AT 29/40-120 early deployment issue

[byoungblood]

On my first two flights with a 29/40-120 case I picked up here a few months back I've had two consecutive early deployments with the motor. First was with a G53J, second with a G64W, both running 10 second delays, undrilled.

First flight with the G53, the 'chute deployed almost immediately after motor burnout. Fortunately no damage to anything, (FG Darkstar Jr) but as I disassembled the motor, there looked like scorch marks alongside the delay liner, but it didn't appear to propagate all the way up to the delay o-ring.

Second flight this past weekend, on the G64, the 'chute deployed while the motor was still burning, shredding the shock cord. Chute and nosecone came down on its own, the body plopped down (fortunately away from everyone) in the plowed field, remarkably with only a tiny zipper to show for it. Post flight, this time there showed no signs of flame around the delay liner. However, the body tube was filled with dark black soot, like the ejection charge didn't burn fully. Delay o-ring was intact.

Thinking a problem with the delay well, I measured it with a set of calipers and found no obvious anomalies. However, all of the delays that I've put into this case have felt a little loose and have required a wrap of tape to get them snug. I've got another load for the case, but am a little reluctant to fire this motor again.

Any suggestions, thoughts?

 

[rharshberger]

On my first two flights with a 29/40-120 case I picked up here a few months back I've had two consecutive early deployments with the motor. First was with a G53J, second with a G64W, both running 10 second delays, undrilled.

First flight with the G53, the 'chute deployed almost immediately after motor burnout. Fortunately no damage to anything, (FG Darkstar Jr) but as I disassembled the motor, there looked like scorch marks alongside the delay liner, but it didn't appear to propagate all the way up to the delay o-ring.

Second flight this past weekend, on the G64, the 'chute deployed while the motor was still burning, shredding the shock cord. Chute and nosecone came down on its own, the body plopped down (fortunately away from everyone) in the plowed field, remarkably with only a tiny zipper to show for it. Post flight, this time there showed no signs of flame around the delay liner. However, the body tube was filled with dark black soot, like the ejection charge didn't burn fully. Delay o-ring was intact.

Thinking a problem with the delay well, I measured it with a set of calipers and found no obvious anomalies. However, all of the delays that I've put into this case have felt a little loose and have required a wrap of tape to get them snug. I've got another load for the case, but am a little reluctant to fire this motor again.

Any suggestions, thoughts?

First flight possibly was a classic example of drag separation.
Second flight could be a motor problem, without pictures it might be hard to diagnose. The delay grain fit was my first guess, but you had already taken the precaution of the tape wrap.

 

[SDramstad]

Make sure the forward closure is tight. They had problems about a year ago with loose fitting forward closures.

 

[GregGleason]

A couple of weekend's ago I flew a G64-10W. I noticed the delay liner was a bit loose in the forward closure, to the point I took remedial action. I took some tape (can't remember if it was masking or blue tape) and wrapped it around the delay liner, cutting the tape so that there were no gaps. Gaps would provide a leak path so I went for an edge-to-edge for the tape. I put the delay grain assembly in and it was snug and seated well.

The flight turned out fine. I'm not sure what would have happened if I had not applied the fix.

Let me say that if I were not an experienced flyer of the 29/40-120 case I wouldn't have been none the wiser and flown it as designed.

Greg

 

[byoungblood]

First flight possibly was a classic example of drag separation.
Second flight could be a motor problem, without pictures it might be hard to diagnose. The delay grain fit was my first guess, but you had already taken the precaution of the tape wrap.

More I think on it, I think you're probably right on the first one. Nose cone isn't rattling around, but if the rocket is turned upside down and shaken slightly sometimes the nose cone will slip a little. The NC just has such a long shoulder and it feels no differently than some of my other rockets I just haven't given it a second thought. I'll put a wrap of tape on it next time.

I've already cleaned up the case, but outside of all of the soot in the body tube, I'm kind of at a loss. I've almost considered sacrificing a reload and doing a ground test just to see if it acts right.

I've probably launched my 24/40 case 20 or so times now so I'm familiar with the assembly on the hobby line cases, but still follow the instructions every time. It is a used Rouse Tech case, anyone heard of any issues with their 29/40-120s?

 

[Handeman]

I think both flights were affected to varying degrees by the same issue, too loose of delay elements.

I had two G64W motors in a cluster that had blowby of the delay element and both melted the powder well off the end of the forward closures while opening the touch hole to a larger nozzle size. Of course with the flame coming out of both ends of the motors, it destroyed both forward closures and the rocket. I will say Aerotech was great about replacing the closures and the reloads.

The problem is that all the reloads I bought have the same issue, the delay grain is very loose in the well. Once you put it in, if you tip the closure over, the delay element will fall out. I HIGHLY recommend the solution that Greg described and I use. Wrap a piece of 3/4 masking tape around the delay before inserting. I overlap the tape, use and exacto knife to cut through both layers and then pull the end pieces out so the tape lays together without a gap. Again, as Greg described. I haven't had a failure since I started doing that.

Just a note, I haven't bought a new 29/40-120 reload in a couple of years so I don't know if that problem with the loose grains has been fixed or not.

 

[cerving]

I've had the same issue with 29/40-120's, loose delay elements. If it doesn't fit snugly, wrap some masking tape tightly around the delay and trim to fit. Note that the loose delay issue can cause issues even if you don't have any BP in the well, due to propellant gas leakage; the problem that I had was on a DD flight without a BP charge.

 

[rcktnut]

I've been using cellophane tape to tighten up the delays. I guesstimate what I need and if too loose add more. No need to worry about overlap, all motors worked good.

 

[Handeman]

I've been using cellophane tape to tighten up the delays. I guesstimate what I need and if too loose add more. No need to worry about overlap, all motors worked good.

Interesting. do you have issues with the cellophane tape melting in the delay well?

I trim the masking tape because it usually gets too tight if there is much overlap. Is that much less of an issue with the cellophane?

 

[rcktnut]

Interesting. do you have issues with the cellophane tape melting in the delay well?

I trim the masking tape because it usually gets too tight if there is much overlap. Is that much less of an issue with the cellophane?

No problems with melting, spent delays came out clean as if no tape even used. Never even thought about overlaps, my last one I remember adding 3 separate pieces of tape before getting a proper fit. Anything that hangs over the fore and aft edges I trim flush with a scissors.

 

[bill_s]

"Once you put it in, if you tip the closure over, the delay element will fall out." -- I thought that was normal, every reload I ever flown has been like that and I've never done anything about it or had a problem. (Knock on wood) ... I do always leave the forward closure a bit loose until the rear is on.

P.S. anything about the delay and its spacer assembly? It is always the hardest part about assembling the things. Also the o-rings and keeping everything clean.

 

[byoungblood]

"Once you put it in, if you tip the closure over, the delay element will fall out." -- I thought that was normal, every reload I ever flown has been like that and I've never done anything about it or had a problem. (Knock on wood) ... I do always leave the forward closure a bit loose until the rear is on.

P.S. anything about the delay and its spacer assembly? It is always the hardest part about assembling the things. Also the o-rings and keeping everything clean.

Never had one so loose that it would fall out on its own until this 29mm case. My 24mm and 29/38mm HPR cases they've always been a relatively snug fit.