38mm Retainer and Centering Rings for Mid and High Power Rockets

TL;DR If you are in need of a 38mm motor retainer for your rocket, this might be a solution for you. It contains variations of centering rings and retainer options to hopefully suit your needs, or at least give you a head-start.

Introduction:

This design contains a 38mm high power rocket motor retainer with cap, ring, and an optional ring with flange. In addition, I have included centering rings that include a blank (mid), one with two, 6mm holes (forward), and one with eight, 3mm holes that pairs perfectly with the retainer ring with flange.

Heads up:

I currently do not have a 38mm motor case to test this with. I tested it with an Aerotech 29/240 case with a 38-29mm adapter. While I have tried to make this work, there may be some tolerance issues. If so, please let me know. Also, I will be getting some motor cases for 38mm and 54mm so this shouldn't be a question in the future.

Background:

I am currently designing a scratch-built rocket similar to the LOC 3-in Nuke High Power Rocket (HPR). I wanted to build everything that I can with 3D printed parts, to demonstrate that 3D printing is viable for HPRs. I have had great success with printed retainers in the past but most of them borrowed from others' designs. These are 100% original.

Parts Details:

- Retainer Cap - This is a bigger cap than you might see from others. I made it beefy so that it could withstand some of the heat since it's on the business end of the rocket. This should extend from the rocket by ~ ¼ to ½ in. past the aft of the body tube or however you prefer. It has a recess that will allow for Aerotech motors and can accommodate a 38 to 29mm motor adaptor with ring.

- Ring - This is a simple, threaded ring. It should be mounted flush with the aft centering ring.

- Ring with Flange - This part is designed so you should not have to glue/affix/adhere the ring to the aft centering ring with anything but hardware. I designed it to use 3mm machine screws that mate with heat-pressed inserts for the aft centering ring. At the time of this publishing, I have not completed assembly of this feature, so be careful how you assemble (see printing suggestions). Please note that this is undersized and cannot/should not be used as a centering ring. It is intended to be attached to the aft centering ring so that it can be replaced if necessary. You could also use a plywood or fiberglass aft ring and drill out the holes.

- Centering Rings - You don't have to use any of these by their designation. You can you use the Mid CR for all your needs. I just tried to provide some options

- Forward Centering Ring - This ring has two holes for eye bolts or your favorite recovery method. You could print three and run 1/4x20 all thread through all to stiffen the fin can.

- Mid Centering Ring - this is just a plain, 3in x 38mm CR to use as you will. I typically will use this to “pinch” the find between it and the aft CR to give extra surface area for strength.

- Aft Centering Ring - This ring was designed to mate with the Ring with Flange model. You could use heat-press inserts with 3mm machine screws or remix it and include nuts in the design. Printing Suggestions:

You should consider the forces that will be applied to each part in order to ensure the best chance of success. These parts are designed for high power rockets and as such will be under more stresses during lift off and recovery than their low or mid power counterparts.

Centering rings transfer thrust force from the motor to the body tube by restricting the movement of the motor mount tube. To be successful, the rings need to be ridged and well affixed to the body tube and motor mount tube. High temp epoxy is your friend, here.

I recommend printing with PETG for these parts with =>50% infill. Make sure you are getting good layer adhesion. These are not meant to be pretty, they are meant to be strong

Disclaimer:

Please keep in mind that failure IS and option. You assume responsibility for your rockets and what they do when flying. Please exercise critical judgement when using these or any parts for your rockets and if anything seems sketchy, it probably is.

My intention with these parts is to share my design with others so they can take them to the next level or help those new to rocketry get going easier. These parts were designed with LOC/Precision tubes in mind and will probably not work with others. Please check the inner diameter of your body tubes and outer diameters for your motor mount tubes before printing.

Rocketry is a safe and enjoyable pastime and can be rewarding for everyone when done so safely. Please refer to the National Association of Rocketry's (NAR) Safety Code and be familiar with it before attempting to launch any rocket.

Also, you can swing by the Rocketry Forum and ask questions there. Thousands of amateur rocketeers from all over the world live there and are eager to answer your questions. Please feel free to reach out to me if you need assistance with this model