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AT 29/40-120 Failure

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cerving

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Ok, here's a puzzler...

I had an AT Hobbyline 29/40-120 F52 toast the inside of my Madcow GX3 about 1/2 second after ignition yesterday. There was no BP in the well, as it was flying DD, and the well was taped over to prevent the delay grain burn from getting into the body tube (presumably well after burnout). Took apart the motor afterwards...

o Everything was there, and in the right order (i.e. the delay grain o-ring was against the housing)
o Both closures were tight
o The delay grain was definitely facing the o-ring
o The delay o-ring was toasted, so propellant gases definitely breached the delay grain/o-ring seal
o Both of the propellant grain o-rings were intact
o The washer was essentially intact (as much as could be expected from the burn...)
o The kink in the igniter where I bent it and taped it to the nozzle coincided with the top of the grain... it butted up against the taped-over slot (since it was a single-grain "F" load)
o The motor had been thoroughly cleaned and all o-rings greased prior to assembly the night before

I don't see any reason why this should have happened. I've probably flown at least 100 HL loads, and this is the first time I've ever had this happen. Any ideas? Thanks!
 

rharshberger

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How loose was the delay assembly going into the forward closure? Sometimes they are a loose enough fit that a wrap of tape is needed around the delay grain insulator to make it snug in the well.
 

rcktnut

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How loose was the delay assembly going into the forward closure? Sometimes they are a loose enough fit that a wrap of tape is needed around the delay grain insulator to make it snug in the well.
+1 Been having to snug some of the delays up lately, again something we should not have to do.
 

markkoelsch

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Pictures might be helpful. I would recommend sequential pictures when disassembling the motor in case the manufacturer asks in an effort to help determine the cause.
 

sl98

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Did you use any dog barf in this section? I've had dog barf that was not fully ejected smolder and scorch the inside of a BT.
 

Cl(VII)

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If the gases breached the delay grain o-ring they are getting out the forward closure. I have had some loose fitting delay grains too. I just grease the heck out of everything, and haven't had a problem. Should probably build them up with a little masking tape too.
 

CzTeacherMan

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If the gases breached the delay grain o-ring they are getting out the forward closure. I have had some loose fitting delay grains too. I just grease the heck out of everything, and haven't had a problem. Should probably build them up with a little masking tape too.
Yep. I wrap with tape then grease. Tape for fit, grease for seal.
 

djs

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When you say "toast the inside" did it blow the forward closure, or something else happened? Also, was it a stock ignitor?
 

samb

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... I have had some loose fitting delay grains too. I just grease the heck out of everything, and haven't had a problem. Should probably build them up with a little masking tape too.
Yep. I wrap with tape then grease. Tape for fit, grease for seal.
Ok I'm having trouble visualizing this. I always thought, and built accordingly, that any grease around the delay element was to be devoutly avoided. But this is a timely thread because the OP's description sounds alot like my F40 problem last Sunday at NARAM. However I don't recall the delay grain being noticeably loose. I guess it wouldn't take much of gap for hot gases to escape. But that's what o-rings are for, I thought.
 
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djs

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Grease around the delay is ok- it's when you get grease on the delay grain surface itself (front and back). Then you have the problem of the delay grain potentially not igniting.
 

CzTeacherMan

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Grease around the delay is ok- it's when you get grease on the delay grain surface itself (front and back). Then you have the problem of the delay grain potentially not igniting.
+1... Grease around the outside of the delay liner, not on the delay itself.
 

rharshberger

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Ok I'm having trouble visualizing this. I always thought, and built accordingly, that any grease around the delay element was to be devoutly avoided. But this is a timely thread because the OP's description sounds alot like my F40 problem last Sunday at NARAM. However I don't recall the delay grain being noticeably loose. I guess it wouldn't take much of gap for hot gases to escape. But that's what o-rings are for, I thought.
A snug fitting insulator partially keeps gas jets from traveling up the gap between the delay insulator and the delay/case wall, if its loose the gas jet is allowed direct access to the o-ring. Somebody else may have a better description than this but its the general idea.
 

cerving

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Literally toasted and blistered the entire inside of the body tube as the propellant gases escaped out the ejection hole, melted the nylon shock cord that was mounted to the centering ring (anti-zipper mechanism... the AV bay ejection was supposed to be on the other side), and blew it apart at about 50' AGL. The motor was completely intact as far as I could tell. I like the idea of the delay grain being a bit too loose in the housing, it certainly wasn't that tight. Guess that's one more thing I gotta watch out for...

Oh, and it was the stock igniter. I was a little surprised... it was really skinny, like the 24/40 igniters.


When you say "toast the inside" did it blow the forward closure, or something else happened? Also, was it a stock ignitor?
 
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Kruegon

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Based on the description, I'd agree that the most likely suspect would be a loose fitting delay grain.

Also of note here:

Grease on the delay grain surface is only a concern when using motor eject. When using a plugged closure on HP cases, the instructions now actually tell you to fill the forward delay neoprene washer with grease.

I often fly my 29/40-120 with no ejection. But I pour the BP into a tube and place the red cap on top anyway. Between that and taping my delay grain, I've avoided any gas leaks so far. Perhaps filling the forward area on a 29/40-120 with grease, like we do on HP may need to be investigated. Could be the answer to avoiding this in the future. Of course it's only an option when not using motor eject. And it will increase clean up time and effort.
 

tbonerocketeer

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Based on the description, I'd agree that the most likely suspect would be a loose fitting delay grain.

