Step Three
Fin Fabrication
The fins are constructed from three layers of foam core art board overlayed with two layers of fiberglass. Like all the short, fat, cigar-shaped rockets I've designed, the reason for this choice of fin material is to minimize the weight aft of the Center of Pressure. The bulbous airframe moves the CP dramatically forward, so the the choice of building materials (foam aft, hardwood and steel forward) are very specialized, all intended to move the center of gravity toward the nose.
The center piece of foam core is larger, and the two outer pieces have beveled edges. I used 2-inch fiberglass tape to wrap the leading edge of each fin, and with the beveled core the result was a nice, rounded fin edge. Rick picked up on the relief feature of the fins when he made the drawings. I'm glad, because I had missed this detail. It adds a lot of work, but the effect is worth it. I made templates from 1/8 inch Masonite before tracing - there are nine fin pieces total, so using templates assures they are all made the same.
The photo to the right shows the fins with the fiberglass applied, but before the holes were cut. The photo to the left shows one of the fins sanded and ready for installation
I have no idea what function the holes in the fins are supposed to fill, but I like 'em. The holes are cut with a 3.5-inch hole saw. I mixed a paste of resin and glass microfibers and applied this around the inside edge of the holes to add strength back to the fins after the holes were cut. This was later sanded smooth. The foam and paper of the art board also soak up resin, so the completed fin is extremely lightweight and strong.
The same resin and microfiber mixture was used in to build up the 'foot' of each fin. The fins were secured 'foot' up and the mixture was poured into the cavity formed by the overlap of the fiberglass layers, then sanded flat after curing.
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