Method of Internal Filleting... Split Fins.

OK, I'm a fan of internal filleting. I realize that it may add weight, but I've seen how one of my fins took a hard hit (read: a heavy metal
restaurant-type chair tipped over and fell on it) that would have sheared off a surface mounted fin, and (after repairs to the damaged leading edge) will look none the worse for it.

I have a pair of split fin rockets that I'm building now. The Velociraptor, and the Raptor from Binder Designs. With the way that the fins are laid out internal filleting isn't possible. Or is it?

What I'm going to do is this:

Sand all CR so that they can be inserted over the motor tube, and inside the body tube smoothly, yet still be snug. Drill 2 or more holes into the aftmost CR and temporarily add eye bolts to allow it to be inserted into the body tube and then fished out as needed.

If I had opted for the stock configuration, I'd install the mounting equipment to forwardmost CR and use thread tightening compound (or epoxy) to ensure it stays secure.

Glue the forwardmost CR in place on the motor tube, and fillet it. Aft of the forward CR, wrap the motor tube with wax paper to prevent Epoxy drips from forming on it and preventing later CR's from being installed. Slide the assembly into the body tube taking care to ensure that it's in the correct location.

Take other motor tube and CR, and insert it into the top of the body tube, using the dowel to "pin" the motor tubes together. This will hold the motor tube steady while the epoxy cures. Apply epoxy to the back side of the Stock Motor tube to fillet it into place (and seal the rear of the motor tube assembly and prevent venting issue with the ejection charge). Allow Epoxy to set.

After epoxy has set, remove the wax paper from the motor tube, and fit the aftmost CR into place. Remove the dowel, and the spare motor tube. Wrap a small portion of the dowel with wax paper, and insert this into the motor tube to prevent epoxy from dripping into it. Apply epoxy to the front of the forwardmost CR to ensure a solid instillation and complete the seal. Allow epoxy to set.

Next, remove the dowel, and glue in the forward set of split fins, using whatever form of alignment guides you desire. You might want to temporarily use the dowel and the spare motor tube to ensure internal alignment while the fins set up. After the glue has set. Wrap the motor tube with wax paper again, sliding it up to the back of the fin tabs, again preventing accidental drips from preventing the remaining CR's from being installed. Fillet the fins, and allow the epoxy to set. Then remove the wax paper, and slide the next CR into place. Wrap the motor tube with wax paper again, and apply epoxy to fillet the CR into place.

Once that epoxy is set, remove the wax paper, and install any CR's that may be behind the second CR (ahead of the next set of fins) and then protect the motor tube with wax paper again. After that has set up, Glue in the next course of fins, and internal fillet them. Finally glue on the aftmost CR, remove the eye bolts and seal the holes created for them (I'd use bamboo skewers, chopsticks, or dowels).

what I did on my TALON 3 for the forward fins was drill two holes on each side of each fin, then using a corner of a plastic baggie drizzle epoxy into each hole. Of course holding the rocket at an angle for the epoxy to run down the root of the fin one way then the other. As for the holes I used epoxy clay to fill them, sanded them smooth and finished with external fillets.

I'm always under the impression that a little epoxy goes a LONG way! Some guys think that they need tons of it for it to hold.

For the aft fins I did them the usual way leaving the aft CR off and filleting them from the rear.

+1 injecting the internal fillets, I like CrazyJims method using a syringe and slightly fiber thickened epoxy. The syringes I use are the irrigation type with the plastic curved tips. I also use CJs double buttering the root edge when inserting the fins, it almost gives a root edge fillet on its own.

rharshberger said:

+1 injecting the internal fillets, I like CrazyJims method using a syringe and slightly fiber thickened epoxy. The syringes I use are the irrigation type with the plastic curved tips.

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Where do you get those syringes from?

FYI, I added internal fillets to my latest rocket (I'm kit-bashing a Leviathan...stay tuned for the final product), so it is "apples to oranges", i.e., I'm using Tite-bond and you are using epoxy, but adding internal fillets added only 7% more weight. Well worth it, in my opinion, for the added strength (fingers crossed!).

The_Lone_Beagle said:

Where do you get those syringes from?

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Ebay is one source. Also try your local pharmacy. I have gotten them from both places.

https://www.ebay.com/itm/KENDALL-MO...yringes-12mL-50-BX-Glue-Gun-USA-/181755808570

I get mine at a local ranch and home supply, they are less than a dollar each. The brand is Monoject iirc. Nice thing about thr tips is you only have to drill a 1/8" hole or two next under where the exterior fillets go.

rharshberger said:

+1 injecting the internal fillets, I like CrazyJims method using a syringe and slightly fiber thickened epoxy. The syringes I use are the irrigation type with the plastic curved tips. I also use CJs double buttering the root edge when inserting the fins, it almost gives a root edge fillet on its own.

Click to expand...

I use the same method but don't use anything to thicken the epoxy as I let gravity help my Aeropoxy ES6209 settle into the crevices. I sourced the ones in the pic below on-line and bought a couple of boxes. The Luer lock tips works really well as I can insert the aquarium tubing into it and I also have something for the tape to stick to. They are a little big so when they run out I will likely get smaller ones.

https://www.terumotmp.com/ProductDetails.aspx?categoryId=1&productId=208

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I'm a great believer of internal fillets and have found each successive rocket build introduces a few new challenges. I like the wax paper idea.

I am a fan of the Crazy Jim syringe injection method. All of my larger rockets are split fin and after dropping an Ultimate Darkstar booster on the concrete floor with no consequences I have built all my HP rockets this way.

My choice expoxy for internal fillets for fiberglass rockets is West Systems 105 epoxy mixed with a small amount of chopped carbon fiber. Because West Systems is rather runny it is sure to evenly spread the length of the fin edges even with the added CF. It is also very easy to draw & expell from a syringe.

Ditto for the Crazy Jim syringe method. I used it on my Madcow Frenzy and it worked well.

I also used some extra centering rings to lock the fins in place...a bit of extra work but "anything worth building is worth overbuilding".