TABLE OF CONTENTS Many spacecraft, especially those intended for unmanned low Earth orbital missions, pose only loose requirements on orbit insertion accuracy and need no orbit adjustments during the mission. In other cases, nominal launch vehicle insertion accuracies are inadequate, or the desired final orbit cannot be achieved with only a single boost phase, and postinjection orbital maneuvers will be required. Still other missions will involve frequent orbit adjustments to fulfill basic objectives. In this section, we consider simple orbital maneuvers including plane changes, one- and two-impulse transfers, and combined maneuvers.
In the following we assume impulsive transfers, i.e., the maneuver occurs in a time interval that is short with respect to the orbital period. Because orbit adjustment maneuvers typically consume a few minutes at most and orbital periods are 100 min or longer, this is generally a valid approximation. In such cases, the total impulse (change in momentum) per unit mass is simply equal to the change in velocity ? V.
This quantity is the appropriate measure of maneuvering capability for spacecraft that typically are fuel limited. To see this, note that, if a thruster produces a constant force F on a spacecraft with mass m for time interval ? t, we obtain upon integrating Newton's second law
Since F/m will be essentially constant during small maneuvers, and since the total thruster on-time is limited by available fuel, the total ? V available for spacecraft maneuvers is fixed and is a measure...
