Possibly the hardest part of 2-stage rocketry is the integration.  Getting two rockets to seamlessly work together mechanically, do as they're told electrically, and do so in perfect synchrony, takes a level of awareness and system-level thinking far above that of single-stage rocketry.  

My biggest personal contribution to the project, outside of the organization and management of the team with my co-lead Max Goldstein, was in this integration, as well as ensuring the safe separation and ignition of the second stage.  Included in this work is guiding the design of our interstage coupler, a component that is tasked with supporting the bending loads at an extremely high-stress point on the rocket.  Additionally, I was responsible for the design and manufacturing of the second stage avionics bay, fault-tolerant and safe ignition mechanism, and pyrotechnic ignitor, all of which performed nominally on the first attempt.

Launch days for a 2-stage rocket are hectic, and as one of the team leads, I was responsible for overseeing several tasks at once, including recovery packing, avionics packing, final closeout and assembly, and safe ignitor installation into the second stage.  This experience helped prepare me for a world of propulsion test operations, thanks to being exposed to an environment requiring extreme system-level thinking and ownership.

Unfortunately, the sustainer recovery was unsuccessful, and this was traced back to inefficient black powder charges, a common failure mode in high-altitude rockets, and a focus of attention on subsequent high-altitude attempts.