UFO Propulsion Systems

What type of system powers and propels UFOs whether they are genuine ET craft or man-made vehicles from highly classified projects? They cannot be using conventional rockets or jet engines since they are usually relatively silent and exhibit great maneuvering capability far beyond that of any conventional craft. Do they use some type of anti-gravity system powered by free energy? Or do they use something even more exotic?

Read this article by Stanton Friedman for more information.