Re: [ATM] Carbon Fiber for Spider Vanes

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

It will be a travel scope, and the upper cage will be checked baggage, not, 
I hope, as severe as skydiving.  I will be making a spider such that the 
secondary mirror and holder is easily removed, and put back in < exactly > 
the same position.

Rod


----- Original Message ----- 
From: "Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>
To: <atm@atmlist.net>
Sent: Tuesday, April 05, 2005 12:14 AM
Subject: RE: [ATM] Carbon Fiber for Spider Vanes


> Peter and group,
>
> I'm sorry I couldn't respond sooner.  I'm working on a rush job.
>
> Don is exactly right.  My quotes relating to "five times" were a reference 
> to jargon I hear a lot.
>
> Experimenting with carbon fiber is just about the only way an individual 
> can learn how to work with it.  Carbon fiber wets out differently than 
> fiberglass.  Kevlar wets out different than either of those (E-glass or 
> carbon fiber).  They also allow or don't allow resin out of them in 
> different ways too.  There are so many variables in composites.  That is 
> all I was trying to get across.
>
> Rod's planned test makes me wonder if he sky dives with his telescope.  I 
> wouldn't worry as much about the carbon fiber spider vane as I would about 
> glass optics.  Again, if the carbon fiber is made correctly, it is 
> extremely strong and rugged.  Those who say that all carbon fiber is 
> brittle and can't take some abuse are painting with two broad of a brush. 
> I've dropped many a carbon fiber product I was making on my concrete shop 
> floor.  This includes large diameter, cored tubes, smaller diameter truss 
> tubes and even carbon fiber mirror cells.  Their lack of weight helps them 
> a great deal already.  But, if they are made properly, they are very 
> durable.  They are now making almost anything conceivable out of carbon 
> fiber.  From windmill blades measuring up to 40m in length (~131 feet), 
> boat hulls (both military and water sport/yachts), bicycle rims - 
> handlebars - front forks - the entire frame...  The entire underside of 
> the Ferrari Enzo is carbon fiber.  SpaceShip One.  Key areas on the Space 
> Shuttle.  Current and next generation strike fighters.  Other Stealth 
> aircraft.  Landing gear legs for Navy aircraft (ever seen a 30k pound 
> fighter land hard on an aircraft carrier?).
>
> Sorry for all of the examples but there are a great many things that have 
> withstood great punishment.
>
> One possible culprit to the concept that carbon fiber is inherently 
> brittle and therefore has no durability is tent poles.  These are often 
> small diameter, pultruded carbon fiber, hollow rods.  Pultruded carbon 
> fiber is basically unidirectional.  There is no strength in the opposite 
> direction, hoop or transverse direction of the pole.  You can easily take 
> a pair of pliers and shatter the carbon fiber because you are applying 
> force in a direction the rod has nearly zero strength in.  Making products 
> with unidirectional goods allows for greater strength but only in the 
> direction they are laid.  Another attribute of unidirectional is that it 
> does not have the surface impact strength that woven goods have.
>
> I have a page that has general information about composites:
> http://www.dreamscopes.com/pages/glossary.htm
>
> Jerry is absolutely right about controlling the process at every step. 
> Wrinkles are definitely something to avoid.  They are among the easiest to 
> see however.  So once a person gains some experience wetting out 
> materials, they should be able to produce items with no wrinkles at all. 
> Those are gross-type flaws in lay up.  Smaller flaws are things like 
> catching the goods with the edge of a squeegee or applicator and "opening" 
> the fabric (in a woven goods example) in that area.  The same type of weak 
> next to strong effect the wrinkle example illustrates.  For a telescope 
> these kinds of minor flaws, catching the woven goods slightly, aren't of 
> great importance to a person just trying to make something out of carbon 
> fiber for themselves, not for a living.  But these flaws can be 
> potentially fatal in certain applications where the design is pushing the 
> materials to the limit, which in turn pushes the fabrication of the part 
> to the limit.  Aircraft and aerospace applications for example.
>
> Don's list of additional factors are all true.  Try to think as each line 
> that he listed though as an iceberg.  Within each subset are a dozen 
> smaller ones.  It never ends, literally.  This should start to hint to 
