The Defense Advanced Research Projects Agency (DARPA) has formally invited bids for a $155 million Vulture II contract to build and test an ultra-long-endurance aircraft.
The "near full-scale" flight demonstrator is expected to fly from one and three months, but the goal is to prove whether a future aircraft design could remain aloft more than five years.
DARPA launched Vulture more than two years ago. The agency awarded phase 1 contracts in April 2008 to Aurora Flight Sciences, Boeing and Lockheed Martin to complete design studies. Each team has revealed a concept for an unmanned aircraft that uses solar energy to power an electric motor.
DARPA expects to receive proposals based on solar-powered, heavier-than-air aircraft, but any energy source except nuclear power will be considered, according to DARPA's broad agency announcement released on 22 October.
In Phase II, DARPA will select one contractor to build and fly the flight demonstrator for up to 90 days, carrying a 90kg (200lb) payload with a 1KW power supply. The goal for the five-year flight is to carry a 450kg payload and a 5KW power supply.
The flight demonstrator must measure at least 90% of the wing span of the full-scale aircraft, DARPA's announcement says.
DARPA envisions that such a long-endurance aircraft could operate as a recoverable satellite. The vehicle could remain on station, relaying communications or collecting intelligence data, for either military users or "national interests". The latter term typically describes both military and civilian intelligence agencies.