G138T Claims another case

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What load is causing issues with the 24/40 case?

cls said:
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.
Click to expand...
 
Tonimus said:
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".
Click to expand...

aside from the case issue...nice job on the altitude and speed!!!!
 
Thanks! It actually went significantly faster than that. About 630 mph. The 492 mph was based on my Jolly Logic AltimeterTwo, before I discovered I had actually maxed out the accelerometer. OR sim predicts 80 feet less, so I'm confident that our top speed was right around 630 mph.
 
I just flew another G 138 this weekend.... No issues at all.
I did tape the out side of the grain for a snugger fit in the casing.
The plan was to fly 2 more in a cluster but, they failed to ignite.


JD
 
failed to ignite? what did you try to light them with? a bucket of water? :)
 
I thought I could source enough current to light 3 Quick burst igniters with a Perfect flight timer and a lipo battery...
Should have used the e-matches.

JD


DizWolf said:
failed to ignite? what did you try to light them with? a bucket of water? :)
Click to expand...
 
JDcluster said:
I thought I could source enough current to light 3 Quick burst igniters with a Perfect flight timer and a lipo battery...
Should have used the e-matches.

JD
Click to expand...

Ah, thought you were lighting off a ground system. nevermind then :)
 
ATGM said:
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]
Click to expand...

Rex R said:
not to pick nits but, the video appears to show the taped end facing aft.
Rex
Click to expand...

I had a blister yesterday on a certain AT load, thus I'm reviewing other blister information in detail.
I watched that video, and they shove the grain into the case such that the taped side if the grain is on the side of the case that has the Aerotech stamp on it. Then later in the video, they install the forward closure with delay grain on the other side of the case. Then they installed the nozzle on the Aerotech side of the case. I agree with Rex, the video depicts installing tape to the grain on the aft end. I thought the tape was supposed to protect the forward side of the case. Yes?
 
I flew 2 more today, in a 7 motor cluster. No damaged casings.
The liners from both motors.
The tape is 3M high adhesion tape.
2016-09-04 14.31.50-2.jpg

JD
 
Yes he for sure put the nozzle in end where there was more space between grain and case. Tape was on grain forward end of case.
 
crossfire said:
Yes he for sure put the nozzle in end where there was more space between grain and case. Tape was on grain forward end of case.
Click to expand...

You have good eyes. I don't think many people would look at the video to try to discern clearance there. It's a very good point.

I still think the video is misleading, assuming my understanding is correct...

Take a close look at the video at time 4:09 and then 4:44. Keep reference to the case orientation .. AeroTech is on the left in both video segments. At time 4:09 the taped end of the grain was installed such that the tape was on the left, then at 4:44 the nozzle went in on the left.

It's my understanding that the taped side of the grain should be on the forward closure side, not the nozzle side. Is my understanding correct, the taped side of grain should be on the aft closure side?

I'm not trying to beat a dead horse. This issue needs to be crystal clear to all.
 
The pressure is higher on the forward end of the grain. This results in most of the gases flowing through the core towards the nozzle (yeah, Captain Obvious speaking here...) but the pressure differential has it's effect on the outside of the liner too. If there is enough clearance between the liner and the casing, a significant amount of still very hot gases will flow there and damage the casing. Taping the forward(!) end, so that the fit of the grain is snug, will reduce the amount of gases that enter the space between liner and casing to a harmless level.
It is no problem if a small amount of gas is in contact with the casing, because it has a low thermal capacity and will cool down quickly before the casing gets too hot. That's the reason why motors work even if the liners etc. are not sealed airtight. If, on the other hand, the gases are allowed to flow freely over the casing, there will always be "freshly combusted" hot gas in contact with the casing that is way too hot for the aluminum to handle.

In the longer high power casings with 4 or more grains, the seal disc performs the same function.

Reinhard
 
Last edited:
Reinhard said:
The pressure is higher on the forward end of the grain. This results in most of the gases flowing through the core towards the nozzle (yeah, Captain Obvious speaking here...) but the pressure differential has it's effect on the outside of the liner too. If there is enough clearance between the liner and the casing, a significant amount of still very hot gases will flow there and damage the casing. Taping the forward(!) end, so that the fit of the grain is snug, will reduce the amount of gases that enter the space between liner and casing to a harmless level.
It is no problem if a small amount of gas is in contact with the casing, because it has a low thermal capacity and will cool down quickly before the casing gets too hot. That's the reason why motors work even of the liners etc. are not sealed airtight. If, on the other hand, the gases are allowed to flow freely over the casing, there will always be "freshly combusted" hot gas in contact with the casing that is way too hot for the aluminum to handle.

In the longer high power casings with 4 or more grains, the seal disc performs the same function.

Reinhard
Click to expand...

I concur. The last two motors I flew showed signs of hot gasses getting past the forward seal and leaking down the outside of the liner. From this and a few other threads, I've learned a lot about the internal pressure distribution, and where the seals are most critical.
I look at the forward seal area more critically now... perhaps a little more grease on the outside of the grains, liners and liners with seal disks. Also make sure the liners are cut straight and have no cracks or burrs that might promote a leak.
 

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