The cato to end all catoes!!!

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Did you rocket have a recording accelerometer? Taking a look at the acceleration curve can help distinguish between an over pressurization and some other type of motor failure.

Just curious, how can you tell the difference between the two?
 
In brief, no accelerometer, never used the case before, with or without the adapter. If it overpressurized, then was my igniter too big, or the dowel I inserted it with?

I got the motor out last night, but no pics yet. It is shrapnel above the top ring of the adapter, perfect below. Nozzle is still inside, but looks okay.
 
Just curious here, how does a burn through do that to a body tube??

Burns through but doesn't dump the contents of the case. propellant stays inside and road flares out the top.

like this-
18969811404_1a2ac6c11c_b.jpg
 
In brief, no accelerometer, never used the case before, with or without the adapter. If it overpressurized, then was my igniter too big, or the dowel I inserted it with?

I would hazard a guess from the photo posted that the ignitor and dowel are long gone.
 
Just curious here, how does a burn through do that to a body tube??

Remember that the rocket also lawn darted.

There will also be some pressure containment inside of the tube if the motor has nowhere to go, as well as an initial pressure wave/debris spray as the motor let go while under pressure.

I have another 6000 CATO in my Bad Attitude. The pressure was immediately relieved because the AeroPack gave up the ghost immediately. See pics from Dave McCann:

dan-3937 by DizWolf, on Flickr
 
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Remember that the rocket also lawn darted.

Are we talking about my rocket? My rocket did not lawndart. It ascended about 400', then the motor blew. It separated, and the main deployed. The upper half came down gently under main. The booster burned up from the M motor blowing out the front.

The guess on why it spaghettied is that the epoxy holding the fiberglass together must have melted, reducing the tube to its windings.

The booster was on fire when it landed, and the field on fire after that. Thankfully some friend stomped it out, because I was trying to get my kids lunch. :facepalm:
 
Are we talking about my rocket? My rocket did not lawndart. It ascended about 400', then the motor blew. It separated, and the main deployed. The upper half came down gently under main. The booster burned up from the M motor blowing out the front.

The guess on why it spaghettied is that the epoxy holding the fiberglass together must have melted, reducing the tube to its windings.

Ahh okay, never mind. I think this pretty much guarantees you had a burn through.
 
Ahh okay, never mind. I think this pretty much guarantees you had a burn through.

So then why is the casing only melted at the top? The rest of the casing is fine. In fact, I considered having it re-cut and notched down to a 4800 Ns.
 
So then why is the casing only melted at the top? The rest of the casing is fine. In fact, I considered having it re-cut and notched down to a 4800 Ns.

Why wouldn't it be fine? A localized hot spot developed and burned through. The exhaust then travels through the newly formed hole and the nozzle. The remainder of the case is now actually seeing less pressure than a motor functioning properly.

Check the lower snap ring groove for any expansion, but if there isn't any I'd surmise the case is fine in a shortened application. Hell, send it to me and I'll shorten it for you.
 
Welcome to the club!!!!!
You have now appeased the rocket godz for many, many flying seasons with your offering.

My favorite rocket after landing, only the fins remained intact..IT looked like a cigarette that had burned it's total length in the ashtray.LOL

poster92211698.jpg

Edit: Neil took pic at Red Glare many moons ago.

Just remember this....I would rather lose my rocket spec-ta-cu-lar rather than have it hanging in a tree or drift off to netherland.

AND you got all your good stuff $$$$ back intact. Those in attendance at the launch WILL remember your flight above all others!:smile:
 
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Why wouldn't it be fine? A localized hot spot developed and burned through. The exhaust then travels through the newly formed hole and the nozzle. The remainder of the case is now actually seeing less pressure than a motor functioning properly.

Check the lower snap ring groove for any expansion, but if there isn't any I'd surmise the case is fine in a shortened application. Hell, send it to me and I'll shorten it for you.

I may just do that!

I think I am following you. So the burnthrough was only at the top grain. So my next question is why. Sorry if you have already covered this. I am a bit slow sometimes.
 
I may just do that!

I think I am following you. So the burnthrough was only at the top grain. So my next question is why. Sorry if you have already covered this. I am a bit slow sometimes.

Either a bad liner (poor layer adhesion, alignment of spirals between different layers, bad resin content, unnoticed crack) or a path for gases to travel out of the liner and in-between the liner and case. I suggest people use a lot of grease to occupy any air space in a motor to prevent this. Like...bathe in grease. A tub of grease is cheap compared to a case.
 
Either a bad liner (poor layer adhesion, alignment of spirals between different layers, bad resin content, unnoticed crack) or a path for gases to travel out of the liner and in-between the liner and case. I suggest people use a lot of grease to occupy any air space in a motor to prevent this. Like...bathe in grease. A tub of grease is cheap compared to a case.

