Getting the basic fin cut out

The fins are created from 2.4mm thick G10 plate. I created a clipped delta design and created a jig that I would use to sand the three fins down to the correct IDENTICAL size. Below is a picture of this jig.

I have used a Steel right-angle bar to sand up against. Here are some pictures of the fins.

Profiling the fins

I wanted the fins to have a profile suitable for less than speed of sound. While they will travel greater than speed of sound for some motors, for the majority of the flight they will be travelling below the speed of sound. I also choose this design because it is relatively simple to produce.

I did all this work by hand. I started by drawing the boundaries of the sanded regions. I even drew a line down the thickness (half way) on leading and trailing edge.

Here are a tonne of photos showing this process.

Surface Preparations for Epoxy

It is very important to do all fin preparations BEFORE we attach them to the air-frame; it is a lot easier to do.

I sanded the entire surface of the fins with Grit 60 sand paper. This is so that the epoxy fillets adhere well to the fins and also so the CF Fin-to-Fin material adheres properly.

Little cuts were made on the root of the fin every 10 mm, approx 1 mm deep with a hack-saw to encourage a good bond to the air-frame.

I also scribed 45 degree incisions every 10 mm from the root, to ~7 mm up from the root. This provides extra place for the epoxy fillet to ‘grab’ on to the fins. I used the following “tool” to do this.

Here is a photo of one of the fins I scored.

I also used a hack saw to cut 1mm deep incisions into the root of the fin every 10mm. No photo available.

The Jigs

I created some jigs using my 3-D printer to hold the fins in place. I had a few goes at getting ones that would slip on easily…but not too easily. This is so the fins could not flop about.

You may wonder how I avoided JB-Weld getting on to the Jig. Well, I have a gap between the air-frame and the inner surface of the Jig. So I was able to carefully insert the fin with minimal or no JB-Weld getting on the jig. In some cases the fin did stick a little to the 3-d print, but it was easily pried off.

Attaching the Fins

I used JB-Weld Epoxy to attach the fins, just like I did for the previous rocket build. I didn’t have the luxury that I had then of being able to see through the air-frame to look at the adhesion of the epoxy. But that is okay. I only attached one fin at a time, giving it 24 hrs for the epoxy to cure.

Photos

Here are some photos of the process.

The Fillet Epoxy

I created the fillets using West Systems. The products used were:-

• 105 Epoxy
• 206 Hardener
• 413 Filler  (this supersedes 403 filler)
• Masking tape

Tool used to shape the Fillets

I created a special tool (pictured below) to help shape the fillets. This tool was created using my 3-D printer. It was shaped to give a fillet of radius approx 6 to 7mm.

The tool was created in FreeCad  using a cylinder of radius of 7mm sliced at 45 degrees and attached to a “handle”. This allows me to drag the tool along the surface at 45 degrees, knowing that the curvature of the fillet is approx 7mm.

Here is a photo of it:-

The Procedure

I measured 105 Epoxy and 206 Hardener materials by volume using a syringe. I passed the 413 through a sieve to remove the lumps. Then I introduced 413 filler in small quantities, until it’s consistency was that it JUST held its shape. Then I loaded this Epoxy into another syringe and squeezed it into the fin roots and shaped them.

Creating the fillets

Because the fin fillets are so small, I created ALL the fillets in one go.  I actually ran out of epoxy and had to create a small second batch. I have it a few days to cure. Below are some photos of me preparing the rocket for filleting, by taping it up.

Below are photos of the finished job.

Sanding it down

The finish wasn’t flash, so I decided some sanding was required so that bumps to show through the tip-to-tip. NOTE: The aim was not to sand it down to completely remove the holes; The Tip-to-tip will handle this. I used a AA battery with Grit 60 sand paper.