RE: [ATM] Carbon Fiber for Spider Vanes

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

Doug,

Foams, for composite's, come in different densities.  Typically as the 
density goes up so does the psi the material can take.  Even low density 
foams, in my experience have fairly high psi's.  30 psi rings a bell on 
some fairly lightweight foam that I have.  A "bump" into a ladder isn't 30 
psi unless you really form tackled your OTA or all of the force of the bump 
is concentrated in a very small area, like the tip of a Phillips head screw 
driver.  But wood would fair even worse in the screwdriver example.

Again, if the skins were produced correctly, they have much more strength 
than you would expect.  Use a woven good on the very outside layer though, 
don't use a unidirectional.  They don't have much impact strength like the 
woven's do.

Breaking a skin on one side of the laminate and in a small area will be 
similar to wood.  It won't affect things much.  The scope isn't going to 
collapse.  If you do puncture or somehow break off skin in an area there is 
a fundamental routine to fixing such things.  Any fiberglassing video or 
book should have the technique listed.  Sanding the area and feathering the 
edge, then applying layers of fiberglass that are larger and larger 
compared to each other.  The new laminate layers feather into the feather 
you did on the damaged composite.  If you do the technique well enough and 
repaint, you won't be able to see and it will have almost no affect on 
strength, when you're done.  Again, these aren't bridges.  They hold 
relatively light objects.

The bottom line is that the composite, if made and designed even remotely 
well, will weigh very little and will have little inertial mass.  Swinging 
the back end of the OTA around is different.  That's where the mass 
is.  But small bumps and whatnot might mar the surface but it is unlikely 
they will do anything else.  If it does break a skin, just fix it as 
outlined above.

This all holds true if you using foams I'm familiar with.  But there are 
foams that can be compressed together fairly easily but once skinned are 
amazingly strong.

Skin thickness can be very thin and still perform exceedingly well.  Make a 
few samples.  Think of things in a scientific manner, use scientific 
method.  Sample A has two skins on each side, sample B has three and sample 
four has four.  Or two, four and six per side each sample.  Whatever you 
want.  Once they're cured, go at them with your tests.  Drop the same item 
on them from the same height.  It's very crude but it should give data that 
is useful.  Use that data as a guideline for what you feel you're 
comfortable with.

Thank you.

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



>I've been following this thread with some interest, and have considered
>fiberglass/foam occasionally, but haven't actually used it yet.  One thing
>I that holds me back is concern about lateral strength.  For example, if I
>made upper cage rings from carbon and foam, I understand that I can build
>them stiffer and lighter than plywood, and they should perform well in
>use.  Now what happens when I'm setting up and bump the cage against the
>ladder.  Plywood takes a pretty good hit without damage, and a fair amount
>of damage without losing strength.  If I have a thin composite layer over
>foam, I would think that it will damage easily, and that this would have a
>big effect on the overall strength.  Do I end up destroying the whole
>piece because of a small hit?  Would I be better using glass instead of
>carbon, and making the laminations thicker anyway?
>
>Thanks,
>-Doug

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