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

["matt" <mariusrf@bellsouth.net>]

Please see some inline answers and comments:

-----Original Message-----
From: artbianconi@blast.net <artbianconi@blast.net>
To: Rod Shea <RodShea@comcast.net>
Cc: atm@atmlist.net <atm@atmlist.net>
Date: Thursday, November 18, 2004 1:20 AM
Subject: Re: [ATM] Composites are NOT hard to use, just messy


>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.
>

Art, the reason CF would work is not ultimate breaking strength, but
deflection under those minimal loads a telescope sees under normal use .
Even lightweight structures deflect under their own load . Airplane wings
aren't engineered to be so stiff that they wouldn't deflect  0.1mm , but if
telescope parts start having this type of deflections, the scope gets the
shakes , doesn't hold collimation etc.
Before making stronger and more rigid parts, you need to spend more time
thinking what the efforts are and what you are trying to achieve . Again,
not ultimate breaking strength but preventing things from working like
optical flexure displacement stages .


>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!
>


Art, I have never seen anyone bagging a tube the way you describe it .
That's not the way carbon tubes are vacuum bagged . Always the back of the
laminate due to inside tube that supports the laminate is exposed to
atmospheric pressure . There's no bang whatsoever. Inside the tube there's
free contact with the atmosphere. You just seal the vacuum bag against the
edge of whatever tube you're using as support for the laminate . Air
pressure from the top of the laminate is balanced and cancelled by the air
pressure on the inside of the tube . People have used any form shape and
size tube, from simple cardboard that is washed out after the laminate cure,
to foam, to metal mandrels that expand during high temp baking/curing and
are cooled down rapidly by blowing coolant through the middle after curing
is complete. NO BANG .

>> >
>> > 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

Aluminum truss tubes are known to be a better conductor than CF composites .
You'd need to take the same precautions for a CF scope as for a truss tube
in case of T-storms, unless you were just jesting.

best regards,
matt tudor


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