G138T Claims another case

Had my fastest and highest successful flight yesterday. A number of milestones, including my L2, but generally I like low and slow. Anyway, we slapped together a Hobbyline 29/40-120 case loaded with a G138T. Went to 3,429' at 492mph. Slightly tangled streamer on recovery, but trying to fit 10 feet of streamer in a LOC Lil' Nuke isn't easy. No damage to the airframe. Here's the case, though.





I did load this up exactly per the instructions, including liberally greasing the outside of the "liner".

This seems to be a common theme with this motor. Anyone have a solution to prevent it?

Whats it look like on the inside? If there is no damage on inside, I would keep using it for anything but the the G138T reload. Orient the heat stressed section toward the nozzle where there is less heat and pressure during the burn.

The G138T really could use a metal seal disk like the HP 29 mm reloads use.

Here's the inside. Same color damage, just larger.

The only thing that you can do is: wrap the grain in some tape.
As said before, you can't use a seal disc here because the grain has no real liner.

JD

Seal disk won't work. The reason is also the cause for the faux liner. They are trying to shove too much motor into too small of a case. They needed every Ns they could get which means the most propellant.


I'm kind of worried to fly my G138...

codysmith said:

Seal disk won't work. The reason is also the cause for the faux liner. They are trying to shove too much motor into too small of a case. They needed every Ns they could get which means the most propellant.


I'm kind of worried to fly my G138...

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If worried do not fly it. Really simple.

codysmith said:

Seal disk won't work. The reason is also the cause for the faux liner. They are trying to shove too much motor into too small of a case. They needed every Ns they could get which means the most propellant.


I'm kind of worried to fly my G138...

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I'd go ahead and fly the motor. If the casing is damaged, AT will replace it. While I have heard of heat damaged casings and experienced it myself, I've never heard of a CATO with this motor.

I had one of these actually blister a casing and they replaced it. My rocket wasn't damaged in any way except the motor was a bit hard to remove due to the blister on the casing.

On the other hand, I'd take the reload off your hands if you were closer.:wink:

Oh, I wasn't aware that AT would replace it. Not sure what to fly it in but it will fly

I refuse to fly those crazy high thrust G motors. Every one of them seems like it is trying to tiptoe around L1 certification status, and winds up with questionable technical status as a result.

Make a trade, because someone out there wants it!

FYI: You do need a L1 cert to fly any motor that has more than 80 Ns average thrust......


JD

watermelonman said:

I refuse to fly those crazy high thrust G motors. Every one of them seems like it is trying to tiptoe around L1 certification status, and winds up with questionable technical status as a result.

Make a trade, because someone out there wants it!

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I'll be flying it. With my NAR and TRA Jr. Status, I can fly L1 motors with a mentor. Basically, I am L1 as long as there is a L1 adult with me

I emailed Aerotech yesterday, we'll see what they say.

JDcluster said:

FYI: You do need a L1 cert to fly any motor that has more than 80 Ns average thrust......


JD

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Oh, right; I knew that. Well, I have no idea why they make such compromises for these weird G motors then. G80 works great, as do the H options in that range.

watermelonman said:

Oh, right; I knew that. Well, I have no idea why they make such compromises for these weird G motors then. G80 works great, as do the H options in that range.

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Because it's a lot of thrust in a small, lightweight case. I loved the launch, just not what it ended up doing to my hardware.

Fromthe discussions I recall, they tried making it an H138, but that used a even thinner casting tube...and those tests failed. They thickened it up and wound up without enough propellant for an H. So, we get this, which I think is still a compromised design. I've flown a few, but won't do any more of them. I wish they'd just make a G80 reload, and a core burning green. Love the loads, but the slot burners torch the liners every time. Maybe convolute liners, but then the cost skyrockets.

I still love the G138 and fly it more than any other reload for my 29/40-120 case.

Given that we already have G reloads (for this case) in the 50-80 N (avg thrust) range, I don't think a G80 would add much, so I appreciate the additional variety of this reload.

emckee said:

I still love the G138 and fly it more than any other reload for my 29/40-120 case.

Given that we already have G reloads (for this case) in the 50-80 N (avg thrust) range, I don't think a G80 would add much, so I appreciate the additional variety of this reload.

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So how do you prevent the issue at hand?

Tonimus said:

So how do you prevent the issue at hand?

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You do not prevent it. The motor seems to mess with cases so I would consider this a compromised design.

Tonimus said:

So how do you prevent the issue at hand?

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Unfortunately, there is really no way to prevent it. The only real way to prevent it is to not fly the motor in question. I'd say it's a 50-50 proposition. It will either damage the casing or it won't. Even if it does, you're covered. AT will replace it. They won't replace the reload unless it CATO's in some manner but they will send you a new motor.

I have flown a total of 4 of them. One blistered the casing just below the forward closure which AT replaced. The other 3 worked fine and didn't cause any damage or discoloring of the casings.

However, would I ever fly any more of them???

That I am not sure of. It's a beautiful motor, I really loved the flame on it but risking damage to the casings each time is something I'm not sure I really want to deal with.

Unfortunately the G138 is a max load for this case so this happens more than we would like. We are looking into solutions to solve this.

I've used nine so far without any case damage.

ATGM said:

Unfortunately the G138 is a max load for this case so this happens more than we would like. We are looking into solutions to solve this.

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Im cool with it. As said---its a max load barely shy of an H packed into a 40-120 hobby case.:grin:

After speaking with Karl he suggested wrapping the forward end of the grain with a piece of masking tape. I just finished a video on the G138 that will show the method that Karl is referring to. We built and tested several of them using the tape method and that seemed to work pretty well. We didn't have any discoloration after the testing.

[video=youtube;hd4gRUB1eUU]https://www.youtube.com/watch?v=hd4gRUB1eUU[/video]

Noted. I may risk another case.

Another option; would be to offer it in a plugged variation only.
The delay can make it worse for the casing.


JD

I've been sitting on the remaining four G138s trying to decide what to do with them as there was no surety that blistering would not occur again (https://www.rocketryforum.com/showthread.php?121649-G138T-Blistered-Case&highlight=g138).

Thanks, ATGM for helping keep this issue alive enough for some action. Now, with a official visible remediation from the source, the level of confidence in firing this load just went way up!

All three of my 29/40-120 cases are discolored and battered... But they keep working! Don't worry about it just keep flying.

The 24/40 cases tho are a different thing. Several are bulging at one end and have spat the rear retaining ring. Can't trust those.

not to pick nits but, the video appears to show the taped end facing aft.
Rex