aerotech case blistering

Anyone who regularly uses the G138 able to comment on, either one in how many flights tend to blister the case, or how many of those reloads could be expected on a single case?

K1103?

I've flown a few of them...discolored the insides, outsides fine. They came up with something about taping the grain, I'd look up the exact fix.

Personally I think they went a bit too far on it. I'm done flying them, and I'm waiting to see what new stuff they come out with (ATGM has hinted at some new ones..)

The K1103X and other loads should not wear out a case.

The G138 is the oddball.

Yep, I had a G138T blister a casing. AT made good on it and sent me another.

Now it seems the fix for this is to wrap a single layer of masking tape around the forward end of the grain before inserting it into the casing.

Haven't flown one in awhile though.

After having a case dis-colored by a G138T, someone reminded me that, because there is no liner, the directions say to use a generous layer of grease on the outside of grain before loading it into the case. Since I started doing that, I have not had any problems with case blistering.

No problem with the 1103X.

You may recall a discussion on the subject in this thread: https://www.rocketryforum.com/showthread.php?121649-G138T-Blistered-Case&highlight=g138.

I still have a few G138 reloads and will likely use them with additional masking tape per the Aerotech notice that came out not long after my experience. Percentage-wise on blistering, I'm 1 for 2 so far.

This is good to know as I have an unused G138 in my motor box.

I had a K1103X destroy a seal disk. Melted the o-ring groove right off of one edge. As far as the G138, I had it blister a case, then Aerotech released a youtube video showing how to wrap it in tape to prevent case blistering.

Tonimus said:

I had a K1103X destroy a seal disk. Melted the o-ring groove right off of one edge. As far as the G138, I had it blister a case, then Aerotech released a youtube video showing how to wrap it in tape to prevent case blistering.

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Was there evidence of blow by at the liner/seal disk junction. Wondering if you had a good seal, faulty liner, etc? Do you have a picture of the damage?

No blow-by. The top part of the seal disk didn't show any odd coloring or damage.

That seems odd to me. If it did that you would think there had to be some flow at that location. It would have scorched the o ring on the seal disk, possibly the liner too.

Did you send a picture of this to Aerotech?

markkoelsch said:

That seems odd to me. If it did that you would think there had to be some flow at that location. It would have scorched the o ring on the seal disk, possibly the liner too.

Did you send a picture of this to Aerotech?

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Seal disks won't last forever

markkoelsch said:

That seems odd to me. If it did that you would think there had to be some flow at that location. It would have scorched the o ring on the seal disk, possibly the liner too.

Did you send a picture of this to Aerotech?

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While a leak will basically guarantee thermal damage, it is not necessary. If the propellant burns hot enough for a long enough time, it will damage exposed aluminum. The ridges that have melted on this seal disc are notable for being rather "peripherally located" with less thermal conductivity to the bulk of the seal disk. They are also exposed to the thermal radiation in contrast to the forward ridges on the other side of the o-ring. Therefore, these are the parts of the seal disc the will heat up the fastest during the burn.

ATs seal discs are somewhat odd cases, because they are the only aluminum pieces that are exposed to the chamber conditions without thermal protection. If I had to guess, this is because they were substituted by AT for the forward insulator at a later time and therefore there is not enough space for both the seal disc and the forward insulator. Because they are not significantly load bearing it is not a problem if they loose temper, so they can get hotter than the rest of the casing.

It appears that Propellant-X burns hot enough to approach the thermal limits of this aspect of ATs design. Makes me wonder how well Propellant-X would work in the similarly built 75mm (4G+) and 98mm casings, with their increased burn times.

Reinhard

markkoelsch said:

That seems odd to me. If it did that you would think there had to be some flow at that location. It would have scorched the o ring on the seal disk, possibly the liner too.

Did you send a picture of this to Aerotech?

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tbonerocketeer said:

Seal disks won't last forever

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I flew a M1939 for my level 3 flight. I used a older Dr Rocket 98/10240 case with a new seal disc. After the perfect flight, I was cleaning all the parts and found that my seal disc had a burned spot through the front lip and toasted the o-ring in that spot. Dr Reese looked at it, and said keep using it. However, I erred on the side of caution and bought a new one. If I can find it, I'll snap a pic and post it here.

Adrian

I remember one time it was posted (forgot where though) that Propellant X was shredding the phenolic nozzles when tested in the 75mm casings.:wink:

My guess is it contains quite a bit of aluminum which would raise the combustion temperature substantially.

Reinhard said:

It appears that Propellant-X burns hot enough to approach the thermal limits of this aspect of ATs design. Makes me wonder how well Propellant-X would work in the similarly built 75mm (4G+) and 98mm casings, with their increased burn times.

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That definitely sticks out at me as well. The only option in those sizes is going to be DMS, at least for the foreseeable future.

Yeah, I sent the pic to Aerotech and they sent a new one. I know they don't last forever, but there should be some sign of wear before it goes caput. This one was perfect before the flight. I remember the condition so well, because this was my first flight with a seal disk and I thoroughly inspected all parts and followed the directions to the letter.

Yesterday I flew a H250G-M in my never used Dr. Rocket 29/240 case. I had two problems with the motor:

1. Motor Ejected at 6.5 seconds, significantly earlier than the 10 second delay associated with the -M medium delay.
2. The Forward Seal Disk leaked hot gases and caused a slight blister in my never before used case. I'm sad about that.


Here's the AT Assembly Drawing for the H250G-M:
View attachment 299571

I inspected the components and found that the leak appeared to have occurred between the Forward O-ring and the Forward Seal Disk.
The Seal Disk O-ring did show signs of gas leakage, but not nearly as much as the Forward O-ring.


