CF composition construction questions

Dave

Producing something for so specific a function, it would be useful to know a bit more about the function. With 24" diameter, I assume this is a reflecting telescope, so I'm not sure why 'Obviously, lighter is better' ... astronomy's not my field; what do folk generally use for this? I'd have thought that (within reason) the greater the mass of the structure, the steadier the image.

Regarding composites, I have become a huge fan of the materials/layups with integral resin infusion mediums - we use two kinds for different purposes. Saercore, or Rovicore (and the others I don't know about) are random mat glass, either side of a knitted plastic stuff. It's remarkably conformable, and it works particularly well in closed moulds. We have used it under a bag, however - in which case you can regulate its thickness to some extent with more or less vacuum. In your case you could apply some carbon either side if you want.

The other material is Lantor Soric (similar to Corecell, I gather), which is a non-porous polyester material with hexagonal flow channels for the resin. You simply lay the Soric between your reinforcement (be it glass or carbon) and, when it's done its job as an infusion medium, the hexangonal ribs of resin act as a core, significantly adding to the strength of the composite.

Both these materials make the infusion process delightfully easy, and cut the cost of 'consumables'.

And regarding the suface finish from a bag - on complex shapes you apply lots a surplus bag to reduce the risk of bridging, and the result can be decidedly wrinkly. With a simple shape like your cylinder, though, you could prevent the wrinkles by stretching the bag quite taut; you will, however, get the texture of the outside reinforcement. If you don't want that, although it would be a bit more fiddly, you could lay your cylinder up on the inside of the tube.

 

Christopher,

Thanks for the post. I'll take a look at the materials you listed.

Mass on a telescope is good (neglecting portability, mine will be housed in an observatory) if it is centered in the pier and mount that hold the telescope in place. A lighter tube is more easily moved and held stable by a given mount compared to a heavier tube. There are two commericially available mounts I am considering and they are rated to 150 lbs payload for photographic use. A 20" scope of the type I am building is pushing those limits so I need to conserve weight where I can. There are no "off the shelf" mounts available with larger capacities and custom mounts are big $$$$$.

The tube will be approx 24" diameter by 56" long. At one end it will house a mirror and a metal mirror cell that weigh a combined 75 lbs or so. At the other end, it will house a smaller mirror with support structure that weighs around 15 lbs. The tube will be cradled by large circumferential tube rings that will be mounted to the telescope mount. Take a look at http://www.rcopticalsystems.com/20inch.html to get an idea of the type of telescope I'm building. I could go with a truss but the cost for materials and machining all of the parts is quite a bit more than the cost of materials for a carbon fiber tube.

I just need to ensure that the tube is rigid enough to support the optics and be held securely by the rings. I'm not sure how many layers of CF at a given weight is adequate. I know that aluminum tubing is available and that it is fairly thin stuff... like 1/10" to 1/16". It seems like 1/16" thick CF may have enough strength but I would think it would be fairly brittle and easily damaged. The tube will be drilled at a number of points to allow bolts to attach the mirror cells to the tube. I think that 1/8" thick walls should be plenty strong and thick enough to support the mounting bolts for the optics. I had planned on using a couple of layers of Kevlar/CF as the outer skins for aesthetic reasons and that should help provide a certain level of toughness for the tube as well.

I am concerned about the compressibility of honeycomb core material as the bolts will need to be fairly tight (pulling tension across the diameter of the tube) to rigidly hold the mechanical assemblies that hold the mirrors. I don't want to be puncturing the surface of the tube as I tighten down bolts.


Dave

 

That's a concern I agonize over as well with some of my projects. If you aren't concerned about fatigue resistance and such, the easiest way to do it is to drill the hole through the outer surface of the structure, then take a piece of wire (like a paperclip), bent at a 90 degree angle and insert it into a drill. Let the paperclip spin around inside the core of the structure. Then take a vacuum or compressed air and suck/blow the particles out. Then you can pour/inject resin, or a slurry into the area, making a hard point. After it cures, you drill the hole again and mount to the hardpoint.

 

possible tube?

Toot- a slightly more sophisticated procedure, but similar in concept, is used to hold gearboxes and engines onto some helicopters. Done well, solid inserts are insanely strong.
In my experience, washers are ALWAYS necessary when bolting to carbon, regardless of the type of load.
1/8" would be what I'd expect if you're making this solid, no core. On a tube that size I'd want to use perhaps a 1/2" overexpanded core. The extra thickness makes it a LOT stiffer! I'd hate to have to recollimate the scope every three days because the tube flexes. Off the top of my head, I'd be tempted to say 1/2" Nomex overexpanded honeycomb sandwiched on each side by four layers of ~300g (9oz) carbon in epoxy. This wall would have a Young's modulus of around 9.3 GPa (depending on core density)- yes that's only 13% of aluminum's modulus, but it's 18 mm thick thanks to the core- compared to 3 mm of aluminum, that's 216 times the geometric stiffnesss at 13% of the modulus, or a total of 28 times stiffer than 3 mm aluminum wall (comparing flat plates, not tubes, but the comparison should hold...). This tube, at 24" dia, would weigh around 8.5 lb per foot of length. Sound like what you're thinking of?
Oh- if you do the cf/kev outer layer, make the carbon axis of this stuff follow the length of the tube and the kevlar follow the circumference, this way the carbon will be of the most benefit....

 

Washers are important, as the composite structure, regardless of the type of fabric, will not take well to localized stresses.

Washers are ok because they spread the stresses over a larger area. Other ideas for mounting hardware include:

after drilling the hole, coat the rivet/bolt/whatever with epoxy before tightening it down. After drilling through the FRP, you will have strands of fiber that have debonded. This bit of extra glue helps to rebond those areas and prevent stress risers. Not as good as a washer, but it's another option.

Even better, add a small piece of resin-coated fabric over the hole that you intend to mount into. Then push the bolt/whatever through the fabric, but without tearing the fabric... simply force the tows to realign to one side or the other of the bolt. Rewet it, let it cure. Once it's cured, the fibers will transfer the load around the bolt hole. This is far superior to a washer where the surface is not flat. A washer on a rounded surface, for example, isn't really going to work well. This approach, however, can be quite messy.