How to build a fiberglass rocket, part 9: attaching fins

Exciting times! I’m ready to attach the fins.

If you’ve never put together a rocket before, well, I’m baffled that you are reading this blog. But typically, a rocket will have either 3 or 4 fins, which are placed symmetrically – equally spaced out, in the 360 degrees around the center. If 3 fins, then they’d be 120 degrees apart; if 4 fins, then 90 degree spaces.

This rocket has 3, and then another 3 aligned above them for a total of 6, each spaced out by 120 degrees. I think having 6 in this arrangement is purely aesthetic, as opposed to just having 3. Who knows?

I’ve used this two part epoxy before (resin and hardener) in one or two previous steps with this rocket construction. But this is the first time I’m using it in larger quantities.

rocket with fins attached, lying on workbench and partly suspended in air
back that up

Basically, I used a fin alignment guide (which I found online for free and printed out) to ensure that the fins were aligned properly, spaced exactly 120 degrees apart all the way around. I then prepared some of the epoxy and applied it with a popsicle stick (sophisticated technology), applying it to the edges or “roots” of each fin as if I were buttering a piece of toast. I inserted each fin into its slot, where the fin edge or root with the epoxy is pressed up against the motor mount inside. This is the first of several steps to ensure the fins are securely attached, starting with the interior.

To hold everything in place while this initial round of epoxy cures, I had a couple options. I saw some fancier solutions that other people have done involving using jigsaws and drills to cut out holes in large plywood sheets, and lots of vises and clamps.

That seemed like a lot of work, so I just used some rubber bands and propped the rocket/ fins up against some heavy objects like cans of paint or bricks while it cured (making sure nothing could move, and that the fins were aligned perfectly according to the alignment guide).

For a closer look at the epoxy, I included this photo as well, since it’s critical to this and the next few steps. I used West System epoxy as it was highly recommended, and it works great. The 105 is the epoxy resin, and 205 is the hardener. Each comes with a pump, and you just combine one pump of each into a mixing cup, and mix thoroughly for several minutes. It begins to cure pretty quickly, and the chemical reaction causes it to get extremely hot as it cures (to the point where it will burn you if you touch it, even through the plastic mixing cup, and steam is visibly coming off the top).

mixing epoxy on workbench
mixing two-part epoxy

For most of the fin attachment points, I’m also mixing in some chopped carbon fiber (pictured here as well) which is, in certain places, injected inside with a syringe. The carbon fiber greatly strengthens the epoxy as it cures.

Next, I’ll continue using the epoxy to attach the fins via this injection method, along the inside. After that, a final application of carbon fiber-infused epoxy on the outside of the rocket to create fillets (i.e., just a narrow strip of epoxy along each area where the fin touches the outer rocket body, shaped into a curve to minimize drag).

Things are really coming along – with the fins finally attached, it’s starting to look like a rocket!

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