Keller Cliffton from Zipline thought making the drones would be the hardest part. It was only 15% of the problem. The company spent 9 months figuring out delivery for one hospital. Weeks of all-nighters. Building inventory software in a Rwanda shipping container. Then the 2 AM call came: "We delivered the blood to the roof.” It was 100 feet off due to a bug in the code, and a nurse had to climb up on to a roof to get it, but the patient’s life was saved. Keller: "We got a 3-star Yelp review.…