We did grease it up really well with Super Lube. The liner was a standard phenolic from ARR, which we have used before. So maybe a bad cut, or the cut made the spiral start to unwrap? I guess it makes a strong case for switching to the convolute liners.
 
I'd find out where the burn through was. If at the sholder to the fwd closure, it could be a poor fit. So the liner material might not be the issue. If at an area between grains, then definitely the liner.
 
I'd find out where the burn through was. If at the sholder to the fwd closure, it could be a poor fit. So the liner material might not be the issue. If at an area between grains, then definitely the liner.

Again, I will post pics soon. But it looks to me like it was at the top of the motor. So, either I had bad hardware, or I had a bad cut or not enough grease on the liner. That is what I am getting from the comments.

Here is a quick and dirty mock-up.

blowncase.png
 
The liner is not intended as a pressure vessel, just an insulator. It can leak around the closures without damage. Grease on the whole liner helps prevent cracking, especially with a loose liner. But, it is ok to leak at static pressure without lots of gas flow and not cause heat transfer into the case.

Where grease is important is at the forward end to help prevent the flow of hot gasses circulating around the closure/case/o-ring joint. The liner alone doesn't prevent this. (Although Aerotech tries using the liner as a seal on the top end with a seal disc, leaking only at the nozzle end). The fwd closure joint also has the highest pressure within the casing. With excess heat softening the case around the forward closure, it will give way there first. For long burns, it's also good to have an insulator on the face of the forward closure (with or without a smoke grain).

Even if you have a good insulation interface there at the liner overlap at the forward closure, it can burn through due to a poor o-ring seal. Incorrectly sized o-ring, too much gap in the case ID vs closure OD, cracked or sheared o-ring, or previously bulging or stress from an earlier overpressure/over-temperature situation (a near cato not realized).

Some clues can be found based on when in the burn the cato happened:

An early overpressure during the burn can split the casing anywhere. Nozzle plugging, giant igniter, erosive burning, soft/spongey grains, uncured grains, etc will cause that. But, there's not enough heat transfer at that point to soften the casing anywhere. It can walk a snapring out, shear a nozzle, shear at a snapring groove, or split open the case.

About 1/3 to 1/2 the expected burn time, that's when an unexpected progressive burn will cause a cato. If the case stays well insulated, this can blow similar to what I described above in the early cato situation. Or, if there's softening of the case due to poor insulation, it will burst there, usually at the forward end (highest pressure point dynamically). The cause of the unexpected progressive burn is usually a lack of grain spacers, where the faces of the grains inhibited each other under acceleration (as I explained in an earlier message in this thread). You'll see this as a normal flame that gets angry really fast! The pressure will go to 2x or 3x the design value.

Anywhere through the burn, a cato could be due to unexpected high burn rate. Depends on the grain design, but 1/4 the way is typical. Propellant cracks or voids, low density foamy grain, propellant not adhering to the casting tubes, or plain old high-g shearing of propellant (especially bigger diameter with large web thickness). The failure will depend on the weak spot in the casing design and how much heat transfer has happened, as with the other circumstances.

So, I guess it's a miracle that any rocket motor works! :wink:
 
True! We don't use grain spacers. We scallop the top of each grain down toward the core so that they are not lying flat on each other. We then glue everything into the liner with a mixture of, well, something that starts with an 'R' and something that starts with a 'D'. (Don't want this thread moved to the Research forum!) This worked perfectly well twice before, but maybe we got lucky.
 
True! We don't use grain spacers. We scallop the top of each grain down toward the core so that they are not lying flat on each other. We then glue everything into the liner with a mixture of, well, something that starts with an 'R' and something that starts with a 'D'. (Don't want this thread moved to the Research forum!) This worked perfectly well twice before, but maybe we got lucky.

Coning one side of each is a well-testing method. Could your <redacted> adhesive have oozed onto a grain face and inhibited one or more of them?

You had mentioned mixing/matching closures with a casing from a different source. If there was a poor match, it could be a leak that started it all. I've measured various manufacturer's ID's and OD's and have been surprised at the variations (even within one manufacturer). You should have 15% to 20% compression of the o-ring when seating against the casing. Some people say less, but I think they're confused with the requirement of 2% to 5% stretch of the o-ring when placed over the closure's groove.
 
So, if I am convinced that we slathered it with grease and am sure that both O-rings were installed, then could poorly manufactured hardware be the cause?
 
It certainly could be bad hardware.
It could also have been several other things.

How loose was the forward closure when you slid the case over it?
 
Could it be a gap between the liner and the forward bulkhead? Or a liner that was cracked?
 
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