I’ve grown pretty decent over the years at creating composite aerostructures. From my first attempt at fiberglassing a body tube to the latest projects with prepreg and vacuum bagging, I figure it might be fun to share my version of creating composite structures with the world.

Fiber Selection

The common thread (ha!) to all of these techniques is the fiber itself. Choosing a fiber is the most important part of the layup process, as it’s what does all the work. Epoxy itself has little to no strength, so it’s important to choose the proper fiber and put it in the right direction. The goal is to maximize strength in the place you need it — on body tubes, fibers running the length of the tube are a good idea, since the tube takes lots of axial loading, whereas on fins, fibers running across the fins from tip-to-tip are a good idea since they add stiffness in that direction. A good compromise is to select a fabric with fibers running in two directions, which also makes life a lot easier by tending to stay together and behave in a fashion as you’d expect.Physical fiber selection can be a complex process, but my general rules of thumb are as follows:

• Fiberglass is excellent for most hobby rocket work. It’s strong, easy to work with, cuts well, and is cheap. The main drawback of fiberglass is its strength-to-weight ratio — it’s lower than the other common fibers available to hobbyists. But it’s much stronger than plain unreinforced materials, and it’s dirt cheap, making it the first thing I reach for in most cases. Fiberglass is also useful on top of other materials, especially Kevlar, since it doesn’t act funny when sandpaper hits it.
• Kevlar is an interesting material choice, in that it has a slightly higher strength-to-weight ratio than fiberglass, and is very forgiving in terms of bumps and jolts. It is very flexible in a layup (I’ve smacked Kevlar parts on the ground and they bounce really well…), and has the added benefit of making your rocket bulletproof. That one goes over well at parties. Kevlar is significantly more expensive than fiberglass, and due to its bulletproof nature, is close to impossible to cut without the proper equipment. It’s also slightly more difficult to wet out — while fiberglass turns clear when successfully wetted, Kevlar turns a darker shade of yellow (think banana vs. Post-It yellow) when saturated. It’s a visible difference, but subtle. Oh, and Kevlar is impossible to sand; even with a full coat of high-build primer, one nick of the fiber with sandpaper will make your rocket fuzzier than 70s shag carpet, except it’s nearly impossible to get rid of the fuzz. For Kevlar laminates, a layer of light fiberglass over the top helps tremendously for finishing.
• Carbon Fiber is an awesome fabric that finds use mostly in high performance projects designed for extreme velocity or altitude. It has the highest strength-to-weight ratio of the three common fibers mentioned here, and is also tremendously stiff. The drawback to stiffness, of course, is brittleness; this characteristic can be mitigated somewhat through the proper choice and cure schedule of resins. Carbon fiber is also the most expensive fabric of the three, and as such sees limited use in most hobby rocket projects. However, proper application of carbon fiber in a high-performance airframe results in a structure that is stronger than steel, and lighter than aluminum. It’s the ultimate choice.

A few other notes on doing composites:

• Fibers don’t like to bend around tight corners. Tight corners are not only difficult to stick fabric into, they also generate stress concentrations in the fibers that ultimately weaken the finished product. Practically, when doing fin layups, this means lay down fillets first, and put cloth over them, so that there is a smooth transition from the fin to the tubing. When doing a nose cone layup into a mold, add a bit of epoxy/chopped Kevlar into the corner where the shoulder meets the part, or (better yet) vacuum bag the part to hold the cloth into the tight corner.
• Always sand stuff you’re planning on bonding, and clean it off with acetone. Really. The slick surface of G10 fins is nearly un-bond-able, so it requires serious scuff sanding before epoxy will stick to it, and masking tape uses a silicone-based adhesive, which repels epoxy like nobody’s business.
• On a related note, masking tape makes decent cheap compression tape over small layups. It’s possible to (quite easily) remove the tape while the epoxy is “B-staged”, or when it has cured to the point of being un-sticky, but not fully cured. (Go poke the leftover epoxy with a stick to see whether it’s there or not. If it’s not, and you use your finger, it’s a mess to get it off.)
• Composite reinforcement isn’t that difficult — try it, you’ll like it. And when done properly, you can make ridiculously light rockets that are extremely strong.

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Posted December 26, 2007 in Rockets. Add Comment / Trackback