G138T Blistered Case

I have had this happen once.. Unfortunately ended up losing the rocket, but they promptly replaced the case and the motor - also a G138. Burned through the case wall about 1/2 an inch below the forward closure threads.

From what I've read, it sounds as if the G 138 might require a seal disc?


I have flown about 2-3 without issue.
(Knock on wood)

JD

I also had a G138T blister the case. When this reload was announced, it was mentioned that it wouldn't use a liner. With that being said, I was a bit afraid issues like these would pop up.

I personally think this motor needs a liner. Maybe not a seal disk but most definitely a liner. I know a liner would mean a tad less propellant and N-secs but would probably yield a more reliable motor and less reusable-turned single use motor casings.

While this IS an awesome motor, I probably won't be in a big hurry to fly any more of them.

I think this load shows people really like full G motors. Stuffing one into a 120 case may not be the best way to provide that. I'd absolutely be down for getting new hardware if they released a 29/160 and 38/160 cases. Cut them to be under hazmat too...

Unlikely, but a boy can dream.

DizWolf said:

I think this load shows people really like full G motors. Stuffing one into a 120 case may not be the best way to provide that. I'd absolutely be down for getting new hardware if they released a 29/160 and 38/160 cases. Cut them to be under hazmat too...

Unlikely, but a boy can dream.

Click to expand...

Actually the original goal was to develop a high power reload for the RMS-29/40-120 Hobby Line case. AT came real close, but didn't quite make it, just not enough internal volume. The G138T develops 157.1 Ns with 70 grams of propellant. It needs about 6 Ns more propellant to be unambiguously listed as a H- motor.

https://www.aerotech-rocketry.com/uploads/5f501498-bb2b-4022-b6cd-001ac1d6eb8b_hp-g138t_nar_cert.pdf

The casting tube used is 1.005" o.d x 0.950" i.d. x 3.902" long. By changing the i.d. by 0.01" you gain or loose 0.95 cc of propellant. The specific impulse of the propellant is 2.25 Ns/gm. The propellant density is typically 1.6 to 1.7 gm/cc. so you gain or loose 2.25 x 1.65 x 0.95 Ns ~3.5 Ns per 0.01" of liner i.d. change. thickness.

https://www.aerotech-rocketry.com/u...1dc-791f5c173b2a_29-40_hp-g138t-14a _assy.pdf

https://www.rocketmotorparts.com/Paper_Motor_Liner,_29mm/p1577809_7795930.aspx typical.

https://www.specificimpulse.com/images/ISP_Portfolio.pdf

I'll guess the original design called for an even thinner casting tube. RCS lists a casting tube with a 0.975 id which could add an additional 8.7 Ns of propellant which could have been the original casting tube, but that wouldn't have provided enough insulation if the 0.950" i.d. liner is sometimes blisters the casing.

https://www.rocketmotorparts.com/29mm_Casting_Tube,_1000__OD/p1577809_7781876.aspx

I am just speculating that this was the original design, but it is a logically possible.

Bob

I did the certification testing for these motors and the thinner liners/casting tubes were test fired and they did exceed 160Ns of total impulse but 2 of 3 casings were trashed. Had no issues with the slightly thicker liner/casting tube variant which was the configuration that was certified.

John

The H 238 isn't much more in the terms of Ns....
Coming in at 165 Ns.

The only real difference is that the G 138 is a single grain, hence the
HAZ-MAT requirements.

JD

After looking at my two 29 mm x 40-120 cases;
There is some loss of dye to the anodizing at the forward end of the casing.
Each case has about 1 firing each.
The next time I load up one of these I'll be sure to reverse the case so the forward end is aft.

One possible fix might be to: either double up or glue the forward insulator in place to get a better gas seal.


JD

JDcluster said:

After looking at my two 29 mm x 40-120 cases;
There is some loss of dye to the anodizing at the forward end of the casing.
Each case has about 1 firing each.
The next time I load up one of these I'll be sure to reverse the case so the forward end is aft.

One possible fix might be to: either double up or glue the forward insulator in place to get a better gas seal.


JD

Click to expand...

I think I'm just moving to use H238T's in the 29/180. Same price, less worry. G64's and G76's are great rides for the 40-120.

JDcluster said:

The H 238 isn't much more in the terms of Ns....
Coming in at 165 Ns.

The only real difference is that the G 138 is a single grain, hence the
HAZ-MAT requirements.

JD

Click to expand...

Not quite the same. The G138 is designed for the RMS-29/40-120 Hobbyline casing. Getting 160+ Ns into a 120 Ns model rocket motor casing wasn't possible.

The H238 is designed for the larger RMS-29 High Power 29/180 casing. Getting 175 Ns in a 180 Ns casing isn't hard.

Bob

bobkrech said:

Not quite the same. The G138 is designed for the RMS-29/40-120 Hobbyline casing. Getting 160+ Ns into a 120 Ns model rocket motor casing wasn't possible.

The H238 is designed for the larger RMS-29 High Power 29/180 casing. Getting 175 Ns in a 180 Ns casing isn't hard.

Bob

Click to expand...

From an engineering standpoint they're worlds apart. Once you slap them up in a rocket however, aside from the much bigger punch off the pad, the flight isn't going to be too terribly different. My Vulcanite sims out to around 1700' at 250 mph on both.

