Cold Gas Reaction Control System
Rendering of main components for the RCS. They are packaged tightly inside the HAPP and are not easily visible after assembly.
what's under the hood?
The cold gas reaction control system sets the HAPP apart from other high altitude balloon projects. The RCS is what provides active stabilization while the HAPP is at extreme altitudes with almost no ambient atmosphere.
If you've seen videos of an Apollo Command Module from the 1960s firing it maneuvering rockets, then you've seen an RCS in action. The HAPP RCS works on the same principle, only more safely, as it uses cold gas rather than hypergolic rocket fuel!
The RCS starts with two carbon fiber tanks pressurized with air to 4500 PSI. From the tanks, air pressure must be regulated down before it reaches the solenoid valves. The HAPP uses two-step regulation. On the neck of each tank is a stack consisting of a miniature high pressure regulator (HPR) and low pressure regulator (LPR), resulting in a final output in the range of 100 PSI. Pressure downstream of the LPR is measured with a transducer.
The tanks feed two valve packs, each consisting of 6 high-speed, high-flow solenoid valves. When configured for stabilization around the vertical (Z) axis, valve output can be combined for higher flow rates to the jet nozzles. When configured to fly in 3-axis stabilization mode, valve output can be split and routed to the appropriate nozzles. The picture above shows the RCS configured in Z-axis mode with three valves per nozzle.
Final design for Z-axis nozzle packs along with several failed iterations. This is what innovation looks like...
Ultimately, air is routed to the nozzles, which are mounted on the exterior of the aeroshell. The nozzle design is optimized for a high-altitude flight regime to ensure maximum efficiency when it counts. Nozzles come in matched pairs of two per rotational direction on each axis. Due to the complex geometry of the nozzle packs, they are fabricated using high-accuracy stereolithopgraphy 3D printing.
Underneath the carbon fiber air tanks is an extremely lightweight, multi-material impact absorption cradle. The various layers include low density neoprene, expanded polystyrene (EPS), and expanded polyurethane (EPU). The EPU chemistry was tuned to achieve an ideal density during molding.
The tanks are anchored onto the rigid carbon fiber Tank Deck (not shown above) using black nylon straps with velcro and M5 anchor bolts. The straps are just visible in the picture.
Check out the blog for additional technical details and the evolution of the final design.