To strengthen the fins, a fillet was put between the fins and the airframe, and then two layers of carbon fibre added over the top of the fillets. The fillets are made of two different parts: (1) epoxy clay at the front and back of the fillets (black), and (2) epoxy lightened with fillite in the centre of the fillet (gray). The epoxy clay is extremely strong and easy to shape, but is also very dense. It was only used at the front and back of the fillet, where it will not be covered by the carbon fibre.
After the fillets have been laboriously sanded smooth, carbon fibre was added over the fillets. When rockets fly at high speed the fins are prone to ‘flutter’, where the fins bend and twist at a high frequency. This can result in broken fins,. so some strengthening is required. We used two layers of 200gsm uni-directional carbon fibre, and Sicomin epoxy. The fabric is orientated so the fibres run down one fin, across the body tube and up the other fin, and this prevents the fins from fluttering.
The fabric is wetted out with epoxy and covered in peel ply (red) to help release excess epoxy and give a smooth finish. The peel ply is a plastic film that is designed to not stick to epoxy, and it is perforated with a series of small holes to allow excess epoxy to escape.