As I’ve written extensively about before, it’s absolutely critical that your rocket has a good motor retention system. You don’t want the motor falling out of the bottom of the rocket. For one thing, it may land on someone’s head. More importantly, if the motor falls out, your rocket will not perform as intended.
There are different types of motor retention mechanisms, ranging from the extremely simple (e.g., wrap it with masking tape so it’s a tighter fit) to relatively simple (e.g., metal clips or hooks to keep it in place), to slightly more complicated but significantly more reliable hardware, such as a machined aluminum motor retainer. That’s what I went with for my previous rocket (54mm motor mount) and that’s what I’m using again here (75mm).
The main question for me was: how am I going to attach it? The aluminum motor retainer has pre-drilled holes for screws, but generally you would attach it directly to the closest centering ring, which would be the “aft” one, or the one closest to the bottom of the rocket.
With this rocket, everything is fiberglass, and I wasn’t sure about drilling this many small holes in a fiberglass centering ring (“CR”). The ring is extremely thin, and the screw inserts would extend far beyond it instead of sitting comfortably inside it. Plus, I needed something to fill a gap between where I wanted my aft CR to sit, and where the motor retainer would be located.
So for multiple reasons, I decided to buy and use an additional wooden CR (two, actually, as I needed to fill a 1/2″ space and each wooden CR is exactly 1/4″ in width).
It worked out well. I used wood glue to attach the two CRs together, and then after it dried, I drilled holes and inserted each of the metal screw inserts. I then attached the aluminum motor retainer to the wooden CRs with screws, as pictured here.
I was then able to attach the whole thing – additional CRs and metal retainer – to the motor mount tube using epoxy.
To be honest, this was one of the easier and more straightforward steps in the assembly of this rocket. This type of retainer is reliable, though, and now I have one less reason to worry when I launch.
There are still plenty of other things to worry about, plenty of things that can go wrong – sometimes catastrophically – and I’m sure at least one of them will.