After six years of development, the full size version of the U.S. Air Force Global Observer UAV reconnaissance aircraft had its first test flight. With a 56.5 meter (175 foot) wingspan and four electrically powered propellers, the aircraft can stay aloft for over 160 hours (a week) while flying at 21 kilometers (65,000 feet). Carrying about 400 kg (880 pounds) of vidcams and communications gear, the one ton Global Observer uses liquid hydrogen to run a generator that powers the four props and the sensors.
Global Observer is basically a powered glider, thus it cruises at about 250 kilometers an hour. There have been attempts to build long-endurance UAVs like this using solar panels in the wings. But those aircraft are not as robust, nor can they carry as heavy a payload (or generate as much power) as those using liquid hydrogen, although the solar powered aircraft can stay in the air longer. Global Observer UAVs, costing about $40 million each, would be replacements for space satellites, as well as providing the usual "persistent" observation of terrain below.
Global Observer may never make it into service. There are reliability and durability issues that more testing and tinkering will answer, one way or another. The last five years have been spent scaling up the original, smaller, models of this unique aircraft type. The final version may even be a little bigger. But if all goes well, Global Observer could be operational in 5-10 years.