Aerotech K700 54-2560 ruptured during flight

[JLebow]

I just completed my first year of high power rocketry, flying with Oregon Rocketry. In 14 flights, my rockets climbed a total of 65,000 feet. I’m hooked.

I had an issue with one of my flights last weekend. I launched my Madcow/Rocketry Warehouse Adventurer 3 (7 feet, 3 inch diameter) with an Aerotech 54-2560 and a K700 white lightening. This was the 4^th flight for the rocket, but the first flight ever for this case. I was expecting about 8,000 feet. I reached 5,000 feet on a K513 a few months ago. The motor was low on power right from the start. After 15 seconds I reached apogee at 3,600 feet, and the drogue chute deployed, while the motor was still burning AP. At 45 seconds, a good amount of smoke was still bellowing and I was worried I was going to need to use the fire extinguisher, but at 60 seconds the motor finally was out, the main deployed and the rocket landed softly.

The motor casing ruptured. The hole is 1” x 0.75”, located 4.5” from the aft closure. Rotated 180 degrees and also 4.5” from the aft closure is a 1” round dimple, about 0.5” deep. There is a crack running the full 360 degrees around the casing, running through the dimple and rupture, with very thin sections of aluminum barely keeping the casing from becoming two pieces. The damage is located about where grain 1 and 2 join up. About 1.5” to 9.5” from the aft closure (centered on rupture) is heat induced discoloration. The middle 7 inches is flat black, transitioning to a golden straw color at both ends of the heat zone. Inside the motor, the phenolic liner is completely burned away in the same sections that show the discoloration. In one of the pictures you can see about an inch of the liner still intact right next to where the nozzle was located. At the forward end, the seal disk has been sucked into the casing by about 4”.

There was nothing unusual that I noticed about the motor assembly process. I can verify that both closure o-rings were installed and they appear to still be intact. All 6 gains fit properly in the liner (no slop and not a tight fit between the grains and the liner). The seal disk was a good fit, with the seal disk o-ring requiring just the right amount of force to seat snugly. The grains were flush to the liner, and the aft closure engaged with one rotation to go to get the aft closure fully screwed down.
The senior club members were not sure exactly what happened, but the leading guess is that the ignitor slipped down to the lower grains during ignition, and the rest of the grains were delayed in starting, and burned for a long time under low pressure. Is it possible for some grains to burn, without the others igniting in short order? Anyone have any ideas?

What information should I include when I ask Aerotech for their opinion?

 

[ChrisAttebery]

I'd contact AT ASAP. Don't disassemble it until you talk to them. Karl may want to see it as is.

 

[AdAstraPerAspera]

I saw an igniter slip to the aft end of a large HP motor before, it wasn't pretty. The motor never even got enough pressure to lift the rocket of the pad. It sat and burned the back end of the rocket. Aluminum dripped out on to the ground as the case melted. I believe this was at an LDRS in upstate NY years ago.

 

[Kallahan11]

Did the kit have O-ring grain spacers and if so did you use them? I know on my M1600 it came with them but the instructions did not say anything about them. I knew what they were and what they were for so I installed them and everything worked well. I bring this up because it happened at a grain interface and if there is not enough space between the grains you can torch holes through the case. Make a warranty claim with Aerotech and fill out a MESS report https://www.motorcato.org/

 

[Nick@JET]

That looks like the liner was cracked or weak spot to me, looked just like my 98mm.

I flew an M1939W and then liner looked like it had a Molding mark and I didn’t think it was a crack but that was the spot of the burn through and blistered the case. I could barely get it out of the motor mount- AT was great about it - Charlie and Karl took care of right away and now I closely inspect my liners.



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[djs]

That looks like the liner was cracked or weak spot to me, looked just like my 98mm.

I would agree with this, plus the discoloration up the side of the case looks like some sort of backside burning.

Grain spacers being left out wouldn't have caused this, in my opinion.

 

[ColumbiaNX01]

thats sucks, but sounds like a personal problem. just kidding contact aerotech.

 

[JLebow]

Thanks everyone for responding. I just sent off an email to Aerotech.

Did the kit have O-ring grain spacers and if so did you use them? I know on my M1600 it came with them but the instructions did not say anything about them. I knew what they were and what they were for so I installed them and everything worked well. I bring this up because it happened at a grain interface and if there is not enough space between the grains you can torch holes through the case. Make a warranty claim with Aerotech and fill out a MESS report https://www.motorcato.org/

This reload did not have grain spacer orings. There was a cross section drawing of a motor in the kit that did show grain spacer orings, but a note stated that grain spacer orings are not used on all motors. The spacer orings are the same size as the forward seal disk, only one oring of that size was in the kit. Also, with the seal disk installed in the liner, the six grains fit with the last grain flush with the other end of the liner. Any oring spacers in this reload would have resulted in the grains sticking a fair way out of the liner, and it would have been impossible to start the threads on the aft closure.

