A method is presented by which interplanetary trajectories may be calculated extremely rapidly and to a degree of accuracy suitable for design studies. By splitting the three-center problem associated with interorbital ballistic transfer into three one-center segments which overlap somewhat at their junctions, solutions may readily be found by machine iteration. The extreme speed of the method enables wholesale amounts of worthwhile numerical results to be obtained. Examples presented in the paper include velocity requirements for trips to and from Mars during the year 1960. Considering tha t the orbits of Earth and Mars are elliptical and mutually inclined, the Hohmann min imum energy transfer criterion disappears, and the occurrence of several local minim u m energy trajectories is observed; this phenomenon is expected to be typical of all interplanetary orbit studies.