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Above: Hermes 1 nose cone mold (4 layers of 1" thick MDF, sanded and finished with gel coat), manufactured by Raul Largaespada. 

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Overview of Nose Cone

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Required Materials

  • 1" thick MDF (medium-density fiberboard)- how much???
  • epoxy?
  • pins (what size? length?)
  • gel coat?
  • fiberglass
  • moar epoxx
  • Partall Paste #2
  • PVA (polyvinyl alcohol) Film
  • Spray gun
  • lubricant
  • scissors
  • sandpaper (400, up to 1000 and 2000 grit)
  • Phenolic
  • Steel
  • Bolt
  • Washer
  • Bandsaw
  • router

Design

The shape of the nose cone for Hermes 2 is a hypersonic optimum (AKA 3/4 power series, fineness ratio 5.5:1) with a 2" straight section for the payload coupler to fit into. For the first attempt, we applied ablative coating (Fibreglast Systems 3000 epoxy with 15% by weight of phenolic microballoons) straight into a mold that we routed out of MDF, and used 10 layers of fiberglass (5.7 oz S-glass) on either side of the mold. The mold was prepared with wax and Fibrelease before the ablative coating was added, and vacuum-packed after the fiberglass layup was complete. The thickness of the fiberglass was around 0.125". The ablative coating broke off in several parts when the part was released from the mold, so for the second attempt (flight nose cone), we applied the ablative coating after taking the part out of the mold.

Fiberglass Nose Cone Manufacturing Procedure

Overview of Nose Cone Design

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Talk about mold materials used (MDF, gel coat), why fiberglass used

Hypersonic optimum not tangent to linear section so chose some radius in Solidworks (should calculate minimum radius to avoid shock waves)

Overview of Mold Design

Manufacture Mold


Each half of the mold: 5 layers of 3/4" MDF (total thickness: 3.75"). Since the radius of the nose cone is 3", this gives us an extra 0.75" on the bottom. The top view of the mold measures [ x ], accounting for space on either side of the nose cone

Alignment pins, pry slits

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Manufacture Mold

To route the mold we are using the router in Gelb (next to Todd's shop). You will need an STL file of your nose cone mold. Make sure to wear a dust mask while routing and use the vacuum because a LOT of dust will be generated.

Required Materials:

  • 3/4" or 1" thick MDF (we used 3/4")
  • Epoxy (WHAT KIND)
  • Pins (??)
  • Ruler
  • Router
  • Table saw or Bandsaw
  • PPE: safety goggles, gloves

Procedure

  1. Created Create design of mold in Solidworks (using "Solidworks mold tools" --> helpful tutorial: https://youtu.be/yqROZFStz6c) and save as an .STL file.
  2. Calculated Calculate how much MDF needs to be used (based on thickness, dimensions of nose cone). Make sure to account for extra area for pry slits and alignment pins. 
  3. We want the mold to be very regular (90 degree angles) so that alignment when routing will be easier. Cut MDF using a table saw or a bandsaw (where can we find table saw?)
  4. Glue layers together (using what?)
  5. Alignment pins and pry slits? (what are pry slits? what order should this be done in? how many pins?)

Route Mold

  1. Download Mastercam: http://ist.mit.edu/mastercam
  2. Plot a path to cut the mold using Mastercam
  3. Go to Hobby Shop to get help from peeps
  4. Take notes of helpful tips and add to this Wiki article
  5. When routing:
    1. Make sure the router is routing the correct shape- pause if necessary.
    2. Lots of sawdust will be generated, so follow the router tip with a vacuum to get most of it.

Mold Preparation

  1. Sand the mold, starting with 400 grit and working up to 1000 and 2000. This will take a long time!
  2. Apply 5 layers of gel coat (respirator required)

Nose Cone Layup

  1. Cut fiberglass layers
  2. Mold release/lubricant (more than you think you need!)

Nose Cone Tip

  1. Cut the MDF using a bandsaw and glued together (using what epoxy?). The piece of MDF we used was 25" x 97" so it required two people to cut it on the bandsaw because it was so large. It would be better to use a table saw to get the sides more even so that aligning the mold on the router will be easier.
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  2. Mark where pry slits and alignment pins go. 

Nose Cone Layup

Required Materials:

  • Fiberglass (5.7 oz S-glass)
  • Epoxy (Fibreglast 3000)
  • Wax
  • Fibrelease
  • Phenolic microballoons
  • Spray gun
  • Squeegees
  • Acetone
  • Epoxy boats
  • Popsicle sticks
  • Spear tool
  • Scissors
  • PPE: gloves

Procedure

  1. Cut fiberglass layers and vacc bag (HOW BIG) the day before layup (smoothed fiberglass in mold, taped down, drew lines, accounted for tab and cut out 20 identical)
  2. Applied Fibrelease
  3. Prepped ablative coating (15% by weight phenolic microballoons --> SHOW 3 EQUATIONS USED)
  4. Degassed ablative coating mixture (put it in a vacuum pump for ~20 minutes, or until bubbles stopped forming). Make sure it's in a large container because it expands a lot when you degas it.

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  1. Applied ablative coating to mold (not recommended, this should be done afterwards next time)

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  1. Started layup --> first plies weren't wet enough, so make sure you're not skimping on epoxy

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  1. Applied starting from the tip, lining up one end flush with the mold (use same side for each half --> align it well the first time because it's difficult to cut)
  2. Trimmed edges

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  1. Smoothed SLOWLY and don't put down more fiber until bubbles are gone
  2. Attached halves, make sure fibers not caught in sides
  3. Shined light on one end
  4. Used rod tool to smooth fibers from each end

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  1. Applied two extra strips along seams (two on each seam) and smooth with tool and by hand for good measure. Cut end of strip so it's not on linear coupler section because this will make it easier to sand later
  2. peel ply difficult to apply, use blue tape
  3. use cone shape for rest of vacc bagging materials (taped narrow end to rod and pushed it in, worked pretty well)

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  1. Waited until vacuum pump dropped to around -27 inHg. It took a while for it to drop because we had to seal some gaps in the vacuum bag. (In the picture below, the vacuum wasn't quite strong enough)

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Lessons Learned:

  • Spear tool (two pieces of disposable squeegee attached using 5-minute epoxy to a long rod) works well for smoothing fibers on the inside of the nose cone
  • Make sure fiberglass plies are wet enough or you risk delamination
  • Do not use Fibrelease, just use wax
  • Don't put the ablative layer on the outside --> apply if after the layup
  • When vacuum bagging, use cones instead of two separate halves because this will be difficult to apply
  • Make sure the outside of the mold is epoxied and waxed so that fiberglass doesn't overhang and stick to either end (makes mold release more difficult)
  • Make smaller, and more pry slits so that mold release is easier
  • Take the time to vacuum tape WELL so that you aren't looking for leaks later
  • Consider using Vaseline for mold release after all (messy but it might help avoid delamination)

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