Sunday, September 11, 2016

Aeroshell Part 1: Plug

It's been a long hot summer, and it may seem from the lack of blog updates that the project has petered out. Not true! I've spent the summer learning how to fabricate aerospace-quality custom carbon fiber shells to use as the aerodynamic body of the HAPP. This is the "skin" that will cover the carbon fiber "skeleton" I developed earlier.

It took a lot of trial and error and a lot more time than expected. I estimate I've got over 250 hours invested in various trials, most of which ended in failure. Sure, I could have paid thousands of dollars to have a professional fabricator do the work, but where's the fun in that? Plus, I picked up a few more 21st century fabrication skillz.

In the end it boiled down to this. I selected the following method of fabrication:

  1. Make a scale model of the aeroshell. This is known as a plug.
  2. Using the plug, make a fiberglass mold that's a negative image of the model.
  3. Using the mold, form the various layers of carbon fiber and other materials to create the actual part that will fly on the HAPP.

So it's going to be plug, mold, and part. Ready? Here's the plug...

Plugs are useful because it's often easier to shape or sculpt a model of the part rather then try to directly create a negative-image mold. The plug can be made out of a variety of materials, such as styrofoam, wood, or clay. I know, because I tried them all!

The final plug I used consisted of a wood skeleton covered with automotive styling clay. The wood pieces were cross-sections of the desired shape. I cut the main outer radius of the cross-sections using a router mounted to a giant protractor.

In this photo you can see the outline of the entire aeroshell  - it's a scale model of the Apollo command module from the 1960s. I chose this shape due to it's known stability in supersonic flight regimes. This is important because the HAPP will go supersonic as it descends in free-fall after the balloon bursts at 30Km altitude, only to slow down as it descends to thicker atmosphere.


Giant protractors rule!

As the aeroshell will be fabricated in two pieces - a lower "heat shield" and an upper shell - I separated the heat shield cross-sections from the uppers and mounted them on a base board. This was the first step in shaping the plug.





Cross-sections mounted, I tried to fill in as much of the volume as possible with cheap fiberglass and expanding polyurethane foam. I could have filled it all in with clay, but good clay is quite expensive and also heavy - it probably would have required over 200 pounds of clay had I not used glass and foam.

One little trick I developed was to include a small steel shaft in the center of the plug. This provided an axis about which I could rotate a wooden guide fixture and confirm whether the finished profile is correct. You can see this guide fixture in the following image.


Clockwise from top left:
(1) Laying fiberglass over wood cross-sections.
(2) Wetting out the glass with resin.
(3) Checking glass profile versus target profile.
(4) Filling out the volume with polyurethane.

Next came the surface clay and final shaping. I tried a variety of modeling materials and had several false starts.


Some good, some not so much

After good advice from a friendly professional automotive designer (thanks, Richard!) I settled on Autostyle Clay by Chavant. It can be shaped, machined, and coated. However, before forming the clay, it does need to be warmed up over 100F (40C) or so, which I accomplished using the kitchen oven.


Baking some tasty treats

Bit by bit the clay went on. I pressed and formed the clay into the desired shape, constantly checking and scraping clay with the wooden guide fixture mounted on the central steel shaft. This fixture rotated freely and allowed me to confirm the profile was precisely correct.


Clockwise from top left:
(1) Filling out the profile with clay.
(2) Scraping with the wooden fixture.
(3) Lots of bits after scraping!
(4) Ready for coating.

The last phase of preparing the plug entailed getting a smooth, glassy, and hard epoxy gel coat onto the clay. The gel coat gives a durable surface for creating the fiberglass mold. To do the gel coat right requires multiple coats. I applied three coats with a compressed air spray gun. In between coats, I filled in low spots with automotive body filler and then sanded with a progression of grits from #180 to #2000. Eventually the surface attained a nearly-flawless look and feel which I enhanced by buffing with automotive polishing compound.

To orient yourself, note that you're looking at the bottom of the craft. This would be the heat shield on an Apollo capsule. The plug is approximately 1 meter in diameter.


From the top, each row shows successive layers
of gel coat with filler (reddish color) and the
result after sanding to a fine finish.

At last the plug was ready and I could progress to the mold fabrication phase. This blog post may read like a nice linear story, but it encompasses perhaps 100 hours of reading, trial and error, and tedious model construction. It was fun to do... once.

Next post: Fabricating the mold.

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