To fabricate the Coldfire motor tube, a phenolic tube of slightly smaller diameter was used as a "mandrel".  It was wrapped with newspaper to build it up to the proper diameter. Then covered with wax paper.  Two layers of carbon fiber sleeve from Aerosleeves were then stretched over the "mandrel" and saturated with epoxy.  After the epoxy cured, the newspaper mandrel was easily removed.  This left a very strong and light weight carbon fiber motor tube.

3.0" ID flexible phenolic tubing from Giant Leap Rocketry was used as the starting point for creating the custom sized motor tube.  It was wrapped in many layer of newspaper until the desired outside diameter was achieved. 

Each layer of newspaper was rolled on as tightly as possible and taped in place.

Once the proper diameter had been achieved, a layer of wax paper was rolled on to protect the newspaper from the epoxy that was going to be applied to the carbon fiber sleeve.

The seam on the wax paper was sealed with one length of Kapton tape.  This tape is a polyimide film, it is very thin and like the wax paper will not allow epoxy to soak though. Kapton tape is used in the electronics industry and is available at places like Digi-Key.

Once the "mandrel" was complete the next step was to cut a length of the carbon fiber sleeve.  This sleeve comes in arbitrarily long lengths from Aerosleeves.  I used their 4" ID biaxial sleeve.

The sleeve was slid over the mandrel and then pulled tight. Tension on each end caused the sleeve to contract and pull down snug against the surface of the mandrel.  The sleeve was cut slightly longer than the mandrel so that the ends could be taped down onto a pipe running through the center to maintain tension.

This photo shows a PVC pipe running through the center of the mandrel and suspended between two saw horses.  The ends of the carbon fiber sleeve are taped down to the PVC pipe.  A black nylon tie-wrap is used on each end to chinch it down tight and maintain tension within the sleeve material over the mandrel.   The second layer of sleeving is also placed onto the PVC pipe just to the left of the mandrel.  It will be slid on once the first layer has been coated with epoxy.  Click on the photo to get a closer look at all this.

A foam roller was used to apply West System epoxy to the sleeve.  It was a bit difficult to tell when the sleeve was properly saturated with epoxy since it does not turn transparent the way fiberglass does.  After the epoxy was applied, the second layer of sleeve was bunched-up and carefully slid over the top.  It was then stretched out and pulled tight the same way as the first layer.  More epoxy was applied to the second layer with the foam roller.

This is a roll of release coated shrink tape also available from Aerosleeves.  It is 2.5" wide and only 0.002" thick.  This tape shrinks when it gets hot so it can be used to squeeze out excess epoxy from the sleeve.

The shrink tape was carefully wrapped around the epoxy coated carbon fiber tube and taped into place at each end. A heat gun was then used to help shrink the tape and the excess epoxy was squeezed out the edges where each layer of the tape overlapped the previous layer.  Although this worked, it would have been better to put the tube into a proper curing oven to uniformly heat the tape and cure the epoxy all in the same step. 

Once the epoxy had cured, the shrink tape was pulled off. At that point it became apparent that the excess epoxy had left small indentations around each seam where the shrink tape layers overlapped.  What happened was the excess epoxy had been squeezed out, but it tended to puddle on the tape and these puddles created lumps that pushed down into the surface as the tape shrank.   Basically the newspaper layer method created a mandrel that was so soft, these lumps pushed into the paper and created the indentations.   The indentations are surface imperfections that would be very undesirable on a body tube but are of no consequence on this motor tube since they will not be visible.

The motor tube was then removed from the PVC pipe and a hose clamp was positioned near one end.  The hose clamp served as a reference edge for a hack saw that was used to cut off the end of the tube.  The hose clamp reference edge ensured the end was cut perpendicular to the long axis of the tube.

All in all, this tube turned out great.  It is very strong in the long axis and the HyperTEK flight tank fits perfectly.

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