Figure 7.33. Thrust and propellant rate vs. specific impulse [ASPL, 2000].
During a typical operation, two parameters are varied while keeping the power constant: thrust and exhaust velocity (i.e., Isp). Therefore, during an interplanetary mission, most of the trans-planet trajectory (the portion of the trajectory from near Earth to the planet) can be traveled at a constant and moderate thrust, with a modest but useful acceleration and with high Isp. When the spacecraft must slow down to reach its final destination (e.g., for planetary orbit capture), thrust may be increased, reducing capture time at the expense of a lower Isp. According to the information available [Chang Diaz et al., 2000; Ilin et al., 2000], this concept is capable of an Isp = 104 s with a thrust of 1,200 N, increasing to 3 x 105s with a thrust of order
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