This is a brief but interesting piece on the future capability of two drones to conduct autonomous aerial refueling. Caveat: the paragraph following the photo comes from the website of the manufacturer, Northrop Grumman.
Northrop Grumman RQ4 Global Hawk
Autonomous Aerial Refueling
“Northrop Grumman is leveraging its synergistic partnership with NASA Dryden to execute the DARPA KQ-X program, which has demonstrated technologies that should enable autonomous high altitude fuel transfer between two Global Hawks, extending flight endurance. The engineering and development effort resulted in the first HALE formation flight, a dual-ship close formation with one UAV flying 30 feet from the extended refueling drogue of the other UAV. Northrop Grumman is responsible for all engineering and design/modification of both aircraft.”
You can read more here:
RQ4 Global Hawk