Now that the fins are tacked, let's reinforce the airframe-to-fin and motor tube-to-fin joints. Remember, this is adding weight to the wrong end of the rocket, so a little goes a long way here...keep it light but strong!
External Fillets
It is wise to start on the external fillets. If there are any gaps for epoxy to leak through and you start with the internals, you may have a mess to clean up! Thus, let’s get a good seal on the outside, then make a mess of the inside!
If there are any cavernous gaps between the fins and the edge of the airframe, it may be wise to fill them, as even the *best* filleting epoxies will sag over unsupported areas. As mentioned, the fins fit the slots perfectly; however with some bevels on the fins, there are gaps at the leading and trailing edges.
These gaps are filled with epoxy filled with a bit of cabosil. All sides are done with a single batch of epoxy.
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*Unless you have a super-stout filleting blend, perform one set of external fillets at a time to avoid sags*
Clean the area to be bonded with a tack cloth to remove any dust or debris.
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I'm using Rocketpoxy for the external fillets. Mix up a batch, plop it into place along the fin-to-airframe joint. *if you're concerned with excess, tape off the areas just outside the fillet edges to keep things clean; I skipped that step here.*
Begin to smooth the epoxy with a filleting tool. Some use round rods, dowels, pipes, or oily fingers; I prefer the West System sticks mentioned earlier. For the first pass or three, the stick is held at a shallow angle relative to the length of the airframe; resulting in a deep fillet and a fairly even distribution of epoxy.
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After the first few passes, the angle of the magic stick is increased, resulting in a smoother fillet, less epoxy, and the desired result. The steeper the angle, the more epoxy you're pulling around, and the more that may be removed from the fillet (that's a good thing!). Any epoxy that spills over the edge of the stick is removed by carefully scraping the other end of the stick along the spillover. For reference, I used a 20 gram batch of epoxy for each pair of fillets, with roughly 4 grams leftover from the scrapings, so roughly 8 grams per fillet.
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The end result looks like this. Allow to cure in position so gravity doesn't cause your fillets to sag. As thick as Rocketpoxy is, it will sag courtesy of gravity until it reaches the "green" stage.
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Internal Fillets
I'm using the mid cure (12-20 minute cure) epoxy for internal fillets. The fin-to-motor tube joints and the fin-to-airframe joints will be epoxied in three batches; one for each 120 degree rotation.
Remove the aft centering ring by pulling on the two screws installed earlier. Epoxy is mixed and a moderate amount is poured into the locations shown with red circles below.
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The Booster is placed in a nose-down position so the epoxy may flow down the length of the joints thanks to gravity's fine work. A thin dowel is used to help the epoxy along its path when and where necessary.
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After the epoxy beads have traveled the length of the fin's root, the rocket is returned to a horizontal position to allow the epoxy to self level and settle into the joints evenly. Allow to cure.
Repeat for remaining two sides.
More to come...