bit passes over the launch site twice a day, producing two launch windows per day.
7.3.8 Orbital Maneuvers: Changing Orbital Altitude Once a spacecraft is on an orbital trajectory, it is in free flight, and keeps coasting on that trajectory due to gravity and inertia. If you want to change the orbit, you need to perform an orbital maneuver. To maneuver requires delta v, in other words, you must fire thrusters and have enough propellant for your desired delta v. One aspect of an orbit that you might wish to change is orbital altitude. You can either fire the thrusters in the direction of your orbit, to increase velocity which also raises the apogee, or you can turn the spacecraft and fire the rockets opposite your direction of travel, to slow down and lower the apogee. To raise the apogee of an Earth orbit, thrust is applied along the direction of flight at a point in the orbit that is 180° away from the desired new apogee. Reducing a point in the orbit is done in a similar fashion. Thrusting to reduce speed will reduce the altitude of the spacecraft 180° away from the point of the burn.
7.3.9 Changing Orbital Inclination An orbital inclination change requires a burn, to occur in one of the two points where the initial and desired orbits intersect. In general, inclination changes take a very large amount of delta v to perform, and most mission planners try to avoid them whenever possible to conserve fuel. Instead, getting a satellite into the right orbital plane is typically achieved by launching a spacecraft directly into the desired inclination, or as close to it as possible so as to minimize any inclination change required over the duration of the spacecraft life. The simplest way to perform a plane change is to perform a burn around one of the two crossing points of the initial and final planes. The engine must be fired perpendicularly to the initial
Figure 7-12. An engine burn in the direction of travel (prograde) at perigee raises the orbit’s apogee. (NASA) direction of travel. The delta v required is the vector change in velocity between the two planes at that point. If both orbits are circular and have the same radius, the delta v for the inclination change is proportional to the orbit’s velocity and the inclination change between the two orbits. The equation reads Δv = 2v × sin
Δi . (2)
In the case of elliptical orbits, maximum efficiency of inclination changes is achieved at apogee, where orbital velocity is the lowest. In some cases, it can require less total delta v to raise the satellite into a higher orbit, change the orbit plane at the higher apogee, and then lower the satellite to its original altitude.
7.3.10 Orbital Rendezvous A rendezvous is a maneuver during which two spacecraft approach at a very close distance. Rendezvous requires a precise match of the positions and orbital velocities of the two spacecraft, allowing them to remain at a constant distance. Sometimes it is desired to dock them, which is bringing the spacecraft into physical contact and creating a link between them.
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