Need help fixing cluster MMt

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Well-Known Member
Apr 17, 2003
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I messed up big time. While I was painting my 3x Orbital Transport I damaged one of the four motor tubes.
What happened was I took a dowel and wraped masking tape so it would fit snug. But when I inserted the rod into one of the four tubes I did not place it all the way inside the motor tube.

I then placed the booster in my bike stand so I could paint it. Afetr painting I went inside to do other thing and I came out the tube had kinked and ripped.

Attached is a photo of the rocket in the stand raedy to be painted.
As you can see in the last post the rocket is bending as I took the picture. When I came back out the tube had colapsed and I brought it in and tried to remove the tube.
I was able to get most of the aft tube but most of the tube is still stuck to the forward centering ring. seephoto.

What can I do to save this rocket?
How do I get the rest of the tube out of the booster?
Do I plug it and fly on three motors?
Do I insert smaller tubes and insert C's?

Please help:(
Here is what I would do:

Because the remaining three motor tubes are buggered, replace them all. I'd use a small diameter drum sander, 80 grit. Chuck it into a drill, or use a Dremel with barrel sander, and start removing the motor tubes. After the tubes are out sand out the aft centering ring, carefully. Use an extension for the drum sander and sand the forward ring out, carefully. Build another 4 cluster motor mount assembly. Use epoxy internally and slide your new motor mount into place. It will be some work but this is a great looking rocket and will be worth the time.
If those other MMT's are still strong enough to hold the motors, (remember that the stiffness of the motor will hold the MMT's rigid when inserted), you might be able to get away with just repairing and replacing the single tube.

Start by cleaning out all the damaged area to leave a clean attachment surface for the new tube. Cut the appropriate length and attach the engine hook as you did for the others. To attach the new tube, I would put a large excess of epoxy on the forward centering ring around the outside of the tube hole, then quickly place the tube into position and turn the model vertically, nose-down. This will allow the excess epoxy to flow around the tube and form a solid contact joint/fillet. If possible, spread a little epoxy on the joints between the new tube and the other adjacent tubes, so they can lend support to each other. Finish it off with a solid epoxy fillet around the aft end of the new tube, and it should hold against your 24mm motors.

My theory...

After looking at the remaining tubes I decided to follow Swimmer's advice and rebuild the MMT.

Iused my dremel and my drum sander attachment for my hand drill and cleaned out the old MMT. I was unable to remove the forward ring from the BT because it was epoxyed in to the reduction chamber. Therefore I will shorten the new MMT to account for this.

Here is a pic of the BT with the MMt removed. You can see the forward ringwith the reduction chamber the feads into the BT 60 tube.
Wow.... great job.:) Going to be a very nice repair. Be sure to post pics as you repair.
WOW! One of the neatest repairs I've seen to date! You did a realy good job of dremeling out the motor tubes ! Now just replace the tubes with new ones :)
I spent the night building the new MMT to place in the booster. This time I have decided to forego the motor clip as the retaining device. I will use a bolt and washer to hold the motors in place.

In the attached photo, the tubes are glued to the forward MMT ring. The rear ring is not glued in place. I will paint the tubes before placing them in the booster. I think that I can paint them much easier that way then after they have been glued to the booster.
In this photo, the MMT is in place but not epoxyed to the BT. As I said before I want to paint the tubes before glueing.

One question I have is should I put a layer of epoxy in the area of the motor tube to protect it from the ejection blast. I did line the reduction chamber with aluminum tape and coated it with epoxy. I also lined the BT-60 stuffer tube with nomex.(?)

I guess that I just answered my own question. I will coat it with epoxy. I plan to use D/E's to fly her.
I have made progress with the new MMt. I have primed the mount and I am ready to epoxy it to the booster. In the attached pic yoy can see that it has been primmed. I aft ring in shown in possion but it has not been glued.

You can also see that I epoxyed the forward ring with a good ammount of material.
In this pic you can see that also epoxyed a male/male coupler in the center of the MMT so I can use a screw and washer to keep the motors in place during the ejection phase.
Well the deed has been completed. I used two hour epoxy on the foward ring and let it set while the booster sat in an upright postion. After it set, I then used thick CYA to seal the aft ring. You can see the screw in place that will be used to retain the motors.

Now I can continue with the painting.

What did I tell ya?? You did a fantastic job on this repair. It was a lot of work, I'm sure, to remove and replace the assembly but the results are worth it. Keep us posted.