Below is the detailed procedure for how the first flight candidate fin can was manufactured. A phenolic tube (insulation from motor case) was overwrapped with 3 layers of carbon fiber (structural support). After the fins were manufactued (see "Hermes 2 Fin Design"), they were attached to the tube and 6 plies of increasing size were added to each of the 4 sides of the fin can in the tip-to-tip layup. After vacuum-bagging and room temperature curing for 24 hours, the fin can was oven-cured. Post-processing (sanding, painting, etc.) was not done because we wanted to manufacture a second flight candidate fin can to make some improvements, namely concerning the misalignment between the phenolic leading edge and the tip-to-tip plies.
For each step, there is an overview of the process, followed by required materials for this step and then numbered substeps. Some safety notes:
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, 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). We only had two people for the tube layup which was difficult because you need to keep turning the tube in order to keep adding more carbon fiber, so make sure you 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.
Required Materials:
Did root bond using 5-minute. fin can jig needs to be improved
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.
Sand fillets
Procedure:
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 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 (in CSAIL, for example, the laser cutter did not fit the largest cutout so we had to do it by hand).
Quentin designed CF cutouts (6 sizes --> small to large)
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) | 16" x 20"  |
| 2 | |
| 3 | |
| 4 | |
| 5 | |
| 6 (largest size) |
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
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) | 20g |
| 2 | 22g |
| 3 | 23g |
| 4 | 26g |
| 5 | 33g |
| 6 (largest) | 39g |
| Total mass of all cutouts (6 sizes x 4) | 652g |
For the tip-to-tip layup, we used a total of 6 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 708g.
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
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)
Check the cure cycle for whatever epoxy you're using. In this case, we used System 3000 High Temp Epoxy, and the cure cycle is detailed below. Note that the epoxy changes from clear to amber-colored after the cure is completed.
Need 2-3 people, not just one person, and do it during the day time!
Cure Cycle for System 3000 epoxy
Required Materials:
Procedure:
Vacuum bagging materials were difficult to remove, but not impossible
Used Dremel to get rid of CF on trailing edge and either end of tube (in future, taper trailing edge of G10 to get smoother layup quality)
Sanded CF on leading edge
