Aerospace engineers have been putting a lot of effort into developing electric propulsion systems for aircraft but even after over half a century of work they keep hitting a wall. Electric motors, for all their advantages, depend on an outside power source to function. And that's where things go pear shaped.

If an aircraft propelled by electric motors is powered by, for example, batteries, that's a real problem. Batteries have an energy density that is 1/50th of aviation fuel, so it takes 50 pounds of batteries to do the work of one pound of aviation fuel. Worse, as aviation fuel is burned up, the aircraft gets lighter. A battery-powered aircraft remains the same weight from takeoff to landing, which is not good.