The Fiberglass Follies
It's a very small world. Someone that I used to work with--and who, several years before that, sold a small telescope to a friend of mine--has done extensive work with carbon fiber composites for aircraft.
Anyway, he tells me that the polyester resin style of fiberglass that I am using is much less stiff than the epoxy kind--and for the intended purpose (repairing car fenders and such) that is probably a good thing.
Also, as my wife suggested, and I surmised, fiberglass stiffness is highly dependent on cross-section, and scaling up the diameter of the tube requires scaling up the wall thickness as well. As a result, four layers of fiberglass cloth for a 1.75" ID tube is far less stiff than four layers of fiberglass cloth for a 20.4" ID tube.
I had thought about contacting Sky Valley Scopes about a tube because they used to make honeycomb composite fiberglass tubes that were very strong, and very light. See this example of a remarkably strong and light tube. The honeycomb composite use a very strong honeycomb layer in between two thin skins of fiberglass. You get all the benefits of the strength of the honeycomb and its lightness. (The XB-70, to my knowledge, was the first to use this approach, with stainless steel honeycomb sandwiched between titanium skins.)
Unfortunately, Ken Ward, who ran Sky Valley Scopes, reached a point where his arthritis prevented him from continuing to make the tubes. His wife Judi forwarded his instructions on how he made these tubes to me. It sounds like something that I could ruin a lot of materials before I got good at it!
Anyway, I have abandoned the idea of making a fiberglass tube for the Big Bertha rebuild. As soon as National Metal Fabricators opens for business Wednesday I am going to order up aluminum tube sections for this. I will also return the unused container of polyester resin--but not the fiberglass cloth. I may continue to experiment with making fiberglass tubes, using epoxy instead of polyester, and the honeycomb material that you can buy from operations like Aircraft Spruce and Specialty.
I have been tempted for some time to rebuild the 3" f/4.5 reflector that I built some years ago. Because of the scarcity of parts in this size, there are a number of compromises to it. I used a larger tube than I needed (4" ID) because the only mirror cell that I could find had that as an OD for the base plate. The diagonal was bigger than it needed to be, because of what I could buy at the time, and the limitations of a high profile focuser. Now that I can machine the parts that I need (especially in this inky-dinky size), I may do a complete rebuild, with an optimally sized diagonal, tube, and focuser. And maybe I will replace that heavy piece of PVC that is the current tube with something made of fiberglass. Even the polyester resin should be capable of making an adequately stiff and light tube, based on the 1.75" ID experiment.