Re: [ATM] Composites are NOT hard to use, just messy

[artbianconi@blast.net]

Hello Rod,

If I may, I would like to suggest that you exercise your learning 
curve with fiberglass first. It's easier to work with; 
dramatically less expensive and essentially overkill as a 
structural medium, especially in a low stress environment like a 
telescope.

If you were contemplating a critical load bearing spar on a 200 
knot aerobatic airplane, where you are pulling plus or minus 8 
G's, I'd not question it's use even though many such aircraft 
still do such manuvers safely with Sitka Spruce spars. 

It's unfortunate that carbon fiber has such esthetic appeal. Many 
argue for it's use even though the underlying motive is more 
appearance and ego than functionality. I will admit that I too 
love that hatched pattern. Still, when I build the prototype 
components, I will do so with Bidirectional fiberglass. I can't 
afford not to. And, it would likely still be more than for the 
loads the scope will see.

Part of my approach is driven by the fact that I have never built 
a truss type telescope and mistakes are inevitable. To make a 
mistake with fiberglass is not nearly as painful to one's wallet 
as the same mistake with CF. Go check the prices, You'll see what 
I mean in an adrenalin heartbeat.

http://www.Fiberglast.com is amongst the best resources for first 
timers and experienced users that I can think of.

Another reason is that the stiffer the fiber, the shallower the 
angle at which you must wrap it around a sharp edge. If you take 
a layup of fiberglass and orient it so that the plies are 
perpendicular to that sharp edge, it will fight you. It won't 
like what you are asking it to do and what you will wind up with 
is a balloon edge. It will look as if resin ran down the side of 
the part and collected in a meniscus at the bottom edge. In 
conventional layups (no vacuum bagging) you get around that by 
orienting the lay up so that the fibers intersect that sharp edge 
at a shallow angle. Problem solved? Not quite.

To be able to use that trick, the fibers still must be oriented 
in a manner consistent with the load paths the system will see 
and changing angles to accommodate an edge might not be 
consistent with the load paths.

In a conventional tube, I prefer the plies of a Bi-Directional 
cloth to be at a 45 degree to the center axis so that weave forms 
a double helix as it wraps around. It makes for better torsional 
and beam strength without adding the weight of extra plies. Will 
the plies fight wrapping around a sharp edge at 45 degrees? That 
depends on the weight of the cloth. 

Of course, if you can vacuum bag a tube without crushing it, then 
this becomes a non issue as the bagging material will force the 
ply to conform. Is crushing a consideration? You Betcha!

A 10 inch diameter tube made for an 8" f-8 mirror will be perhaps 
75 inches long in it's untrimmed state. It will have a surface 
area of about 4500 square inches, each of which will have to 
support 14.7 psi at sea level if you vacuum bag it to high 
numbers. That's 66,000 plus pounds of compression or about 33 
tons! With those loads, I suspect you will need a mandrel! Pop 
that puppy into an autoclave and add a few atmospheres of 
pressure. Better stand back and hold your ears for the loud 
"Bang!" Been there done that!

I like to use PVC sewer pipe or steel hot water expansion tanks. 
The outside of a discarded hot water heater can be perfect 
provided it's not textured. Or, you can build up a foam core from 
1 inch insulation foam (Home Depot) hot glued in layers to a 
steel pipe and then hot wire the foam to a circular shape using 
an electric "cheese slicer" wide enough to cut the form. 

Foam cores add dramatically to the strength of composite parts 
with only a slight increase in weight. However, they compromise 
the tube's ability to anchor fasteners. CG's, with all your 
components, must be calculated with extreme care so that you can 
substitute other core where the tube will be fixed to the mount. 
You'll also need robust core material for mounting Telrads, 
focusers, etc. 

How will you mount that thing when it's finished? Are you using 
those age old, aluminum cast rings like I have on my Criterians? 
Or will you use steel straps and discover one day that the rivet 
head crushed the tube (a CAVE) when it was overtightened by some 
one who was not thinking . That's what happened to the CAVE on my 
12.5  f 8.

