How to build a fiberglass rocket, part 3: airframe prep

My first couple of posts related to the Darkstar Extreme were just recapping my progress in high power rocketry to date, and outlining everything that’s needed in order to build this particular rocket. But now I’m finally ready to begin assembly.

As mentioned previously, the “kit” basically includes all of the rocket airframe parts (shown below), along with some nylon recovery harnesses and miscellaneous hardware (steel screws, nuts, washers, forged eye bolts, and quick links). The first thing I did after unboxing everything was soak the fiberglass pieces in water for 24 hours, to remove any remaining mold release agent. In other words, as the proud parent of a new rocket, one of the first things you should do is to give it a proper bath.

fiberglass rocket airframe parts, soaking in water pre-assembly
exciting, fast-paced action!

After rinsing off and drying each piece, I moved everything out to the workshop. Time to begin construction of the workshop’s inaugural rocket.

Just to provide some overall structure for what I’m planning to do here: the idea is to assemble the fiberglass airframe, but in a way that allows it to separate at multiple key points in the future. In certain places I’ll use epoxy to permanently attach pieces together, but in several other locations I’ll need to measure and drill holes, and then insert small nylon screws (“shear pins”) which are strong enough to hold the pieces of the airframe together, but which also have the ability to shear in half when sufficient force is applied (e.g. a small controlled explosion), allowing the rocket to separate and a parachute to deploy.

fiberglass rocket airframe parts on workbench
marking the airframe for future drilling

To help visualize how all these pieces go together, the major components of this airframe (from top to bottom, when the rocket is standing vertically on the launch pad) are: the nosecone (grey, with aluminum tip) permanently epoxied to a 6″ coupler; a 24″ payload section; an 11″ coupler which serves as an electronics bay; and a 52″ booster section. There is also a 1.5″ band or ring that fits around the e-bay/ coupler, and six fins (three larger, three smaller). Inside the booster section is a motor mount with a smaller 75mm diameter and 4 centering rings.

fiberglass rocket airframe pieces on workbench, pre-assembly
labeled for your viewing pleasure

Here you can see a lot of measuring, marking, and drilling on the airframe. More specifically, there are 3 holes drilled in the nosecone/ coupler and the payload section, which can then be secured together (and later separated) with shear pins. Another 3 holes and shear pins connect the “bottom” of the e-bay/ coupler to the long booster section. And then three more holes – this time plugged with steel screws serving as rivets – connect the “top” of the e-bay/ coupler to the payload section. These steel rivets ensure the payload section does not separate from the e-bay during flight, but they allow diassembly on the ground by removing the rivets, if needed.

fiberglass rocket airframe parts, pre-assembly
crude but effective sanding technique

Finally, in the middle 1.5″ of the e-bay/ coupler, I marked the location of the band or ring that will be secured to the coupler with epoxy, shortly after this.

Aside from measuring, labeling, and drilling, I also needed to sand many parts of the fiberglass airframe. In general, it’s helpful to sand anywhere that epoxy will be used to ensure better bonding. This includes a few external areas (like the one pictured above), as well as the areas on the centering rings where they will touch the motor mount and the booster section; the edges of all six fins, along with the areas that the fins will touch on both the motor mount and booster section, and so on. Lots of sanding here with coarse (60 grit) sandpaper.

So begins the thrilling assembly of the Darkstar Extreme.

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