Do I really need that aft centering ring?

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blackwing94

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I'm building a fiberglass Madcow (wildman) X-celerator. It's a split fin rocket. The motor mount tube that comes with the kit seems to be about a 1/4 inch to short. Or maybe I just built it wrong. Anyway, on the motor mount tube I have the top centering ring followed by 6 through the wall fiberglass fins epoxied to the motor mount. I'm left with just barely enough motor mount tube to put on an aeropack 54mm retainer. If I add in the aft centering ring, the retainer will have very little real-estate to JB weld on to. My question is, with all the fins holding the motor mount in place, do I really need the aft centering ring? Just wondering.

That would make it a Scottish rocket with the up the kilt wind blowing. Without the aft centering ring there's lotsa room to stick a camera in there.
 
As long as the forward CR is sealed to keep in the ejection gasses, and the fins are firmly attached to the motor tube (with internal fillets), the only reasons I can think of is to help align the motor tube, to have an aft CR help act as a thrust plate, and to reinforce the rear of the rocket (preventing damage on hard landings, or knocks...)

My vote... No, it's not needed.

You might want to add a short length of coupler in there to help reinforce the back end just a little, but it's optional.
 
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I think you will be fine without it. If you want a little extra get some of the PML urethane foam, and fill the space between the body, find, and motor mount.
 
The ring keeps fire out of the tube. Look at rockets that have been launched many times the aft ring is roasted.

M
 
Thanks for the replies. I have to say I never even considered that a aft centering ring might be optional in some cases. I've never questioned it.
 
Another time the aft ring comes in handy is during landings when the motor nozzle is the first thing that comes in contact with the ground.
 
I think you will be fine without it. If you want a little extra get some of the PML urethane foam, and fill the space between the body, find, and motor mount.
+1! (BTW- he meant 'fins') I injected foam, cured and trimmed it back and then carefully sealed the back end with JB weld AFTER I mounted the retainer. Mix up your epoxy or whatever and throw it in a sandwich bag-viola'! An ersatz icing bag making everything easier to control. Just don't cut too big a hole in the corner and everything will stay neat.
 
+1! (BTW- he meant 'fins') I injected foam, cured and trimmed it back and then carefully sealed the back end with JB weld AFTER I mounted the retainer. Mix up your epoxy or whatever and throw it in a sandwich bag-viola'! An ersatz icing bag making everything easier to control. Just don't cut too big a hole in the corner and everything will stay neat.

Yep, I meant fins. Damned spell check on phone.
 
Foam sounds like a fun plan. I've never used foam before. This is a 4" rocket so I would need to fill 24 inches of body tube (minus the motor mount tube) with foam.

After I slept on the problem, I am considering making the motor mount hole in the centering ring a little bigger so it fits around the outside of the retainer. I would have to epoxy/jbweld them in at the same time.
 
Problem solved. As advised, I enlarged the ID of the centering ring to include the retainer.

There are few problems in life that can't be solved by the judicious application of a Dremel. :cool:
 
The Madcow X-Celerator is from Rocketry Warehouse, not WIldman, FYI.

I had the same problem with mine, years ago before Madcow took over. I complained to RW, and they sent me a longer MMT at their expense. Might be worth a call to Mike.
 
Use it, the ring, and spray with BBQ paint as an ablative..
 
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