G138T Blistered Case

Just drag raced two similar rockets using the Aerotech G138T-14 in the 29/40-120 case with the delay drilled out for 10 seconds. When the cases were being cleaned two significant items of concern were noted as follows.

Here are the parts for each of the motors at time of disassembly. Are there any indications from the internal components that would explain what happened?


Near the top of the Aerotech case there was a discoloration and a raised rough area where the metal appeared to have been effected - is this what some call a "blister?"


The Rouse Tech case had blotchy discoloration along the top of the case and part of the forward closure.


Does anyone have experience with these kinds of anomalies? The inside of the cases were clear with no apparent damage. Has there been structure damage to either case to the point where is should be retired from service?

I think that is indeed a blister. I think I might remember there being similar problems with early G138s, but I could be wrong. It looks like the anodization is gone in places on the rousetech motor, and to my untrained eye, it looks like there's probably been structural damage to the cases. Perhaps another motor might be enough to cause an all out burn through.

Nate

I assume the Rouse-Tech one wasn't blotchy before? To me that looks more like someone either did a terrible job anodizing, or used an overly harsh cleaning solution, not that the motor overheated. I'd keep using it as long as it's not distorted at all.

The black Aerotech one is definitely blistered and should be warrantied.

I suspect you may have placed the forward o-ring between the grain and the forward insulator instead of on top of said insulator.

Either that or the grain was cut too short from the factory.

I would have no problems using either case. The hotspot doesn't look life threatening, I have several cases with them I use regularly.

jimzcatz said:

I would have no problems using either case. The hotspot doesn't look life threatening, I have several cases with them I use regularly.

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This is bad advice.

Contact Aerotech. Any raised area is certainly a concern and will lead to failure. Unsure on the discoloration, but it is not ideal for sure.

This seems to be a recurring problem. I had the same type of case discoloration after flying a G138T reload. Not really surprising, I suppose, comparing the G138T thrust and impulse to other Hobbyline reloads (157 Ns versus 118 Ns for a G64W). The G138T is a wolf in sheep clothing, so to speak.
View attachment 250676View attachment 250677

Since there was no blistering, I probably will continue to use the case, but not with another G138T and maybe only in rockets that I don't like so much.

WoShuGui said:

This seems to be a recurring problem. I had the same type of case discoloration after flying a G138T reload. Not really surprising, I suppose, comparing the G138T thrust and impulse to other Hobbyline reloads (157 Ns versus 118 Ns for a G64W). The G138T is a wolf in sheep clothing, so to speak.
View attachment 250676View attachment 250677

Since there was no blistering, I probably will continue to use the case, but not with another G138T and maybe only in rockets that I don't like so much.

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Do you grease the outside of the casting tube?

I've flown several, and only had the anno coloring fade on the interior of the case. I do grease my liners, not sure if that matters. dye fading I wouldn't worry about at all. If the metal raised...that needs to retire.

I wish all the manufacturers would make 160 cases, and have more full G loads (non hazmat, under 80N ave thrust would be great too )

One of my favorite motors!
I have waited years for AT to finally release a full G in this case.

I have also flown a few without issue (knock on wood)....

I just ordered some more from Wildman which might be holding up my order???
Were either of these reloads assembled with grease of any type, if so what type?

Motors should NOT be assembled with Vaseline!
It liquefies at body temperature.

Second question: what type of igniters were used?


JD

I flew my first one of those a couple weeks ago. The black casing seemed to get slightly discolored but overall it seemed to take it fine.

Mr G said:

Just drag raced two similar rockets using the Aerotech G138T-14 in the 29/40-120 case with the delay drilled out for 10 seconds. When the cases were being cleaned two significant items of concern were noted as follows.

Does anyone have experience with these kinds of anomalies? The inside of the cases were clear with no apparent damage. Has there been structure damage to either case to the point where is should be retired from service?

Click to expand...

If you use it again, then I would recommend to put the nozzle end where the blistering is and extra grease at the end.:2:

-John

DizWolf said:

I wish all the manufactures would make 160 cases, and have more full G loads (non hazmat, under 80N ave thrust would be great too )

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I concur .

CarVac said:

Do you grease the outside of the casting tube?

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Yes. Just a thin layer for insertion and post flight clean-up. Are you thinking about the grease facilitating heat spread to smooth out the temperature distribution? Interesting how the hot spots are at the ends of the case, but mostly near the forward closure.

I flew one G138T in a drag race. My case had what appeared to be pitting on the inside near the o-ring. The other rocket had a bulge on the case of about 1/16". Fortunately it was in a 29 to 38mm adapter so it didn't ruin the MMT in the rocket.

I figured I dodged a bullet on that flight. I've never flown a G138T since.

WoShuGui said:

Yes. Just a thin layer for insertion and post flight clean-up. Are you thinking about the grease facilitating heat spread to smooth out the temperature distribution? Interesting how the hot spots are at the ends of the case, but mostly near the forward closure.

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I was just wondering. I don't know how much protection grease provides, but it's a data point.

The blisters are on the ends of the case because that's where the hot gases can easily reach the case. They're on the forward end more than the aft end because the aft end has a big nozzle relieving some of the pressure, while the forward end has higher internal pressure (and gases try to squeeze around outside, that's why the seal disk exists in longer motors).

JDcluster said:

One of my favorite motors!
I have waited years for AT to finally release a full G in this case.

