RE: [ATM] Carbon Fiber for Spider Vanes

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

Peter and group,

I'm sorry I couldn't respond sooner.  I'm working on a rush job.

Don is exactly right.  My quotes relating to "five times" were a reference 
to jargon I hear a lot.

Experimenting with carbon fiber is just about the only way an individual 
can learn how to work with it.  Carbon fiber wets out differently than 
fiberglass.  Kevlar wets out different than either of those (E-glass or 
carbon fiber).  They also allow or don't allow resin out of them in 
different ways too.  There are so many variables in composites.  That is 
all I was trying to get across.

Rod's planned test makes me wonder if he sky dives with his telescope.  I 
wouldn't worry as much about the carbon fiber spider vane as I would about 
glass optics.  Again, if the carbon fiber is made correctly, it is 
extremely strong and rugged.  Those who say that all carbon fiber is 
brittle and can't take some abuse are painting with two broad of a 
brush.  I've dropped many a carbon fiber product I was making on my 
concrete shop floor.  This includes large diameter, cored tubes, smaller 
diameter truss tubes and even carbon fiber mirror cells.  Their lack of 
weight helps them a great deal already.  But, if they are made properly, 
they are very durable.  They are now making almost anything conceivable out 
of carbon fiber.  From windmill blades measuring up to 40m in length (~131 
feet), boat hulls (both military and water sport/yachts), bicycle rims - 
handlebars - front forks - the entire frame...  The entire underside of the 
Ferrari Enzo is carbon fiber.  SpaceShip One.  Key areas on the Space 
Shuttle.  Current and next generation strike fighters.  Other Stealth 
aircraft.  Landing gear legs for Navy aircraft (ever seen a 30k pound 
fighter land hard on an aircraft carrier?).

Sorry for all of the examples but there are a great many things that have 
withstood great punishment.

One possible culprit to the concept that carbon fiber is inherently brittle 
and therefore has no durability is tent poles.  These are often small 
diameter, pultruded carbon fiber, hollow rods.  Pultruded carbon fiber is 
basically unidirectional.  There is no strength in the opposite direction, 
hoop or transverse direction of the pole.  You can easily take a pair of 
pliers and shatter the carbon fiber because you are applying force in a 
direction the rod has nearly zero strength in.  Making products with 
unidirectional goods allows for greater strength but only in the direction 
they are laid.  Another attribute of unidirectional is that it does not 
have the surface impact strength that woven goods have.

I have a page that has general information about composites:
http://www.dreamscopes.com/pages/glossary.htm

Jerry is absolutely right about controlling the process at every 
step.  Wrinkles are definitely something to avoid.  They are among the 
easiest to see however.  So once a person gains some experience wetting out 
materials, they should be able to produce items with no wrinkles at 
all.  Those are gross-type flaws in lay up.  Smaller flaws are things like 
catching the goods with the edge of a squeegee or applicator and "opening" 
the fabric (in a woven goods example) in that area.  The same type of weak 
next to strong effect the wrinkle example illustrates.  For a telescope 
these kinds of minor flaws, catching the woven goods slightly, aren't of 
great importance to a person just trying to make something out of carbon 
fiber for themselves, not for a living.  But these flaws can be potentially 
fatal in certain applications where the design is pushing the materials to 
the limit, which in turn pushes the fabrication of the part to the 
limit.  Aircraft and aerospace applications for example.

Don's list of additional factors are all true.  Try to think as each line 
that he listed though as an iceberg.  Within each subset are a dozen 
smaller ones.  It never ends, literally.  This should start to hint to 
everyone of the true complexity of composites.  This factor has hurt 
composite growth more than any other.  Inconsistency from part to part is 
what the military has been fighting for decades.  New processes and better 
understanding of how strong all of the variables affect the properties of 
the final part are helping push composites more mainstream but it is still 
inherently complex.

If you use room temperature resins, they often have low Tg's.  Glass 
Transition points.  You can read more about this subject here:
http://www.dreamscopes.com/pages/elevcure.htm

Resin content (or fiber content, however you want to look at it) is a 
separate subject from Tg.  Low resin content helps make the part lighter, 
stronger, stiffer, potentially more uniform, etc..  Tg is Tg no matter how 
much the resin content.  Unless you routinely paint your solid tube 
telescopes black and leave them "sunning" in the Arizona summer Sun, Tg 
isn't nearly as important as resin content.  You will have far greater 
gains in performance by reducing resin content than you will by switching 
to an elevated temperature epoxy.  You can still paint it black, just don't 
put it in the Sun for very long.  Only your own test on your own part can 
define what "very long" is.  Again, variables.  Is it a glossy finish or 
matte.  Does the paint reflect IR or absorb IR.

Since I'm making composite products for customers, my liabilities and 
concerns are different than if I were making it for myself, especially if I 
were only making one.  You can read some of the reason's why I switched to 
elevated temperature epoxies here:
http://www.dreamscopes.com/pages/history.htm

The current elevated temperature epoxy that I am using can be cured up to 
350F, which gives a Tg of around 350F.  Cooking a ground based telescope to 
350F is not necessary though.  Not for this resin.  The epoxy gains the 
highest property jump between 150F and 200F, for my particular resin.  I 
cook them in my 6' x 12' x 6' oven, which you can see below:
http://www.dreamscopes.com/pages/projects-04/oven/07.htm

A "weekend warrior" doesn't need an oven as large as mine but the oven does 
have to be extremely accurate and controllable.  When I place the parts in 
the oven, I can't heat the oven up faster than 7F/minute.  Same thing for 
the cool down.  It cannot exceed a drop in temp of more than 
7F/minute.  Microprocessors control this form me, as well as the closed 
loop temp probes stationed inside the oven.  In other words, buying an old 
oven for 10 bucks isn't going to give you what you want...  They turn on 
full blast initially, to heat to the set temperature as quickly as 
possible.  This would ruin a part using the epoxy I have.  With carbon 
fiber dry goods costs between $20-$40/yard and epoxy at $100-$150 per 
gallon, ruining a part can be a wee bit stressful.  Now imagine the 131 
foot windmill blade...

Alan-
Yes, the CG definitely moves toward the primary when you reduce the weight 
of the structure.  Cary takes this further by reducing the weight of the 
primary.  Reduce it's weight and you reduce the requirements for the rest 
of the structure.  A trickle down affect.  Compare that to a full 
thickness, plano style mirror with wood and/or metal structure and the two 
are magnitudes different in weight.  Or you can consider swapping metal and 
wood structure for carbon fiber to give an equivalent stiffness of the 
metal and wood parts, but reduce the weight of the primary.  This gives an 
OTA that is stiffer and lighter.  This ability to change and specify every 
aspect of the telescope opens up the telescope design.  An analogy would be 
a refractor: singlet compared to a triplet.  The first has only two degrees 
of freedom while the second offers six degrees of freedom.

Whether a lightweight structure will topple or hold steady is another topic 
that needs further defining so as not to be a generalization.  One of the 
main advantages of truss style OTA's is there lower surface area for the 
wind to react against.  As far as structure, with regard to stiffness not 
related to wind, the truss uses the well documented triangular shape that 
makes extremely strong and stiff structures.  Ever make a toothpick bridge?

Off to bed for a few hours...

Sincerely,
Shane Santi
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833

_______________________________________________
ATM mailing list http://www.atmlist.net/