Page 84 - Intro to Space Sciences Spacecraft Applications

Propulsion

Figure 3-9. Geostationary orbital transfer. The plane-change maneuver is 71
performed out at the apogee of the transfer orbit to minimize the Av required.

Geostationary Transfer. Figure 3-9 illustrates the transfer of a satellite
from a typical low-earth "parking" orbit of 28.5" inclination to a geosta-
tionary orbit.
The first maneuver establishes the spacecraft on a transfer orbit which is
an ellipse that lies in the same plane as the parking orbit with its perigee at
the same altitude as the parking orbit and its apogee at geostationary altitude.
This should be recognized as a Hohmann transfer orbit described in Chapter
2. Note that in order to intersect the final desired equatorial geostationary
orbit, the apogee (and, thus, the perigee) of the 28.5" inclined transfer orbit
must be located over the equator. As the satellite approaches this point over
the equator, a perigee kick motor (PKM) is fired to inject the spacecraft into
the transfer orbit from the parking orbit. The change in velocity required at
this point can be found from the relationships given in Chapter 2.
At the transfer orbit apogee, two changes to the orbit must occur in
order to establish a geostationary orbit. First, the elliptical transfer orbit
must be circularized at the geostationary altitude. If this is done as a sep-
arate maneuver within the same plane as the transfer orbit, the Av required
would be the same as for a Hohmann transfer. However, to be geostation-
ary, the orbit must also be equatorial, so the plane of our spacecraft orbit
must be changed from 28.5" to 0". The Av relationship for a simple plane,
given earlier in Chapter 2, is repeated here to illustrate an important point:
Ai
"simple plane change = 2v sin - (2 - 22)
2
This relationship reminds us that plane changes performed at low
orbital velocities minimize the required Av. That is why, in geostationary