DARPA considering a plane that would fly non-stop for five years
How does DARPA think that the Vulture should be powered? There are currently three options under consideration. The first one is nuclear and - wait, this is not under consideration. The second one is to refuel the aircraft in flight. This favors fuel cells because they are more efficient and reliable than internal-combustion and gas-turbine engines. The other option is solar energy, either solar thermal, which is unproven, or solar electric, which is a proven technology.
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