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G53-7FJ Delay Failure

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clarinet

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Yesterday I flew a rocket using an AT G53-7FJ reload and the ejection charge fired much too early. The propellant ignited and burned normally, but the ejection charge fired only about 2 seconds after the propellant burned out. At that point the rocket was about 400-500 ft up and moving at high speed. Fortunately there was only minimal damage to the rocket which I have already repaired.

I don't understand the reason for this failure. I am sure I assembled the motor correctly as I have done dozens of times before with reloads using 29/40-120 cases. I used the 7-sec delay element as supplied with the motor and I did not drill it. The burned-out motor and casing appear normal to me without damage or deformities, and the delay element and propellant were fully burned. I have I entered this info in the NAR database of motor failures.

I would be happy to know if anyone has any insights as to why there was a 2 sec delay instead of 7.
 

rcktnut

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How did the delay insulator fit in the forward closure? They have been loose lately, I flew a G64-7 last weekend and when loading it up had to add a wrap of cellophane tape to the delay insulator to get a tighter fit in the well. The delay insulators have been very loose on the last few loads I used for the 29/40 -120 and I had to add tape to them all lately. With a loose fit the gasses can blow by the insulator and prematurely light the ejection charge. This problem has been brought up before on previous threads.
 
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clarinet

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Jeff - Thank you for the suggestion. I have seen this discussed before. The fit was a little loose on this one so I may try the tape idea in the future. The odd thing is I've had loose fits before without a problem. I thought the delay o-ring was the primary seal preventing exhaust gases from reaching the ejection charge, not the tightness of the delay element. Also, if loose fit was the problem I would have thought the exhaust gases would ignite the ejection charge almost immediately - not after the propellant had burned out.
 

Rocketclar

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I had a cato in 2018 with a G64-4 probably due to loose delay element. Not only did it destroy my G-Force, it sheared the top of the reload off!!! I, too, filed a MESS rpt and contacted AT about it at the time.
 

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cerving

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I've had a couple G53's in which the flame apparently breached the delay grain seal and ignited the ejection charge immediately after launch, both of them damaging the rockets they were in. Call me susperstitious, but I won't fly a G53 anymore.
 

clarinet

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Aerotech tells me that when the forward closure is secured the delay o-ring is compressed sufficiently to stop combustion gases from reaching the ejection charge powder, and that leakage around the delay element should not be a problem. I am sure I built the motor correctly, but I wonder if there was sufficient compression of the o-ring. There wasn't much resistance to tightening the forward and aft closures, unlike the smaller 24/40 reloads where I always encounter resistance when tightening those closures.
 

rcktnut

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One would certainly think that the delay o-ring would prevent this from happening but evidently it isn't. I became aware of the problem a few years ago when a friend assembled a G64 in the field and he noticed that the delay insulator was loose in the delay well. His rocket never left the pad the motor burned thru the top of the forward closure, powder well was gone, looked a bit worse than the one in post #6. I still had a bunch of the single grain G-64's left and all the delay insulators fit good. Now with all the 2 grain G64's all the insulators have been loose. I taped them all when assembling and never had a problem. If loose delay insulators aren't the problem what is?
 

clarinet

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I'm wondering if there was adequate compression of the delay o-ring in the motor. I didn't feel much resistance when tightening the closures. I assembled it correctly and put in all the parts - the three o-rings, forward insulator, etc. I've flown G53's a number of times before without a problem.
 

Handeman

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I had a similar issue that @rcktnut described. I got a new order of 29/40-120 reloads and the first thing I did was fly two G53-7FJ in a dual motor rocket. Both delay grains were loose and could fall out of the forward closure if you tipped it up. I built both the same as I had for a 100+ before that. Both blew past the delay grain, lit the powder, and melted the powder cups off the forward closures. Needless to say my rocket was a roman candle and burned to ash on the ground.

Every 29/40-120 I've had since, that had a loose delay grain, got a wrap of 3/4" masking tape and I never had the issue again.
 

clarinet

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This seems like a design flaw to me - a seemingly minor issue like a loose delay grain resulting in a CATO. The possibility of a loose delay grain and what to do about it is not even mentioned in the manufacturer's instructions. Ideally the delay o-ring should prevent the problem, but maybe the o-ring compression is not always sufficient to do that.
 

rcktnut

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This seems like a design flaw to me - a seemingly minor issue like a loose delay grain resulting in a CATO. The possibility of a loose delay grain and what to do about it is not even mentioned in the manufacturer's instructions. Ideally the delay o-ring should prevent the problem, but maybe the o-ring compression is not always sufficient to do that.
Not a design flaw rather the supplier of the delay insulator tubes not having the tubes quite up to spec. It doesn't take much to get them to fit properly and not a big deal to fix it. Like you said though problem should be mentioned one way or another.
 

n3tjm

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The firmly believe that issue is with the catalyst AT uses with their faster burning motors. The catalyst contaminates the delay grain, causing it to burn faster. If the delay grain has "yellowed" don't use it. At one point I seen Aerotech reloads have the delay grain in its own "static" bag, but I have not seen them do that for a while. If the reload is over a year old, I'd suggest you replace the delay grain. Aerotech does sell them as delay kits. I have seen this happen numerous times with oop G71's and G53's. If the Delay grain was not installed properly, it would of failed shortly after ignition. (Interesting note: Once, after a flight, I discovered I forgot to insert the delay spacer. The flight was perfect though.)
 

clarinet

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I've never had an early ejection ignition problem with any AT 24/40 or 29-, 38-, or 54-mm HPR motors, only with 29/40-120 reloads - specifically one G53 (discussed above) and two G64's in which their ejection charges ignited on the pad.
 

ATGM

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Yesterday I flew a rocket using an AT G53-7FJ reload and the ejection charge fired much too early. The propellant ignited and burned normally, but the ejection charge fired only about 2 seconds after the propellant burned out. At that point the rocket was about 400-500 ft up and moving at high speed. Fortunately there was only minimal damage to the rocket which I have already repaired.

I don't understand the reason for this failure. I am sure I assembled the motor correctly as I have done dozens of times before with reloads using 29/40-120 cases. I used the 7-sec delay element as supplied with the motor and I did not drill it. The burned-out motor and casing appear normal to me without damage or deformities, and the delay element and propellant were fully burned. I have I entered this info in the NAR database of motor failures.

I would be happy to know if anyone has any insights as to why there was a 2 sec delay instead of 7.
It sounds to me like your delay element burned along the spiraling of the casting tube. Path of least resistance, etc.
 
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