Parking Orbit - Rationale For Parking Orbits

Rationale For Parking Orbits

There are several reasons why a parking orbit may be used:

• It can increase the launch window. For earth-escape missions, these are often quite short (seconds to minutes) if no parking orbit is used. With a parking orbit, these can often be increased up to several hours.
• For non-LEO missions, the desired location for the final burn may not be in a convenient spot. In particular, for earth-escape missions that want good northern coverage of the trajectory, the correct place for the final burn is often in the southern hemisphere.
• For geostationary orbit missions, the correct spot for the final (or next to final) firing is normally on the equator. In such a case, the rocket is launched, coasts in a parking orbit until it is over the equator, then fires again into a geostationary transfer orbit.
• For manned lunar missions, a parking orbit allowed some checkout while still close to home, before committing to the lunar trip.
• It is needed if the desired orbit has a high perigee. In this case the booster launches into an elliptical parking orbit, then coasts until a higher point in the orbit, then fires again to raise the perigee. See Hohmann transfer orbit. In this case the use of a parking orbit can also reduce the fuel usage of an inclination change, since these take less delta-V at high elevations.

The figure shows the first two reasons. For this lunar mission, the desired location for the final firing is originally over southern Africa. As the day progresses, this point stays essentially fixed while the Earth moves underneath, and this is compensated for by changing the launch angle.