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.

For each step, there is an overview of the process, followed by required materials for this step and then numbered substeps. Some safety notes:

• A respirator is required for all steps involving sanding fine particles like carbon fiber.
• A respirator is required when using carbosil before it is completely mixed into the epoxy. The respirator can be taken off once the carbosil is completely mixed.
• A respirator is not required when only using Systems 3000 epoxy. 
• Gloves are required when sanding carbon fiber to prevent small splinters

SUMMARY OF FIN CAN MANUFACTURING

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. 

** During this overwrap, we didn't leave any space on either side of the phenolic tube as a bagging surface. Because the carbon extended to both edges of the tube, we had to use an inner bag that we bagged the outer bag to. The bag used was also a low-temperature one that did not fully survive the cure. These bagging issues were fixed for Dusty and Crusty overwraps. 

Required Materials:

• Phenolic Airframe Tubing PT-6.0, 6.007" ID (https://publicmissiles.com/product/airframes
• Carbon fiber
• System 3000 epoxy (resin + hardener)
• Layup jig (in this case, two tables and a rod)
• Wax paper
• Squeegees
• Popsicle sticks
• Epoxy boats or paper cup (for mixing epoxy)
• Flash tape and painter's tape
• Scissors/Pizza Cutter
• Ruler
• Acetone and shop towels (to clean up epoxy on squeegees)
• Gloves
• Safety goggles
• Vacuum connector
• Fabric spray glue (usually scotch 77)
• Vacuum tape (yellow)

Procedure:

1. Kit your materials to the sizes listed above. 
   1. Make sure to use high-temperature materials. 
   2. Kit on a table with fresh butcher paper. Clean surface is important
   3. Wear gloves. 
   4. Use the pizza cutter and the absurdly large shears for the carbon fiber. 
   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. Now it's time for the tube layup. Prepare the layup jig (make sure it's stable, put a pole over it and wrap it with wax paper so that once you start the tube layup, the tube doesn't stick to it)

3. MIX EPOXX, DO LAYUP
   1. For systems 3000, it's 100 parts of the resin to 18 parts of the hardener. 
   2. Make sure to keep track of how much epoxy was mixed. 
4. When the carbon fiber is on the tube, wrap the tube with:
   1. Release film
   2. Peel ply
   3. Breather
   4. Vacuum bag (vacuum connecter)
5. Put the part under vacuum. Check for leaks. The vacuum gauge should read around -26 mm Hg. 
6. Let tube cure at room temperature for 24-36 hours
7. Place part under vacuum into the oven and follow 12-hour cure cycle. Feed the GREEN high-temperature hose through the side of the oven to maintain vacuum during cure.**

**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 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.
   

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. 
   
5. Apply the epoxy in the fillet area, being careful not to go outside the tape. 
   
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. 
   
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.

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 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). 

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

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. *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

Procedure:

Prepared vacuum bagging materials

Vacuum bag was too big (32" x 38")--> make smaller next time

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??

Required Materials:

• System 3000 High Temp Epoxy Resin + Hardener
• Squeegees
• Popsicle sticks
• Epoxy boats or paper cups
• Paint rollers (LINK)
• Acetone
• Shop towels
• Scale
• Respirators
• Gloves
• Large sheet of Mylar (to protect surface from epoxy during layup)

Procedure:

Mark centerlines on tube and CF cutouts

CF didn't meet up in certain places (ends of tubes, some were barely large enough to cover the fins)

Only had one ply going all the way to the end - saw exposed G10

Triangle of exposed area near the fins --> adjust size of cutouts

Below is a table showing the mass of each cutout size. One of each size was measured, so this is technically not an "average" mass, but we expect that since the laser cutter was used to make the cutouts, the mass of each size is more or less the same. 

For the tip-to-tip layup, we used a total of 9 boats of epoxy. Each boat weighed 118g, not including the mass of the epoxy boat itself, so the total mass of epoxy used for the tip-to-tip layup was 1062 g. In reality, after vacuum packing, some epoxy is removed (sucked out through the peel ply). Work should be done in the future to get a better estimate of how much epoxy is removed during the vacuuming process.

Ply Mass Log: (includes wax paper & some painters tape!!!)

PLIES AS ASSEMBLED: 

SIDE A: 1A, 2A, 3A, 4A, 5A, 6D

SIDE B: 1B, 2B, 3B, 4B, 5B, 6B

SIDE C: 1C, 2C, 3C, 4C, 5C, 6C

SIDE D: 1D, 2D, 3D, 4D, 5D, 6A

Epoxy Boats Log

Hermes III Test Fin Can CF-Epoxy Ratio Estimates.xlsx

Carbon fiber mass fraction: 0.408

Epoxy mass fraction: 0.592

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

Procedure:

Vacuum bag was 32 x 38" (this is TOO BIG, make smaller next time. For the test fin can though the bagging was too SMALL, so find a good middle ground)

Used layup jig to support tube

Make sure to run fingers along fillets so that they don't bunch up and end up with creases after the room temperature cure

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

Need 2-3 people, not just one person, and do it during the day time! make a nice graph

Cure Cycle for System 3000 epoxy

• 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:

1. Sanded CF on leading edge so that only phenolic remains (careful not to remove the phenolic)
2. Cut off ends of tubes using Multimaster tool or Dremel
3. Sanded high spots on tube and fillets, but be careful not to oversand or this will decrease the strength of the fillets
4. Filled in low spots with Bondo and sanded (Bondo is way easier to sand than carbon fiber. Consider filling in the low spots with epoxy instead of Bondo to increase strength of fin can, but remember that this will take longer to sand)
5. Manufactured aluminum transition piece to match the outer diameter of the fin can with that of the motor case (used a lathe --> it's a simple part but it took a couple tries)
6. Epoxied the transition to the motor case, then slid the fin can on, and the Propulsion subteam attached the boattail 

CHANGES FOR NEXT TIME: 

• Do plies by 1, 1, 1, 1, 2, 2, 2, 2,.... instead of 1A,2A,3A...1B,...
• Log how much epoxy per side of the fin can 
• Update sizes of vacuum bag, bleeder, peel ply, etc. 
  • "A pain to get in the bag and I don't think it will distribute the pressure from vacuum evenly"
  • Correct shape and size
• Weigh the tube, then weigh the fin can after cured with bag removed 
• Calculate the volumes of materials 
• Sand the area of the fin can where the fin will attach to ensure proper contact during root bond
• Root bond with Systems 3000, not 5 minute