Big Bertha centering ring problem

Hi,

Today we had a disappointing attempt at launching my sons Big Bertha on its maiden voyage. As I was slipping the engine in the centering ring gave way and the tube was free to move within the main rocket tube.

I think I may have forgotten to reinforce that bottom ring with extra glue on the bottom.

We tried launching it anyway and it went up about 8 feet and came right back down and the engine burned out on the ground. As a result, the bottom centering ring is destroyed so I removed it. It looks to me like I can replace the centering ring and the rocket should fly again although the tube is slightly warped from the heat of the engine misfire.

How should I go about making a replacement centering ring? Any ideas on how to make the connection a little sturdier would be appreciated.

Thanks in advance. Steve

Estes sells or at least used to sell replacement centering rings, either as a set of centering rings or in larger products. Other rocket supply companies may also sell them, but I'm not sure who sells what nowadays.

However, you can make one yourself reasonably easily. If you don't have something that draws circles, I'd just draw the outer circle by tracing the diameter of the main tube, and the inner circle by tracing the engine mount (after measuring carefully to place the inner circle in the center--drawing two perpendicular chord lines and measuring the distance along them is probably the best way to go here). Obviously, tracing the outer diameter of each tube will give you a circle that's a little bigger than what you need, so cut out inside the line, and you may need to trim it a bit.

There are also computer programs out there to allow you to print out templates, but I don't know if any of them are freeware.

As for attaching it more securely, my advice is to make sure you use plenty of glue (wood glue is probably best for this) on both rings. After you get the centering rings in place on the engine mount tube, deposit glue around the entire circumference where the ring meets the tube (do it on the front side, so it doesn't mess up the appearance of the model from the exterior). Once that glue is dry and you're ready to place the engine mount into the rocket, spread a line of glue around the entire diameter of the main tube, then insert the front end of the engine mount assembly, and then, once the front ring is in, apply another line in the tube, to glue in the back ring. You can also place glue directly on the rings before inserting them, but that tends to squeeze glue out around the edges, making a mess.

In any case, if you have this rocket for long, you'll probably have to replace the engine mount occasionally, sometimes requiring new centering rings: this is especially likely when it's a humid day (or the rocket gets wet, say in snow) and you forget to take the engine out before it swells with humidity. I've had the mount on my Bertha-ish rocket destroyed a couple of times this way, but I've never had it give way under power.

Scott Orr

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Here is a script I discovered awhile back. It is for generating transitions from one body-tube diameter to another, but by using a length of "0", one can generate a centering ring for whatever inner and oute diameter one needs. It is a simple .ps (PostScript) which virtually any printer on Earth will recognize and print. Just edit the relevant lines using your preferred text-editor, and resave it. After printing the file, just tack-glue it onto card-stock, cardboard, wood, etc., and use is as a guide to cut out your ring or transition. Note - you need to save *all* of each line, even the %'s on the beginning of a line, as they are comments.

Here is the script:

Here is also one, by the same author, for generating "body wraps" to use as fin alignment guides.