Fiberglass 101

Text and photos by John Wahlquist.

Just read Jeff's summary of how to glass a rocket and I like it. Just thought I would add another technique to the hopper. Specifically, the method PML recommends when they sell you a fiberglassing kit with one of their birds. It goes like this:

1). Clamp a broom stick to your work surface so that you can slide the rocket on and it will be supported while you work.

2). Insure that your fins are mounted and filleted before proceeding. Sand all airframe components that you will be glassing (fins, body, fillets) and then remove the sanding dust. I wipe mine down with pure (no lanolin) Isopropyl Alcohol. This removes the dust and any surface oils from hands, etc.

3). Slide the rocket onto the broomstick.

4). Cut your glass pieces to cover the section of the airframe you will be glassing. Start with pieces to cover between the fins and then go on to the pieces that will cover the remaining airframe. Tubular sections are easy as they require simple rectangles. For more complicated areas like the fin can, try using paper or cloth pieces to help establish a pattern for cutting the fiberglass cloth. And, if you can, cut the cloth on the bias so that it has a little more give while you are positioning it (if you are not sure about this step, ask your wife!). Cutting on the bias improves the flexibility (drape) of the glass and reduces fraying at the edges (and subsequent clean-up and sanding) and at the overlaps.

5). Draw a single straight line along the airframe to help position the larger rectangular pieces of cloth.

6). Starting with the rear-most glass pieces, lay them out on some newspaper and very lightly fog coat them with 3M's 77 spray adhesive (or equivalent). You can also apply a very light mist of adhesive to the airframe (it shouldn't be needed but you can - just remember that too much adhesive can cause problems latter).

7). Position the first piece on the body overlapping the fins. Adjust the position and slowly press the glass onto the body tube, working from the center out to the edges. Avoid wrinkles. If you get a wrinkle, gently lift the glass fabric to release the wrinkle and continue smoothing the glass into place. Once the glass is smoothed onto the tube and fins, rotate the tube and apply the next piece. You may find it necessary to cut the glass near the edges of the fins so it will lay properly on both the fin and the tubing. If so, cut the glass with a pair of very sharp scissors (not your wifes sewing scissors - you do want to survive this process) or a single edge razor blade. Repeat for all the pieces neede to cover the fins.

8). Now that you have the fin can covered, it's time to cover the forward section of your airframe. Take the rectangular piece you cut, you know, the one to cover from the fins to the forward end of your body tube. Lightly spray it with adhesive and line up one edge on the line you drew on your body tube. Lightly press the glass down along the line and then start to slowly smooth and press the glass down as you rotate the body tube. Don't worry about slight fraying at the edges, you can take care of that later.

9). Once you have the basic airframe covered it's time to mix and apply the epoxy. While polyester could be used, a good finishing epoxy will make for the strongest rocket. Following the manufacturers directions, mix up a small batch of epoxy (rule of thumb - only mix what you think you can apply in one-half the stated potlife). Once the epoxy is mixed, immediately start brushing it onto the fiberglass on your rocket. Try to apply and work in just enough epoxy to fill the valleys between glass strands. Start with your overlaps and saturate the surface but don't go for a totally filled surface as it will only add weight to your project and cause you to spend MAJOR time sanding your airframe. Work quickly to apply epoxy to all the glass surfaces you have prepared. Avoid shifting the glass and work carefully to prevent bubbles from forming. If you get a bubble, try to fill it with epoxy and then press it down to the airframe. If you think that the epoxy might kick-off too quickly, try placing it in the refrigerator for 20-30 minutes immediately before mixing. This will slow down the reaction some and buy you a few extra minutes.

10).Once you have coated the airframe leave it in a warm area (warm means 60F or warmer) and allow it to cure. As it cures it will go through a stage where it has gelled but isn't cured solid. This is a good time to trim some of the defects (runs, sags, frayed edges, ect.) but be careful to not peel up the edges of you glass job. Once done, allow the epoxy to cure per the manufacturers recommendations.

11).Now comes the most fun of all. Finishing! To start, those ugly overlaps can de filed down using a rasp (yes, rasp with no W). You then sand the fiberglassed areas until you just begin to expose the glass fibers. One neat thing that you can do with epoxy/fiberglass - you can use your power sander but be careful, if you sand through the glass fiber all your efforts have been for naught.

12).Once smoothed, remove the sanding dust and apply a finish coat of epoxy or body filler (ex. Featherfill) to your rocket to smooth out those less than perfect surfaces. Continue to finish your rocket in a normal fashion from here.

Is this the best way to finish a rocket? Well, it sure ain't how we do things at work (Northrop Grumman Corp.), but it does make a strong airframe as my experience at the last launch showed (PML Nimbus flown on a J350 had a main chute deployment failure, came down from altitude - 5299.5' don't you know - and impacted in the HARD section of the east side of the lakebed - damage: some ingrained dirt on the fin can and scratches on the nosecone). As always, remember to protect yourself from the chemicals you will be using. Gloves at a minimum, and make sure you have good ventilation in your work area. And when using epoxies, isopropyl alcohol makes a very good clean-up solvent.

Hope this helps some.