ENGINEERING NOTES 1235 quence, e.g., low-, high-, low-thrust. This result can be derived directly from the preceding theory. The field-free solution to the primer vector isThis is a straight line in three-dimensional space which passes through the origin if p(0) and p(0) are parallel (they must be parallel in the one-dimensional case). Thus, the magnitude of p, which is the distance from the line to the origin, is as shown in Fig. 2. Clearly, the switch functions Ki are largest at the beginning and end of the transfer corresponding to periods when high thrust is optimal. The optimal sequence, high-thrust, low-thrust, coast, low-thrust, high-thrust, follows directly. Notice that trajectories on which p(0) and p(0) are parallel require a coast in some region about p = 0. This is not, however, required if p(0) and p(0) are not parallel.