Aerotech J 420R-M Failure

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Permalink Reply by Bryan Sparkman on January 14, 2008 at 7:50pm

Do you have any pictures? And did the case actually rupture, or did it just burn from both ends (forward closure failure)? I've built plenty of J-motors in the 38/720 case so I have some experience. Sorry about your rocket, I lost one too this weekend.

-Sparky

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Permalink Reply by Kevin Davidson on January 14, 2008 at 10:56pm

I have been talking about this in another club.
I appears that it was a Rocket failure and not the motor.

Apparently the cavity created between the MM rings blew apart to release the pressure overload.

I will be filling these with foam in the future.

THanks for your interest.

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Permalink Reply by Bryan Sparkman on January 15, 2008 at 2:18am

Hmm, OK. I suspected that something of that nature could be the culprit from the high-G takeoff. I watched your video and the only thing that doesn't fall in line with that theory is that there is no "pop" of the ejection charge after the shred. I would have expected the motor to still be functioning after the shred and fire the ejection charge at 10 seconds (assuming the delay was for 10 seconds). However, it is plausible that the shred dislocated the BP from the forward closure well, and that could explain the lack of a "pop". Did the forward closure show any signs of a blow-by? Either way, it probably wasn't a motor manufacturer defect. Nice videos though. Sorry about the rocket.

-Sparky

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Permalink Reply by Mark Koelsch on January 15, 2008 at 9:00pm

Kevin, the exploding airmass theory seems a little strange to me. At what altitude would you say it blew? How much did the rocket weigh?

The motor itself is intact? Stupid question, but did you grease the orings and gaskets within the motor?

Mark

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Permalink Reply by Kevin Davidson on January 15, 2008 at 9:55pm

Altitude was 2 to 3 sec off the rail.
Height?
Weight was 3.3 lbs.
It was LOC 3" tube with G10 fins attached through the wall.
The motor was in good condition when I recovered it. An expert on sight looked at the motor but not the rocket. He did think that there was some evidence that the flame could have gotten buy the forward closure and ignited the BP. I do not see evidence of flame in the motor mount tube forward of the motor.
In the future I will fill the cavities with foam or drill relief holes.

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Permalink Reply by Steve Shannon on January 18, 2008 at 1:49pm

Keith,
It sounds like the rocket disassembled itself between 2 and 3 seconds after liftoff? The J420 burns for 1.5 to 1.6 seconds. Is it possible you had either drag separation or pressurized the chute compartment (sounds like maybe you didn't have vent holes?) An early chute opening would leave you with a zipper and could result in the fins detaching due to inertia. Did you have a zipper?

Like Mark, I don't understand what you meant about overpressurizing the space between centering rings either. Was the motor mount tube still in the rocket body? Was it intact?

The worst thing for me is to not understand why one of my rockets failed. Let us know what you decide, please.
Steve

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Permalink Reply by Kevin Davidson on January 18, 2008 at 6:43pm

Steve
Yes there is a small zipper and the chute was torn apart.
the aft end of the rocket was blown apart . I did have a hole in the BT where the chute is to prevent the the nose being pushed out. When I built the rocket and put the fins in that crated spaces between the aft MM center ring and the middle ring just forward of the fins I believe this area was could have ruptured due to the atmospheric change.

This is what I have left

Kevin

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Permalink Reply by Mark Koelsch on January 18, 2008 at 9:35pm

Kevin, I doubt it had to do with air pressure. If the can is sealed, the air pressure difference from a couple fo thousand foot change is not going to be adequate to cause it to explode. Secondly, if you had holes for a motor retainer through the aft end, they would help bleed off the pressure.

I think looking at this your fins came off due to flutter, and as they did so they took chunks of the airframe with it.

How thick are the G10 fins, how were they attached, and did you have any other laminate over the fins?

Mark

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Permalink Reply by Steve Shannon on January 18, 2008 at 11:36pm

I agree with Mark that pressure in the fin can (between centering rings) should not have caused this. I would suspect (as Mark has mentioned) the possibility of fins shredding or at least vibrating badly, especially long ones like that which would have possibly severely weakened the airframe and fin can. I would also suspect that the weight of the motor yanking against the aft centering ring and motor retainer could have pulled the aft end of the weakened air frame apart pretty spectacularly when the chute opened at nearly max velocity. We may never know.
Was this the first time you had flown this rocket?

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Permalink Reply by Kevin Davidson on January 19, 2008 at 9:54am

I think you could be right but there is no way to tell for sure. I was hoping to find someone with a video of the event itself but no one has posted anything.
I have flown this rocket many time with other motors usually I launch the H series motors but it was time to move up to a J and I guess I need to build better rockets.

I Will Be Back

Thanks for the inputs.

Kevin

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Permalink Reply by Steve Shannon on January 20, 2008 at 11:26am

I'll bet you build fine rockets. If you don't engage in some destructive testing from time to time you really don't know where the weak points lie.

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Permalink Reply by Kevin Davidson on January 20, 2008 at 12:43pm

Thats true but it is still painful to watch all your hard work turn to confetti.

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