Adapting a 98mm Rocket for a 75mm Motor

Below is a link that tells the story with pictures and words. In a nutshell, my AMRAAM bay was 98mm x 36" and the top of the motor mount was capped with a 1/8" thick G10 Bulkplate and 1/2" Ply Bulkhead. I wanted to use the new Animal Motor Works 7600ns Motors, which is a 75mm motor 41" in length. Hence, I had to figure a way of drilling out the top bulkhead in my 98mm Mount so a 75MM Motor Mount Adapter and it's 7600ns Motor would fit. Here is how it was done:

https://www.rocketryforum.com/ctulanko/album/index.php?dir=amraammod

Hope this helps someone in the future...it taught me to "overbuild" motor mount length, so the bay for my HARM is 42" deep AND uncapped!

Enjoy,

Carl

The step-by-step photo instructions (provided in your hyperlink) is very informative. My question is, why did you cap your motor mount tube (MMT) in the first place? If you have positive motor retention, you normally shouldn't need to have a "thrust block"-type arrangement for your motor. Why not leave the top part of the MMT open, so that you can select various lengths of RMS rocket casings?

Good questions, dluders.

During my initial design, I had assumed the model would build heavy; it's just the way I build (strength adds weight and my theory is you can never overbuild...if so, just use a bigger motor.) Moreover, I had planned that the lower closure of the motor casing would perform the majority of lifting stress, but wanted to relieve that somewhat by adding additional support between the top of the MMT and the "Cap" in question.

After the build was underway, I decided on an AeroPack Retainer, which removed the remote possibility of the RMS rear closure digging into the bottom plywood CR, as the retainer distributed the pressure across it's larger mounting surface. Overkill was built in, but alas was never needed or used and I was optimistic about Aerotech getting back in business. Now, more than a year later, they are still not producing so I decided to modify my bird for the AMW motors, as they are readily available.

Needless to say, my 75% HARM AGM-88A was designed and built with lessons learned and uses a Hawk Mountain 98mm Fiberglass Motor Mount long enough to accomedate longer motors, all the way up to the AT N4800 if desired. It is also compatable with the longer AMW motors. The problem still existed though, how to successfully drill a 3" hole dead center through 3/4" of glass and wood...there, I got an idea...and lucky.

Carl

Dude! I knew I hung around you for something. Very ingenious thinking! I have some 54 kwik switches that I'll be using that contraption for.

Shane