My first CATO

[terryg]

After 5 years of flying mid and high power I finally had my first CATO last week end. Of course it happened on the first flight of a new design that I finished just a few days earlier. It was a experimental design whose goal was to try to maximize the whistling sound of a high power launch. I used a 29 mm H250 mojave green purchased within the last month (I have another one still in the package that I will examine). I built the motor the night before and there were no particular issues. The grains were not too tight and it assembled without any unusual trouble. Upon launch the motor had a side burn thru the side of case just under the seal disk. It cleared the launch pad and finally set off the ejection charge after an interesting semi hover.

 

[terryg]

Neither the forward of aft closers were damaged and the proper o rings were in the right positions. The burn thru was aft of the metal seal disk and end up welding it into the tube and created a half moon shaped hole in it which I suspect is what set off the ejection charge eventually.

 

[Donaldsrockets]

Wow, that's weird.

I wonder if by chance there was a crack in the liner.

If everything there checks out, it could possibly been a flaw in one or more of the propellant grains. When you check your other H250G, try to see if you could shine a light down through the cores. If not, then I'm sure a wooden toothpick would work well in finding any potential voids (holes).

I flew an H250G back in July with no problems at all. It kicked my 5 foot tall 3 inch diameter BSD Sprint up to 2,000+ feet in a hurry and due to the large fins, it whistled VERY loudly.

 

[terryg]

I speculate from the damage that the phenolic liner failed just under the seal disk and the blow torch effect melted the side hole. I am debating the best method of repair. At this point I think I will drop a 29 mm motor mount into the damaged 38 mm mount and build up and patch the side. I will be filing a MESS report and contacting Aerotech, but I wanted to take time to document it first.

 

[Pantherjon]

WOW!

Donald I think hit on it, and what happened was a liner failure..It also could have been a bonding failure in that particular grain and the casting tube creating more burn area and the eventual torch out the side of the case..Hopefully AT will square you away..

Keep us posted..

 

[DexterLB]

That's brutal!

 

[Pem Tech]

WOW!

.Hopefully AT will square you away..

Keep us posted..

The one failure I have experienced with AT was a G77 LMS case splitting. They replaced the motor without a problem and even followed up in person at NSL that year.
Two thumbs up for AT customer service.

 

[kelltym88]

The O-ring in the forward section seems to be pretty big. Now, I am no expert, but isn't the skinnier of the O-rings supposed to go in the forward section, and the thicker one in the rear around the nozzle?

Were the O-rings provided all the same thickness? I tried to access the AT website, but it has changed and for some reason it is loading really slow for me, but I was wanting to see if there were any instructions for that reload.

But if that were the wrong O-ring, would that have happened? Could it have allowed just enough of a gap to cause that malfunction? Let me be clear, I am in no way suggesting that you assembled the motor incorrectly. But that kind of CATO does not happen very often.

 

[hognutz63]

The O-ring in the forward section seems to be pretty big. Now, I am no expert, but isn't the skinnier of the O-rings supposed to go in the forward section, and the thicker one in the rear around the nozzle?

Were the O-rings provided all the same thickness? I tried to access the AT website, but it has changed and for some reason it is loading really slow for me, but I was wanting to see if there were any instructions for that reload.

But if that were the wrong O-ring, would that have happened? Could it have allowed just enough of a gap to cause that malfunction? Let me be clear, I am in no way suggesting that you assembled the motor incorrectly. But that kind of CATO does not happen very often.

The thicker O-ring is the forward O-ring.

https://www.aerotech-rocketry.com/c...-RMS_Instructions/29mm/29_240g_in_20052-1.pdf

Eric

 

[kelltym88]

Thanx. That's what I was looking for. OK, my next question would be did you put the O-ring on the seal disk? And if so, then what happened is probably as has been suggested, a liner failure, or a grain issue.

 

[SparkyVT]

I had a similar CATO about 18 months ago on the same motor. See the attached pic. Instead of a burn through, the side ripped open and ejected the forward seal-disk. AT said that I assembled it incorrectly and pinched the the O-ring ontop of the seal-disk. Personally, I thought the motor went together pretty loose and didn't tighten up until the last quarter-turn or so. Short liner??? Either way, I got a new case from them.

-Sparky

 

[terryg]

Yes, the seal disk had the proper thinner smaller diameter O ring installed and it seemed to seat into the end of the phenolic tube without issues. The tube had a sticker on it that warned to peel the grain liners if needed for proper fit. Normally I have had to remove this sticker to insert the liner into the case, but this time it slid right in easily without doing that. The grains slide into the phenolic with little force after removing the serial # labels. I am not sure where the forces are applied to the phenolic tube as it is screwed shut. I will have to look at the position of the parts after assembly on the next one I build (after I get a new case ). Since the mojave green burns hotter and is more energetic it seems to burn thru liners pretty consistently in the hobby line case (with no damage to case however). I am going to add one additional step to my usual motor assembly steps. I will examine the ends of the liner and use the cleanest cut end for the seal disk.

 

[n3tjm]

I personally have not had AT motor fail in that area but I have seen it happen several times.

 

[terryg]

I sent the case back to aerotech for analysis and they sent me a replacement case and seal disk. Based on their analysis:

Probable failure mode: O-ring failed to seal due to chips in the liner material just below the liner end. This allowed hot gases to penetrate past the seal and compromise the aluminum case

This is very useful information, since this is something we can easily check for when we are assembling reloads. I will add this simple check to my routine assembly procedures. I would like to thank aerotech for the replacement case and for finding the most likely cause of the problem. :clap:

 

[rocketsmith]

I speculate from the damage that the phenolic liner failed just under the seal disk and the blow torch effect melted the side hole. I am debating the best method of repair. At this point I think I will drop a 29 mm motor mount into the damaged 38 mm mount and build up and patch the side. I will be filing a MESS report and contacting Aerotech, but I wanted to take time to document it first.

I would use a 38mm case as a mandrel and patch the motor mount tube with fiberglass. Just wax the outside of the case with parafin wax and fiberglass right on the motor surface. Then pull out the case and sand any interior imperfections smooth with a dowel wrapped in sandpaper. Then you can fill in the hole in the airframe and you are good to go. I have done this before and it worked out well.