Now that we have a free day, it's time to build our demo Blue Tube rockets. We are building 2 at the same time. One will be 3" and the other 4". We are using a fincan system on these, so we will slot the aft ends of the body tubes all the way to the bottom and slide the fincans in. Shock cord attachment points will be taken care of by placing an eyebolt in the top centering ring. After the fincans are in place, we will spend some time making fillets and attaching rail buttons and such. The rockets will get painted fins and nosecones only, the body tubes will be sprayed clear so that the tube can show through. After all, we want it to be obvious that these are Blue Tube!

So, let's get started (remember, click on the thumbnails for larger views and more pictures):

The fins are scratch built using plywood and carbon fiber. They will be two layers of plywood with a single layer of carbon sandwiched in the center. Fin shape was determined based completely on looks and suspected performance. We like trapezoidal fins for their durability and past 'impact resistance' in our fleet, so we will use that on these as well. Fin size was determined based on comparison to other rockets we have built and flown. If additional stability is needed post build, we can always add nose weight.

The fins were cut to rough shape and laminiated. After laminiation, they were sanded to final shape and tapered on the leading and trailing edges. We do not taper the outboard edge as this helps the fin corners resist breaking and adds to the airfoil appearance. We have decided on a 38mm motor tube in the 3" and a 54mm in the 4", so centering rings were cut using these dimensions. The fin cans were then built using Don's custom home brewed fin-jig.

Next up, we had to slot the Blue Tube to accept the fincans. This was our first experience working with the new material and we were eager to see how it would cut, machine, sand, and glue. The 3" tube went onto the fin-jig to be marked for slots. Using the same jig as was used to build the fincans should assure us that the slots lined up with the fins. After marking, the tube went into the slotting jig where it could be held on the bed of the milling machine. The slotting jig aids in holding the tube still on the table of the mill and also helps keep the tube in round once the slots weaken it. After slotting in the mill, the ends of the slots were opened up the bottom of the tube using a small hand saw.

The tube cut very well. It is very similar in behavior to plastic. A good sharp tool cuts it right out of the way and leaves a clean edge. A dull tool will tend to leave a jagged edge sticking up, but that cuts right off with one pass of an X-Acto knife. The stuff is truely unique. Sometimes when cutting it, there is a smell of paper, but the chips that are left behind seem to be more of a plastic. I also burned some of the shavings that were left behind from milling the tube. The burn rate was VERY slow. It is unlikely that the material would burn much at all had it not been turned into tiny little shavings. What does that matter? Not really sure.

Time to install the fincans. We attached the shock cord to the eyebolt and secured it in place. To protect the nylon cord, we use a kevlar sleeve about the length of the body tube. Things can tend to get a little messy when epoxying the fincan in place, so we wrap the shock cord in wax paper far enough up to protect it during building. Once we are finished with the epoxy, the wax paper will be removed and the shock cord will be clean as new. We previously measured the distance to the front centering ring to know how long the slots should be, so we place a generous amount ot epoxy right along the top of the slots and push the fincan in until the bottom centering ring is flush with the bottom of the body tube. More epoxy is placed around the bottom centering ring and the body tube is clamped tight to the ring with a hose clamp. A coat of epoxy is also applied to the joints at the root of the fins.

The fins are now in place and have had some time to cure in the oven. It is time to put fillets on and have these rockets about finished. We start by placing tape on both the body tube and the fin at the proper distance away from the joint as determined by our spoon. Next, we mix up some peanut butter thickness epoxy using faring filler. The epoxy is globbed on the joints between the tape and smoothed with the back of a spoon. The tape is removed and back in the oven it goes.