High electric potential systems, which can be used to get reactionless propulsion by means of gradiental electic field are known from T.T.Brown research work. Main ideas patented in USA # 3,187,206 filed on May 9, 1959.

    The Biffeld-Brown effect describes the active force, which is created in the flat capacitor to the positive plate side if the voltage is above 50 kV. Dr. Brown did not explain the reason of the force,  which he named as "X-force" in text of the patent. In my opinion, this force is a result of the asymmetry in orbital motion of the electrons in dielectric atoms due to the electric field between the plates. In other words, this asymmetry leads to the centrifugal force gradient and non-zero linear component of this force.

Later this idea  was developed by Jean-Louis Naudin, France. But his theory use pulsation of voltage.