I'll check out the motorcato site. I'd never heard of it before. Luckily I hadn't needed to.

 

[tfish]

Also, with the seal disk installed in the liner, the six grains fit with the last grain flush with the other end of the liner. Any oring spacers in this reload would have resulted in the grains sticking a fair...

Was there room left in the liner for the nozzles's shoulder? The above makes me think, No.

Tony

 

[markkoelsch]

Thanks everyone for responding. I just sent off an email to Aerotech.



This reload did not have grain spacer orings. There was a cross section drawing of a motor in the kit that did show grain spacer orings, but a note stated that grain spacer orings are not used on all motors. The spacer orings are the same size as the forward seal disk, only one oring of that size was in the kit. Also, with the seal disk installed in the liner, the six grains fit with the last grain flush with the other end of the liner. Any oring spacers in this reload would have resulted in the grains sticking a fair way out of the liner, and it would have been impossible to start the threads on the aft closure.

I'll check out the motorcato site. I'd never heard of it before. Luckily I hadn't needed to.

Yes, fill out the MESS report. It send the failure info to the three testing organizations. If you have questions about the site or info needed shoot me a PM.

I had a liner fail on my first L3 attempt. CTI M1830. Blew a hole through the casing, motor mount, Aeropack tailcone retainer. The hole in the casing was 2"X1.5". I feel your pain. I fixed the rocket (a little shorter) and L3 achieved.


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[markkoelsch]

Was there room left in the liner for the nozzles's shoulder? The above makes me think, No.

Tony

Great question Tony


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[AdAstraPerAspera]

Was there room left in the liner for the nozzles's shoulder? The above makes me think, No.

Tony

Aerotech uses axial compression, and has no shoulders to fit inside the liner, unless you count the seal disk.

 

[tfish]

Aerotech uses axial compression, and has no shoulders to fit inside the liner, unless you count the seal disk.

I agree with you on this. But only the 29mm and smaller nozzles don't have a shoulder. This thread is about a 54mm motor. The nozzles on AT 54 mm motors do have a shoulder that needs to fit (all the way) inside the liner.

Tony

 

[jd2cylman]

I just assembled a K700 for MWP. The nozzle did indeed go into the liner and fully seats. There are not any grain spacers provided. With the nozzle fully seated in the liner the seal disc seats at the other end with a little space. Any added o-ring spacers would make the grains too long to fit in the seal disc. As to the carnage above... I hope my motor fares better... No offense meant to the OP.

 

[3stoogesrocketry]

I just assembled a K700 for MWP. The nozzle did indeed go into the liner and fully seats. There are not any grain spacers provided. With the nozzle fully seated in the liner the seal disc seats at the other end with a little space. Any added o-ring spacers would make the grains too long to fit in the seal disc. As to the carnage above... I hope my motor fares better... No offense meant to the OP.

From the OP , I would agree that the motor ignited at the bottom of the motor . Also seeing as it was a very slow burn , im guessing the first and second grain ignited at the interface , and that caused the liner to burn through and destroy the case . I would be interested to know what the nozzle throat size was after the burn . Is it possible a K1275 nozzle was supplied by mistake ? Im guessing no , and leading to a improper ignitor ignition . Stuff happens .

Eric

 

[JLebow]

Was there room left in the liner for the nozzles's shoulder? The above makes me think, No.

Tony

I must have remembered this detail incorrectly. This picture shows a thin clean spot just inside the liner that lines up with the proper depth of the nozzle shoulder. The really clean section lines up with the aft oring.

I just assembled a K700 for MWP. The nozzle did indeed go into the liner and fully seats. There are not any grain spacers provided. With the nozzle fully seated in the liner the seal disc seats at the other end with a little space. Any added o-ring spacers would make the grains too long to fit in the seal disc. As to the carnage above... I hope my motor fares better... No offense meant to the OP.

No offense taken. I hope your flight is great. I posted here to learn from folks with a lot of experience, and that is what is happening.

From the OP , I would agree that the motor ignited at the bottom of the motor . Also seeing as it was a very slow burn , im guessing the first and second grain ignited at the interface , and that caused the liner to burn through and destroy the case . I would be interested to know what the nozzle throat size was after the burn . Is it possible a K1275 nozzle was supplied by mistake ? Im guessing no , and leading to a improper ignitor ignition . Stuff happens .

Eric

I think your theory makes the most sense to me. I am waiting to hear back from Aerotech. The diameter of the used nozzle is 0.55" or just under 14 mm.

 

[cherokeej]

Please lengthen your message to three characters.

OK.

Bugs Bunny, Daffy Duck, and Quickdraw McGraw.

There. Three characters.

Don't light the bottom of the motor.

 

[markkoelsch]

One thing you may want to consider for future flights. I larger and longer motors it is common practice to tape the igniter to a thin dowel and insert it into the motor. This should keep it in place long enough to properly ignite the motor.


