In my first semester at Duke, I worked in a team to develop a durable, impact-resistant frame for a hexacopter drone. The client was a Duke team competing in the $7 million dollar Shell Ocean Discovery XPRIZE competition. Their challenge was to map 250 square kilometers of the sea floor autonomously within 24 hours. The problem was, their frame was designed as an afterthought and would break when the motors were cut off at about 5 feet above the ground. Our design objectives were to create a frame that would survive the 5 foot drop onto grass (at a minimum) with a target survivable crash of about 15 feet. The frame needed to be reusable, quick to manufacture and assemble and be as lightweight as possible to conserve fuel.
I developed a shock-absorption mechanism built into the three legs that contacted the ground. Drawing inspiration from Apollo Lunar Landers, we imagined a crumple zone in the leg. Feedback from our client led to the additional design criterion that no parts would need periodic replacement, so a sustainable solution was found. We cut the vertical outer-most carbon fiber rods approximately a half inch shorter than would lead to flush, normal connections at the joints. This allowed us to insert a spring outside that would absorb force before transferring load onto the carbon fiber tubes.
With only a few weeks remaining in the semester, our client approached us with spare motors, propellers, and avionics. We were asked to give our prototype full flight capabilities. We succeeded, and were able to perform flight and crash tests. We cut power to the motors to the degree the safety system would allow, leading to a near-free fall condition, and the drone survived from a drop of approximately 25 feet.
