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Darmok and Jilad at Tenagra
This is the fifth in a series of bulbous rockets, the third variation made from the molds I created from the TRFKAB airframe. The name is an obscure reference to a Star Trek: Next Generation episode. (I wasn't an especially big fan of this particular show. The selection of this specific name is more related to timing than anything like an enduring passion for Star Trek.)
The airframe of DAJAT is the same as TRFKAB but it is inverted, making it less retro and giving it weird, alien appearance. The pods on the fins are roughly the same shape as the airframe. I will make a cone for the front end and another one for the aft end. The aft cone will be removed for flight, but I think I might leave it attached at the pad and let it blow off when the motor comes up to pressure. It will be more unique that way and make everyone look twice to figure out how it is supposed to work, and which end is supposed to be up.
Click [HERE] for flight photos!
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Full view
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Oblique view
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The rocket will be 45 inches in length and just under 15 inches in diameter. Each fin pod will be 18 inches in length and just under 6 inches in diameter. The line drawings have no colors on them because I haven't figured out what I want to do for a paint scheme yet.
I used a wood lathe to turn a single fin tip pod to serve as a plug. It's a big monster. From this I made a two-part fiberglass mold, which I will use to make the three pods.
The two halves of the pod mold. I damaged the plug pretty badly trying to get it out of the mold, so I don't have a photo. Always take your photos first! But you can trust me, it looked pretty slick before I wrecked it.
In fact, it looked almost identical to this one. This is the first part to get pulled out of the mold. It's always a relief extract the first part. I was amazed at how light this part is, especially compared to the solid wood plug.
I made the airframe with two layers of 1.5-ounce glass. I knew this would be sturdier than past airframes I've made, but I had no idea. It is a lot heavier than I had intended!
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Here are all the molded parts posed together. The paper hat that the airframe is wearing is the start of the nose cone, just like the ones I made and documented [HERE!].
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This is the pattern for the fins. There will be a 3/8-inch plywood core that will extend from the motor mount to the outside edge of the pod. I will cut slots in both the airframe and the inside-facing edge of each pod. The wood-grain
ends depicted in the drawing are the sections of the fin core that will extend through the walls of either the airframe or the pod. The white section in between will have 3/4-inch styrofoam glued to each side, which will then be shaped and covered with a fiberglass skin.
I will also likely fill the area inside the pods with expanding foam.
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I cut one plywood fin core and one fin slot, then slotted one fin pod, then paused to take a photo of how they look together. Pretty close to my orginal concept drawing.
After I cut this first fin slot, I decided that maybe it wasn't such a bad thing that I made the airframe thicker than I had intended. The strength of the thinner shells comes from the the axial reinforcment of the complex curves. I slice right through an entire airframe half three separate times when I mount these fins, so the extra heft in the skin is a real advantage.
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And here it is with all the fins attached. The plywood fins have a semi-span that is 1/2-inch wider than they would have if I had followed the original drawings exactly -- just in case you are keeping track of such things.
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Front view. These photos have a surreal, underwater quality.
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Aft view. They're balugariffic!
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Gettin' Jiggy Wid It!
I used some scrap plywood to make a couple of jigs for aligning the various parts while they were being epoxied together.
The idea is similar to the one already documented [HERE!] but you'll notice that the jigs are circular instead of triangular or square. That's because I cut them using the methods documented [HERE!]. This is no more difficult, but it is important to mark all your lines before you cut out the centers.
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The big one holds the fins in place while the epoxy cures.
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This one holds a single pod in place while the epoxy cures.
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Epoxying the fins into position.
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The pods are epoxied into place.
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I cut out six pieces of 1-inch foam, one for each side of the plywood cores. For the record, 1-inch foam is really only 7/8-inches thick.
Each piece needs to be shaped before it is glued into place and the fiberglass is layed over the top of it. So I made another jig to rough out the correct contour with my hot wire foam cutter.
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Just slip the foam into place and run the wire along the rails.
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Move the rails to the opposite side of the jig to trim side two.
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The foam pieces were then glued to the plywood core using carpenter's wood glue, and I did some additional shaping using a pneumatic grinding tool with a 2-inch sanding drum and 50 grit sandpaper. After that, three layers of 6oz. fiberglass cloth wetted with epoxy finishing resin went over the top.
I filleted each joint with a paste made from epoxy resin and acrylic micro balloons. I also applied this same paste to the leading and trailing edge of each fin. Then, all this resin was left to cure for 36 hours. I spent the next day cleaning my garage instead. If you had ever seen my garage, you'd know why I had to take a full day to clean it.
I used the pneumatic grinding tool with a 2-inch sanding drum and 100 grit sandpaper to clean up the fillets, then I applied vinyl spackle over all the recently fiberglassed areas. After it dried I started sanding. And sanding. Then some more sanding. The photo to the right is after the sanding and just before DAJAT got its first coat of primer. (I can never tell just how much more sanding is needed until I see the rocket with primer on it.)
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Front view of DAJAT with primer.
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Oblique view of DAJAT with primer.
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The curvature of the fin contours don't show well
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in these photo, but here are some closeups anway.
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Carefully lining up the aft cone to bore the launch rod hole.
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The same for the nose cone.
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This is how the rod passes through the aft cone.
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And the same rusty 1/2-inch launch rod in the nose.
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I installed countersunk magnets into the tailcone cap. Magnets will also be epoxied onto the airframe to hold the cone in place during display at the pad. The three 1/2-inch vent holes in the center are to keep the ignitor burst from popping it off prematurely.
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The motor mount uses the same scheme as all of my bulbous rockets. A G-Wiz flight computer rides in the cavity between the motor tube and the 4-inch stuffer tube. The shown avonics mount is interchangeable with the other bulbous rockets and slides into the slot in the aft centering ring.
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This is what DAJAT looks like in chrome.
My original plan was to paint DAJAT with this chrome paint and then cover it with a green clear coat. Both paints are made by Duplicolor and the instructions expressly stated that they two paints were compatible and formulated to be used together. Unfortunately, the chrome effect gets dulled dramatically when the green clear coat is applied. (It looks great over real chrome, though!)
Even worse, this chrome paint is alot like the metal paints of old -- the shine rubs off pretty easily. It can't be waxed, either. So even though this looks really cool, it probably won't look like this for long. But for now, it looks pretty awesome!
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Click [HERE] for flight photos!
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