Case Failure Questions

Hi, I’m new the hobby and this my first time posting to the forum. I was hoping someone in the community could provide some guidance.

My first HPR rocket is a Madcow 2.6 Patriot. The rocket was a straightforward build and I added I think about 7-8 oz. of lead to NC to get the CG to the spot recommended which I verified. Fins all appeared true. I used ½ oz sinkers for the lead and they looked to be centered in the epoxy in the NC.

My first flight on an Aerotech G80 had a complete motor failure on the pad. The entire rocket burst in to flames. The remains appeared to show a nozzle/case separation. My suspicion is I may have dropped the motor and cracked the case/nozzle. The rocket was a mess but I was able to rebuild it with a new Kevlar shock cord, paint, etc.

My Second flight did not go any better. The rocket left the pad but slower than I would have expected. This was on a CTI G115-13A. The sim estimated 1300’ but the rocket never became stable, corkscrewed slowly to only about 200 feet and which point it nosed down and lawn darted onto a blacktop path. The ejection looks to have fired correctly not that it mattered as the entire flight lasted about 5 seconds. The NC was trashed , cracked a fillet and I may have lost 1” of the airframe but it looks to be salvageable if I decide to rebuild. I have some theories about the failure I was hoping to investigate with the community:

1. I flew this on a ¼” launch rod. I thought it was clean and the rocket appeared to move smooth on the rod. But it didn’t appear to jump as off the rod as much I would have thought. The sim estimated 45/ft/s. off the rod but maybe it didn’t reach that. My thought is the rocket should have eventually got stable with some velocity and altitude but it definitely never did. Should this be the case? I had a Wildman Eagle Claw mini on the same day fail to achieve altitude and stability in a similar fashion fall and blow it’s ejection late on the ground. Perhaps dirty rod or tight lugs? I’m thinking of just moving on from rods to rails and being done with it.

2. I had a clear case failure on the 2G CTI 38mm case and I believe this to be the major cause of the failure. See attached pics. I’m imagining this may have caused the corkscrew failure to achieve stability. I can see a burn through in the case, the phenolic and the o-rings. My question is what I may have done or not done in the assembly that may have caused this? When I first loaded the motor in the case it threaded very smooth. But the second or third time it started to thread very tight. Perhaps the o-ring got torqued up and caused the case failure? Are there any tips here? How tight should this the motor be?

Any guidance or tips in order that I can prevent and fix this next time is appreciated!

Todd

A few questions.

1) how deep are you installing ignitors?

2) on the 38mm, do you mean the second or third motor you're installing or are you installing then removing, the reinstalling the motor?

I see that you used a 2-grain motor case, but a 1-grain motor. Did you use a spacer? I don't see one in the photo. Phil L.

David, I believe I seated the igniter all the way. I bent the lead slightly before installing all the way. One note, the I did lose some of the igniter material on the lead just as I was installing on this last flight. The igniter had about 50% of the material missing but appear to work . .
When I say 2nd or 3rd time, I meant I had installed and removed the same motor prior to flight, so I was sort of test fitting. When the O-rings were well lubed they'll threaded smoothly. But after the 2-3rd time the threads started to get tight. So I'm not sure if I messed up the o-ring or perhaps didn't tighten enough.

Phil, good catch, yes, I did use a 1g spacer in the 2g case, with a 1g grain. I didn't show a pic of the spacer. Perhaps I installed the spacer incorrectly, but it appeared to only fit one way.

The threads on the pro38s are plastic. The case is metal. It looks like they got cross threaded, which is a common problem. Best to install them once and not take apart until after it's fired.

Thank you for this. The threads didn't feel "right" when I re-installed the motor the 2nd or 3rd time but it didn't feel cross threaded. It went on the first time very smooth and I would have thought it would go back on easily enough. But you're right in that the plastic threads are seemed very soft so I could see how this would happen. If I cross threaded this I would expect this would have failed at exactly the place in the case as it did. Bummer of a mistake, live and learn! Thanks for the help!

OK, glad to see your response. Please note I was just asking a question here, not to imply anything. I think David is correct, although I've never tired to remove and reinstall one of these motors. The threads are just plastic, and I've wondered sometimes how they hold up to the pressure. I do like the CTI motors! Phil L.

Here you can see me screw in a used nozzle on my 38mm 2g case incorrectly doing this does destroy the threads after this it was impossible to thread the nozzle correctly back in. Check your threads on the nozzle that caused this case failure if they look like the one in this picture then that may be the cause but any way. i have never had a case failure and I'm almost old enough i can do the tripoli mentoring program...

Almost old enough*** sorry my spelling