Coldfire Fin Attachment

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The photos below will illustrate some of the construction techniques that were used to create the Coldfire booster section and in particular how the fins were attached to the motor tube. The design of the booster section can be seen in the drawing at the left.

View booster section design drawing

The photo above shows the carbon fiber motor tube, three centering rings and three fins. The left most centering ring is made from aluminum and has threaded holes in it that will be used to allow small screws to hold the motor thrust plate in place. The two remaining centering rings are standard aircraft plywood rings 3/8" thick. The fins are fiberglass

The three fins are made of G10 fiberglass sheet that is 0.125" thick. They are a custom size and were purchased at Giant Leap Rocketry. I only had to cut the small slots for the centering rings and sand the leading and trailing edges.

The booster aft centering ring is made of aluminum and has threaded holes in it that line up with the holes on the thrust plate. This ring was custom machined at a local machine shop based on the drawing at the left.

1/2" wide Kapton tape was used to cover each of the threaded holes on the aft centering ring. Both ends of each hole were covered to avoid accidentally getting epoxy into them. After the booster section was completed the Kapton tape was easily removed from the end of the ring that was exposed out the bottom of the motor tube.

Kapton tape is a polyimide film, it is very thin but will not allow epoxy to soak though. Kapton tape is used in the electronics industry and is available at places like Digi-Key.

The Kapton tape was trimmed with a razor blade to cover no more of the centering ring than necessary. This allowed more surface area on the ring to be bonded with epoxy into the booster section.

This is a fin alignment fixture. It is a simple assembly of some wood pieces on a plywood base. I use the clamps to hold the fins to the upright pieces. The upright pieces are precision aligned (accounting for the 0.125" thickness of the fin stock) at 120 degree spacings and screwed to the base with wood screws.

The motor tube fits over a centering ring in the middle of the fixture. This fixture will hold the fins at the proper angles and keep them aligned while the epoxy cures.

		This photo shows how the root edge of each fin was roughed up with a small sanding wheel to make sure the epoxy and Kevlar strips would adhere well.
	All of the parts were dry fit in the fin alignment fixture. Then Hysol epoxy (described below) was applied to all of the mating surfaces.

Three types of epoxy and an applicator gun before assembly. Click here to see a larger image.

Applicator gun and cartridge fully assembled. Click here to see a larger image.

The epoxy used to secure the fins and centering rings is Loctite Hysol E-20HP. It is a high strength industrial adhesive with high sheer strength and resistance to peeling. It can be purchased at McMaster-Carr (part number 6430A19) for about $12 for a 50ml cartridge. An applicator gun (part number 74695A71) and a mixing nozzle (part number 74695A12) are also needed. The gun uses a ratchet scheme to push a plunger into the cartridge. The plunger pushes out the correct 2:1 ratio of resin and hardener. The mixing nozzle is a long plastic tube with a spiral type internal construction that causes the two streams of liquid to intermix before exiting out the tip. There is no measuring and no hand mixing necessary. The 2:1 mix ratio is always correct. The nozzle also makes it extremely easy to apply the epoxy exactly where it is needed. All of this speeds up the work and cuts down on wasted epoxy. This system is very handy and a real pleasure to use! The gun is about $25 and the mixing nozzles are about $1 each. The mixing nozzle mounts onto the front of the cartridge with a type of bayonet scheme. It can be easily removed and is disposable. Once removed, the cap can be replaced onto the front of the cartridge. It too uses a bayonet mount scheme.

The photo on the left above shows two other types of epoxy adhesive that use this same scheme. The E-120HP gives a longer working time (120minutes). It is a non-sagging, aerospace grade with superior resistance to rapid temperature changes. It has high shear strength and resistance to peeling. The E-60HP is a metal and plastic bonder with high sheer strength and peel resistance. They are part numbers 6430A24 and 6430A28 respectively. There are many other types of epoxy available for this system as well.

Close up of the Hysol epoxy applied to each fin root and each centering ring. The applicator nozzle keeps it all very neat.

After the epoxy had cured, the motor tube was removed from the fin alignment fixture. At least now it's starting to look like a rocket!

The next step was to cut some reinforcing strips for the fin root attachment. These yellow strips are 3/4" wide and about 6" long by 0.026" thick. They are 8 oz Kevlar cloth from Thermostatic Industries, Inc. The part number is 8K-60.

A good way to cut Kevlar is with the special shears (that look like scissors) available from CST for $17.95. Part number E814.

West System epoxy was brushed onto the fin roots as well as the motor tube areas next to the fins where the Kevlar strips were going to be placed. The Kevlar strips were then saturated with the same epoxy and laid into the fin root joint.

Close up of the first two strips laid into the fin joint.

The second set of strips have now been added.

Wood sticks were clamped into place to hold the Kevlar strips firmly into the fin root joint while the epoxy cured. Strips of wax paper were used to keep the wood sticks from bonding to the epoxy.

Now we just wait for the epoxy to cure and then do it all again on the next side.

The Black-and-Decker Workmate platform seen here makes a nice fixture for holding the whole assembly. It can be opened up to allow a fin to drop into the slot. The slot also holds the motor tube quite nicely.

The Kevlar fin strips have now all been added. So has the fourth centering ring about two inches above the top of the fins. The booster body tube can also be seen in the left side of this photo. It has been fiberglassed and has an anti-zipper band installed on the top end. The bottom of it has also been slotted for the fins.

It's a perfect fit!