How to: Mid-High Power Fillets

Here is how I make my beautiful, smooth as a baby's butt, strong as a mule fillets, with no sanding! OK, some sanding could make them better, but you get the drift. This is just my way, it is not the cheapest, easiest, or strongest way, but it works for me and I have not yet cracked a fin fillet using this method. As they say, there is more than one way to bake a cake.

What you need:
Epoxy (any type will do, I still use El Cheapo 30 minute epoxy.)
Milled Fiberglass (for extra strength and a higher viscosity(linky))
Protection (Gloves, particle mask because of the milled fiberglass, newspaper, etc.)
Smoothing tool* (a used Estes motor, a washer, a plastic spoon- something to smooth out the fillet with)
Masking Tape
Sharpie
Something to mix the epoxy (weapons of choice: Popsicle sticks and a paper plate)
Sandpaper or a Dremel (optional)
Soapy Water (optional)

The Materials
Note: Incredibly messy table not necessary.

1. Rough up the junction between the fin and body tube exterior, using 60-80 grit sandpaper or a Dremel. This helps the epoxy bond to the two parts and creates a stronger fillet. After doing so, rinse and dry the parts in water with or without soap to get rid of any dust or oils from your hands in the fillet area. This also helps improve the bond between the fillet and the rocket.

2. Get all of your materials ready: Lay down some newspaper to protect your table, put on some gloves to protect your hands, a particle mask for your lungs, etc. Always wear protective gear! Epoxy and milled fiberglass are both very nasty materials. Tip: having napkins handy to wipe up any Oopsies will certainly be a plus.

3. Mark the tube using your smoothing tool and a Sharpie. Liberally coat the end of the smoothing tool with Sharpie ink, and then run it down the tube as if smoothing out a fillet. This shows you where to lay down the masking tape (next step).

Masking tape "dams" for the epoxy.

4. Put down masking tape along the lines you just made and on the bottoms of the fins too (left). This catches any stray drips of epoxy, leaving you with a drip-less fillet. Depending on when you take the masking tape off will determine how much sanding you have to do later. If you take it off too early, it might drip, too late and it will leave little ridges on the edge of the fillet that you will have to sand down.

5. Mix up a batch of epoxy- I usually try to mix a little less than what I think I need, because I would rather mix new epoxy than waste some. Add some milled fiberglass after the epoxy is mixed a little bit. You don't have to worry about too much or too little, but the manufacturer recommends a ratio of about one part fiberglass to eight parts epoxy, which sounds like a little less than what I use, but you could probably even do a ratio of 1:2 if you wanted to, but it would be very difficult to work with. 1:6 or 7 seems to be what I use, but it really doesn't matter.

6. Once you put the fiberglass on top of the epoxy, mix it all up until it is evenly distributed and it all looks uniform. Now to the actual filleting!

7. Slather some epoxy on the area to be filleted, and smooth it out once with your tool. Do not smooth it down multiple times, because that will create little bumps/ridges in the fillet where you started smoothing anew. Take the tape off as soon as possible, and let the epoxy set. Then you can move on to the next set of fillets!

Voila, the final result! These fillets seriously feel like glass.

* You can use just about anything to make the fillet smooth. Small cylinders such as used Estes 18mm motors can be used, and large washers for larger fillets. You want your fillet to be big enough to strengthen the fin, but small enough so it doesn't look ridiculous, add unnecessary drag, or both. In order to find the right one for the job, just place a candidate on the joint between the fin and the body tube, and judge if the fillet width is too big, too small, or just right (see below).

How the smoothing tool works. The area between
the tool and the fin/body junction is where the fillet will be.