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Making Clustered Rocket Motor Mounts

Article by Paul Nortness

In my previous writings, I have discussed cluster rockets and how they work. Today, I would like to show you how to build a clustered motor mount.

This is considered an advanced rocket building technique, so if this is your first experience with rocket building I recommend checking out my tutorial on basic rocket building first.

To follow along with this tutorial you will need a ruler, hobby knife, pencil, yellow wood glue and 5 minute epoxy glue. I will be building a cluster mount for three 24 millimeter motors.

I placed an order for parts with Balsa Machine Services. The parts list for this mount is three 3.75 inch BT-50 tubes, three 24 millimeter thrust blocks, three standard length engine hooks and two laser cut adapter rings. Once complete, this motor mount will fit into a BT-80 based rocket. When designing a clustered rocket, remember to use a body tube that is big enough to accommodate your cluster.

cluster motor mount

The first thing we want to do is mark 2.5 inches from the bottom of the body tubes. Take the hobby knife and make a little cut at the 2.5 inch mark. These slits will be for the engine hooks. Test fit the hooks to make sure you have cut a big enough slit. Now, take a flat piece of wood (I find wood coffee stirrers work great) and mark 1.25 inches. Apply yellow wood glue to the tip of the stick. Insert the stick into the top of the tube (the end closest to the slits) until the mark you made is flush with the end of the tube. Now spread the glue on the inside of the tube with the stick. Once you made a ring of glue around the inside of the tube, put in the thrust block.

cluster motor mount 1

Mark a spent motor casing 1 inch and use it to push the motor block into the tube, stopping at the 1 inch mark. Alternatively, if you do not want to mark a motor casing you can just leave the engine hook in the slit and press the thrust block in until it rests against the tab on the end of the hook. Don't forget to repeat this process for the other motor tubes.

cluster motor mount 2

Now, we want to mark the tubes for gluing. Press the tubes against a door jam and make a line on the opposite side of the engine hook slit. These lines will help us find the right spot to glue the tubes together. Once you have your lines, apply a bead of yellow wood glue along the line of one tube. Press a second tube into the glue so the lines match up. Let the glue set for about 15 minutes just to make sure nothing shifts. It is important the tubes get glued together straight so they fit properly into the adapter rings. After the glue sets, glue the third tube on so it makes a tiny body tube pyramid.

cluster motor mount 3 cluster motor mount 4

Alright, now the fun begins! It is time to swap out the yellow wood glue for epoxy. Epoxy glue is much stronger and will be able to tolerate the stresses three "D" motors will put on the mount. Insert the engine hooks. Take your first adapter ring and slide it over the top of the motor mount. Begin to slide the ring down towards the hooks. Once you get to the hooks, make sure they are straight and push the adapter ring over the hooks. Make a ring around the three tubes 1 inch from the bottom, carefully avoiding the metal hooks. Now, slide the adapter ring down onto the ring of epoxy. Now apply glue along the joint on the opposite side.

cluster motor mount 5 cluster motor mount 6

Our last step is the upper adapter ring. Put a ring of epoxy a half inch from the top and slide the adapter ring onto the mount. Push it into the glue and apply glue to the joint.

cluster motor mount 7

The construction of the three 24 millimeter cluster motor mount is complete. The only thing left to do is install it into the rocket. Because of the high amount of thrust involved, a motor mount like this should only be installed using epoxy. This mount could easily rip right off if it is installed with yellow wood glue. Building clustered motor mounts are the same regardless of the size. Just a suggestion, Estes "Big Bertha" is a perfect test bed for your first cluster and fits three 18 millimeter motor mounts like a glove!

Have fun and happy clustering!

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