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[AdAstraPerAspera]

I agree with you on this. But only the 29mm and smaller nozzles don't have a shoulder. This thread is about a 54mm motor. The nozzles on AT 54 mm motors do have a shoulder that needs to fit (all the way) inside the liner.

Tony

My bad, I just flew a 38mm this past weekend and it had no shoulder. I forgot that the larger motors do have one. It's been awhile since I flew anything that big on Aerotech.

 

[Loki Research]

Sorry to stick my nose in here, but I'm just trying to help. Did the rocket sit with the motor burning on the pad for several seconds before it lifted off? If it did, then I would agree that the igniter slipped down. If the igniter slipped down to the faces of the 1st & 2nd grain, it would take several more seconds for enough propellant to light in order to produce the required thrust to lift the rocket off the pad.

If it took off normally, shortly after the button was pushed, and then burned for a really long time, it is much more likely that the liner failed shortly after ignition which put the holes in the case. Then, having much lower chamber pressure, it took much much longer to consume all the remaining propellant. I ask because I don't recall reading how long the rocket sat on the pad.

 

[JLebow]

Sorry to stick my nose in here, but I'm just trying to help. Did the rocket sit with the motor burning on the pad for several seconds before it lifted off? If it did, then I would agree that the igniter slipped down. If the igniter slipped down to the faces of the 1st & 2nd grain, it would take several more seconds for enough propellant to light in order to produce the required thrust to lift the rocket off the pad.

If it took off normally, shortly after the button was pushed, and then burned for a really long time, it is much more likely that the liner failed shortly after ignition which put the holes in the case. Then, having much lower chamber pressure, it took much much longer to consume all the remaining propellant. I ask because I don't recall reading how long the rocket sat on the pad.

It just so happens that my friend captured the launch and some of the flight on video. The camera was on a tripod aimed at the pad, and midway into the flight he picked up the camera and caught the second half of the flight. I posted the video on YouTube.

[video=youtube;etpfymZb2jE]https://youtu.be/etpfymZb2jE[/video]

There was only about 1 second delay from first smoke to lift off.
25 seconds into the flight, the rocket reaches apogee and the drogue chute deploys
36 seconds after lift off, fire was still visible (unfortunately not captured on video) from the rocket. Someone in the crowd shouts, "It's still burning. The motor is still burning."
48 seconds (1:11 of the video) the motor makes a crazy bellowing noise.
1:09 main deploys, still lots of smoke
1:23 smoke reduces greatly

 

[mpitfield]

After reviewing your video, it appears to my eye that the boost right off the pad seems low for that config. I launched my 3" Darkstar, which was close in specs to yours, a 79" rocket weighing 14.5 lbs, and it shot off the pad with noticeably much more intensity. I would say that whatever happened started almost immediately and likely got worse as the flight progressed. I would have thought that a liner defect would have taken at bit of time to have any depreciable effect on the case, at least not instantly for that motor and case.

Below is a link to the flight. It only contains on-board video and stills, but to my eye you can tell that it achieves a relatively high velocity much more rapidly than what yours did. For comparison, you can see the flight and rocket specs in the credits, yours would have likely achieved something similar had the flight been nominal.

[video=youtube;N4L0TyW7xV8]https://www.youtube.com/watch?v=N4L0TyW7xV8[/video]

 

[JLebow]

After reviewing your video, it appears to my eye that the boost right off the pad seems low for that config. I launched my 3" Darkstar, which was close in specs to yours, a 79" rocket weighing 14.5 lbs, and it shot off the pad with noticeably much more intensity. I would say that whatever happened started almost immediately and likely got worse as the flight progressed. I would have thought that a liner defect would have taken at bit of time to have any depreciable effect on the case, at least not instantly for that motor and case.

I can confirm your observation with some data. I have video of this rocket on three motors. Playing back in slow motion, frame by frame, and capturing the time to traverse the 10 foot launch rail, I have an average speed for the first 10 feet of flight. This is not the speed at the top of the rail, but the average speed over ten feet, starting with the rocket at rest on the pad. Speed off the rail could be estimated at twice the average velocity.

K700 - 21 MPH, Apogee at 3659 ft
K513 - 47 MPH, 5278 ft
J415 - 27 MPH, 4289 ft

 

[Loki Research]

it is much more likely that the liner failed shortly after ignition which put the holes in the case.

Just to clarify my statement. The liner of course did fail which resulted in the case damage, but I wasn't trying to imply that the liner was the initial cause of the motor failure. It does appear from the video however that it was not a misplaced igniter.

 

[JLebow]

Quick update. I just heard back from Karl at Aerotech. He says that it was "definitely cause by a cracked or defective liner." They are replacing the case and the reload. I can't say enough good things about the warranty department at Aerotech.

Now to search the forum on rebuilding the motor mount tube in my rocket. The filament wound epoxy tube is now mostly just filament.

 

[JLebow]

And more photos taken by club members turned up online. Here they are.