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

[artbianconi@blast.net]

> 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, 

The current X series aircraft have forward swept wings and built 
in instability that is so "on the edge", that it takes three 
computers to help the pilot "fly by wire" No human being has the 
response speed to keep the wings from going divergent. The 
critical factor in keeping the plane in one piece is stiffness 
and the guys at De Sault (they build the Mirage and Falcon jets) 
who I spoke to were striving at controlling deflections down to 
0.005 on a span 0f 25 feet under a load of 8 G's.

> 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 .
> 
The cell I am building is designed for stiffness with low weight. 
Strength has never been an issue. I know where the loads are. As 
I pointed out to Ross, fiberglass is quite capable of dealing 
with the stresses of the typical OTA. I am not working to some 
ultimate, theoretical ideal. There's no point in doing so. Best 
justification for that is that there are people reading this who 
have built very good telescopes out of wood,  a structural 
material whose geometric stability is compromised just with 
humidity changes. It seems to work anyway. 

This demand for theoretical perfection is not part of my thinking 
Matt. As a designer, I learned a long time ago that design and 
compromise are inevitable bed mates. 

> >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 .

I did say he'd need a mandrel didn't I? Matt, you insist on 
making comparisons of how the big boys make parts in an autoclave 
that looks like the pressure hull of a nuclear submarine. Of what 
value is that to Ross or any other ATM who's trying to build a 
tube in his wife's kitchen when she isn't cooking? Designs and 
the process for making them have to be developed in the context 
in which they will be built and the materials available. Ross, 
and others like him, are in the real world, scrounging for 
surplus stuff, building their own ovens. The odds of his ever 
making an autoclave at home big enough to contain a one piece 
tube are so remote as to be beyond practical consideration. 

During my learning curve I did a lot of things trying to learn 
how this stuff works and saw lots of grossly distorted parts 
after the molds were twisted by the process. Some gave up more 
suddenly. One male mold, a cowling for an aircraft engine, could 
not handle the compression and failed dramatically. We thought 
the autoclave had failed. We anticipated that but tried it 
anyway. It's how people learn. I push the envelope until it 
breaks. In the arena where I play the most, I must know. There is 
too much at stake.

I am not disputing anything you said. What I am asking is that 
your use of material absolutes be reined back a little and 
adjusted for the guy on the street who has neither the funds nor 
the resources to optimize materials to their design limits.
> >> >
> >> > 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
>

I am aware of that Matt. I've flown in aluminum airplanes in IFR 
conditions with lightning all around me. I've experienced St 
Elmo's Fire and could feel the electrical tingling in the 
controls. Fortunately I was not the pilot at the time. I was too 
busy trying NOT to soil my underwear!

That statement was more tongue in cheek than anything else. I 
can't imagine why an astronomer or his prized scope would be 
outdoors in a thunderstorm! What could he look at?

Thanks

Art
 
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