Today I undertook reinforcing the airframe tube and two coupler tubes for my next rocket. Like Flying Colors this rocket is being constructed from LOC/Precision 54mm kraft paper tubes reinforced with 3" light biaxial sleeve fiberglass from Soller Composites.
I started by fiberglassing the airframe tube as I did the practice tube that will be the second airframe tube in the new rocket. I once again used the treated shrink tubing to squeeze out excess epoxy using the process described on the Soller web site. I was able to strip the shrink tubing off by late this evening and I'm very happy with the results. Clearly the best of all the tubes I've done to date.
Once I had the airframe tube hung up to cure I started work on the coupler tubes. As can be seen elsewhere in this blog, I used biaxial sleeve to reinforce the coupler tube for Flying Colors, but while successful it was not a process without its problems. I had issues with the fiberglass pulling away from the inside tube wall if any force was applied that would pull towards the outer ends of the tube, as if the sleeve were a Chinese finger trap. It also kept pulling away from the tube at both ends where I had folded the sleeve back over the outside of the tube. These two problems tended to feed on each other as trying to correct one tended to result in the other being exacerbated.
While it is likely that using a vacuum bagging process would cure these issues, I do not have the equipment to implement such a process at this time. I have spent a fair bit of time thinking about the trouble I'd had and had come up with a possible solution that I decided to attempt implementing for these coupler tubes.
I started my new process by taping wax paper around the outside of the coupler tube in order to minimize transfer of epoxy to the outside of the tube. The wax paper was trimmed to match the ends of the coupler tube. Next the fiberglass sleeve was inserted into the tube. One end of the sleeve was folded back over the outside of the tube and secured with masking tape. This was done in order to restrain the sleeve within the tube while the remaining work was performed. The other end of the sleeve was not secured, thus essentially eliminating the possibility of the finger trap effect occurring, so long as I didn't pull on the sleeve from the unsecured end. I then used a disposable fiberglassing brush, available at your local hobby shop, to apply the epoxy to the sleeve, working from the end of the tube with the sleeve secured to it. I made sure that any force applied to the sleeve was a pulling motion towards the end of the tube I was working from. This forced the sleeve to expand against the wall of the coupler tube. Once I had all of the fiberglass wetted with epoxy I inserted a rolled up tube of waxed paper and then expanded it to contact the wall of the tube. I then inserted an uninflated balloon, purchased at the local party supply store, inside the tube, stretched the balloon to reach both ends of the tube, and inflated it with my air compressor. The balloon helped hold the fiberglass against the tube while the epoxy cured, while the waxed paper served as a release to keep the balloon being bonded to the tube. When the epoxy had cured to the gel stage the balloon was popped and the internal waxed paper removed. The fiberglass then received a rough trim and the external waxed paper was also removed. This process was duplicated for the second coupler tube and the tubes are now being allowed to fully cure before any further work is done.
The results are considerably better than I achieved on Flying Colors' coupler tube, with the fiberglass well bonded from end-to-end. There are signs that the internal waxed paper wrinkled and trapped some of the excess epoxy. Use of a proper release directly on the balloon would likely alleviate this issue. The balloons were also only able to exert a limited amount of force since the expanded outside the tubes as more air was blown into them. This limitation means that they didn't squeeze out as much excess epoxy as I would have liked. It is likely that the only cure for this would be to use a proper vacuum bagging process, however, use of a long balloon in place of the round balloon I chose to use my yield better results. I used the round balloon as I wanted the heavier walls such balloons tend to have. The photo album should help clarify the process.
Photo Album: 2011-05-07, Fiberglassing New Rocket
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