In 1983, engineers at General Electric experimented with an "unducted fan" engine. Without the external casing, airflow through the blades increased, delivering more power for the same amount of fuel. The thing was loud, but the company pressed on because the trick could reduce fuel consumption by as much as 26 percent. Then fuel prices dropped, gas guzzling became acceptable, and GE mothballed the project. Now that airlines are again conscious of fuel costs and carbon, the idea is back, and new tech is making it feasible.