Hermes 2 Flight Candidate Fin Can #2

Things to fix for Hermes 3 Fin Can in GREEN!!

Summary of things to fix for Hermes 3 Fin Can:

• DESIGN
  • Talk to USC about how they design their fin can (also ask for manufacturing tips)
  • Reach out to MIT faculty (esp. Prof. Mark Drela for layup advice)
• Tube layup
  • Overwrap the fin collar in CF (at least 3 layers). We used a fiberglass-overwrapped tube because the carbon fiber tube layup had bubbles and we ran out of time, but using carbon fiber would have been way stronger. 
  • Make sure there are no bubbles, and account for things going wrong
  • Do a practice run so the people doing the layup have prior experience
• Root Bond
  • Don't use 5-minute for the root bond. Make a jig that can support the fin/fin can so you can use a stronger epoxy. Even though most of the strength of the fins comes from the root fillet, it's still important to have the root bond as strong as possible
• Root Fillets
  • Consider an inner fillet of proline and outer fillet of epoxy to add strength (and two different types of fillets might make it more difficult for a fin to come off because of fin flutter)
  • Use System 3000 epoxy so that it increases in strength after the oven cure. Using West Systems saved time, but the epoxy may have become brittle after the oven cure because it has a lower glass transition temperature.
  • Degas the epoxy to remove bubbles before applying it to the root.
  • Make sure the fins are sanded where the epoxy will go to improve the bond (rougher surface = more surface area to bond to)
  • Make the fillets larger (reference pictures below to see how large the fillets were). Larger fillets = STRONGER.
• Tip-to-tip layup
  • MORE LAYERS OF CARBON FIBER
• Static load test
  • Don't test the flight fin can, but make sure whatever you're testing is identical so we know for sure that the flight fin can works
  • Make individual fins and test them (for example, bond a fin to a board with the same root fillet as the flight fin can, and do a CF overwrap and then test that. That way, you don't have to make a whole fin can but you can get some useful data)

Procedure Overview

Below is the detailed procedure for how the second flight candidate fin can was manufactured. insert more detail

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 and phenolic
• A respirator is required when using colloidal silica before it is completely mixed into the epoxy. The respirator can be taken off once the silica is completely mixed UNLESS the epoxy you are mixing it into also requires a respirator.
• A respirator is required when using West Systems 3000 epoxy
• Gloves are required when sanding carbon fiber to prevent small splinters
• When using the Dremel, use safety goggles that completely cover your eyes to protect from stray projectiles
• Use safety goggles that completely cover your eyes when sanding so that fine particles do not irritate them

SUMMARY OF FIN CAN MANUFACTURING

1. Tube Preparation

The fin can fits over the motor case (at least, it's supposed to...). Since it didn't quite fit, we sanded the phenolic tube using a flapper wheel, and then overwrapped it with 3 plies of carbon fiber. The phenolic tube acts as insulation from the heat of the motor case and is not structural, so carbon fiber needed to be added to bear the structural loads. During the tube layup, the phenolic tube was held by a pole over the layup jig (much like a pig on a spit). Have at least three people helping with the tube layup.

Required Materials:

• Phenolic Airframe Tubing PT-6.0, 6.007" ID (https://publicmissiles.com/product/airframes)  Get a stronger phenolic tube (canvas, instead of cardboard, for example) 
• 60-grit sandpaper
• 220-grit sandpaper
• Carbon fiber (HOW MUCH)
• System 3000 epoxy (resin + hardener)
• Layup jig (wooden jig, metal pole, nuts and bolts to secure jig)
• Mylar
• Squeegees
• Popsicle sticks
• Epoxy boats or paper cup (for mixing epoxy)
• Painter's tape
• Scissors
• Ruler
• Acetone and shop towels (to clean up epoxy on squeegees)
• Gloves
• Safety goggles
• Respirator

Procedure:

1. Sand inside of tube using a flapper wheel (attaches to a drill)
2. Periodically see it if fits over the motor case. If not, repeat step 1.
3. Once it finally fits, sand with 220-grit sandpaper to give it a smoother finish.
4. 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 Mylar so that once you start the tube layup, the tube doesn't stick to it)

5. Carbon fiber is difficult to work with because it snags and frays easily, so make sure whatever surface you're using is completely cleared off. Also make sure that this surface is large enough for the entire piece of carbon fiber, because the fiber will warp if it overhangs. 

   	Length (in)	Length + Offset (in)
   Outer diameter (OD) of tube
   Circumference of phenolic tube (OD x π)
   Length of phenolic tube

6. Mark a (INSERT DIMENSIONS) area on the carbon fiber using painter's tape and cut along the tape. It's much easier to cut on tape than cutting the fiber directly, to avoid snags and make sure the dimensions are correct. The dimensions were determined using the table above. 

7. MIX EPOXX, DO LAYUP
8. Let tube cure at room temperature for at least 24 hours
9. Sand tube

-layup had bubbles so we used a fiberglass-overwrapped tube instead

2. Root Bond and Root Fillet

Required Materials:

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?) 

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 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.

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 

Lessons Learned

• Make the phenolic slot thicker to account for fit between phenolic and G10
• When doing the tube layup, make sure there are no bubbles (use heat shrink to help with this)
• Make sure the fin jig layers are properly spaced and insert the fins and make sure the tip-to-tip distance is identical before doing the root bond
• Take care to do root fillets correctly the first time so you don't have to touch them up later
• Cooling the fin can to room temperature during the oven cure takes longer than you think it will
• Don't spray glue carbon fiber to wax paper until after it has been laser cut (wax paper will catch fire in the laser cutter)
• Make sure the cutout squares are large enough so that there won't be tape on the cutouts after they are laser cut (removing tape is difficult and causes frays)
• If any pieces of the cutout are still attached to the carbon fiber, don't pull on it- use scissors to cut or you'll get lots of frays
• Make sure you have enough wax paper
• Make sure you have SHARP scissors for the layup