On the validity of field directed propulsion

The concept of ``field directed propulsion,'' which was suggested by Spenny et al., and Nagashima in 1994, is based on the magnetic force induced by ``uniform'' geomagnetic field. Through an analytical investigation on the force and torque, we present that this concept is principally invalid for their original utility. We show, however, that their idea on magnetic shielding is applicable to the improvement of propulsive performance of ``magnetic sail'' and/or controlling performance of ``magnetic torquer.''