A wheeled vehicle pushes against the earth. For a given power level, as speed goes up, the amount of push goes down, which is why we have gears. The top sustained speed occurs when forces from friction, etc. balance the reduced push force.
A rocket pushes against its own exhaust, and increases the speed and momentum of the unburned fuel as it accelerates. The amount of fuel is limited, which constrains the time it can accelerate, thus top speed.
A jet pushes against the air moving through the engine. The mass flow increases as speed increases, especially in the supersonic realm when air cannot move out of the way. The top sustained speed is limited by heating from friction and compression, primarily in the engine intake. This is why there is so much interest in SCRAM jets, because there is much less intake compression, thus less heating, and a much higher top speed.
The Impeller drive, as described, has none of these limits. So where does the power come from?
Current theory states that empty space is not nothing, but rather a sea of virtual particles. I don't understand the theory, but IIRC, some minor experimental results indicate this may be real. Some speculation says it may even be possible to tap into that energy, although the uncertainty formula of momentum x position, also rendered as energy x time makes it unlikely. Given Heisenberg, it is not possible to say the cosmos started as a quantum fluctuation using current science because the time such energy can exist is extremely small.
Speculate that these empty space energy particles actually exist. Speculate further that the virtual particles have a large spectrum of velocities. Then speculate that the impeller drive sets up conditions where it reacts ONLY with those virtual particles whose velocity is close to that of the drive at that given instant. The impeller drive is then pushing against these, and in effect getting its power from the "empty space" energy. The power that runs the drive is not producing the acceleration directly, but goes into setting up the conditions which "link" into those particular virtual particles.
This is pure speculation, of course, since we are talking about fiction.
