THE DARK ART OF COMPOSITES

Part One (Materials)

In composite laminate construction, fibers are impregnated with liquid plastic resin mixed with a hardening catalyst. When the resin cures to a solid, the fibers are fused together to create a rigidity, strength, and flexibility that neither material has independently.

It took quite a while for me to build up the courage to use composites in the reinforcement of rocket parts. There seemed to be something mystical about it, and not knowing the secret chants left me apprehensive about trying out this unfriendly voodoo for myself. This article is the first in a two-part series explaining this dark art of composite construction as it is used in rocketry. This page contains a description of the important materials and their attributes. The real black magic is described [HERE!].

Fibers

The most common composite reinforcement material is fiberglass, which comes primarily in the form of woven cloth or pressed mat. Generally, woven fabrics are described in weight per square yard while non-woven fabrics or mats are described in weight per square foot. Fiberglass cloth comes in two basic types -- E glass and S glass. S glass is the more modern fabric, stronger and slightly more expensive. One layer of 6-ounce S glass yields the same strength characteristics as 9-ounce E glass, saving 3 ounces per square yard per layer.

There are many other fibers that can be used in place of fiberglass -- modern materials like graphite (or carbon) and aramid (such as Kevlar) are two examples -- but cotton, nylon, or even burlap can be used if you are so inclined.

Graphite is used in applications where a higher strength-to-weight stiffness is required. Aramids are impact resistant but also heavier than glass and not as stiff as graphite. Polyester resin doesn't stick well to aramids or graphite, so youll want to use epoxy with these fibers. Aramid and graphite fibers resist sanding, so if you use one of these in your laminate youll also need to cover it with a top layer of something more sandpaper-friendly. The biggest trade-off is price -- aramid or graphite will cost up to ten times as much as fiberglass for what is, in most rocketry applications, a very marginal benefit.

Resins

There are basically two types of resins -- polyester and epoxy -- and each has their advantages and disadvantages. I wont try to explain the mysterious alchemy behind each catalytic reaction. The end products look and feel pretty much the same.

Polyester is relatively inexpensive so it is very common in industrial applications. The downside is the nastiness of the vapors -- very, very bad juju. You need good ventilation, and its a good idea to use an organic vapor mask so you don't have to inhale the polyester fumes even if you use it outdoors. Its possible to build up intolerance to the fumes and suddenly suffer an intense allergic reaction. Polyester resin cant be used over Styrofoam because it contains styrene, a powerful solvent that attacks foam. This ingredient has its uses, however -- the glue that holds the fibers of a fiberglass mat together is dissolved when polyester resin is applied, so mat and poly resin work very well over intricate contoured surfaces, especially in the making of molds.

Epoxy resin wont dissolve the glue in fiber mat, so you are limited to woven cloth when using epoxy resins. On the plus side, epoxy will not dissolve Styrofoam parts and it is generally less brittle than polyester. It is more expensive but the fumes are much less noxious, making it markedly more pleasant to use. Make sure you choose a low-viscosity resin for making laminates and save the thick stuff for bonding purposes.

One last note on resins; they arent interchangeably compatible. Epoxy can be used to bond hardened polyester parts together, but not the other way around. Try to use polyester resins (including Bondo®) over composite parts made with epoxy and it will eventually debond, or may even inhibit the catalytic reaction of the hardener and not cure at all. I learned the hard way about this incompatibility and it was a mess of supernatural proportions.

Enchanted Dust & Crystal Balls

Nearly any powdery material can be added to resin to increase viscosity or create other desired properties - sawdust, silica, concrete, or even metal filings are used for various purposes. If you want strength, try milled or ground fiberglass for a substance that cures rock-hard. This works well for bonding parts and for forming the sharp angles that fiber cloths resist. Microballoons are tiny hollow spheres made of glass, phenolic, or acrylic. Microballoons look like a fine powder, effectively used to add air to your resin. It will reduce the strength but makes the resin thicker, lighter, and more sandable. Glass microballoons are the cheapest. Phenolic is the lightest but is also twice as expensive. Acrylic is in-between, but is vulnerable to solvents and can only be used with epoxy.

And theres MORE!

For the thaumaturgical sorcery of composite construction, click [HERE!]!

In the mean time, dont let your karma run over your dogma....