Carbon Fiber Structure and Aeroshell
from cardboard to carbon fiber
No system on the HAPP underwent as dramatic of an evolution as the structure. Early prototypes were constructed of cardboard, Styrofoam, plywood, hot glue, and the occasional bit of scrap aluminum. The final version consists almost entirely of custom-fabricated carbon fiber. Most of the joints are bonded together to reduce the need for heavy screws and other hardware.
The core structure is arranged in three circular decks surrounding a central strut. Each deck was CNC-machined from rigid carbon fiber plating. Starting at the bottom, there's the Propulsion Deck, Electronics Deck, and ELS Deck. The location of each deck along the strut was carefully engineered to ensure the HAPP has a well-placed center of gravity and remains aerodynamically stable as it free-falls back to Earth.
The core is designed to be extracted in one piece from the aeroshell for service and repairs. It attaches to the aeroshell with 6 Velcro straps arranged around the rim of the ELS Deck.
Everything below the ELS Deck is contained inside the aeroshell after prelaunch assembly. Above the ELS Deck, three triangular strakes taper upwards to the 360-degree primary camera. The strakes provide aerodynamic stability during the free-fall phase of the mission.
To facilitate bonding of the various carbon fiber components, 3D-printed assembly jigs were created from a "negative image" of the core's CAD model. These jigs held various pieces in place as the epoxy cured.
The main strut is 86 centimeters in length.
The overall geometry of the aeroshell is a scale model of NASA's Apollo Command Module. When correctly balanced, this shape is aerodynamically stable during transonic and supersonic flight, regimes the HAPP may experience during free-fall. It was soon apparent that a thin veneer of semi-rigid carbon fiber was the ideal material, but it took several months (and plenty of wasted carbon fiber) to develop a do-it-yourself manufacturing process for parts this large; the aeroshell is one meter in diameter.
The aeroshell is fabricated in two pieces, an upper and a lower. The three-step manufacturing process entailed sculpting a full-scale model of each piece using automotive styling clay and then coating with a hard gel coat (the "plugs"). Fiberglass molds were formed directly over the plugs, which were then extracted and discarded. Layers of carbon fiber fabric were placed into the molds and covered with airtight vacuum bags. Finally, air was drawn out of the bags through one port while liquid resin was injected through another. After curing, the parts were removed from the molds and trimmed. This type of process is commonly known as vacuum infusion.
It sounds straightforward, but perfecting this process and producing good parts consumed many months. The result was a complete aeroshell that has a total weight of about 1700 grams, looks terrific, and is tough as nails. With such a low weight, the HAPP is essentially a hollow flying wing.
The upper and lower aeroshells are secured to one another with 8 small velcro straps.