Overview

The objective of this fin can is to attempt the first potential flight candidate fin can for Hermes III. This fin can implements the lessons learned from the first fin can. It includes steel leading edges, a different laminate stack up, and the correct fin geometry. 

This fin can is the second in the series of 3 fin cans for Hermes III (Rusty, Dusty, and Crusty.)** This is the story of Crusty.

**Originally, Dusty was supposed to be the 2nd completed fin can, but a series of unfortunate root bonding accidents resulted in Crusty being completed before Dusty. Therefore, despite the fact that the overwrap for Dusty was done a week before the overwrap for Crusty, Crusty is now referred to as the 2nd fin can.

Tube Preparation

1. Kit the necessary materials for the overwrap layup. The sizes listed in the table below are given such that there are 3 layers of carbon fiber that run 22" long on a ~19" circumference tube. 22" is chosen so that there is one inch of exposed left on both sides of the 24" phenolic tube, which will be used for the vacuum tape. The breather, peel ply, and release film are sized to wrap around the tube with several inches spare for vacuum tape. 

   1. Kitting set up: 

      Overwrap Kit Material	Sizes
      High temp vacuum bag (pink, no holes)**	30" x 24"
      Breather cloth (white, fuzzy)	22" x 24"
      High temp peel ply (blue nylon or white polyester)	22" x 24"
      High temp release film (red with holes)	22" x 24"
      Carbon fiber (3 layers, (0/90))	22" x 60"
      Wax paper	24" x 60"

       

      1. Two tables in Gelb lab, with fresh paper laid on top. 

      2. Make sure to wear gloves when cutting everything. 

      3. Be weary of stray pieces of painters tape from the wax paper touching the carbon fiber because this could cause fraying. 

      4. Use the sharp shears for the breather cloth. Everything else can be cut using the pizza cutter. 
      5. For the carbon fiber and wax paper: Kit the wax paper first by laying two 12"x60" sheets next to each other. Tape the two together either directly side by side with no overlap or with two inches of overlap. Use scotch 77 to spray a light layer of spray glue on the wax paper. Make sure to rub out any bubbles of spray glue material that globbed up on the wax paper (any glue globs on the wax paper is an area where there won't be epoxy). Roll out the carbon fiber on a freshly cleaned table such that there's at least 60" of length. Place the wax paper on the carbon fiber, glue side down. Gently flatten the wax paper over the carbon fiber with your gloved hands. Use the pizza cutter to trim the carbon fiber to size. Weigh the carbon fiber and wax paper combination. 

2. Sand the tube with low grit sandpaper to roughen it up.

3. Do the layup. 
   1. Grab a pole and wrap it with wax paper. This pole will slide through the tube and hold it up during layup. 
   2. Secure the pole ends and the tube with two tables, and add weights or something to make sure someone can't accidentally knock the pole off. 
   3. Cover the area beneath the tube with lots of paper/cardboard/etc. Just make sure that any epoxy drips are caught so that the floor isn't covered in epoxy.
   4. Mix Systems 3000 epoxy. Make sure to record the amount used. 
   5. While holding the tube in place, gradually unroll the carbon fiber onto the tube, while pouring epoxy over it. Use squeegees to spread the epoxy. Be careful not to use too much epoxy because this will reduce the overall strength of the part. 
   6. Continue unrolling the carbon fiber from the wax paper until finished. Be careful to keep the same amount of tension on the carbon fiber throughout its length to prevent warping. 
      1. If you notice warps or areas where it doesn't look like the fiber is going in the 0/90 degree direction, pause for a moment and try to massage the fibers back into place. 
   7. The epoxy has a two hour pot life, so hopefully you've finished the layup in 90 minutes and now have time to bag.
4. Vacuum bag
   1. Wrap the part in peel ply
   2. Wrap the part in release film 
   3. Wrap the part in breather. Consider using two layers of breather because our layups are pretty wet. 
   4. Wrap the part in the vacuum bag. Be sure to add vacuum tape to the ends of the tube.
      
