UFO's main means of achieving FTL speeds is not by controlling gravity or warps or anything like that. They work by a controlled negation of inertia or mass. By eliminating mass, and thus inertia, the basic laws of physics can be circumvented allowing temendous velocities and accelerations. The mass is usually not negated completely or even a single molecule would stop the ship. The negation of inertia must be controlled depending on the mass density of the local environment and the velocity of the ship. The greatest speeds can be achieved in interstellar space and even greater speeds in intergalactic space where particle density is at it's minimum. Of course the ship still requires a form of propulsion but that's a secret. At least that's how my ship works.