Above: Hermes 1 nose cone mold (4 layers of 1" thick MDF, sanded and finished with gel coat), manufactured by Raul Largaespada.
Overview of Nose Cone 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
Shape, materials, length, fineness ratio, tip dimensions and material, washer, bolt, method for attaching thermocouples, etc.
*remember that you need TWO halves of the mold.
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
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
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
- Created design of mold in Solidworks (using "Solidworks mold tools" --> helpful tutorial: https://youtu.be/yqROZFStz6c) and save as an .STL file.
- Calculated 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.
- 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.
- 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
- 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)
- Applied Fibrelease
- Prepped ablative coating (15% by weight phenolic microballoons --> SHOW 3 EQUATIONS USED)
- 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.
- Applied ablative coating to mold (not recommended, this should be done afterwards next time)
- Started layup --> first plies weren't wet enough, so make sure you're not skimping on epoxy
- 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)
- Trimmed edges
- Smoothed SLOWLY and don't put down more fiber until bubbles are gone
- Attached halves, make sure fibers not caught in sides
- Shined light on one end
- Used rod tool to smooth fibers from each end
- 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
- peel ply difficult to apply, use blue tape
- use cone shape for rest of vacc bagging materials (taped narrow end to rod and pushed it in, worked pretty well)
- 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)
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)
info from Raul L. (manufactured NC mold for Hermes 1):
"
So the first step would be to make a mold of the nose cone in solidworks using solidworks mold tools, which I believer are an extra set of tools you’ll have to load in. Some googling should help you find those. For using them, I recommend looking up some tutorials on YouTube. Here’s the one I used last year: https://youtu.be/yqROZFStz6c
The final part should be one half mold piece since the two halves should be identical. There should be some extra area around the mold, with the alignment pins and pry slits I mentioned earlier. I’m attaching my mold files from last year to give you an idea of what it should all look like.
Once you’ve got the mold in solidworks, you’ll need to plot a path to cut it out using a router using Mastercam, which is a program similar to solidworks but used for certain machining applications. The router is essentially a spinning drill bit that goes around 15000 RPM and cuts out shapes by translating the drill bit in X, Y, and Z Using a hydraulic control system. It’s a pretty sizable machine and you can make some very big stuff on one. For the Mastercam, basically you’ll select surfaces on the mold and then define the path the router will cut in order to machine those surfaces. You might be able to find more specific guides online, but I definitely recommend going to the guys in the hobby shop for additional assistance. Dave Robertson in Gelb might also know. Here’s the link to download it form IS&T: http://ist.mit.edu/mastercam.
For the actual routing, I did it in the hobby shop (and again the guys who work there were a great help), although there’s also a router in Gelb that I believe Dave Robertson is in charge of. If you can’t learn the router/Mastercam from Dave then your best bet is the hobby shop. Definitely take notes on what they say about operating/using Mastercam so you can work more independently.
When routing, a lot of sawdust is generated, so you’ll need to follow the router tip with a vacuum to try and get most of it. Besides that, you can mostly relax and let the router do its job, although you’ll have to keep an eye on it to make sure It’s doing the right thing. Don’t be afraid to pause it and check. It helps to have the mold be very regular and rectangular so that it’s aligned properly with the router and CAM model.
Once the router’s done, you’ll have to sand the mold to get a really clean finish. This takes a while. Longer than you think. Start with rougher sandpaper (400 or so) and work your way up to 1000 or 2000. Once it’s sanded smooth (you’ll really be able to feel how smooth it is) there’s some sort of white protective coat you’ll need to apply in thin even layers. I forget exactly what it’s called, but applying it requires a respirator and some patience. I used about five layers on last year’s mold.
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