Summary of things to fix for Hermes 3 Fin Can:
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:
SUMMARY OF FIN CAN MANUFACTURING
| 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 | 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 | 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 |
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:
Procedure:
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 |
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.
-layup had bubbles so we used a fiberglass-overwrapped tube instead
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:
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:
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!!
*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" |
Required Materials:
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
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:
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.
| Layer Size | Dry mass of one layer |
|---|---|
| 1 (smallest cutout size) | 26 g |
| 2 | 26 g |
| 3 | 26 g |
| 4 | 26 g |
| 5 | 32 g |
| 6 (largest) | 35 g |
| Total dry mass of all cutouts (6 sizes x 4) | 1026 g |
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.
Required Materials:
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
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
Required Materials:
Procedure:
