LESSON ONE: Constructing the fin assemblies

One of my first thoughts when I planned recreating the original prototype for a How-To Classroom project was how I was going to strengthen the fin area of the rocket. In the original, the nose cone and altimeter bay area was very strong and able to withstand the abuse I was dishing out, but the fin area left me concerned.

I built the original with 1/8" plywood cores and attached the stock styrene fin shells to the plywood with cynaoacrylate glue, but after a few flights, one of the fins experienced a hard landing, bending a fin until the plywood core snapped. Replacing the core would be a nightmare, so I ended up replacing all of the fins and cores with .093 G-10. It was a suitable job, but I missed the signature diamond airfoil of the original fins.

So for this version, one of the first things I researched was a way to make suitably strong fins with the diamond airfoil shape. My ultimate solution was to make a mold of the styrene fin shells and use it to make my own fiberglass versions. The new fiberglass fin shells would then be attached to .063" G-10 cores.

To make the fin shell mold, I started off by cutting off the styrene spin thrusters off of the fin shell sheets and then attached the fin shell sheets to a backing plate. My backing plate was a scrap piece of faux marble used in counter tops. I selected it for its smooth, flat surface. I attached the fin shell sheets to the backing plate with clear packaging tape, like the kind you use to ship a package. Its smooth, shiny surface would allow the fiberglass mold to release cleanly when it was set up.

I placed a 3/4" masking tape "dam" around the perimeter of the backing plate to catch excess resin when I laid up the mold, and then applied eight coats of regular automobile carnuba wax to the styrene fin shells and the backing plate. Then using West Systems 105 epoxy, I applied three layers of 6 oz. fiberglass cloth and five layers of 12 oz fiberglass matte. I ended up with a mold that was approximately 1/4"-5/16" thick, which would be very strong for what I was going to be doing.

In the photo above, you can see the mold. It turned out extremely well for such a low-buck home project, and the diamond airfoil shape is very visible. Once I had my fin shell mold, it was time to start laying up fin shells. With three Honest John's going to be built, I would need 24 fin shell halves: four left hand shells and four right hand shells per rocket. Since I could make four fin shells per run, it would take a total of six production runs to get them all done if I didn't have any turn out bad. I had my fingers crossed.

To make the fin shells, I planned to use two layers of 6 oz. fiberglass cloth backed up by one layer of 12 oz. fiberglass matte. Most people are familiar with what fiberglass cloth is, but some have never seen fiberglass matte. Fiberglass matte is random strands of fiberglass that is "matted" together. The random orientation of the glass fibers allows fiberglass matte to have a lot of strength when the resin has set. This layering approach would allow the fiberglass cloth to give the outside of the fin shells a strong uniform surface in a relatively thin matrix, while the fiberglass matte backing up the cloth would add additional strength.

Honest John fin shell template for cutting fiberglass cloth and matte.

So to start laying up fin shells, I would need to cut two layers of 6 oz. cloth per shell and one layer of 12 oz. matte, or 48 layers of 6 oz. cloth and 24 layers of 12 oz matte. Whew! To get started, I needed to cut an appropriate template to cut the layers out with, so I took an old flight card and started whacking. Careful measuring, marking and cutting will result in a more usable template, and the dimensions worked out to be as those indicated in the template pattern shown to the right.

Once I had a usable template, I got out my 6 oz. fiberglass cloth and my 12 oz. fiberglass matte, placing them on two sheets of flat cardboard for cutting. I used a regular disposable razor knife like used for box opening, which one can pick up at the local dollar store, and a metal straight edge. If you don't have a metal ruler, but you have a carpenter's combination square, you can use the ruled straightedge from the square. This is what I used.

Laying the template on top of the cloth and matte, place your straightedge over the template and leave a fraction more cloth and matte so you don't clip off the edges of your template. You don't want your template consistently shrinking after each cut or you will find your fin shells doing the same. Just cut slightly to the outside of the template. 72 pieces took me a while, but it was good to get it done.

With the shell fabric cut out, you can turn your attention to preparing the mold. I used 4-5 good coats of carnuba automotive wax, Turtle Wax in this instance, to wax the mold. You could use anything that would allow the shell to separate from the mold: some use PVA release agents, some use PAM, some use liquid hairspray, I use Turtle Wax. The better you cover and wax the mold, the easier the pieces will release and the longer the mold will last.

Honest John fin core template for cutting from G-10 fiberglass sheet.

Once the mold was ready, using West Systems 105 resin and the corresponding hardener, I would mix one "shot" in throwaway cups. My one "shot" was from the gallon kit, which represents one depression of the pump plunger. My throwaway cups were washed yogurt cups, perfect for fiberglassing. Using a 1" disposable paint brush, I paint the bottom of one fin shell with resin and lay in a layer of 6 oz. fiberglass cloth, tapping it down with the end of the brush. I repeat this for the remaining three fin shells in the mold.

