Yet another tale of motors going boom.

[Mikus]

I had a new anomaly this weekend too. A 24 MM F36 in my Executioner, well... didn't work so good.

Same deal, immediate launch followed by an almost immediate chute deployment. Rocket cartwheeling thru the sky under thrust zippering as it went.

No casualties other than to my pride and the bird. All motor bits were accounted for after motor assembly but I could have put the delay in backwards I guess. The casing looks and cleaned up fine, but I suspect the flightworthyness of the forward casing as it was very sooty and now discolored. The delay liner was as you can see, a bit charred.

The resultant condition of the motor mount was a bit of a surprise after opening up the rocket. At least the casing stayed in place as evidenced by the crushed motor mount tube and blown aft centering ring. But the motor mount blown into two??? *scratches head*

So experts, what did I do wrong???

 

[troj]

The resultant condition of the motor mount was a bit of a surprise after opening up the rocket. At least the casing stayed in place as evidenced by the crushed motor mount tube and blown aft centering ring. But the motor mount blown into two??? *scratches head*

So experts, what did I do wrong???

The motor mount wasn't blown in two; it was burned in two.

Check the hole in the forward closure -- my guess is it's larger than how it started out, due to erosion. The motor was spewing thrust out both ends, which turns the forward closure into a small, inefficient, nozzle, and erodes the heck out of that opening.

As for what went wrong, it could be an error in delay assembly, it could be a problem with components, it could be a void within the delay itself.

-Kevin

 

[Mikus]

Yes it is larger and somewhat irregular in shape as well. :y:

Check the hole in the forward closure -- my guess is it's larger than how it started out, due to erosion. The motor was spewing thrust out both ends, which turns the forward closure into a small, inefficient, nozzle, and erodes the heck out of that opening.

 

[blackjack2564]

I can't tell for sure, but it looks like the delay spacer was on the O-ring side of the delay. It is supposed to be on the lower side towards the grain, pushing the delay face tight to the front of the closure. If this is the case, it would account for what happened. There would have been a gap that allowed the BP to fill the space and poof!.

 

[kelltym88]

Perhaps Gary at AT could modify the instructions by CAPITALIZING THAT SEGMENT. NOTE: MAKE SURE THE DELAY GRAIN SPACER FACES DOWN TOWARDS THE AFT PORTION OF THE CASING. Or something like that.

Bummer dude, that was a really nice paint job on the Executioner. Love the Metallic Blue.

 

[Chrisn]

Perhaps Gary at AT could modify the instructions by CAPITALIZING THAT SEGMENT. NOTE: MAKE SURE THE DELAY GRAIN SPACER FACES DOWN TOWARDS THE AFT PORTION OF THE CASING. Or something like that.

Or improve the drawings, which are supposed to speak 1000 words

 

[Pem Tech]

Way back when, just after getting back into the hobby I had a somewhat similar problem with a 24/40 case. The first two attempted to flights on the case ended the same as you described. The fore closure was slightly discolored but it was otherwise unharmed. Also roasted a beautiful 6" scratch built Friede doing ejection tests.
:shock:

The only abnormality before flight was that the fore and aft closures were very difficult to screw into place. After picking up a Aerotec closure wrench, and screwing the closures fully into place, there have been no problems. Now, I don't know if this has anything to do with your poor toasted rocket but I hope it does help.

 

[JAL3]

Also roasted a beautiful 6" scratch built Friede doing ejection tests.
.

I wondered whatever happened to that one.

 

[Pem Tech]

I wondered whatever happened to that one.

The 6" Friede is on display in my office at work, she is doomed to being a shelve Queen.

 

[TheAviator]

Your casing should be fine. The 29-40-120 casing I bought about two years ago had a CATO on it's first launch because I misplaced the forward seal disk. 3 foot brilliant white flames out the top of a LOC Norad. It was cool, and I recovered most of the rocket in tact, so I didn't lose much. The casing has flown on a variety of motors since, including G64's, G76's, and F40's. So, by my experience your casing should be fine to fly again.

 

[hardinlw]

If the delay assembly is installed with the spacer forward, then all of the black powder can filter down into the gap thus created. When the delay element burns through, it will just go poof out the nozzle and not deploy the chute. Installed as intended, most of the BP stays in the well and pops the chute out.

The damage indicates that significant hot gasses (way more than the ejection charge) were vented forward. This would happen if somehow the delay element burned through while the propellent grain was still burning and allowed the forward closure to act like a nozzle as someone has already suggested.

 

[ttabbal]

re: delay installed wrong...

I did that on a 54/426 redline motor. Not pretty. Burned most of the lower section of the rocket, and on it's first flight no less. I was not a happy camper. I've become much more zen about such things as I've flown, and crashed, more rockets.

