Composite fin can 54mm rocket

Making the fins

  1. Get a sheet of G10 fiberglass of 2mm thickness. I bought mine, a 250x250mm section from Rebel Space.
  2. Carefuly mark with a scriber the correct fin contours on the sheet. I used this fin shape – 54mm fin template 04.08.09 – JvdB.
  3. Use a new fine tooth saw to cut the fins accurately.Get a new one as the blade tends to get blunt after a while probably due to the glass material.
  4. Square the cut edges with a piece of sand paper on a table.
  5. Sand the surface of the smooth G10 with P60-100 sand paper / linnen.
  6. Bevel the fins with a sanding block.

Making the tube

  1. Take a sufficient length of body tube.
  2. Square a piece of polyester paper.
  3. Use the polyester paper to wrap a single layer around the tube.
  4. Markt the overlap and cut 1mm shorter so the butt to butt seam has 1mm clearance.
  5. Test fit the paper over the body tube and check if there is a 1mm clearance.
  6. Wax the outside of the aluminium body tube 3 times and do the same with the inside of the polyester.
  7. Mix 150 gr of epoxy
  8. Total mass GF = 125 gr
  9. Epoxy used 60 gr??? use small batched 60grmax
  10. Epoxy sand layer: 18 gr epoxy, 1 theaspoon glass bubbles, 1 theaspoon aeorosomething.
  11. Work in progress
  12. Final mass GF tube before sanding: 208gr.
  13. Length glassfiber 5x diamter * pi = 848mm + 50mm overlap = approx. 900mm.

Tacking the fins in place: 

  1. Cut the tube slightly longer and sand to length. Use and old (machined square) casing to make the ends square. I used 4x the OD of the rocket for the base tube. The root of the fin is 108mm (2x OD) and is placed 10mm from the bottom. I want to laminate 3 layers of 200gr/m2 glassfibre cloth tip-to-tip. With an overlap of 20mm each at the front to make a even built up of layers. 1st layer = 40mm overlap, 2nd layer is 20mm overlap en 3rd and final layer is flush with the root of the fin. So: 10 + 108 + 40 = 158mm which makes for 50mm normal tube in front of the added tip-to-tip layers.
  2. Mark the position where the fins will need to be placed. Wrap a paper around the tube. Align / square it and carefully mark where the overlap starts. Measure the length (the actual circumfence of the tube) and divide by the number of fins and add markings at each fin position.
  3. Use a aluminium angle section and mark the fins positions over the length of the tube with a permanent marker. Mark the position of the top & end of the fins so you will know where to add the epoxy.
  4. Use a square and a office clip to attach the fin to the square. The further the square is from the tube the more accurate the placing will be. Make sure the clip with clamp in the center of the square. Use a 10mm block the help to get the fin 10mm from the end. Use a second square to position the fin parallel to the tube.
  5. Use the template as can be found here to align the casing as well as the square holding the fin. Don’t use the centerline but align the square with the outer line of the fin.
  6. De-grease the fins and tube with aceton or similar product.
  7. Mix up a batch of 1,5gr – 5min epoxy to a piece of A4 paper folded 3 times.  Apply a small bead of epoxy (for example with a nail) to the tube covering the length of the fin (rolling the nail and tamping slightly). Use the paper with epoxy to dap the paper against the fin root.
  8. Position the fin against tube and do not move for 5min.
  9. Remove the clip and start the same procedure for the remaining fins.

Adding fillets:

  1. Make sure you take your time to mask of all area’s where you don’t want to have the epoxy smeared on. This will take some time during preparations but in the end clean up is easy. I used the aluminium angle section again to mark the lines where I needed to apply the tape. Radius of the fillet was 5mm therefore all the tape was positioned 5mm from the root of the fin. The front and end of the fin I used 10mm spacing.
  2. Use aceton or similar to de-grase the surfaces.
  3. Make a batch of filler. I tried to use the paste as suggested by Polyservice. but this ended up more like a play-doo / chewing gum (but lighter) consistincy with a very fine texture. It did not not stick well but could be rolled in my hand into a ball or bead. Because it is dry, finishing can be tricky. When filleting with a dowel I saw some rimples in the finishing due to friction. For obtaining a nice shiny paste I would propose to use only halve of the suggested glass bubbles and aerosil
    • 6,0gr epoxy (base + hardner) + thorough mixing
    • 0,3gr aerosil + mixing
    • 2,0gr glass bubbles, added in 2x 1gr portions + mixing
    • Total weight 8,3 gr – used (04.08.2009) 4,5gr for 6 fillets.
  4. Roll a bead and apply the filler into the corner of the fin/tube and press it with your fingers.
  5. Use a 10mm diameter dowel against the corner and squize out the excessive filler. Remove the excess filler so it can be used again. If the masking tape is applied accurately the dowel and tape edge will intersect.
  6. Remove the tape, touch up where necessary and let harden for 24hours.

Tip-to-tip lay up:

Let’s say the fiberglass section is 0,2 x 0,2m = 0.04m2. Actually it is a bit smaller but ending up with some spare epoxy is better rather than running short. We want to achieve a 50/50 glassfiber epoxy ratio. The fiberglass that I used is 160gr/m2 plain weave 1X1 (not good for curved surfaces). Ergo: 0,04m2 x 160gr = 6gr per section. We want to apply 3 layers of fiberglass thus in total 3×6= 18gr glassfiber –> 18gr epoxy. Add a bit of safety and 21gr epoxy should do the job. 14gr base, 7gr hardner. Don’t forget that already a lot of safety was calculated because of the larger than actual surface.  05.08.2009  – Actual fiberglass weight was 13,5gr, total epoxy used: 8,5gr.

  1. Use some 240 grit sand paper and the 10mm dowel to clean up the fillets. Especially near the end at the fin root. I sanded perpendicular to the root at the front and end of the fin to get a 5mm radius which I sharpend for better aerodynamics.
  2. Cut the fiberglass according to the template 54mm fin template fiberglass – 06.08.09 – JvdB. Cut the 3 (or possibly 5) pieces overlapping each other by 20mm (I, II & III). Please see the pictures below (numbers could be actually numbered the other way round).
  3. De-grease the tube and aluminium fins with aceton.
  4. Cut the hairs on the brush to 10-15mm length. I discovered that a pair of HEMA (local store) scissors with serrated blades works superb for this. The shorter hairs work better for wetting out (impregnation) / penetrating the cloth with epoxy.
  5. Apply a light coat of epoxy on the surface of the tube & fins. Lay the shortest cloth of glassfiber over the wetted section and align the slit in corners with the fillets. Carefully tamp & wet out the cloth with the brush, only applying a light coat of epoxy if necessary. Make sure this first layer is 100% perfect because once the second layer of fiberglass cloth is applied you can not change the layer underneath.
  6. Apply the remaining layers in a similar fashion each subsequent layer overlapping the previous. Use the brush to round the edges over the fins for full surface adhesion / lamination. Lightly brush off  the excessive epoxy from the surface or dapp with some paper.
  7. After the lay up has hardend, sand down the excess with a 60 grit sanding block. Use the block over and away from the edge otherwise it will delaminate the lay up from the aluminium fin.

Finishing:

Sand down all high spot with 60 gritt sand paper, use the my own sanding formula with 9gr epoxy