Page 85 - Intro to Space Sciences Spacecraft Applications

72 Introduction to Space Sciences and Spacecraft Applications
transfers, the plane change is done out at geostationary altitude and not at
the parking orbit altitude. In actual geostationary transfers, an apogee kick
motor (AKM) is used to perform both the circularization of the transfer
orbit and the plane change simultaneously, which results in a savings in
Av required over performing separate maneuvers.
PKMs and AKMs may be either liquid or solid fueled. Because only a
single bum of known Av is required at the "kick" points of a geostation-
ary transfer, solid motors provide a simple way of performing this func-
tion and are commonly used. However, if higher performance or a restart
capability is required, a liquid-fueled motor may be more suitable. Dif-
ferent PKMs and AKMs as well as currently available orbital maneuver-
ing systems and launch vehicles are presented in Chapter 8.
Before leaving this subject, it is worth noting that the former Soviet
Union's major launch site, the Baikonur Cosmodrome in Khazakstaan
near the city of Tyuratam southeast of Moscow, is at 45.6" N latitude, as
shown in Figure 3-10. Due to range safety reasons and the proximity of
China to the east, allowable launch azimuths result in a minimum orbital
inclination of 51" from Tyuratam. It should come as no surprise that the
Russian Mir space station is in a 51" inclination orbit. Also, as with the
Apollo-Soyuz mission, future joint U.S.-Russian missions, including the
international space station, will probably be conducted in 5 1 O inclination
orbits, as the penalty paid by the United States to launch into a higher than
28.5" inclination orbit is far less than that which would be required by the
Russians to change the plane of their orbit from 51" to 28.5". As a last
note, the French Ariane launch vehicle has become a major launcher of
satellites into geostationary orbit. Much of this success is due to the fact
that the Ariane is launched from the Guiana Space Center near Kourou,
located at 5.2" N latitude on the northeast coast of South America. Since
only a small plane change is required to establish a geostationary orbit
from this site, more of the payload launched can go into spacecraft weight
and less to propulsion systems making the Ariane attractive to many com-
mercial geostationary satellite providers. Figure 3- 10 shows this launch
site along with the locations and latitudes of some of the world's other
current major orbital launch sites.

Station-keeping and Attitude Control

In Chapter 2 we ignored the many sources of perturbations that con-
stantly act on the orbit and orientation of a spacecraft. If it is important