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Method for Creating
Centering Rings and Bulkheads
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By Peter Stanley

Introduction:

Using “Engine Mount Template Generator” and thin corrugated USPS Priority Mail boxes (Both are freely available.), you can quickly and easily create small to mid-sized custom rings and bulkheads. You’ll also need a printer, spray adhesive, and a hobby knife.

Software:

I found this freeware suite of rocketry tools online a few years ago. The programs are old and windows based, but I’m currently running them on Windows XP without problems. The author states that this can be freely distributed [EMRR Tools Page].

The tools in the zip file are the following: an Elliptical Fin Template Generator, an Engine Mount Template Generator, a wrap around fin guide, and a shroud designer. To use this software, extract all the files (including the .dll file) into a folder somewhere. You can create shortcuts to the files, or just run them directly from that directory. Engine Mount Template Generator (EMTG) is the program used for this tip.

Materials:

  1. USPS Priority Mail cardboard size 0-1096S. This is a very thin corrugated cardboard designed for shipping VHS and related sized packages. You can order this totally free (even free shipping) from the USPS website. It is good to have this kind of stuff around anyway in case you to ever need to ship something Priority Mail. Here is the link to the actual box http://shop.usps.com/cgi-bin/vsbv/postal_store_non_ssl/display_products/productDetail.jsp?OID=1003571 In case that ever changes you can go to http://www.usps.com and browse to Priority Mail shipping supplies. There are probably other boxes that have the same thickness, but I know this one works.
  2. Spray adhesive, such as Duro All-Purpose Spray Adhesive.
  3. Hobby knife

Overview of Process:

  1. Print centering ring or bulkhead template using EMTG, onto plain paper.
  2. Rough cut around the template with scissors, and affix to a piece of O-1096S (or similarly thick cardboard) using spray adhesive.
  3. Cut out with hobby knife.

Step-by-Step:

 

Centering ring:

In this implementation I am creating a ring that will fit between a BT-55 and BT-60 tube, which I needed for a tailcone transition.

  1. Determine the size you need. The inside circle needs to be the BT-55 exterior diameter and the outside circle should correspond to the BT-60 interior diameter. The outside diameter of a BT-55 tube is 1.32 inches. The inside diameter of a BT-60 tube is 1.59 inches.
  1. Open EMTG and enter the following parameters, and click PRINT

  1. Your output should look like this

 

  1. Cut out the image and affix to Priority Mail box using spray adhesive.

 

  1. Cut out the inside circle first, then the outside circle. The best way I have found to do this is first start with an up and down sawing motion using a fresh hobby knife blade. Follow the line slowly, rotating the piece around until you have made one complete revolution. Then carefully drag the blade over the cut, rotating the piece until you have cut through completely.

I could have used this method for the other rings in the assembly, but I happened to have them already.

Bulkhead:

In this implementation I am creating a bulkhead that will fit inside a BT-60 coupler tube for a payload bay. The inside diameter of my BT-60 coupler tube is 1.524 inches.

Use the same technique for centering rings, except change how setup EMTG. See below.

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