At least that's what I THINK happened, no way to prove it from what was left of the delay section. Be happy you did it to a 24mm case, I think that CATO cost me about $150 when all was said and done.

Putting a warning about that in big red letters wouldn't be unappreciated. It's one thing I like about CTI motors over Aerotech. You can't really mess that up. But I do like building motors, it's one reason I play with Hybrids as well.

 

[Fade_to_Black]

I almost did that at GLRMR a couple of weeks ago. It's been so long since I built a 40/120 motor that I put the delay in upside down. I realized my mistake when I started to pour in the BP and it ran down into the forward closure. I quickly disassembled the motor and rebuilt it the right way. :blush: All worked perfectly after that.

 

[Mikus]

Thanks. She was one of my favorites too.

Bummer dude, that was a really nice paint job on the Executioner. Love the Metallic Blue.

 

[Mikus]

That is what it looks like, but how would this happen? Would/could a reversed delay assembly cause it?

I got the chute back in one piece, its not even scorched. And the autopsy shows that the fire damage is limited to the area around the ejection charge - I never saw any fire come out of the top of the rocket. :shock:

The damage indicates that significant hot gasses (way more than the ejection charge) were vented forward. This would happen if somehow the delay element burned through while the propellent grain was still burning and allowed the forward closure to act like a nozzle as someone has already suggested.

 

[shreadvector]

That is what it looks like, but how would this happen? Would/could a reversed delay assembly cause it?

I got the chute back in one piece, its not even scorched. And the autopsy shows that the fire damage is limited to the area around the ejection charge - I never saw any fire come out of the top of the rocket. :shock:

Without asking an expert like Gary from Aerotech, we can make a guess and we will either be correct or incorrect.

My guess: If the spacer is above the delay, when the motor comes up to pressure there will be a HUGE pressure in the combustion chamber. This will push against the delay. Normally, the delay is resting against the upper o-ring and the top of the delay housing inside the forward closure and it cannot deflect - it in fact seats and seals. If the spacer is above the delay, the massive pressure will push the delay up and into the spacer. This will crush the spacer and the o-ring will move, buckle etc. result: very immediate creation of a leak path for the propellant combustion gas ouot the top of the motor through the ejection charge hole in the forward closure.

My personal question is, how large can this hole erode before the forward closure is no longer safe to use? I would think that if the o-ring could still seat properly inside the delay well, it might still be safe.

 

[Garoq]

Without asking an expert like Gary from Aerotech, we can make a guess and we will either be correct or incorrect.

My guess: If the spacer is above the delay, when the motor comes up to pressure there will be a HUGE pressure in the combustion chamber. This will push against the delay. Normally, the delay is resting against the upper o-ring and the top of the delay housing inside the forward closure and it cannot deflect - it in fact seats and seals. If the spacer is above the delay, the massive pressure will push the delay up and into the spacer. This will crush the spacer and the o-ring will move, buckle etc. result: very immediate creation of a leak path for the propellant combustion gas ouot the top of the motor through the ejection charge hole in the forward closure.

My personal question is, how large can this hole erode before the forward closure is no longer safe to use? I would think that if the o-ring could still seat properly inside the delay well, it might still be safe.

If the positions of the spacer and delay are reversed the forward closure will leak immediately. This is one of the most common assembly errors, next to reversing o-rings with the HP 29mm reloads.

The hole can be enlarged and still seal, however I would be more concerned about the aluminum losing its strength from overheating.

 

[kelltym88]

Perhaps Gary at AT could modify the instructions by CAPITALIZING THAT SEGMENT. NOTE: MAKE SURE THE DELAY GRAIN SPACER FACES DOWN TOWARDS THE AFT PORTION OF THE CASING. Or something like that.

Gary,
Could this be done?

 

[Garoq]

Gary,
Could this be done?

Yes, but there are a lot of instructions to revise. :y: It would be have to be an ongoing project. We always thought the illustrations were pretty clear.

 

[DAllen]

No, I think AT instructions are fine albeit a tad wordy. The diagrams are fine IMHO although they seem a bit dark on occasion but that is an issue with the printing process rather than a illustration problem. When I d/l the instructions in pdf they look great.

-Dave

 

[Mikus]

We always thought the illustrations were pretty clear.

I looked again last night and they're crystal clear to me. I'm pretty sure that I had it in correctly but the results would appear to disagree. If I had built that motor at home the night before, like I usually do, then I'd be proclaiming a big fat "Nuh-uh". But I did build it in the field and am aware that mistakes are more likely out in the field.

Well it was my 1st reload incident in a whole bunch of launches so I'll keep an eye on it. Hopefully I won't have to talk about it happening to me again. :clap: