Structure: Initial Core Design

Note from the future: The following post shows what I thought was the final structural design. Little did I know that I would need one more major redesign before finishing the HAPP. See this post for more details.

My blogging has slowed down recently but the HAPP project has not! Among other things, I've been working on the main structure for the flight hardware, as I'm not going to try and fly the plywood and cardboard prototypes I've been using for the early development work.

The punchline first: Main internal structure is complete. Here's the overall view. To get an idea of the finished HAPP, imagine an Apollo Command Module capsule wrapped around this carbon fiber skeleton. A big rounded "heat shield" goes on the bottom (for the HAPP, an impact shield) and a conical section rises from the jet arms up to the apex.

Internal structure with four decks

The structure is organized into four decks. Each deck consists of a circular piece of carbon fiber plate that was cut by water jet. All decks can be adjusted for position along the central strut. This will be critical for ensuring the HAPP's center of gravity is perfectly aligned with the jet arms. It took me multiple iterations to develop a mounting system for the decks and jet arms that was:

  • Adjustable (fine-tuning center of gravity)
  • Repairable
  • Lightweight
  • Professional-looking

Here's my final set of design notes before starting the build - one step up from the back of a napkin (but not a big step).

How science gets done :-)

Starting at the bottom is the Tank Deck. It holds the 90 cubic inch, 4600 PSI carbon fiber air tanks with high pressure regulators, all nestled into impact-absorbing cradles. The white cradles consist of an outer layer of hard polystyrene and an inner bed of flexible expanded polyurethane (seat cushion material!). The cradles are the result of multiple trials using different polyurethane blends and custom molds. You can get an idea of the molding process from the following photo. I literally used the tank - covered with release wax - as the mold insert, and I poured the liquid MDI polyurethane directly around the tank. After it expanded and cured I cracked the mold open like an Easter egg.

Left: Final version of mold with tank inside.  Center: Liquid PU poured into mold, sealed with cap.  Right: Multiple iterations of PU chemical blends to achieve right amount of "cushion."

Here's a view from below the Tank Deck so you can see how it's attached to the central strut. I used an aluminum ring inside the central strut tube and bolted brackets through the tube and into the ring. The deck sits on top of the brackets and is bolted to them. The ring and bracket system is part of the Carbon Erector Set from Rockwest Composites.

Here's the ring and bracket used in the upper decks so you can see better how the system works.

Moving up we have deck 2, the Propulsion Deck. This deck contains the mounting system for the jet arms, the twelve solenoid valves, the low pressure regulator, and the pressure transducer. In the photo above I've set a few valves on the deck and strapped the LPR to the main strut. These will be connected and positioned later when I run the pneumatic tubing out to the jet nozzles. Here are some close-ups of the Propulsion Deck and jet arm mounting system. One cool feature is the 3D-printed mounting rings that sandwich the deck and provide support for the jet arm inboard brackets.

Jet arm outboard bracket (red anodized aluminum)

Jet arm inboard brackets with upper mounting ring (dark 3D-printed resin with through bolts)

Jet nozzle attachment brackets (red anodized aluminum)

Deck 3 is the Electronics Deck. It contains the two Arduino Mega flight computers with the guidance IMU and other instrumentation, as well as the three LiPo battery packs. I've set some of these on the deck for now but will attach them all later.

Deck 3, Electronics. There will be a total of  2 Arduino Megas and 3 LiPo packs.

Looking at the bottom of deck 3, Electronics. LPR is strapped to the main strut.

The topmost deck, deck 4, is the Earth Landing System. This consists of two of these parachutes (one shown in the photo below) and this pyrotechnic deployment system controlled by the Arduino flight computer.

At the apex is the main structural connection for the balloon umbilical. It's made from black anodized aluminum and mounted directly into the central strut tube. The steel wire and suspension rings shown in the photo are not flight hardware - they are temporary attachments to monofilament suspension from my ceiling in the lab.

Finally, compare this flight hardware with one of the early prototypes used for developing the controls system. You've come a long way, baby!

Next up is the custom-molded outer aero shell and impact-absorbing foam. Hope to post sometime during the next several weeks...