> everyone of the true complexity of composites.  This factor has hurt 
> composite growth more than any other.  Inconsistency from part to part is 
> what the military has been fighting for decades.  New processes and better 
> understanding of how strong all of the variables affect the properties of 
> the final part are helping push composites more mainstream but it is still 
> inherently complex.
>
> If you use room temperature resins, they often have low Tg's.  Glass 
> Transition points.  You can read more about this subject here:
> http://www.dreamscopes.com/pages/elevcure.htm
>
> Resin content (or fiber content, however you want to look at it) is a 
> separate subject from Tg.  Low resin content helps make the part lighter, 
> stronger, stiffer, potentially more uniform, etc..  Tg is Tg no matter how 
> much the resin content.  Unless you routinely paint your solid tube 
> telescopes black and leave them "sunning" in the Arizona summer Sun, Tg 
> isn't nearly as important as resin content.  You will have far greater 
> gains in performance by reducing resin content than you will by switching 
> to an elevated temperature epoxy.  You can still paint it black, just 
> don't put it in the Sun for very long.  Only your own test on your own 
> part can define what "very long" is.  Again, variables.  Is it a glossy 
> finish or matte.  Does the paint reflect IR or absorb IR.
>
> Since I'm making composite products for customers, my liabilities and 
> concerns are different than if I were making it for myself, especially if 
> I were only making one.  You can read some of the reason's why I switched 
> to elevated temperature epoxies here:
> http://www.dreamscopes.com/pages/history.htm
>
> The current elevated temperature epoxy that I am using can be cured up to 
> 350F, which gives a Tg of around 350F.  Cooking a ground based telescope 
> to 350F is not necessary though.  Not for this resin.  The epoxy gains the 
> highest property jump between 150F and 200F, for my particular resin.  I 
> cook them in my 6' x 12' x 6' oven, which you can see below:
> http://www.dreamscopes.com/pages/projects-04/oven/07.htm
>
> A "weekend warrior" doesn't need an oven as large as mine but the oven 
> does have to be extremely accurate and controllable.  When I place the 
> parts in the oven, I can't heat the oven up faster than 7F/minute.  Same 
> thing for the cool down.  It cannot exceed a drop in temp of more than 
> 7F/minute.  Microprocessors control this form me, as well as the closed 
> loop temp probes stationed inside the oven.  In other words, buying an old 
> oven for 10 bucks isn't going to give you what you want...  They turn on 
> full blast initially, to heat to the set temperature as quickly as 
> possible.  This would ruin a part using the epoxy I have.  With carbon 
> fiber dry goods costs between $20-$40/yard and epoxy at $100-$150 per 
> gallon, ruining a part can be a wee bit stressful.  Now imagine the 131 
> foot windmill blade...
>
> Alan-
> Yes, the CG definitely moves toward the primary when you reduce the weight 
> of the structure.  Cary takes this further by reducing the weight of the 
> primary.  Reduce it's weight and you reduce the requirements for the rest 
> of the structure.  A trickle down affect.  Compare that to a full 
> thickness, plano style mirror with wood and/or metal structure and the two 
> are magnitudes different in weight.  Or you can consider swapping metal 
> and wood structure for carbon fiber to give an equivalent stiffness of the 
> metal and wood parts, but reduce the weight of the primary.  This gives an 
> OTA that is stiffer and lighter.  This ability to change and specify every 
> aspect of the telescope opens up the telescope design.  An analogy would 
> be a refractor: singlet compared to a triplet.  The first has only two 
> degrees of freedom while the second offers six degrees of freedom.
>
> Whether a lightweight structure will topple or hold steady is another 
> topic that needs further defining so as not to be a generalization.  One 
> of the main advantages of truss style OTA's is there lower surface area 
> for the wind to react against.  As far as structure, with regard to 
> stiffness not related to wind, the truss uses the well documented 
> triangular shape that makes extremely strong and stiff structures.  Ever 
> make a toothpick bridge?
>
> Off to bed for a few hours...
>
> Sincerely,
> Shane Santi
> Dream - Telescopes & Accessories, Inc.
> http://www.dreamscopes.com
> 610 - 365 - 2833
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/ 


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