Centering Rings


 The Thirfty Rocketeer blog continues...

 

My very first rocket ever built was the Freedom III, which was a skill level 2 rocket without much sophistication. In fact, the clerk tried to talk me out of it, as I was an adult with some wood working skills.

But I remember being SO SLOW and CAREFUL as I built it, as I had no idea what I was doing, nor how it could go wrong. (In fact, the things that DID go wrong were as a result of my choice of the wrong glue, --not the fault of the kit.)

Now, I recall being amazed that the punch-out centering rings would fit so smoothly, so securely into the body tube, that I was surprised to see the rocket coming together.  Later on, it dawned upon me that EVERY rocket must have some form of centering rings and that it would be a common design element.

I recently purchased a box of Big Red Max, the up-sized version of the classic Der Red Max. Everyone I spoke with urged that if you were going to use larger/stronger motors, you HAD to replace the card stock centering rings with Plywood rings, as well as fins.

Not knowing much about the kit, I went ahead and invested another $20 bucks or so in the pre-cut fins and centering rings package that was available. For the most part, they are exactly like the card stock except for this:

First, the forward centering ring has a very narrow "slot" pre-cut into it, the right size and shape for a thin ribbon. It took me a moment or two to realize this was for a shock cord, and I wondered why after an upgrade, they still used the flat rubber shock cord from the kit. Surely this could be a weak link.

Second, the plywood fins are beautifully pre-cut, and precise... they are not three pieces glued together.  They are entirely built of one piece and this probably makes them stronger.

Third, these improvements make the motor mount more secure, especially if you use epoxy.

Now, I thought I had BSI 15 minute epoxy on the shelf, but it turns out I replaced two empty bottles with BSI FIVE minute epoxy and THIRTY minute epoxy. .  I know that the 5 minute sets up incredibly fast, and is more brittle than the 30 minute variety, which is much stronger, but takes longer to cure. 

As a result, I'm building with the 30 minute BSI epoxy, though I wish I had 15 on hand. I don't.

Now the assembly is straight forward, almost child's play... and I started in with the first centering ring and moved onto the second one with my 30 minute epoxy fastening them in place.  Just prior to doing the third one, I froze, and realized I had committed an error.

It's common knowledge that if you want to filet the inside junctions of the through-the-wall fins and motor mount, you must leave the rear centering ring off until later.  I had started with it, and done the second slotted ring as well.  


Quickly my brain searched for an answer. Surely I had not painted myself into a corner AGAIN?

It was too late to try to pull off the rings and start over. I had to either go on from here and give up on the internal epoxy, or switch to wood glue, or something.

Then it dawned on me.  I could upend the motor mount, and insert it wrong end first, allowing me to epoxy the hell out of the insides before it dried and sealed up.

That is, if you number the centering rings from the BOTTOM up as installed, they would be 1, 2, 3  and you could glue them in that order.  But if you want access to the insides, and you would install them in the order 2, 3 then glue all, and 1 last.

The only variation this causes is a lack of a slot in the top centering ring for the shock cord. And a green spacing ring has been installed on the wrong side of the center ring. As a result, the pre-cut plywood fins won't fit quite as tightly.... but they're going to be epoxied in anyway, so who cares?   I think I could sand a little of the fin away to make room for the green ring, and again, the epoxy will fill anything that is not snug tight.

At least, that's my plan.

 

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