      1. Slide the vacuum connector into the part. 
   5. Pull vacuum and listen for leaks. You're aiming for -26 mm Hg. 
5. Let it cure at room temperature for 24-36 hours. 
6. Bring into the oven to cure for an additional 12 hours. 

Root Bond and Root Fillet 

The root bond refers to the bond between the bare fiberglass fins and the overwrapped phenolic tube. The purpose of this bond is to ensure that the fins are attached perpendicular to the tube. This attachment is very weak and will break if you knock it, so be very careful handling the fin can after the root bond. We use systems 3000 for it (probably overkill because it's a dumb weak bond) and a root bond jig made from acrylic. Schedule 36 hours for the root bond. 

The root fillet refers to the fillet of epoxy between the fiberglass fin and the CF tube. The fillets are important because they're the real structural attachment between the fin and the tube. Once you have the fins bonded in the right position, you beef them up with a fillet. Using Systems 3000 for the fillet means you have to add Cabosil or Colloidal Silica to thicken up the epoxy because it's a laminating epoxy. Make sure you're consistent with the amount of thickening agent you use. Also really important to note that each fillet takes 24 hours to cure, and you can really only do 2 fillets at a time. At the proper rate, this process should take 7 days. 

Required Materials:

• CF overwrapped tube
• Systems 3000 epoxy (resin and hardener)
• Acrylic fillet radius tool 
• Gloves
• Respirator
• Cabosil
• Popsicle sticks
• Epoxy boats or paper cup (for mixing epoxy)
• Painters tape
• Ruler
• Acetone and shop towels (to clean up epoxy on acrylic tools)
• Vacuum 
• Root bond jig
• Sandpaper (220 grit)

Root Bond Procedure

1. Measure the outer diameter of the overwrapped fin can. Use this measurement to size the diameter of the hole for 4 root bond jig plates. 

   Root Bond Jig Dimensions	Size
   Diameter of collar	6.27"
   Width of slot	0.131"
   Length of slot
   Diameter of rod

2. Measure the thicknesses of the fins. Use this measurement to size the slots for 3 of the 4 plates. The bottom one should not have slots in order for the fins to rest on it.
3. Use the threaded rods to evenly stack the plates on the root bond jig, and use a level to make sure that each plate is aligned properly. 
4. Prepare the fins
   1. Sand both large faces of the fin and the root face until you can no longer see the shiny, non-stick top coat. Use low grit (like 60 or 80) and finish with higher grit (like 220). Make sure you've thoroughly cleaned it before sanding with some IPA.
5. Apply Systems 3000 epoxy on the root chord of the fin preform (Without the leading edge attached), and slide the fin preform into the slot of the root bond jig to attach. 
6. Tape popsicle sticks each slot where the fin ends so that the fin can be held in place.
7. Leave to room temperature cure for 24-36 hours before attaching leading edge and starting fillets.
   
