Procedure Overview

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:

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, 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:

  1. Sand inside of tube by hand with 60-grit sandpaper (this took a long time: next time use a flapper wheel, 60-grit). 
  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. Cut out a (INSERT DIMENSIONS) piece of carbon fiber. This was determined using the table above. 



Tube layup: 3 layers of carbon fiber (cut piece slightly longer than necessary for some wiggle room), did not oven cure because of time constraints and whole thing oven cured in the end. 

 

-Used Systems 3000 epoxy but didn't oven cure after layup

2. Root Bond and Root Fillet

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. 

3. 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). 

Quentin designed CF cutouts (6 sizes --> small to large)

Cut out carbon fiber squares and spray glue to wax paper( don't use too much spray glue)

Lasercut CF cutout - went well but largest cutout size did not fit in the laser cutter

Make sure to label each piece

 

4. Preparation of Vacuum Bagging Materials

Prepared vacuum bagging materials

5. Tip-to-tip Layup

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 SizeDry mass of one layer
1 (smallest cutout size)20g
222g
323g
426g
533g
6 (largest)39g
Total mass of all cutouts (6 sizes x 4)652g


6. Vacuum Bagging

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)

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.

8. Post-processing

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