Page 61 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecraft Applications
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ly with the conditions for the desired orbit at that point, then the desired
orbit is achieved. If these conditions are not met, some kind of orbital
maneuver may be required.

Orbital Maneuvers

The basic purpose of the launch system is to carry a payload from the
surface of the earth into space. However, in most cases, the launch system
may not place the payload exactly into the desired orbit or may require
more than just the launcher stages to achieve the desired orbit. Also, after
establishing an orbit, adjustments may be required so that the orbit will
conform with some desired orbital characteristic, such as period or eccen-
tricity, to modify the present orbit to a completely new orbit, or merely to
compensate for perturbing forces to maintain the desired orbit.
Once again, as with orbit establishment, it is simply a matter of ensur-
ing that the spacecraft has the proper velocity (both magnitude and direc-
tion) at the proper orbital radius at the proper time. This is usually accom-
plished by adjusting the spacecraft’s orbital velocity magnitude or
direction until the prescribed conditions are met. This is where the term
delta v (Av) comes from when discussing orbital maneuvers.
Orbit adjustments generally fall into two types: in-plane velocity
adjustments and plane changes. Addressing all possible combinations of
maneuvers between different types of orbits is beyond the intent of this
book; however, some basic maneuvers shall be presented to illustrate the
general process.

Single-Impulse Maneuvers. Most orbital maneuvers can be considered
impulsive (instantaneous) because the actual propulsion system “burn” is
relatively short when compared to the typical orbital period. This allows
us to assume that the change in velocity during an orbital maneuver occurs
at a single point. This also tells us that the initial orbit and final orbit share
at least the point at which the maneuver is performed. If the initial and
desired orbits share at least one point, then a single-impulse maneuver is
all that is required to change from one orbit to the other. The Av required,
which may be an in-plane adjustment, a plane change, or both, can easily
be computed using the velocities (magnitude and direction) of each orbit
at the common point.
If the initial orbit and the desired orbit do not intersect, it will not be
possible to transfer between orbits with a single maneuver. Instead, an