   1. If it's really cold, it'll take longer than 24 hours. The temperature dependence means this isn't an exact science. Be very sure that the bond has finished curing before handling the fin can. It'll break your heart if the fins fall off after 24 hours because you'll have to redo the root bond and wait another 24 hours and you're already behind schedule and i just 
8. Attach the leading edges
   1. Move the plates of the root bond jig such that you have enough room to attach leading edges, but you still have a plate securing the bottom of the fin and a plate securing the tube at the top. Having these plates here is useful in preventing someone from knocking the fin off. 
   2. Clean the inside of the leading edges. I used a paper towel soaked in IPA and a slotted screwdriver to stuff as much of the paper towel into the leading edge slot as possible, and then I ran the paper towel through the slot with the slotted screwdriver. This method removed a lot of gunk that using compressed air didn't. 
      1. You could probably use a cotton swab, but I couldn't find those in Gelb. 
   3. Clean the entire surface of the fin and the carbon fiber wrapped tube with a microfiber cloth and some IPA. Make sure there isn't gunk/oil on the part.
      1. Microfiber is extremely important because using a shop towel or any other paper towel will shred. 
   4. Do a dry fit with the leading edge. It should be a comfy, snug fit, but not one where you have to use a ton of force. You might have to see-saw a little. 
      1. If it doesn't fit initially (in my case, it never did), then sand the hell out of the fin while it's sitting in the root bond jig. Be extremely careful while sanding because it is truly heartbreaking when a fin breaks after the root bond. 
         1. Use 40 grit or something low. Removing material as fast as possible is the goal. 
         2. After sanding, make sure to thoroughly clean the leading edge surface of the fin preform to remove any dust. Microfiber cloth and some IPA. Multiple times. 
   5. Mix some Systems 3000 epoxy (100 parts resin to 18 parts hardener)
      1. Don't mix up the caps. 
      2. Clean the containers after you pour. It takes 2 seconds with a paper towel. 
   6. Using a small popsicle stick or a needle, cover the inside of the leading edge slot with Systems 3000. Make sure to also cover the surface that goes up against the fin can. 
   7. Insert the leading edge onto the fin, and make sure to have some painters tape handy to tape the leading edge to the fin to prevent it from sliding downwards. 
      1.  Careful that the tape doesn't end up somewhere the epoxy is because then you've cured painters tape to the leading edge. 
   8. Leave it for 24-36 hours.

Root Fillet Procedure

1. Add in cabosil in small amounts and mix until a peanut butter consistency is achieved.
   1. We used 9% cabosil for Crusty (i.e. if you have 100g systems 3000 resin and18g hardener, then it's 9g cabosil)
2. Place the epoxy cup in the vacuum chamber, and wait 2 minutes. The epoxy should rise and then fall after the air bubbles have been pulled out. 
   1. 2 minutes was not enough. The fillets in Crusty were still very bubbly. See the Dusty procedure to figure out how to remove bubbles. 
3. Mark the radius of the fillet on the fin and on the fin collar with a sharpie, and then cover in tape. In this case, Crusty had a 0.75" fillet, but I'm reusing the Rusty pictures. 
   
4. Apply the epoxy in the fillet area, being careful not to go outside the tape. 
   
5. Use the acrylic fillet radius tool to remove excess epoxy. Be sure to swipe perpendicular to the root to ensure that the same amount of epoxy is being removed each time. 
   
6. Let it cure at room temperature for 24-36 hours. 
7. Place in the oven in the vertical position to oven cure for 12 hours. 
8. Sand the fillets.
   
   

Tip to Tip Cut-Outs

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 repeat 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, fiberglass can not be cut on a laser cutter, but carbon fiber can, as long as 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 (the CSAIL laser cutter has a width of 18", too small for the final layer of the tip-to-tip cutouts, but the one in MakerWorks and N51 is big enough.) 

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 
  • Parchment paper is miserable to work with, don't do it
• Scissors
• Sharpies
• Ruler
• Gloves
• Pizza cutter

Procedure:

*Make sure to wear gloves whenever working with/handling carbon fiber, or you will get small splinters that are not painful but very itchy!!

1. Roll out two rolls of wax paper next to each other. They're 12" wide each, so overlap until you reach the desired width and cut to the desired length, as specified in the table below. Use painters tape to attach the 2 sheets together. 

2. Lightly spray the wax paper with 77 fabric glue. 

   1. Important that this is extremely light. You'll be tempted to use a lot because it helps stick to the CF better, but anywhere there's a large amount of 77 means there won't be any epoxy. I don't know about you, but I'd rather the fin can be held together by epoxy than 77 any day. 
   2. Use to finger to spread out any large bubbles of 77/ 
3. Lay the sprayed wax paper on the carbon fiber, glue side down. 
   1. The carbon fiber table set up is important. Make sure you've rolled the CF onto a clean table that's big enough to cut on. Also make sure you dewarp the fiber before you lay the wax paper down. 
   2. Check that you're laying down the wax paper on CF that's properly oriented (i.e. cutting a 45 is different from cutting a 0/90) 
   3. Gently flatten the wax paper over the carbon fiber with your gloved hands. 
4. Use the pizza cutter to trim the carbon fiber to size. 
5. Use the table or whatever other means to keep track of the kitting. Figure out some way to organize the piles of CF
   1. I made envelopes out of the butcher paper and labeled them with the size and orientation. This was helpful to transport the CF and to prevent additional nasties from getting on them. 

