Wonder where unmanned aircraft are going? Bigger, smaller, higher, longer? Take a look at Qinetiq's solar-powered Zephyr, which has just completed a 54h flight, beating the official world unmanned endurance record of 30h 24min, set by Northrop Grumman's jet-powered Global Hawk in 2001.
A fragile-looking thing, the hand-launched Zephyr is at the vanguard of efforts to extend the persistence of UAVs beyond hours to days, weeks, even years. Solar power, hydrogen fuel, even in-flight replenishment are being researched. The most extreme of these efforts is DARPA's Vulture project to demonstrate technology for a UAV that stay aloft for five years.
I'm not joking, check out this video...
Now that's what I call a stretch goal.
But why all this effort, and how can it be feasible? The why is easier than the how.
Military, and ultimately commercial, users are looking for UAVs that can operate like satellites yet are as flexible as aircraft. Cheaper to launch. Easier to reposition. As ultra-reliable and self-sufficient as a satellite, yet able to be refuelled and upgraded like an aircraft.
Solar soarer - Qinetiq's Zephyr
Instead of endurance, users want persistence - the ability to stay in one place in the sky and stare at one point on the Earth indefinitely - for missions where the gap in coverage when a satellite is out of sight or an aircraft is on the ground can be critical.
Making the task a little more feasible is the fact that the likely early applications involve single-minded payloads - missile-defence sensors or communications-relay repeaters. Vulture may have to stay aloft for five years, but it only need carry a 1,000lb payload.
It's a long way from Zephyr's 54h to Vulture's 44,000h, but the journey has begun. The first step along the road to persistence will be from the Global Hawk's 30h-plus to endurances of seven to 10 days - a step that will require different propulsion and operational concepts.
Hydrogen-burner - Boeing's HALE
Qinetiq's Zephyr uses solar cells. Aerovironment's Global Observer uses hydrogen fuel cells. Aurora Flight Systems and Boeing are pursuing piston engines modified to burn hydrogen. And DARPA, NASA and others are working on autonomous air-to-air refuelling.
But Vulture is a different animal. It is hard to get your head around the implications of designing and testing something that will take off and not return for five years. But the satellite folks have been doing it for decades, and their products don't come back - not in one piece.