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How To Make A Model Rocket Motor Retainer

Article by Paul Nortness

Reloadable Motor Systems, also known as "RMS", give the rocket enthusiast greater choice of rocket propellants. Initially, the cost is rather high due to the metal case that you need to purchase to use the reload kits.

The cheapest case available is over $40.00 and can go as much as $800.00 (yes, I just said 800 bucks!). So you can imagine what a bummer it is if one of these cases gets lost.

Take a look at our how to reload an RMS motor video here.

Motor retention is important in any form of rocketry and even more so when using a reloadable system. Today, I will show you how to build a screw on retainer out of household products.

I will be building a retainer for a Aerotech 24/40 RMS case. The case is the same width and slightly longer than an Estes "D" motor.

rocket-motor-retainer

The nice thing about this case is I can use it in any Estes rockets that fly on "D" or "E" power with little to no modification. What I need to complete this build is a 12 ounce bottle of Gatorade (Gatorade not required, but a great thirst quencher!), 5 minute epoxy, 100 grit sand paper and a BT-50 body tube. For tools, I will need to use a Dremel with a cutting wheel and a sanding drum, a hand drill with a 3/4 inch blade bit and my trusty hobby knife.

rocket-motor-retainer-1

First thing I do is drink the Gatorade. This will empty the bottle and allow me to cut the threaded top off, and it is great on a hot day! Make sure the bottle is clean and dry before proceeding. I cut off the little plastic ring with my hobby knife to get it out of the way. Once dry, I will use the cutting wheel on the Dremel to remove the screw top from the bottle. BE CAREFUL! The Dremel more melts the plastic than it cuts it, and the melted plastic will be very hot to the touch. The cutting wheel will leave a pretty ugly line behind so I am going to need to hit it with the sanding drum. Smooth down all the rough places and knock off all the extra burrs of melted plastic left behind from the cutting wheel. The cut doesn't need to be completely flat, but the sides need to be flush.

rocket-motor-retainer-2

Once I have used the sanding drum to clean the piece, I will test fit it onto my BT-50 body tube. This will serve as the motor mount tube. Satisfied with the fit, I am almost ready to glue it into place. I just want to lightly sand the inside of the ring with my 100 grit sand paper. Why not do this with the sanding drum earlier, you may ask. Well, that is simple. All I want to do is create a rough surface for the glue to adhere to and the sanding drum would actually remove material. So, I lightly sand the inside until the surface has been roughed up and has some grooves.

rocket-motor-retainer-3 rocket-motor-retainer-4

Now, it's time to glue. I am using a two part epoxy glue. I want to mix equal part epoxy with hardener. I will do my mixing on a paper plate and use a wooden craft stick as my mixing and applicator. Once I have thoroughly mixed the epoxy and hardener, I am ready to begin. I just apply a bead of epoxy along the outside edge of the body tube and slide the ring over the tube until it is in place. This is five minute epoxy, so we should be able to handle it shortly. I mixed a little too much epoxy, so I will use the extra glue to put on my centering rings at this time. Because of the extra power E and F rocket motors have, I have switched out the fiber rings for plywood rings. I will need to ensure the rocket is stable since I have added extra weight to the tail.

While the epoxy is drying, let's take care of the lid. Using my hand drill with 3/4 inch blade bit, I want to make sure I am dead center in the middle of the lid. Going very slowly, I will drill a hole. I do this step in the garage using a piece of scrap wood to drill into. If the edges are rough, I can hit it with a grinding stone on the Dremel. Afterwards, I should have something that looks like this:

rocket-motor-retainer-5

Now that I have the lid taken care of I can test everything.

rocket-motor-retainer-6

And with the motor and end cap installed...

rocket-motor-retainer-7

So there you have it, a screw on retainer for reloadable motors. The great thing about this project is the cost. I was able to get a 6-pack of 12 ounce Gatorade bottles for $4.99. That is six motor retainers for 5 bucks! Compared to the commercially available retainers that sell for $15.00, I just saved myself 10 bucks! A couple things to remember though, as before mentioned I have added a lot of weight to the tail of the rocket. Because of that, any time you make a modification like this you must always make sure the rocket passes the stability test. Also, because the RMS case has a lip on the aft enclosure that holds the motor from moving forward, a thrust ring was not necessary. If you decide to modify this for use with black powder motors, just install a thrust ring and you will be fine.

Until next time, have fun and happy building!

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