...
Says it takes 6 hours to cure, but if you're in a time crunch you can apply a new fillet every 3-4 hours.
Sand fillets
Procedure:
3.0 Progression of the Carbon Fiber Cut-outs Design
Last year, the tip-to-tip design called for 6 layers of 3 distinct sizes, 2 plies per size. This allowed for a tapering effect on the surface of the fins and the tabs at the top added extra layers above the fins (WHY?)
This first design had much room for improvement, however. The upper tabs going in both directions made aligning the layers more difficult during the layup. To fix this, the upper tabs only extend in one direction such that they are double the width of the fin-to-fin distance. In addition, the fin can extends below the fins themselves so a bottom tab was added to each layer. Another issue was that the taper between layers was too quick, creating actual bumps at each ridge where the layers size changed. To fix this, the design was changed to 6 distinct layers, 1 ply per size, which were offsets of the largest size which covered the entire tip-to-tip surface such that the leading edge and tip edge for each layer are half an inch apart and the trailing edge is a quarter of an inch apart. The final addition was an inch long offset on the largest layer on the tip edge and the trailing edge, the sections which will not have phenolic, to make sure that the fins are completely covered when the last layer is added. These considerations led to the fin design below:
This design was used for the first fin can flight candidate. The layup for this fin can revealed other issues with this design. Firstly, the ridge between the phenolic edge and fins was unfavorable so in the new design, there was no tapering from the leading edge of the fin. In addition, there were some gaps in the carbon fiber between the lower tabs so a half inch tab was added to the lower tab of each layer (tabception). This was to ensure that each layer would meet in that section of the fin. Another issue seen in the layup is that there were sections in the layup with only one layer of coverage, a possible site for improper heat maintenance. To fix this, the second to last layer of the layup is the same shape as the last, only lacking the extra offset on the tip and trailing edge. These changes resulted in the design below:
Further analysis of the design gave three more changes before creating the next fin can. The first was a simple change; the upper tab was made slightly larger, 10.0" instead of 9.8", to make sure that each layer reached the next tab over. Next was the issue of carbon fiber not reaching the phenolic leading edge so as a precaution, the leading edge was offset by a quarter inch on each side. Though this would cause some excess carbon fiber to lie over the phenolic and this excess would need to be cut for each cutout, the extra time in doing so would allow us to cut to exactly the leading edge with the carbon fiber. The last change before the next lay up was the removing the taper on tip edge, mostly because we saw no need for it and only needed to see a smooth taper on the trailing edge, these changes resulted in the design below, the design used in the Hermes 2 Flight Candidate Fin Can #2:
3.1 Carbon Fiber Cut-outs for Tip-to-tip Layup
The carbon fiber cutouts are the pieces of carbon fiber used in the tip-to-tip layup, which consists of laying plies from the tip of one fin, over the tube in between them, over the tip of the next fin (and repeating for and repeating for the other three sides). The preparation for the tip-to-tip layup involves cutting out squares of carbon fiber and attaching them to wax/parchment paper using spray glue. The purpose of the paper is to prevent the carbon fiber from warping when it is being handled. Then, the actual shapes are cut out using a laser cutter. In general, fiber glass can not be glass can not be cut on a laser cutter, but carbon fiber can, as long as the the fiber is dry (i.e., not "pre-preg", referring to sheets of fiber that have already been impregnated with epoxy). Make sure that whichever laser cutter you use can fit the largest cutouts (in CSAIL, for example, the laser cutter did not fit the largest cutout so we had to do it by hand).
Quentin designed CF cutouts (6 sizes --> small to large)
the CSAIL laser cutter has a width of 18", too small for the final layer of the tip-to-tip cutouts).
Required Materials:
- 5.7 oz 2 x 2 twill weave carbon fiber (https://www.fibreglast.com/product/3K_2_x_2_Twill_Weave_Carbon_Fiber_Fabric_01069/carbon-fiber-fabric-classic-styles)
- Blue painter's tape
- Adhesive spray glue (3M or something like that, should be a red can)
- 2-3 rolls of wax paper or parchment paper
- Scissors
- Sharpie
- Ruler
- Gloves
...
*Using tape, mark squares of carbon fiber (24 squares, 4 for each size cutout, making sure there is enough room to fit the cutout with ~1 inch margin on all sides) using the following table:
Cutout Number Dimensions of CF square 1 (smallest size) 1418" x 20" 2 1518" x 20" 3 1618" x 20" 4 1718" x 20" 5 18" x 24" 6 (largest size) 20" x 24" - Cut along tape and label which size cutout it is (1-6) on the tape so you keep them organized later when laser-cutting.
- Prepare 25 (24 for the cutouts, one extra) pieces of wax or parchment paper (if paper is too small, tape the pieces together so that they are the same size or slightly larger than the carbon fiber squares
- On the extra piece of wax/parchment paper, use spray glue to test to see how far you should hold the glue from the wax/parchment paper so that when you spray, the paper just barely sticks to your finger. If you use too much spray glue, the carbon fiber will be difficult to remove from the paper and the fibers will be warped, ruining the tip-to-tip layup.
- For the rest of the 24 pieces of paper, lightly spray the wax or parchment paper with spray glue. Use your finger to see if it just barely sticks to the paper.
- Carefully take a piece of carbon fiber (one of the squares you cut out earlier) and lay it gently onto the wax/parchment paper, smoothing out any imperfections and making sure the piece is square (and not warped into a diamond or other shape).
- Laser-cut carbon fiber
- Select proper settings for carbon fiber
- Select proper file (make sure the size of the carbon fiber square matches with the cutout file- this is why labeling the carbon fiber earlier is important)
- Place carbon fiber in bed of laser cutter
- Make sure there are no loose pieces of wax/parchment paper curling up, as this will get in the way of the laser-cutter
- If you have room, weigh down the carbon fiber (but only after testing the path of the laser cutter, or you may damage it if it runs into the weight)
- Turn on the exhaust
- Cut! You may have to cut twice, so make sure you don't move the carbon fiber or the cutout will be messed up when you cut it for the second time.
- If you see small areas smoldering (meaning slightly glowing, NOT a flame- fire is bad), keep the laser cutter door shut and wait until it extinguishes. Opening the door would cause oxygen to enter and the smoldering to get worse.
...