   2. Tip to Tip Ply Number	Orientation	Size	WP Kitted/Total	CF Kitted/Total
      1 (smallest)	+-45	20x22	4/4	4/4
      2	0/90	20x22	4/4	4/4
      3	+-45	20x22	4/4	4/4
      4	0/90	20x22	4/4	4/4
      5	0/90	20x22	4/4	4/4
      6	0/90	20x22	4/4	4/4
      7	0/90	20x26	4/4	4/4
      8 (largest)	0/90	22x26	4/4	4/4

6. Laser-cut the carbon fiber. Have some friends help. 
   1. Pull out the CF sheets from the envelope, and place one sheet into the bed of the laser cutter. 
   2. Gently remove the wax paper from the sheet. Dewarp the sheet with your gloved hands.
   3. Turn on the air. Turn on the other air. Have a mentor help you if you haven't used this machine before. 
      1. If they're freaking out because it's carbon fiber, tell them this is just fabric and it's fine. CF with epoxy in it is a no-no, but the plain fabric is fine. 
   4. Directions for using the MakerWorks laser cutter: 

      1. Cutting Carbon Fiber on the Epilogue in Makerworks

         • speed 10%
         • 100% power
         • 50 Hz frequency
         • 600 ppi

         Exporting from Solidworks:

         • right click on the face you wish to export and click "Export to DXF / DWG"
         • Specify the proper name and file location, and set the file type to dxf
         • The Left Panel should say "DFX / DWG Output"
         • Ensure "export" is set to "faces / loops / edges"
         • Ensure "export options" is set to "Single File"
         • Click the green check mark at the top of the panel
         • The file will be exported!

         Printing a DXF File:

         • Open the dxf in your favorite vector graphics editor (e.g. Adobe Illustrator, inkscape, Corel draw)
         • In Illustrator
           • Set your document to the size of your piece
           • Place your piece in the top right corner of the laser cutter
           • In illustrator, select file > open and navigate to your dxf
           • In the window that appears, make sure "original" size is selected
           • Hit okay
           • It may complain about not having a text file - just hit okay
           • Move your drawing to the top right corner of the file
           • Delete extraneous lines (solidworks will include a "only for educational use" decal)
           • Select the drawing and set the line width to "hairline" or 0.001" if that isn't an option
   5. After about 20 seconds post-cut, open the bed. Then slowly pull up the excess part of the CF that was cut. 
      1. You might hit a small snag or two. Just use a pair of scissors to snip it. 
   6. Throw out the excess
   7. Lay the wax paper, glue side down, on the CF cut out. Gently remove it from the bed and place it on a nearby table. 
      1. Have someone mark the centerlines of each of the CF pieces by dotting a silver sharpie down the line. 
         1. If you're laying down a ruler to help you measure/mark, make sure you've cleaned it.  
      2. Have someone label each piece by its ply number. Mark this multiple times on the wax paper and dot it on the actual CF ply. (Dotting it is important because if you drag the sharpie on it, it'll warp the fibers)
   8. Lay the next piece in and cut.

Tip-to-Tip Layup

The t2t layup refers to the process of applying epoxy onto the dry CF cutout plies, laying them on the fin can, trimming the wet plies, and then bagging. The prep takes about 2-3 hours. The actual layup itself should be 2 hours (epoxy pot life) but it usually takes us 4 hours from first pot to full vacuum. The more prep you can do before you mix the first epoxy batch, the better. 10 minutes of prep can save you like 2 hours of layup time. If Drela taught us one thing, it was to be extremely prepared. 

Preparation

Layup:

Epoxy: 1312.9 grams