Re: ATM Carbon vs. Glass fiber for tube

["Dave Pitou" <dpitou@ix.netcom.com>]

Hello,

        For a given thickness of layup, and equal fiber/epoxy ratio's, carbon
fiber is about twice as strong as good glass cloth, and 3 or 4 times
stiffer. It weighs about 1/2 of what the glass weighs, and It costs 5 to 10
times as much. With the correct choice of fiber and fiber directions a tube
can be designed with essentially a zero coefficient of thermal expansion in
one direction, (lengthwise), but it will still expand diametrally. In my
experience the carbon fiber is easier to layup since it wets out easier
than glass cloth, but it seems to cause a little more itching.

        If you are planning on covering the cordboard tube with a few wraps of
fiber I'm afraid the tube won't be stiff enough not to sag. Telescope
components are almost always 'stiffness limited', that is you care more
about flexure than strength. The stiffness of a tube is dependant on the
wall thickness and the material it's made of, so just making the tube out
of carbon doesn't automatically result in a stiff tube. The wall thickness
of a 6" x 6 ft. tube should be about 1/8" for decent stiffness irrespective
of the material used. If you use a .013" thick carbon cloth the tube would
require about 10 wraps, or about 10 yards of cloth, which would cost about
$450. In the way you are planning on making the tube I think the cabon
would be overkill.

Dave
 
> 
> Hello,
> 
> I was wondering what the experts thought WRT carbon fiber versus glass
> fiber for telescope tubing. If I were to take a long, thin cardboard tube
> and cover it with one of these two products, which it the more desirable
to
> use, and why?
> 
> The tube will be 6" i.d., by about 6 feet long.
> 
>