Considering there isn't much difference in the case sizes, the difficulties and risk of designing and using the G138T don't seem worth it compared to the H238T.

Granted the G138T has a cool progressive burn....a pair was dramatic in a cluster, Slow lift off then noticeably accelerated. The effect was less dramatic in a lighter single motor rocket. But that cool worth the risk? I don't know.

I just built 2 of them yesterday.
I did wrap a layer of painters tape around the grain to help insulate it.

Thrust profile is also different.
Since the G 138 is a single grain; unlike most AT reloads: it is also progressive, not regressive.


JD

The core diameter of the G138T grain is also different. It's much smaller than other BATES grains. I'm sure this also has something to do with the progressive burn.

The supplied igniter just barely fits into it.

My Sunward King Tut pyramid LOVED that motor.:wink:

I too really love the progressive burn.

An Aerotech 29/40-120 RMS showed up yesterday from the factory as a warranty replacement for the case that suffered blistering from the G138 flight. Thanks Aerotech for a hassle-free process! We'll have to think twice about how to best protect the case if another G138 flight is in the future.
View attachment 254467

Another option might be: to wrap AL Duct tape around the grain??
The good stuff is suppose to be good for 400 degrees F.

Hopefully my Wildman order isn't to far off either......

I have 6 of them on order.

JD

Another thought for those experiencing the blisters / burn throughs:
Was the igniter inserted all the way to the top of the grain?

Why I ask is it might be part of the problem.

What type of igniter was used?
What size igniter?


On my 2 flights I used just a lighter similar to those cheap chinese orange wired ones.
No dip.

JD

JDcluster said:

Another option might be: to wrap AL Duct tape around the grain??
The good stuff is suppose to be good for 400 degrees F.

Hopefully my Wildman order isn't to far off either......

I have 6 of them on order.

JD

Click to expand...

This is a liner less motor. As there is no liner, the casting tube is the defacto insulator/liner, and it is either not thick enough, or more likely not long enough to prevent hot gas impingement on the aluminum motor casing which is the only way you will blister the casing.

The only 100% solution is a better (thicker and or longer) casting tube which acts a better thermal barrier/gas seal. According to Johnly the original G138T certification motors had an even thinner casting tube to add sufficient propellant to make it a H-impulse motor, and that variant toasted 2 of 3 motor casings. The current casting tube is thicker, but at times does not appear to be adequate.

Please note the difference between the casting tube length in the G138 reload assembly drawing https://www.aerotech-rocketry.com/u...1dc-791f5c173b2a_29-40_hp-g138t-14a _assy.pdf and the liner in the G64 reload assembly drawing https://www.aerotech-rocketry.com/c..._assy_dwgs/29mm_mr_rms/29-40_g64-10w_assy.pdf .

1.) Note there is no o-ring seal between the casting tube and the nozzle in the G138 while in the G64 the liner extends past the nozzle and forms a cavity between the liner, nozzle, casing and threaded nozzle retention ring. The latter design does not have a gas leakage path, whereas the G138 design has no o-ring gas seal.

2.) The forward end of both motors has identical gas seal designs and that's not where the problem occurs.

Draw your own conclusions.

Bob

OP is stating that the problem is at the fore end and not aft ???

All the ones Ive fired cooked the rear of the casting tube, and discolored the case in the aft. No blisters, but it does seem this design pushes it a bit hard.

There is still an o-ring on the nozzle, though the nozzle fit is just pressed up against the grain.
Forward end was discolored on mine. Case not shown here.

On the issue of further preventative measures, Aerotech just offered me the following suggestion:

I would try greasing the liner liberally to prevent possible expansion of the liner tube at its spiral joints....while I'm not a fan of a lot of "grease" this helps if the case is at the large end of the ID spectrum and the lines at the opposite end.....

To my surprise, I actually got to fly some high power last weekend. On a whim I decided to dust off (literally) my old 29/40-120 hardware and fly a G138. This is old hardware mind you, like from 1994 and I honestly can't recall how many times I used it but I'm guessing around 10 times prior to the G138. It's in good shape with most of the printed graphics still legible. Anyhow, I made a good effort to grease up the outside surface of the grain. I don't think it would qualify as a liberal amount but I made sure it was greased up reasonably well. The flight went fine and upon disassembly and cleaning everything looked cool, no blistering or other undesirables. The remains of the propellant grain tube looked to be in really good shape to my surprise. I should have taken a picture but there was only a small that was totally burned up toward the aft end, but otherwise looked like a white greased up tube.

On a side note, I noticed the delay insulator tube was somewhat loose in the forward closure. I don't recall having this issue many moons ago but I remember someone recommending to wrap some tape around it to snug it up a bit. I did just that, using exactly one wrap of masking tape. It slid into the closure with moderate force but nothing over the top. Whether this was necessary I have no clue but I did it anyway...

Here's a video of the flight:

[video=youtube;Elr52mtM7xc]https://www.youtube.com/watch?v=Elr52mtM7xc[/video]

Another thought is to: plug the motor without any delay element.
The smoke delay can and will wreck hardware that is on the verge of being toast.


JD