The Forward Seal Disk was dicolored and had some minor erosion right at the edge on the side that seals with the Forward O-ring:



The liner showed signs of gases leaking between it and the case wall.
The liner was shorter in length at the location that the seal failure occurred. Could this have been a manufacturing problem, or would this be the expected result to a liner where hot gases leaked past a seal? The side of the liner that did not leak measured just under 7 1/16", while the side of the liner where the leak occurred measured just under 7 1/32". The forward side of the liner cut looked slightly slanted.. which is in-line with my measurements.

Forward side of Liner where seal failure occurred:


Side view of Forward side of the liner. The left side was where the seal leaked hot gases. Notice that the left side is slightly shorter than the right side:


The seal failure resulted in a slight blister of my new never-been-used Dr. Rocket 29/240 case.
It's hard to detect, but here's a picture of the blister, which is 0.0075" per digital caliper measurement:


Inside the case, I have not been able to remove a black mark that is at the location of the seal failure. I suspect the black material is a mixture of AP and O-ring material.
I used digital calipers to measure the location of the edge of the leakage mark. It was 0.595" from the forward edge of the case:


Next, I measured the thickness of the components that were installed in the forward side of the case, the forward closure and Forward O-ring. I measured the distance as 0.599", which lines up perfectly with the black mark inside the case where the leak emanated:


This pretty much proves where the leak was. I'm certain that I used plenty of grease and that the O-rings were all well placed, sliding in smoothly. Both closures were fully seated and all felt normal during assembly. My prime suspect is that the liner may not have been squarely cut. If the liner were not cut square to it's length, then it would not evenly hold the Forward Seal Disk against the Forward O-ring. This would provide an escape route for the hot high-pressure gases.

Its amazing that this motor burns out in 0.9 seconds and that this much damage can happen.

I'm not sure if I should use this case with even this slight blister. Perhaps I could make sure the blistered side is nearest the nozzle end, where I understand the pressure and temperatures to be less extreme.

I've flown about 4 G 138's.
The case held up but, did take on some discoloration near the top of the motor.
My theory; either loose fitting delay grains and or the added heat from using a longish delay (not drill down on it).

I wish they would put out a plugged closure for the 29 mm x 40-120 casing.
I know it's meant to be be hobby line but, this is HP load.

JD

post #19 , looking a drawing do you see a chance #10 and #18 o rings are reversed ? , even though used that 0 ring on seal disk to me looks loose ?

Good thought, but it looks right to me. Middle pic in #19 the seal disc o-ring is thinner than the rightmost o-ring (which I assume is the fore o-ring).

michigander said:

post #19 , looking a drawing do you see a chance #10 and #18 o rings are reversed ? , even though used that 0 ring on seal disk to me looks loose ?

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Thanks for asking. I'm certain that I have them in the correction locations. The 1" diameter 3/32" thick Forward O-ring is loose the photos, the 7/8" 1/16" Seal Disk O-ring is installed in the slot only in the second photo. Be aware that the O-rings were damaged/burned thin as shown in the photos.

kevinkal said:

Yesterday I flew a H250G-M in my never used Dr. Rocket 29/240 case.

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An H250G had it's way with one of my cases also. I think I remember reading about a few other cases burn through at the same location.

I would contact AT for certain. This should be replaced under warranty

seth_cooper said:

An H250G had it's way with one of my cases also. I think I remember reading about a few other cases burn through at the same location.

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Wow, that is impressive for a non-research load. I've flown many H250s, but are all pretty old now (4-5 years). Other than thinking the liner was short or an O-ring missing, I can't think of a cause. Did the closures start to feel tight as you screwed them in? That is normal, if there was no resistance, I'd thing a short liner or some missing part was the issue. I don't think I ever seen blow by or any of the more common failures do that kind of damage to a case.

cavecentral said:

Wow, that is impressive for a non-research load. I've flown many H250s, but are all pretty old now (4-5 years). Other than thinking the liner was short or an O-ring missing, I can't think of a cause. Did the closures start to feel tight as you screwed them in? That is normal, if there was no resistance, I'd thing a short liner or some missing part was the issue. I don't think I ever seen blow by or any of the more common failures do that kind of damage to a case.

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I found these 3 photos on rocketryforum.com....all with H250G's

View attachment 299675

View attachment 299676

View attachment 299677

It's amazing what can happen in 0.9 seconds. It would be interesting to know the details for each of those "cases". Correct assembly vs. incorrect etc.
Hope it's far from the norm. I really liked the boost of the H250G.

Ha! The blue case was mine from 8 yrs ago. They told me it was the first reported CATO of the H250G. If I recall, the rear closure didn't really snug up well. I think the liner somehow got cut short, or I left out the fiber washer at the aft end (or an o-ring).

It was definitely an anomaly. I've flown a total 218 Aerotech motors and this was my only RMS CATO. The other was a defective G80T single use. 2 for 218 - thats less than a 1% failure rate. I haven't had a failure of an Aerotech motor since that flight (knock on wood). I have flown the H250G two other times without issues.

The H250G is a pretty hot motor.

The middle red one is mine. I also had another one blister on this reload (all assembled correctly). I believe the key issue is that the end of the phenolic liner has to be perfect. Inspect both ends inside and out and use the best end. It must be totally flat so the seal disk can be fully and evenly seated. If the end is not even, sand it flat and even. Use extra grease on the "O" ring and give it a coating on the outside edge of the seal disk when it is assembled. This reload pushed the design of this case to the max. I have never had any issues with the H210R or any other reloads for this case. Likewise, I have never had a failure with the I245G reload for the 38/360 case.