FIGURE 6.2 EOTV CONFIGURATION The use of very low thrusts also raises an issue as to specifically which trajectory mode should be used. Due to the very low thrust levels and long thrusting periods required of electrically propelled vehicles, the trajectory mode selected for their use generally entails continuous thrusting in geosynchronous space, resulting in spiral trajectories as those shown in Figure 6.3(a). The velocity increment for such a trajectory is approximately twice that of the optimal, high thrust or impulsive trajectory mode. By lengthening the mission timing, however, as shown in Figure 6.3(b), thrusting only in the vicinity of periapse and apoapse of the transfer orbit, it is possible to devise trajectory modes where the velocity increments required of a low thrust vehicle approach that of the optimal high thrust transfer. Thus one cannot choose a specific velocity increment for the disposal options presented here as these require further analysis and cost optimiza-
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