I’ve always been interested in squeezing every bit of performance out of the rockets I build, so I decided that I would take the opportunity to test a performance technique on a kit I bought. The end goal was to practice foiling fins for my next project, which will include 1/4″ plywood fins nearly 24 inches in length. It made sense to start with something a bit easier–1/8″ balsa fins not 4 inches in length. Balsa is a considerably easier material to sand than plywood due to the perpendicular grains of plywood.
The visual theory is pretty straightforward: people like things that look sleeker and more streamlined and generally fall into a hard-to-produce shape category. More significant is the aerodynamic theory. For this case, low speed aerodynamics dominate the equation. Though a rocket indeed travels very fast, a rocket at the scale of this discussion travels well below the local speed of sound, and thus the air around the fin does not experience the effects of compressed air. These effects come into play when a velocity of Mach 0.6 or greater is reached (subsequently, long after the surface mounted fins are ripped off the body tube and the rocket becomes extremely unstable). Even so, the maximum velocity of most hobby scale rockets is only sustained for a short time, thus spending the most time at significantly lower speeds.
- These fins were punched out of a balsa board from the Estes Patriot kit. As per the instruction manual, sand down the fins to make smooth edges. An observation of the fin cross-section will reveal a square fin profile.
- Round the leading edge. Start with a shallow angle on one side and gradually round off the leading edge by increasing the sanding angle until the sander is perpendicular to the fin at the leading edge. Complete the rest of the leading edge in this manner, but reversed.
- Sharpen the trailing edge. This part is relatively easy. Sand the trailing edge of the fin into a wedge shape, equal angle on both sides. It is easiest to sand each side down to half the thickness at the trailing edge, adjusting the length of the wedge depending on the airfoil design you choose.
The whole process took about 5-10 minutes per fin for these fins, leading me to believe that scaling the technique to my next project will be monumentally difficult. Nevertheless, the the fin came out with a nice airfoil-like profile.