RE: [ATM] Carbon Fiber for Spider Vanes

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

Peter,

You are exactly right about cored materials.  They offer the lightest 
solution, if thickness of design allows it.  On larger telescopes I will 
use thin core material in the spider vanes.  But this isn't possible on 
smaller telescopes because it makes the vanes proportionally too thick.

As with everything else in composites, core materials are offered in 
anything you can think of.  There are the basic honeycombs, which are sold 
in whatever thickness, cell size and material you could want.  Fiberglass 
and Kevlar honeycomb are about the same price, while carbon fiber honeycomb 
is the most expensive.  It is $500 a square foot...  But in certain 
aerospace applications they will use it because it is the only honeycomb 
that I know of that is isotropic.  Like a great many composite goods, 
fabrication techniques, etc., the end product is anisotropic.  It does not 
have the same properties in all directions.

Other core materials are foam, balsa and even foam-filled honeycomb.  The 
latter combines the best of both foam and honeycomb.  Some rocket nose 
cones use this AND they are thermoformed.  Very expensive.  Another 
technique presses foam into the honeycomb cells....  This technique is not 
for the faint of heart.  Use a foam with too high a density and you just 
crushed your expensive honeycomb cell walls.

ATMers are not trying to make the absolute lightest, stiffest and/or 
strongest part cutting edge technology allows though.  Using fiberglass 
over a core is an excellent way to build a very rigid structure that is 
also light.  This is the most economical way for a person to build a stiff 
and relatively light structure out of composites.

It is a common misconception though that an end product in carbon fiber 
(for a given stiffness or strength) is more expensive.  Carbon fiber 
(specific weaves, types, etc....) are much stronger and stiffer than 
fiberglass.  My experience shows that the additional laminate layers (or 
skins in the case of a sandwich core application) required for fiberglass 
will use more resin for the initial lay up and will therefore use more 
labor for those additional layers.  Epoxy is expensive and adds weight to 
the part.  So the fiberglass part is heavier and when all costs are 
determined, it is very similar to the cost of using carbon fiber.  This is 
only true when you are holding a specific strength or stiffness, so that 
both have to reach the same value, apples to apples, in other words.  An 
additional benefit for an opto-mechanical system is that carbon fiber has a 
lower CTE than fiberglass.  Certain types of carbon fiber have a negative 
CTE.  That is, they contract when warmed and expand when cooled.

Fortunately for the ATMer they don't have to hire labor or try to produce 
technology leading parts.  So Peter's suggestion and use of fiberglass 
skins over a core is perfect for someone who wants to dabble in composites.

Thank you.

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

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