leak in and pressurize it.

 

[billdz]

Yeah, the rocket already looks bent before it comes off the rail. I don't have any guesses to add, unfortunately.

View attachment 572468

Yes, definitely bent before the rocket is off the rod. It's a mystery.

 

[Banzai88]

I can’t see the video very well on my phone, but it looks like the coupler may be exposed pretty far down on the rail.
Where is the hole in the airframe? Maybe some exhaust was able to leak in and pressurize it.

Yes, definitely bent before the rocket is off the rod. It's a mystery.

Carefully examine the o-ring around the forward closure, and inspect the case. There have been instances of high speed gas cutting blowing by the o-ring and out the top of the clousre BETWEEN the plastic closure and aluminum case that will cause such pressurization. Doesn't take much. You're looking for a teeny tiny nick in the o-ring, might just look a little melted, and a corresponding thin grey or black line along the case about the width of a .7 pencil line, and about as light in color......if that's what happened.

J420 is a 6grain XL motor, and the 38 and 54mm long CTI motors are known 'issues' (certainly the 54s more than the 38s, but CTI has acknowledged in the public CTI thread that there IS an issue with the long motors at the forward closures).

 

[billdz]

Carefully examine the o-ring around the forward closure, and inspect the case. There have been instances of high speed gas cutting blowing by the o-ring and out the top of the clousre BETWEEN the plastic closure and aluminum case that will cause such pressurization. Doesn't take much. You're looking for a teeny tiny nick in the o-ring, might just look a little melted, and a corresponding thin grey or black line along the case about the width of a .7 pencil line, and about as light in color......if that's what happened.

J420 is a 6grain XL motor, and the 38 and 54mm long CTI motors are known 'issues' (certainly the 54s more than the 38s, but CTI has acknowledged in the public CTI thread that there IS an issue with the long motors at the forward closures).

The 38mm case has no forward closure. The o-ring, the delay assembly, the liner, and the case all appear to be undamaged and unmarked. But you must be right, somehow pressurized gas must have gotten up and damaged the airframe, bulkhead, and coupler. And it happened very quickly, right after ignition.

 

[Banzai88]

The 38mm case has no forward closure. The o-ring, the delay assembly, the liner, and the case all appear to be undamaged and unmarked. But you must be right, somehow pressurized gas must have gotten up and damaged the airframe, bulkhead, and coupler. And it happened very quickly, right after ignition.

OK, I mis-spoke....forward delay grain element assembly. It's a known issue in the high pressure long CTI motors, I've been a victim of it several times. Now, no matter what for CTI 38s, I glue in the forward assembly after drilling the delay.

 

[dpower]

If the aft closure let loose, the forward O-Ring will lose its sesl immediately. Cross-threaded aft closure, perhaps?

 

[billdz]

If the aft closure let loose, the forward O-Ring will lose its sesl immediately. Cross-threaded aft closure, perhaps?

If so, that explains what happened to my rocket. Aft closure was not cross threaded.

OK, I mis-spoke....forward delay grain element assembly. It's a known issue in the high pressure long CTI motors, I've been a victim of it several times. Now, no matter what for CTI 38s, I glue in the forward assembly after drilling the delay.

Yes, it happened to me in the past as well, I'll never fly another 6XL. I had thought this was because of the plastic aft closure, but I hear it is also an issue with the 54mm 6XL, which has a metal aft closure.