Page 83 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecraft Applications
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ship between launch azimuth and resulting orbital inclinations from both
the Kennedy Space Center and Vandenberg Air Force Base in southern
California, the two major U.S. launch sites.
Though theoretically unlimited, the actual launch azimuths used by the
United States are restricted to the directions that allow jettison (and in the
case of the space shuttle, recovery) of the initial stages of the launch vehi-
cle over the ocean. Vandenberg is used primarily to launch spacecraft into
polar or the slightly retrograde sun-synchronous orbits, and, for the safe-
ty reasons just stated, launch azimuths are always to the south.

Earth Rotation. In the previous discussions we ignored the rotation of
the earth for simplicity in relating launch azimuths to orbital inclinations.
However, the earth rotates at an appreciable rate which contributes a sig-
nificant initial velocity to the launch vehicle. This initial velocity assists
launches to the east, with the greatest assistance realized for 90" azimuth
launches. This easterly component of velocity must be compensated for in
order to launch into a truly polar orbit, and must be overcome in order to
launch into a retrograde orbit. Launching in any azimuth other than 90"
decreases the amount of payload a particular launch system can deliver to
orbit compared to a direct easterly launch. This explains the popularity of
28.5" inclination orbits for U.S. space missions.

Orbit Adjustment

It was noted earlier that if the launch vehicle does not establish the
desired orbital position and velocity precisely, or if the initial orbit is not
the spacecraft's operating orbit, then an orbit adjustment will be required.
We differentiate orbital adjustments from station keeping or attitude
adjustments by the relatively larger Av requirements of orbital adjust-
ments, as quantified in Chapter 2. Missions requiring orbital adjustments
may incorporate completely separate propulsion systems to accomplish
these maneuvers, while station keeping and attitude adjustment (if per-
formed by a propulsion system) may be accomplished by a system inte-
grated into the design of the spacecraft.
Addressing all possible types of orbital adjustments and their associat-
ed propulsion requirements is beyond the intent of this book. However, an
excellent and commonly performed example exists which can be used to
describe both the magnitude of adjustment required and the typical types
of systems used to perform these maneuvers.