With the rising buzz around drones, some have started envisioning them to become bearers of good news. Or at least the packages you ordered. But while Amazon might have gotten a bit of green light for its delivery drone plans, there remains one technical problem. Drones can't really stay up in the air for long, with some of the best lasting only half an hour. Horizon Unmanned Systems, however, might have a solution in the works. Its Hycopter UAV can fly 2.5 to 4 hours, depending on payload, by using fuel cells instead of lithium batteries.