I have also flown a few without issue (knock on wood)....

I just ordered some more from Wildman which might be holding up my order???
Were either of these reloads assembled with grease of any type, if so what type?

Motors should NOT be assembled with Vaseline!
It liquefies at body temperature.

Second question: what type of igniters were used?


JD

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If you read the AT instructions they say to use Vaseline or similar grease when the supplied tube of grease is depleted. It doesn't matter that Vaseline liquefies at body temp. I have used nothing but Vaseline for over 10 years and for everything D to L motors and on hot days also, when it's a liquid coming out of the tube. It works just fine. The only purpose for the grease on o-rings is to prevent galling during assembly. Once the motor is assembled, the grease has no purpose. As long as it's slippery it doesn't matter if it's liquid or not, it will allow proper assembly.

To answer a couple of questions -
The casting tube was covered lightly with Super Lube as were the closure threads and o-rings per the instructions.
The Rouse Tech case and closures were uniform in color prior to the launch.
The cases were cleaned using baby wipes.
The forward o-ring was between the forward insulator and the forward closure.
The grain was a snug fit as it required some pressure to turn the rear closure all the way closed.

Would the information on these motors be appropriate for a MESS report? There was no malfunction during the flights.

The Rouse-Tech case got hot enough to degrade the dye in the anodizing. The case is fine. The Aerotech case got hot enough to char the motor tube. The roughness is cooked on paper. (You can see the space from the spiral groove.) You probably have ruined the temper in the case, creating a weak spot. It will work, but may fail with high pressure motors.

IMO step 6 in the instructions on liberally applying grease is very important in motor without a separate liner. You must make sure the closures are tight so the forward insulator is in direct contact with the grain casting tube and the O-rings are greased. If you leave out the forward insulator or fail to apply grease to the outside of the casing tube and the o-rings, there is a chance that hot gas will directly impinge on the aluminum motor casing since a seal failure at the forward or aft end of the motor is more likely to damage a linerless reload than one with a separate liner.

Bob

Did I miss something? Liner less reload? I know this motor uses a liner.

jimzcatz said:

Did I miss something? Liner less reload? I know this motor uses a liner.

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It doesn't. The casting tube is in direct contact with the case wall.

I have used this reload once before, and I really dont remember it NOT having a liner. Gettin old I guess.

I don't think there are a whole lot of loads out there with no liners. I will not be using any that don't anymore, even though I grease all my liners. I have the AMW 38/640 case and used quite a few AMW I315 skids with it. Those loads did not have a liner and did not hurt the case (very heavy) but were a total bi^ch to clean up. A whole container of baby wipes would not clean the case up. The inside of the case was just a bunch of char stuck to the sides of the case. Used a wood dowel to scrape the crap out, soak in soap water for an hour or two and then scrub for awhile with a brush. That load was worth the extra work though!

Here they are, single grain...no liners. Wonder why they don't cast them into convolute liners (other than cost, but I think this is a case where it makes sense)

DizWolf said:

Here they are, single grain...no liners. Wonder why they don't cast them into convolute liners (other than cost, but I think this is a case where it makes sense)

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Probably because that would be too thick, reducing the propellant that fits in the case.

2 interesting notes on the grease: the G138 instructions say to 'lightly grease' the o-rings and 'liberally grease' the closures, the threads, and the outside surface of the grain, without mentioning the delay insulator. On the other hand, the G76 instructions say to 'lightly grease' the o-rings, closures, and threads, not mentioning either the delay insulator or the motor liner.

I personally grease all cardboard liners, including the G138. I'm glad to know that that's worked out for me. (Phenolic liners come out much more easily when not greased)

On the other hand, I also always grease the delay insulator. Maybe that's unnecessary.

There might not be any on the market today but,
All original Kosdon 38 mm ( except for the C-slots) had no liner.....
I've flown about 6-8 Kosdon 38's and all you did was drop in the grains.

The problem with using convolute liners: you have to score or sand the inside surface....
otherwise the propellant won't sick to the walls....

JD

rcktnut said:

I don't think there are a whole lot of loads out there with no liners. I will not be using any that don't anymore, even though I grease all my liners. I have the AMW 38/640 case and used quite a few AMW I315 skids with it. Those loads did not have a liner and did not hurt the case (very heavy) but were a total bi^ch to clean up. A whole container of baby wipes would not clean the case up. The inside of the case was just a bunch of char stuck to the sides of the case. Used a wood dowel to scrape the crap out, soak in soap water for an hour or two and then scrub for awhile with a brush. That load was worth the extra work though!

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Sent off an inquiry to Aerotech about a possible warranty claim based on the info in this thread and BAM!, within 24 hours there was a response indicating a new replacement case would be on the way.

Now that's service! Thanks, Aerotech.

I've heard of this happening, but have never seen it in person. I've only ever flown one G138, in one of the Aerotech brand black cases with gold closures. There was no visible damage. I must say, seeing this doesn't make me want to fly another G138, but I probably will anyway. I have one reload left. Now I did fly an H148 Redline in a Rouse Tech case one time that blistered bad. But it was my fault, I totally forgot to use the liner.

I've flown a couple of these without problems. I'll second Bob's post - greasing the outside of the grain/liner is *required* for this reload, unlike all other 29/40-120 reloads. With the thin casting paper, this is an important part of the insulation for this reload.