The things that we take for granted in conventional OTA's require 
a second look and believe it or not, your best ally will be a guy 
who knows structures but nothing about astronomy. He's the one 
who will ask questions the rest of us know not to ask. Like the 
pediatrician in Michael Chriton's book (and movie) Andromeda 
Strain. The "Odd Man Theory" is real. Those who all share a 
common education and who think from a homogeneous paradigm, miss 
things the ignorant guys don't. 

Carbon fiber represents a major break through in structural 
material. I urge you NOT to build a conventional OTA with CF 
using a design that has been driven (and fixed) by structural 
limitations that existed 45 years ago.  Out of the box thinking 
is needed here to really take advantage of this stuff.

And it's still easy to build one, once you have developed an 
appropriate design. 

One man I know built a composite tube for his 8" mirror. Used 
bendable wood as core material in critical places, foam in others 
and used but one skin of 6 or 8 ounce BiD, inside and out. It was 
a first time effort and though he asked a lot of questions, I 
still didn't know what to expect.

Some of the plies weren't oriented as well as I'd have liked but 
1.) I am a perfectionist and 2.) that's an esthetic consideration 
that's of no value and will be hidden by a nice coat of 
polyurethane 2 part epoxy paint. 

What is important is that THIS WAS HIS FIRST EXPERIENCE WITH 
FIBERGLASS! The results blew everyone's mind! Mine too and I'm 
used to this. You could balance it on one tiny finger with all 
the important parts in place!

And you will too. Play first with glass, critique your first one 
with a critical eye. Then put some money into CF. Hope this helps

Good Luck

Art Bianconi


 

> Art,
> 
> I am going to give a CF scope a try a try.  It seems clear, if
> you step back a bit, CF is the lightest, stiffest stuff you can
> use, if you can design it right.  I have the been
> scrounging/ebaying stuff to make a curing oven , and I am
> mostly set up for vacuum bagging, also scrounged.  I envy you
> your autoclave <g>.
> 
> My plan to do a CF spider is/was admittedly because the rest of
> the scope will be CF, perhaps not the best reason to do so. 
> Because of some off list emails, I have been largely convinced
> that steel is the miracle ingredient for spiders, since it has
> the best stiffness and strength per volume.  It is still quite
> tempting to try a spider with unidirectional CF, and some
> bidirectional Kevlar to decrease the risk of catastrophic
> failure.
> 
> Rod
> 
> ----- Original Message ----- 
> From: <artbianconi@blast.net>
> To: <atm@atmlist.net>
> Sent: Wednesday, November 17, 2004 5:05 PM
> Subject: [ATM] Composites are NOT hard to use, just messy
> 
> 
> > Ross, I wish more people would try working with composites,
> > especially those who bust their tuckus to get a 1/12 wave
> > mirror and then stick in a shellacked cardboard tube!
> >
> > Admittedly, I've built a lot of things over the last 30 years
> > with this stuff but experience is not a major factor in my
> > success. The learning curve is not that steep. A lot of ATM's
> > are at the effect of rumors that have no basis in fact.
> >
> > I am committed to exploding the myths and awe that surrounds
> > this material. As soon as my shop is built, amongst the first
> > thing I will do is have a hands on seminar for ATM's in this
> > area.
> >
> > There is only one legitimate downside I can think of to
> > making a big telescope out of CF. If it's something like a
> > 15" f-8, it will be big but super light and easy to move
> > about.

> > BUT,
> >
> > you'd best not be anywhere near it if a cumulous nimbus is
> > nearby. If Zeus throws out one of his presents, the scope
> > will not be the only thing standing there made of carbon!
> >
> > Art
> >
> >> > Ross, the entire telescope (15" f-4.3) is scheduled
> >> > to be made of  carbon fiber.
> >>
> >> Wow, sounds ambitious.  I can see that it undermines
> >> your bragging rights to have a metallic spider in
> >> there.  Since you'll be swimming in carbon cloth and
> >> epoxy I see no reason not to just give it a shot and
> >> go for the curved carbon spider--you can always retro
> >> something more conventional in there if the diffuse
> >> diffraction is annoying.
> >
> >
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> >
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> 
> 
> 
> 


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