Also of note here:

Grease on the delay grain surface is only a concern when using motor eject. When using a plugged closure on HP cases, the instructions now actually tell you to fill the forward delay neoprene washer with grease.

I often fly my 29/40-120 with no ejection. But I pour the BP into a tube and place the red cap on top anyway. Between that and taping my delay grain, I've avoided any gas leaks so far. Perhaps filling the forward area on a 29/40-120 with grease, like we do on HP may need to be investigated. Could be the answer to avoiding this in the future. Of course it's only an option when not using motor eject. And it will increase clean up time and effort.
I've used no motor ejection countless times, and never filled the closure with grease.
 

Kruegon

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I've used no motor ejection countless times, and never filled the closure with grease.
I have as well. I was just offering something for consideration if people continue to see this issue. We should look for anything that can continue to keep our hobby safe and off the government radar. If blow your issues persist, perhaps this could be a next step.

I've always made sure that my delay grains a lightly snug in the closure. It's been enough for me so far. But if others continue to see issues it may warrant us looking at alternative solutions.
 

cerving

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Too bad AT doesn't offer a plugged forward closure for the HL motors... I guess 99% of the people that use them wouldn't need it. I've been tempted a few times to plug the ejection hole with a self-tapping screw, but that would void the certification on the motor.
 

KenRico

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My hobbyline RMS 29 40-120 is showing its age... threads feel gritty and can see some blistering inside too.

Will keep flying til it fails and buy a new one .. on the flipside my RMS cases are aged but great ..

Kenny
 
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Kruegon

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Too bad AT doesn't offer a plugged forward closure for the HL motors... I guess 99% of the people that use them wouldn't need it. I've been tempted a few times to plug the ejection hole with a self-tapping screw, but that would void the certification on the motor.
There's a method listed for plugging a HL case with JB Weld. I believe this was in regards to the 18/20 or 24/40 case. And it's via instruction directly from AT. I wonder if the 29/40-120 case has ever been tested by AT. If so, a manufacturer's authorization for doing such would remove it from the experimental list and move it to a commercially approved method.
 

cerving

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There's a method listed for plugging a HL case with JB Weld. I believe this was in regards to the 18/20 or 24/40 case. And it's via instruction directly from AT. I wonder if the 29/40-120 case has ever been tested by AT. If so, a manufacturer's authorization for doing such would remove it from the experimental list and move it to a commercially approved method.
Hadn't heard that this was direct from AT... I'll have to look into it. I got plenty of JB Weld handy...
 

rharshberger

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Hadn't heard that this was direct from AT... I'll have to look into it. I got plenty of JB Weld handy...
Heres the link, but it only refers to the 18/20 case and endburning loads. http://www.aerotech-rocketry.com/news.aspx?y=2013

IMO permanently plugging the charge well does not constitute modification of the motor as the motor will continue to function as it should minus an ejection charge, really there is little difference between filling the charge well with dog barf and taping over it and JB Weld, they both have the same effect.
 

grouch

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Yea but you are being logical. Some guy somewhere who is illogical will say that it's tampering with how the motor was designed and intended to be used and therefore modified. Then there will be a 7 page argument on it where three or four people will keep saying the same things over again only louder each time until someone finally post a picture of a bunny with a pancake on it's head with great anticipation of responses to his clever post only to check back several times to see that the thread is at the bottom of the pile.
 

cerving

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Heres the link, but it only refers to the 18/20 case and endburning loads. http://www.aerotech-rocketry.com/news.aspx?y=2013

IMO permanently plugging the charge well does not constitute modification of the motor as the motor will continue to function as it should minus an ejection charge, really there is little difference between filling the charge well with dog barf and taping over it and JB Weld, they both have the same effect.
That link goes back to 2013... I don't think they've released a plugged closure for the 18/20. I'll ask AT if the JB Weld plug constitutes an approved method for plugging the 24/40 and the 29/40-120 as well.
 

crossfire

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I can't believe plugging the charge hole would a problem. So if one wraps tape around the delay grain or peels paper off a motor grain isn't that making changes to a certified motor? I don't think so. No need to make a problem out of nothing.
 

djs

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Forward closure questions aside- Cris- can you post pictures of the aftermath? Are you filing a MESS report and/or contacting AT?
 

grouch

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Well played djs. I have effin tears in my eyes. People at work on the same conference call are looking at me sideways. They know I am not paying attention. Damn bunny pancake.
 

boatgeek

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What if you just put a bit of masking tape over the JB Weld and claim it's dog barf under there if anyone asks? I've never seen an RSO pull apart a LPR/MPR rocket before, so it should just fly under the radar. If the worst happens and somebody disassembles the motor after flight, look really puzzled at the JBW and say, "Damn, I've never seen it do THAT to dog barf before..." :)

+1 for pancake bunnies.
 

byoungblood

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At my last launch, I had an early deployment on a G53, roughly about two seconds after launch based on the video I took. Its delay element was a bit loose, it would slip out if I lightly shook the closure. I tried putting a wrap of masking tape around it, but then it wouldn't fit. I took a couple of other delays and they fit normally in the same closure. Next time I guess I'll try a wrap of cellophane tape if I run across another loose delay.
 
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