Overview

The objective of this fin can is to introduce the new members to the layup process, refresh old members to the layup process, and ensure that we acquire all the necessary materials to be efficient in future layups. This fin can is mostly a replica of Hermes II. It lacks leading edges, and the fins used were the wrong size.

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

The format for this page is based on Hermes II Flight Candidate Fin Can #2

Summary of changes from Hermes II Fin Can:

• No leading edges (intentional)
• Incorrect fin size (unintentional)
• Mix up of one of the "6" plies (unintentional)
• Lack of surface prep (intentional)

Procedure Overview

Below is the detailed procedure for how Rusty was manufactured.

...

1. Tube Preparation

The fin can is supposed to fit over the motor case. It never does. If this were a flight fin can, we would sand the inside of it with a flapper wheel until it did, but Rusty is simply Rusty, not Dusty nor Crusty. Rusty starts at a phenolic tube which is overwrapped with 3 plies of carbon fiber. The phenolic tube acts as insulation from the motor case because we do not know the temperature on the outside of the motor case. The carbon fiber overwrap bears the structural loads. During the overwrap layup, a rod was fed through the phenolic tube. The ends of the rod rested on two tables such that the tube surface didn't touch the tables (imagine a pig on a spit.) We had a total of 6 people help with the overwrap layup. 

...

**The hose we actually used at this time was a low-temperature rubber one that started to get soft in the oven, so we disconnected it before it melted. The green high-temperature hose was acquired after this. 

2. Root Bond and Root Fillet

Required Materials:

• CF overwrapped tube
• Systems 3000 epoxy (resin and hardener)
• Acrylic fillet radius tool 
• Gloves
• Respirator
• Carbosil
• 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 

Used System 3000 epoxy. fin can jig needs to be improved --> make sure to space layers evenly, use top layer with smaller slits. Holes were slightly too small so was difficult to adjust

AFTER root bond, attached phenolic LE to G10 fin core (did after root bond because phenolic wouldn't have fit in the fin can jig)

Root fillet (West Systems fast hardener + colloidal silica)

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. Can use a heat gun to speed this up even more.

Sand fillets

Make sure to do fillets right the first time because this time we had to fill them in again (using West systems + fast hardener + silica)

talk about scheduling!! also speed up to tacking w/ heat gun

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?) 

Image Removed

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:

Image Removed

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:

Image Removed

1. to size the diameter of the hole for 4 root bond jig plates. 
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. 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. 
5. Tape popsicle sticks each slot where the fin ends so that the fin can be held in place.
6. Leave to room temperature cure for 24-36 hours before attaching leading edge and starting fillets.
   Image Added

Root Fillet Procedure

1. While wearing a respirator, mix 100 parts Systems 3000 resin with 18 parts hardener. 
2. Add in carbosil in small amounts and mix until a peanut butter consistency is achieved. 
3. 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. 
4. 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, Rusty had a one-inch fillet. 
   Image AddedImage Added
5. Apply the epoxy in the fillet area, being careful not to go outside the tape. 
   Image Added
6. 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. 
   Image Added
7. Let it cure at room temperature for 24-36 hours. 
8. Place in the oven in the vertical position to oven cure for 12 hours. 
9. Sand the fillets.

3.0 Progression of the Carbon Fiber Cut-outs Design

Last year, the tip-to-tip design called for 6 layers of 6 distinct sizes. This allowed for a tapering effect on the surface of the fins and the tabs at the top added extra layers above the fins.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

...

1. *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)	18" x 20"
   2	18" x 20"
   3	18" x 20"
   4	18" x 20"
   5	18" x 24"
   6 (largest size)	20" x 24"

2. Cut along tape and label which size cutout it is (1-6) on the tape so you keep them organized later when laser-cutting. Next steps tell you to attach wax paper, but wait until after laser-cutting to do this because wax paper burns in laser-cutter
3. 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
4. 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.
5. 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. 
6. 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). 
7. Laser-cut carbon fiber 
   1. Select proper settings for carbon fiber (100% power, 4% speed)
   2. 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)
   3. Place carbon fiber in bed of laser cutter
   1. Make sure there are no loose pieces of wax/parchment paper curling up, as this will get in the way of the laser-cutter
   2. 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)
   3. Turn on the exhaust
   4. Cut! (Start with a test piece to make sure it's being cut properly) 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.
   5. 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.

4. Preparation of Vacuum Bagging Materials

Required Materials:

• Peel ply (light-blue)
• Release film (should be light red, make sure it's perforated)
• Bleeder fabric
• Vacuum bag material (Stretchalon 800, resistant to up to 400F)
• Ruler
• Scissors

...

Waited to cut excess off peel ply, release film and bleeder fabric until started vacc bag 

5. Tip-to-tip Layup

TALK TO PROF. MARK DRELA before doing the layup. He will have tons of tips (because the below procedure may very well not be ideal- also do research to see what works for other teams, and look up Jim Jarvis guide on the internet). For example, it may turn out that we should apply the dry cutouts to the fin can BEFORE adding epoxy instead of AFTER. Who knows??

...

This information is so close to being useful but isn't quite there because we can't use rule of mixtures to actually calculate what our elastic modulus. 

6. Vacuum Bagging

Required Materials:

• Vacuum tape
• Vacuum bagging materials from previous step
• Painter's tape
• Layup jig (wooden jig, metal pole)
• Scissors
• Vacuum pump (pump, fixtures)
• Respirators
• Gloves

...

Leave vacuum pump on overnight, making sure window is open to avoid vapor build-up

7. Oven Cure (see attached "Oven Cure SOP" for more details)

1. Seeing as the cure cycle for System 3000 epoxy is around 12 hours, got four people to sign up for shifts (this requires planning in advance!)
2. Positioned fin can vertically as shown and router the vacuum tube through a port hole on the right side of the oven. Positioning the fin can vertically is important so the fins don't splay and become misaligned. 
3. Followed the cure cycle for System 3000 High Temp epoxy. Note that the epoxy changes from clear to amber-colored after the cure is completed.
4. Once the cure cycle is complete and the fin can has cooled to room temperature, put on gloves. Then,
   1. Turn off the vacuum pump
   2. Detach the vacuum fixture from the vacuum tube
   3. Remove the fin can from the oven
   4. Carefully remove the bagging materials and discard

...

• Increase from room temperature to 150F at a rate of no more than 2-5F per minute
• Hold at 150F for 3 hours
• Increase to 250F (no more than 2-5F per minute)
• Hold at 250F for 3 hours
• Increase to 300F (no more than 2-5 per minute)
• Hold at 300F for 3 hours
• Ramp down to 100F (no more than 2-5F per minute) --> decreasing temp takes a lot longer than increasing and lower temps are harder to control
• Do not shut down the oven and leave it to cool down. Note that this takes longer than you think it will (as long as 1-2 hours)

8. Post-processing

Required Materials:

Procedure:

...