How to build a fiberglass rocket, part 4: motor mount

With the airframe of the rocket nearly complete, I just needed to prep the area where fins will eventually go. The rocket is pre-slotted (i.e., it comes with slots already punched out to insert the edge of the fins), but the slots are all too narrow and needed to be sanded quite a bit to widen them.

In addition, I drilled 12 individual holes (one for each side of all 6 fins). Later in this assembly, I’m going to insert the fins into these slots, where their edges will be up against the motor mount tube inside this airframe. I’ll then inject epoxy with a syringe into each hole, and tilt the rocket back and forth to spread it around, ensuring that the fins are strongly secured in place both internally and externally. But I’m getting ahead of myself.

booster section of rocket airframe, with slots for fins
pre-slotted airframe

On to the motor mount tube!

This 75mm fiberglass tube has a slightly smaller diameter than the 4″ rocket airframe. (To be clear, I have no idea why the motor mount tubes are almost universally measured in metric units – 54mm, 75mm, and 98 mm being fairly common in high power rocketry – while the airframe itself is measured in inches. It’s a mystery for the ages.)

There are 4 beige colored fiberglass centering rings: the inner diameter of each ring fits snugly around the motor mount, and the outer diameter of each ring fits inside the larger airframe. The purpose of these rings, as the name implies, is to center the motor mount inside the airframe.

red motor mount with yellow kevlar recovery harness on workbench
motor mount with kevlar recovery harness

The primary goal here is to secure the yellow kevlar recovery harness to the motor mount. Later, I’ll attach a much longer kevlar cord to this one, and the other end of that cord will attach to one end of the e-bay (with a parachute attached as well).

This basically makes sure that the bottom part of the rocket, including the motor, stays linked to the e-bay in the middle of the rocket – and also makes sure that a parachute can deploy, when these parts separate after apogee. Since there’s nothing obvious to hook or attach this cord to on the motor mount, the solution is to simply epoxy it directly to the motor mount.

I measured the width of the cord (1 inch) and marked it on the top centering ring, and then sanded down a 1 inch width on both sides of the inner part of the ring, to allow just enough space for the cord to fit between the ring and the fiberglass tube. About 6 inches of cord are on each side of the tube.

motor mount with recovery harness tucked inside centering ring
before: tucked inside centering ring

After that, I created some very crude “dams” with masking tape since the epoxy is a bit runny before it cures. I put a generous amount of epoxy underneath the cord to bond it to the tube, and then even more on top of the cord, in order to totally encapsulate it.

Here you can see a “before” and “after” picture. I couldn’t quite get all the masking tape off afterwards because some was sealed and bonded (somehow I did not foresee this). But the cord is totally encapsulated. When the epoxy cures, it becomes incredibly hard and is similar to plastic.

recovery harness sealed in epoxy
after: sealed in epoxy

The recovery harness here is now thoroughly secured to the motor mount.

A few notes on epoxy, as this was my first time ever using it. It’s pretty straightforward, but there’s a slight learning curve. I used West System 105 resin and 205 hardener: these are two separate products that come in separate containers with pumps. You add them together (in a ratio of one pump each) into a mixing cup, and then mix them together (I used a popsicle stick) very thoroughly, for several minutes.

Once mixed, the epoxy begins to harden and cure much faster than I initially realized. It also gets very hot, from the chemical reaction – to the point where it’s literally giving off visible steam, and the heat from touching the outside of the plastic mixing cup will burn your fingers.

It’s also a bit runny when spreading, so it really helps to create a barrier or dam with masking tape to keep the epoxy where you want it, as it cures. The tape can easily be removed later.

I was previously used to working with wood glue for cardboard rocket sections and plywood fins, but fiberglass is a whole new experience. Wherever fiberglass pieces need to be permanently attached (e.g., the fins to the rocket body), this two-part epoxy is used, and it’s amazingly strong.

Next up: the nosecone.

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