Pro38-6GXL Liner Ablation - Is This Normal?

In my experience, it is very easy to remove used CTI reloads by hand, so I was surprised to have difficulty getting a used J595 Blue Streak out of its P38-6GXL case. Reprimanding myself for forgetting to grease the case threads, I went to work with vice grips on the nozzle housing and a rubber cap gripper to hold the case. After it finally unscrewed, I found this...

View attachment 153475

I have seen some burn through in CTI liners before, but never to this extent. Over 20 cm of the liner at the nozzle end was gone. Is this a normal result of higher hot gas flow at the nozzle end with the longer motor? Does the liner do anything beside store the grains and make it easier to clean up the case? Seems that as long as the O-rings hold, there is nothing to worry about, but I thought I would ask since this was my first XL reload.

What does the inside of the case look like?

Usually they shrivel up at the top because of the heat from the smoke grain. I've never seen the bottom blown out like that before.

I've had nearly identical burn through on 2 flights with Pro38 6xl. Most of that was left on the inside of the case...Jeroen said it was normal and nothing to worry about.

Nothing to worry about, as long as the case did not get damaged. To facilitate extraction (and insertion) on the longer motors we recommend a squirt of silicone lubricant spray on both ends of the case. It prevents the plastic from sticking.

Jeroen

Thanks everyone.

NikeMikey said:

What does the inside of the case look like?

Click to expand...

There was a layer of char inside the case where the liner had eroded, but no obvious damage. Just more clean up to do than usual. There is a spot of slight brown discoloration ~4cm long by ~1cm wide on the outside of the case that you can see on the picture about 10 cm up from the nozzle end. This is nothing I would normally worry about.

CarVac said:

Usually they shrivel up at the top because of the heat from the smoke grain. I've never seen the bottom blown out like that before.

Click to expand...

Yes. I have seen the hibachi effect on the liner just below the delay grain too, but never so much liner erosion at the nozzle end.

Dipstick said:

I've had nearly identical burn through on 2 flights with Pro38 6xl. Most of that was left on the inside of the case...Jeroen said it was normal and nothing to worry about.

Click to expand...

Jeroen_at_CTI said:

Nothing to worry about, as long as the case did not get damaged. To facilitate extraction (and insertion) on the longer motors we recommend a squirt of silicone lubricant spray on both ends of the case. It prevents the plastic from sticking.

Jeroen

Click to expand...

Really appreciate your responses. Good to know it is nothing to worry about. The motor performed flawlessly; but come to think of it I did notice what could of been liner debris being ejected during the boost. Pro38 6xl was the booster with a Pro38 6G J330 in the sustainer. Motors worked so well I am still looking for the sustainer.
[video=youtube;QYsjYbjz9Ck]https://www.youtube.com/watch?v=QYsjYbjz9Ck&feature=youtube_gdata[/video]

Wow. Awesome upward view. That flew really really straight.

If I were to see discoloration on the outside of a case, I'd assume whatever temper rating the aluminum had is now shot. That is, instead of T6 it probably went to T1, from which it will naturally age to T2 in perhaps a week. But that gives up a lot of room temp strength and stiffness. I don't know the effect of elevated temperature properties as I haven't looked them up. But I wouldn't fly a case showing such an area of discoloration. But all my stuff is EX with pressures generally above commercial loads.

Given the thin wall of these cases, that much liner burned out would also concern me even if there were no discoloration.

Gerald

G_T said:

Given the thin wall of these cases, that much liner burned out would also concern me even if there were no discoloration.

Gerald

Click to expand...

Isn't there a very large margin with these cases, like 300% at which point the threads give out before the case?

If the temper in the case is lost, that margin goes down. Heat aluminum a bit, and 6061-T6 becomes 6061-T0 potentially. Let it age for a while and it will recover a little. Aluminum doesn't tolerate heat very well. At T0, the yield strength (room temp) is at most 8000psi. T6 is at least 35000psi. From a bit of welding info, if left to naturally age, it recovers to perhaps 11000psi from T0. Quite a difference vs the original T6.

Anyway, if it looks to have been subject to appreciable heat, I no longer trust the case. Your choices are up to you!

Gerald

If the case is discoloured on the outside....all bets are off.

Thanks everyone. Given what I have heard and researched, I am inclined to retire the case. That said, has anyone ever had a case fail with a non-defective commercial reload?

WoShuGui said:

Thanks everyone. Given what I have heard and researched, I am inclined to retire the case. That said, has anyone ever had a case fail with a non-defective commercial reload?

Click to expand...

How do you define "non-defective"?

In some cases it would be a bit difficult after the fact to tell if a load was defective!

I have seen Pro38 6GXL blue loads fail - I think 4 so far that I have witnessed. I have not been involved in assembling any of them so I cannot assess "defective".

Gerald

CarVac said:

How do you define "non-defective"?

Click to expand...

I had in mind instances where the case simply gave out under pressure versus a burn through from a nozzle, o- ring or other such reload or loader related failure. Pursuant to Gerald's observations, I can imagine the forensics might not be straightforward. Also, I gather the blue streak propellant is particularly challenging to the 6GXL case. Is that true? If so, is in terms of temperature or pressure or both as they go hand in hand?