A common failure mode of rockets seems to be snapping fins on landing. There are a couple ways to mitigate this problem: avoiding acute angles, using thicker materials, and using stronger materials. In an abundance of caution, I have decided to implement multiple strategies in protecting the fins for my Level 3 project. I started off by laser-cutting 1/4 inch plywood sheets into fin templates. You will notice in the gallery below that there are large cells in the fin templates, which don’t do much to improve the strength of the fin or its aerodynamics. The entire aerodynamic surface of the fin will be wrapped in 6oz fiberglass, so these cells in the fin are really doing nothing but adding to the fin’s weight. Some material is left on the fin template to provide a scaffold for the fiberglass and to maintain the fin’s twisting rigidity. The fiberglass-overlaid wood fin is actually stiffer as a sandwich composite than would be an equivalent thickness wood or fiberglass fin.
This was my first attempt at fiberglassing, so the surfaces look pretty rough, and the weight savings from cutting out cells has likely been nullified by the extra epoxy resin that went into the glass cloth to fill in sagging cloth. A single layer of 6oz E-glass from US Composites was used to wrap around each side of the fin aerodynamic surface. A matrix of thin epoxy from US Composites mixed with fused silica powder was used as the composite matrix. After everything cured, I used an orbital sander to even out the aerodynamic surfaces and applied a metallic gold spray paint.
You might also notice an interesting pattern in the fin tab. The fins are designed to be replaceable in the rare event that one does break. The fin tab region is designed to slide into the motor mount assembly and lock into place. The motor retainer assembly is attached to provide aft fin retention as well as motor retention. The the fin is constrained radially and axially by the large tab at the top of the fin tab and the smaller tab on the bottom. The fin is constrained tangentially by the entire fin tab surface (via the centering rings) and by a line of the fin surface (via the airframe). The fin tab region is not fiberglassed in order to maintain the tight dimensional constraints imposed by cutouts on the centering rings and retainer.
One concern with this configuration is the fin’s susceptibility to flutter. A simple dry fit indicated that there is no degree of freedom for the fin to rattle in its restraint, leaving the primary flutter mode to be the natural flutter mode of the material. Due to the fin’s high stiffness, I do not anticipate this being a major concern. To ensure that the fin will not rattle, I will consider a traditional constraining of applying a temporary wood glue fillet (which can easily be removed with an Exacto knife) between the fin and the outer surface of the airframe. If nothing else, this will create a damper to vibrations that may eat away at the surface of the airframe over time.