54MM Motor Mount Repair

[bchaltraw]

I had a Cato this past weekend in a custom rocket I built. The motor burned out the edge of the engine case and burnt off 2" of exposed MMT. I plan on using a hole saw and cut into the aft centering ring, go a couple of inches up into a good section of MMT, removing the burnt section putting a coupler outside of the MMT, and inserting a new section of MMT. Believe a 2.2 ebay switch band would work for the coupler as it needs to be outside of the motor mount and would support the new MMT section and connect to two pieces so I can put the boattail back on this rocket. I wanted to run it by you all to see if my thinking has a flaw.

 

[bchaltraw]

Before the questions come flying about how many centering rings there are, there are more than enough to support the MMT. Overkill, maybe but when I put a K445 in it, I'd like to make sure there is plenty to put the push in the right direction.

 

[teepot]

I don't see why it wouldn't work.

 

[bjphoenix]

It will require careful surgery to get the old parts out and make room for the new coupler, that's the only new problem.

 

[Zyzzyva1000]

While I think your idea would work (assuming you really want to re-add the boat tail), I had something similar happen. I just ended up cutting off the MMT flush with the final centering ring and changing to a Z clip style retention system (I had put 2 part foam around the MMT where the fin tabs were, so there was no removing anything)

 

[bchaltraw]

I have a wooden dowl that will fit inside of the MMT; I will put the centering drill bit of the hole saw into that and use it as a guide to cut around the MMT and cut a round slot in the aft centering ring. I have 6" of space between the two centering rings. So I will have a solid guide for the hole saw, within the MMT it and the MMT is still good from the aft centering ring forward, so I am going to use a 2.2 body tube, split it to make a coupler, attach it to a good section of MMT and the space left from the cut in the centering ring will be filled with the coupler to give me a place to resecure the fins to the MMT.

 

[bchaltraw]

 

[bchaltraw]

It would not be an Astro Sprint if it did not have a boat tail.

 

[bchaltraw]

The Astro Sprint will rise again!! I put a lot of thought into fixing the MMT being burnt to the rear centering ring; I'm pretty proud of myself; the plan worked! I wanted to keep the boat tail to keep with the Sprint theme. Now I have 2” inside the rocket to slide my coupler to join the new piece of MMT on. I took a 54mm body coupler, put a pool noodle in it, then a dowel that I center drilled to slide the hole saw centering bit into, and went to town.

 

[Banzai88]

Very creative! Great repair!

 

[mrwalsh85]

FWIW, I had to replace the MMT tube in my Scorpius. Since the bottom fin can shattered off, I went ahead and ripped off the body tube. I will splice after it is all done up.

https://www.rocketryforum.com/threads/upscale-estes-scorpius.56999/#post-2393550
Photos explain how you can pull the old MMT (if it is a LOC paper style tube) out and still maintain your centering rings without having crazy surgery. In fact, if you were careful enough, you could probably just soak the MMT tube in water, and it would go a lot easier. But I was in the shop and I am stubborn so I went at it with needle nose pliers and twisted and twisted and twisted until the entire tube came out. A little work with a flap wheel and/or drum sander and you can slide a new MMT in.

Can't wait to get this thing up and flying again!