The recent three-day flight by a Zephyr unmanned aircraft has spurred interest in a Vulture UAV capable of flying for years at a time. Boeing Co., Lockheed Martin Corp. and Aurora Flight Sciences are producing competing designs for a Vulture prototype under a $4 million DARPA contract. The winner of the design phase will then test a model that stays aloft for three months, followed by another model that flies for a year. Experts say power-storage technology -- either regenerative fuel cells or batteries -- will be key to the Vulture's success, but they disagree on how soon the technology might be available.

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