Rocket Team Vatsaas Commander Kip Quasar's Galactic Zephyr

Fins

The large triangular fins are a result of modeling this rocket after the spacecraft featured in those 1950's sci-fi classic films. The materials used and construction of the fins is intended to maximize strength and minimize weight.

Fin Design

The fins are comprised of four core pieces wrapped in a tough fiberglass skin. The aft third has a core of 1/4-inch birch plywood (7 ply). The rest of each large fin is comprised of foam-core art board layered 3/4-inches thick. The composition if this assembly is shown below.

There are nine parts to each fin, excluding fiberglass. Two center pieces, two outer pieces and a bunch of tip pieces that make up the landing pad (details are below).

The aft central fin section is 7-ply Baltic birch. I have constructed fins of 100% foam core in the past, but chose to incorporate some sturdier materials in the core due to the size and mass of this project.

The length of the fin is achieved by extending the plywood core with a piece of 1/4-inch artboard.

The core is sandwiched between two more pieces of 1/4-inch artboard. Once glassed, the diagonal line that extends to the tip of the fin creates a lateral reinforcment that adds to the rigidity of the assembly. The leading and trailing edges of each outer piece has a 1/4-inch bevel.

It is my intention that the fin tenons will run through the airframe's fiberglass skin, supporting foam, and the 6-inch center tube, to be anchored with epoxy directly to the 4-inch motor mount. The space between the 4-inch motor tube and the 6-inch center tube will then be filled with expanding foam. The joint between the airframe skin and the fin will be sealed with 2-inch fiberglass tape and filleted.

Landing Pad Design

Some of the Zephyr drawings shown on this site depict the fins coming to a sharp point. My Level 3 advisor, Andy Woerner, was immediately concerned and pointed out that the pounds per square inch on those fin tips would be in the tens of thousands upon landing. He suggested that I consider clipping off the points and adding a 'pad' that will distribute the landing pressure across a greater surface. The drawings for those pads are directly below. The pad is constructed out of 1" baltic birch plywood. The side reinforcements are birch molding.

Fiberglass Wrap

Bear in mind that the real strength of this composite fin is in the fiberglass wrap, not in the core.

After the assbbly of the landing pads, I applied a layer of 6-ounce fiberglass cloth to each side of the plywood core and wrapped the leading and trailing edges with 2-inch woven fiberglass tape. After assemply of the plywood and foam core pieces, I applied 6-ounce fiberglass cloth to each side, then wrapped the leading edge of each fin with 6-inch wide fiberglass tape. Then the final layer of 6-ounce glass was applied to each side. In smmary, the plywood core has three layers of fiberglass and four layers at the trailing edge. The foam pieces have two layers of glass, and three layers at the leading edge.

Fin Construction

The plywood cores cut from 1/4-inch seven-ply Baltic birch.

The assembled landing pad at the fin tip.

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This photo was taken after the application of the 2-inch fiberglass tape and one layer of 6-ounce glass to the plywood core.

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The foam pieces added to the plywood core make the total fin assembly. These already have glass applied.

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Rolling out the final layer of glass onto the fins. I don't know how other people do it, but I first brush on resin, roll the the glass onto the wet item, then use a spreader to work the resing up through the glass. I then brush on more resin where needed. I believe I use less resin this way.

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I couldn't wait to see how they fit in the airframe, so I cut one fin slot and slid one into place. They look a lot bigger when they are installed.

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