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| Design fin can | Approx. Time Required | # People Required | Notes | |||
|---|---|---|---|---|---|---|
| Fin Design & Manufacturing | 36 hours | The more the merrier. | Manufacturing procedure and detailed breakdown described in "Hermes 2 Fin Design & Manufacturing" | |||
Design/make jigs | ||||||
| Root bond jig | 3 | Allow for tolerance so fins fit, but don't make slots too loose. Make sure you have a top plate. | ||||
| Fillet jig | 3 | Two wooden pieces with a U-shaped slot on which the fin can rests horizontally. Need a larger fillet tool so that the fillets for Hermes 3 are larger | ||||
Tube preparation | ||||||
| Sand tube | 2 | Sand inside with flapper wheel so that it fits over the motor case. This is WAY faster than doing it by hand. Make sure to wear a dust mask or, even better, a respirator | ||||
| Tube layup | 6 | 2-4 | Rough up the outside of the tube before layup. You should have at least 3 layers of carbon fiber. Talk to Drela about tube layup techniques (make sure there aren't any bubbles!) | |||
| Root bond | 1 | 5-minute is ok, but if you have time, use a stronger epoxy. Do not use 5-minute, it's not strong enough!! Make sure the root of the fin is roughed up to improve the bond. | ||||
| Root fillets | Need 4 sets of fillets but it takes 8 rounds cause you gotta do the ends of each side separately (read below for more detail). You'll have to backtrack from when you want to do the layup to when each fillet has to be done by (can speed up using heat gun) Use System 3000 epoxy and make sure there are NO bubbles (degas the epoxy)! Also consider proline fillet + epoxy fillet to add strength. Make sure the area of the fins where the root fillet is applied is roughed up with sandpaper so the bond with epoxy is stronger! | |||||
Layup preparation | ||||||
| Laser-cut CF cutouts | Be careful not to laser-cut over the taped edges of the carbon fiber squares that you use for the cutouts (the tape will be difficult to remove and fray the fibers). Also, don't spray glue wax paper until after laser cutting because the wax paper might catch fire or start smoldering in the laser cutter. | |||||
| Spray-glue to wax paper | ||||||
| Sand fillets | ||||||
| Prep vacuum materials | ||||||
| Layup | ||||||
| Wet plies/weight | ||||||
| Layup | ||||||
| Put on vacuum bagging | ||||||
| If you do the fillets right, you should barely have to sand them. Make sure to wear a respirator whenever sanding fiberglass. | ||||||
| Prep vacuum materials | The shape of each vacuum bagging material (peel ply, breather, bleeder, vacuum bag) should be a square, and you need four squares of each material since there are four fins. The width of the cutouts should be around 4 inches longer than the tip-to-tip distance (not too long or too short). The length of the cutouts should be around 4 inches longer than the fin collar (see images to get an idea of the size). | |||||
| Layup | ||||||
| Wet plies/weight | ||||||
| Layup | ||||||
| Put on vacuum bagging | ||||||
| Vacuuming | Vacuuming | Watch the gauge on the vacuum pump to make sure it drops to at least ________ Did this overnight | ||||
| Oven Cure | ||||||
| Set up oven/vacuum materials | ||||||
| Follow epoxy cure cycle | 12 hours | Make your life a lot easier by assigning shifts (i.e. have 3-4 people helping so you don't have to sit in front of the oven the whole time). Even better, find access to an Autoclave so you don't have to adjust the temperature manually. | ||||
| Remove vacuum materials | This takes a while, but it can be done with patience. Make sure you don't lose track of any of the vacuuming supplies (like the vacuum pump attachment) when you throw out the bagging materials. | |||||
| Clean up edges | Did this using a Dremel right after the fin can was taken out of the vacuuming materials so that people don't get hurt on the sharp edges. | |||||
| Static load test | ||||||
| Design/build jig | Make sure jig is rigid enough (we used 80/20, which bent when the fin can was tested in the Instron) | |||||
| Determine testing loads | ||||||
| Static load test | ||||||
| Analyze data | ||||||
| Post-processing | ||||||
| Sand fillets/outside | Don't sand the fillets too much! Instead, smooth by adding Bondo or epoxy to add material instead of taking it away. (see row directly below for more details) | |||||
| Outer fillets on low spots | We used Bondo to fill in the low spots for Hermes 2. Consider using something stronger, like epoxy (but keep in mind Bondo is easier to sand. If you're going to use something stronger, account for lots of sanding time) | |||||
| Paint | ||||||
| Total manufacturing time |
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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, fiber glass can 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).
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*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" - Cut along tape and label which size cutout it is (1-6) on the tape so you keep them organized later when laser-cutting. 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
- 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 (100% power, 4% speed)
- 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! (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.
- 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.
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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
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")--> 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)
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7. Oven Cure (see attached "Oven Cure SOP" for more details)
Positioned fin can horizontally as shown, because there wasn't enough vertical space in the oven we used. This may have caused the fins to splay (weight of the tube + epoxy loosening --> fins move), so in the future we should position it vertically. (try taking out the rack, for example)
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- Seeing as the cure cycle for
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- System 3000 epoxy is around 12 hours, got four people to sign up for shifts (this requires planning in advance!)
- 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.
- 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.
- Once the cure cycle is complete and the fin can has cooled to room temperature, put on gloves. Then,
- Turn off the vacuum pump
- Detach the vacuum fixture from the vacuum tube
- Remove the fin can from the oven
- 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
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- 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:
- Sanded CF on leading edge so that only phenolic remains (careful not to remove the phenolic)
- Cut off
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- ends of tubes using Multimaster tool or Dremel
- Sanded high spots on tube and fillets, but be careful not to oversand or this will decrease the strength of the fillets
- 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)
- 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)
- Epoxied the transition to the motor case, then slid the fin can on, and the Propulsion subteam attached the boattail
need to: sand creases, sand phenolic down to be flush with CF
Add more fillets once high spots sanded because sanding all high spots removes too much CF
Make fin can transition --> epoxy
Lessons Learned
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