Removable Motor Mount Module

For a new rocket for one of my children, I wanted to create a new rocket with the option to change motors from BP to AP. To that end, I created the rocket to have a removable motor mount module.

This is the first part of the work, which is a 3 x 24mm motor mount.



Greg

The interior has three 4-40 all-thread rods to support the plates, but one of the rods serves as a reference key. I tacked one of the rods in place with CA, then used epoxy to set the location.



It's not very pretty at all, but it works.

Greg

Any updates?

I may have an update this evening.

Thanks for asking.

Greg

GregGleason said:

I may have an update this evening.

Thanks for asking.

Greg

Click to expand...

Well, that was a long evening.

As it turned out, while the concept is cool and logical, the project from a detail design standpoint has been somewhat a challenge. After a few design iterations I think that I have come up with something that has a reasonable chance or working.

Greg

Here is the 3 x 24mm module:






And here are the parts for the 1 x 29mm module. The "Pac Man" supports will be glued to the motor tube and because they are angle cut will be self-aligning along the tube.



Greg

The Key Parts.

Here are the two key parts. The thinner part is glued onto the top of the module. The thicker part is glued inside the fin can.

Here they are apart ...





... and reunited.



A screw will be tapped into the aluminum plate from outside of the fin can.

Greg

Well, the "Pac Man" supports didn't work, so they were removed. They didn't stay in place and just drifted around, so they needed something a bit more positive to keep them in place. Also, it may be a 2-step process for it to work. Step One would be set the "Pac Man's" in place on the motor tube and let cure, followed by Step Two and glue the motor tube assembly to the coupler assembly.

So it was a "teachable moment" going forward on how to construct them. C'est la vie.

Thankfully I was using Aeropoxy structural epoxy rather than the "5 Minute" epoxy, so I was saved by the working time of the Aeropoxy.

It will be strong enough. I hope.

Greg

One of the last steps before the application of the epoxy was to wax, sand, wax the mating parts so that they would not have an affinity to stick to each other. For the wax I used mold release wax and gave 3 coats before the final buff. I then sanded the the bonding surfaces to make sure there was fresh wood for the epoxy to grab.

From left to right, (1) the 3 x 24 mm top ring, (2) the airframe thrust ring, and (3) the 1 x 29 mm top ring.



I used Aeropoxy structural adhesive for the bonding of the parts. I mixed it with a generous amount of milled fiber and a touch of fumed silica to make it a tad thicker. It took about 20 or 30 minutes to apply the epoxy inside the fin can and to move the airframe thrust ring into position.

Greg

Here are the two Motor Mount Modules (MMM) and the fin can curing after the application of the epoxy.

From left to right, (1) the 1 x 29 mm MMM, (2) the 3 x 24 mm MMM, and (3) the fin can (sans fins).



The 3 x 24 mm MMM is sitting inside a section of airframe for two reasons. The first was it was used as a positioning device to center the ring on top of the assembly. After the application of the epoxy and placing the parts together, I pushed them through the airframe section so that it would self-center. The second is that it serves as a stand because the center stud that serves as part of the motor retainer prevents it from being vertically oriented (i.e., it falls over).

Now comes the wait. I'll wait a day or two for the cure to complete and then I will know if I passed or failed this exercise.

Greg

Well, I couldn't wait. I figured that 3 hours in the 100+ heat would have them well on there way to be cured.

So I checked the one that fit the best first. That was the 3 x 24 mm MMM, which was the first one I had designed and fabricated.

After lining it up, I pushed it forward until it locked in place. But I wasn't quite sure that it was fully engaged, until I looked down from the forward end of the fincan. And there it was. Full contact liked I had seen dozens of time before during the test fit process. Well, I guess I was at least one for two.

Now it was time to check the 1 x 29 mm. Like the other one, it locked right in place. Two for two.

Just to get to this point was a big win.

But I am not out of the woods. The tap points need to have the ports created that go from the exterior of the fin can to the aluminum plate.

The taps are for 4-40 screws, and each MMM has it's own location. Therefore, there will be 2 ports on the airframe, about 3/16" apart.

Greg

Here are a couple of pics of the engagement of the MMM's inside the fin can.

You can see the two aluminum plates are together and flush at the top.

This is the 3 x 24 mm MMM:




This is the 1 x 29 mm MMM:



Greg

The final step was breaching the airframe for the threaded ports.

I tried a small drill bit, but got nowhere with that approach. I put the drill in vise grips and pushed, but the point facing was too flat. I was placing so much force that I was afraid that I would snap the bit. It's hard to believe but two wraps of ~6 oz fiberglass is tough stuff.

I then asked my wife if she had a pin or needle that I could use. She brought a pin and I placed those in the vice grips and pushed hard. Finally the pin pushed through. I then used a small diameter round file that reduced to a fine point to gradually open up the pin hole by rotating the file.

Once I had the holes opened, I could try to see if the 4-40 screws would work and if the concept was valid or not. I have experimented enough to know that have of my ideas don't work in the real world. But I had a pretty good feeling about this one.

Here are the results.

The 3 x 24 mm MMM:




The 1 x 29 mm MMM:



Once the screw was in the MMM would not move. After the screw was removed, the MMM came out fine.

I feel gratified that the concept worked, at least to this point. Obviously, the final test will be an actual flight test.

Greg