Page 81 - Intro to Space Sciences Spacecraft Applications

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Introduction to Space Sciences and Spacecraft Applications
tion from the pad. For example, a northerly launch (0" azimuth) would
send its payload into an orbit passing over the poles continuously (ignor-
ing the component of velocity imparted to our launch vehicle due to the
rotation of the earth). This is the polar orbit described in the previous chap-
ter with a 90" inclination. Note that a southerly launch (1 80" azimuth) also
produces a polar orbit. In fact, we could achieve exactly the same (iner-
tially-fixed) orbit as a northerly launch if we simply waited 12 hours for
the launch site to rotate to the other side of the earth, and launched south!
An easterly launch (90' azimuth) from this launch site would establish an
orbit of 0" inclination-an equatorial orbit-circling the earth over the equa-
tor constantly. An orbit with a component of its velocity (vector) in the same
direction as the (easterly) rotation of the earth is known as aprograde orbit.
Launching to the west (270" azimuth) would also establish an equatorial
orbit, but the velocity would be opposite that of the rotation of the earth. An
orbit with a component of its velocity vector opposite that of the rotation of
the earth is called a retrograde orbit. The inclination of our retrograde equa-
torial orbit would be 180", due to the fact that the angular momentum vector
would point opposite that of the prograde equatorial orbit.
Launching from our equatorial launch pad at a 45" azimuth results in an
orbital inclination of 45". Note that launching at 135" azimuth also pro-
duces a 45" inclination orbit. This is similar to establishing a polar orbit
by launching north, or waiting 12 hours and launching south to enter the
same inertially-fixed orbit.
As you may now deduce, from our equatorial launch site we can enter
any inclination orbit directly by simply launching in a compatible azimuth
direction. You may also have noticed that launch azimuths between 0" and
180" result in prograde orbits with inclinations between 0" and 90".
Launch azimuths between 180" and 360' produce retrograde orbits char-
acterized by inclinations between 90" and 180".

Polar Launch Site. Next consider a launch site located on the North Pole
(90" north latitude). The only direction toward which to launch is south
(180" azimuth). This means that the only orbit that can be entered into
directly from this launch site is a polar orbit of 90" inclination. Notice
how the inclination corresponds to the launch site latitude.

Mid-latitude Launch Site. Now consider a launch site located at mid-lat-
itudes, say 28.5" north. If we launch directly north (or directly south) from
this point we would still establish a polar orbit of 90" inclination (again