Repeating the process from above, the second layer of 6 oz. fiberglass cloth is laid down, painting a layer of resin, inserting the cloth, and tapping it down into the mold. I am still on my first "shot" of resin and hardener. Continuing, I lay in the 12 oz. fiberglass matte, which is significantly thicker and more difficult to saturate with the resin. At this point, I am going to quickly need another "shot" of resin and hardener.

The idea is to get the thinnest lamination of cloth and matte as possible, using the least amount of resin. Thicker laminations are weaker and just weigh more. You must overcome the urge to simply keep applying resin and hardener to hurry up and get the matte saturated. Instead, if you will paint the resin onto the matte and then use the brush end to 'stipple' the matte, it will do what you want. Doing this, you are just using the brush to tap the map with the end of the brush bristles, like you were sticking it with a dart. This does two things: it forces the matte and clothe down, compacting it, while the brush bristles work as a siphon to pull excess resin out. Pay special attention around the pointy ends where the cloth and matte could lift away from the mold, creating an air space. Moving the excess resin from the middle of the fin shell to these ends is a good idea — you don't want bubbles in your layup.

Once the layup is complete, you are best to let it set up overnight. No need to rush your work and get rushed results. The next day, using a sharp instrument, you can lift the corner of the fin shells up and lifting, separate them from the mold. I used a straight, sharp wood lathing chisel. Lifted the corner and they popped right out.

As you can see, the fin shells are pretty nice, with the sharp diamond shape we all know and love. After popping them out of the mold, the edges are easy to clean up with 80 grit sandpaper on a sanding block.

The next step is to get them ready for attachment to the fin cores, which will be made of .062" G-10 sheet fiberglass. The two 38mm rockets will use Giant Leap's innovative Groove-Lok fin positioning and attachment system, which puts the fins at positive 90 degrees positions for four fin rockets or 120 degrees positions for three fin rockets. Working inside the styrene fin canister, I wanted all the advantages I could get.

I couldn't use a Groove-Lok, if one even existed, on the 54mm version because the distance between the 54mm motor mount tube and the exterior of the styrene fin canister is so tiny — there wouldn't have been room for the Groove-Lok's fin grooves. So I chose to mount the 54mm version's G-10 fin cores directly to the motor mount tube with additional fiberglass cloth as tip-to-tip reinforcement over the cores. This also dictated that the 54mm version would have to have the fin canister split at the rear to slip over the attached fin cores, where the 38mm versions would not. Because of this, the 54mm version would not have its fin shells attached until after the motor mount/fin canister was constructed, while the 38mm versions would have their fins pre-assembled on the cores before assembling the motor mount/fin canister assemblies.

All of this dictated that I would assemble eight fin assemblies with shells mounted to the cores, while leaving four fin shell sets separate from their cores. As an aside, the 54mm fin cores would have shorter fin tabs than the 38mm version, to compensate for the differing diameter motor mount tubes between 38mm and 54mm motor mounts.

To get the fin shells ready for attachment, I needed to true the back sides, sanding the excess fiberglass matte away to allow for a flat surface to mate to the G-10 core. Because of the diamond airfoil shape, the fin shells would seat about 3/4" around the outside edges. The root would be literally open. Holding the shells for sanding was a task, until I figured out to take two pieces of masking tape and creating a handle.

Using the diamond airfoil as the handle attachment point, I put on a piece of masking tape lengthwise of the break, half on the fin, half folded back. Repeated it for the other side, and putting the two sticky exposed sides together, and had an easy to grasp handle. Trim the excess off so it doesn't get caught in the sander. Using this handle, I held the fins up to a sanding belt and within seconds, the fin was properly surfaced for attachment.

Once the fins shells were surfaced to attach to the fin cores, I cut the cores themselves from .062" G-10 fiberglass sheet. I got my G-10 sheets from Ridout Plastics online at their webstore (http://www.eplastics.com/). They have a good selection and reasonable prices, but more importantly, I didn't have to pay ridiculous shipping or buy full sheets that some online suppliers want you to.

The cores were cut with a table saw using a carbide blade and a bandsaw using a metal blade. Assembly of the fin shells was as simple as sanding a 3/4" band around the outer edges of the cores, painting both the core edges and the fin shell edges with 15 minute epoxy and lining the shell up with the core before applying a couple of spring-loaded clamps to hold them in place until the epoxy sets.

Here you can see eight assembled fins for our 38mm birds and eight fin shells awaiting the 54mm version. You can see the band around the outer edges where the fin shell has made a solid bond with the fin core. Strong yet light, these fins won't have any problem withstanding the abuse they'll get.