Re: ? Problems with CF Spider;Was Re: [ATM] Thin Titanium Spiders & Diffraction Spikes

["Rod Shea" <RodShea@comcast.net>]

Matt,

I have a sinking feeling you are right about a CF spider.  I do notice the 
only ones I have ever seen were ~ 1/8" thick, with a foam core, used to 
raise the sec assembly above the plane of a single ring spider.  I never 
figured this thickness was worth the optical cost.

Do you have any thoughts on my plans to put a layer of Kevlar BID in the 
vanes, and to angle the UNI CF plies similar to the angles in a wire spider?

Thanks,

Rod

----- Original Message ----- 
From: "matt" <mariusrf@bellsouth.net>
To: "Ross Sackett" <rsackett00@yahoo.com>; <atm@atmlist.net>
Sent: Monday, November 15, 2004 9:36 AM
Subject: Re: ? Problems with CF Spider;Was Re: [ATM] Thin Titanium Spiders & 
Diffraction Spikes


> not quite true. Carbon fiber laminate , if designed and engineered 
> properly
> can be significantly stronger than the majority of steels . More precisely
> it can have much higher tensile strength than steel . However , it has a
> twofold problem which could prove to be a show stopper:
>
> #1 it really needs to be designed , engineered and manufactured properly
> for these qualities to happen , or else the laminate will achieve only a
> small fraction of its potential . Among the things that are needed would 
> be
> to use unidirectional carbon tow, properly engineered metal inserts for
> attachment points, vacuum bagged and oven cured , needless to say use a 
> good
> structural epoxy for laminating  . IF these things aren't done properly, 
> the
> laminate will be much weaker than its theoretical strenght and you'll need
> to design in this weakness by using a much larger safety factor , in other
> words overbuilding it and negating any advantages carbon may otherwise 
> have
> if executed properly.
>
> Issue #2 is not so easy to avoid, and that might prove to be fundamental.
> Carbon fiber has tensile strength that exceeds steel but is extremely poor
> in compression. Failure mode is buckling of the laminate and fracturing 
> the
> fibers. Steel and more generally metals do not have this failure mode.
> Another failure mode is puncture . Simply put, you have to not hit, bend ,
> or abuse your carbon spider , because it will only take axial loads. Bend 
> it
> sideways for example to attach it or insert it into the tube and you can
> kiss it good bye.  Hit it with a screwdriver tip and it's done . The
> puncture issue happens especially with thin flat skins , and that would be
> the case with thin spiders . Round thicker walled tubes are more robust in
> this respect .
> So unless you're making a spider that you know nobody will touch for a 
> long
> time (for example a spider that goes into a closed tube in an observatory
> setting) plan on it breaking often and make lots of spares that are easy 
> to
> replace in the field, or don't use carbon fiber for it . Use titanium, 
> steel
> , piano wire, racing bike blade spokes, but not carbon fiber.  Make your
> mirror cell , truss tubes or other scope parts of carbon where it's not
> exposed to these catastrophic failure modes.
>
> best regards,
> matt tudor
>
>
> -----Original Message-----
> From: Ross Sackett <rsackett00@yahoo.com>
> To: atm@atmlist.net <atm@atmlist.net>
> Date: Monday, November 15, 2004 9:08 AM
> Subject: Re: ? Problems with CF Spider;Was Re: [ATM] Thin Titanium Spiders 
> &
> Diffraction Spikes
>
>
>>What advantages do you see for a CF spider?  It
>>certainly would have low thermal mass, but I doubt it
>>will be as stiff as you hope.  Carbon-epoxy has high
>>stiffness for its density, but you'll probably have to
>>laminate it pretty thickly to match the stiffness of
>>conventional steel vanes. (I haven't looked
>>systematically, but having played around with single
>>and double layer BID CF it doesn't seem as stiff as
>>the same thickness of steel.)  There may be a weight
>>advantage, but I suspect matched stiffness for
>>thickness steel has the advantage.  (OK I admit I am
>>being lazy here--if I had my materials tables handy it
>>would be easy to check up on all this.)
>>
>>If you go the CF route you may want to consider
>>unidirectional cloth--the vanes are loaded primarily
>>in tension, and the weft of the BID isn't going to
>>help you much in that case but will add thickness to
>>the weave.
>>
>>Ross
>>
>>
>>
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