Page 199 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecraft Applications
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mon or integrated system for orbital maintenance and propulsion and not
have a separate perigee or apogee kick motor. Therefore, the specific
impulse for both the satellite propulsion subsystem and the orbital mainte-
nance subsystem have the same value, and the systems share some common
tankage and plumbing. Thruster placement, design, and specific fuel mass
may be unique for either orbital maintenance (station-keeping) or orbital
translation (propulsion). Since many spacecraft combine these functions, a
more detailed description of a composite system is explained in this context.

Propulsion and Orbital Maintenance As a Combined Subsystem

In most cases, onboard propulsion systems must be included to allow the
spacecraft to modify its orbit (if necessary), maintain the orbit despite the
perturbing forces it may encounter (orbital maintenance), and sometimes, as
an adjunct, to control the desired attitude of the spacecraft. To accomplish
this, the spacecraft may have a relatively large rocket for orbit modification,
smaller rockets for station-keeping, and tiny thrusters for attitude control.
Associated with each of these are fuel storage, delivery, and control devices.
The propulsion and orbital maintenance subsystems are listed separate-
ly based on a design heritage that recognized that many traditional, mod-
ular satellite designs had completely separate propulsion and orbital main-
tenance systems. For example, some designs used solid-fueled propulsion
systems to translate from the parking orbit (achieved by the launch vehi-
cle) to the final orbit and a separate liquid-fueled orbital maintenance sys-
tem for station-keeping during the life of the satellite. However, many
contemporary designs combine the propulsion and orbital maintenance
systems using a common fuel and storage system and specific and sepa-
rate propulsion and orbital maintenance thruster designs that are unique to
the function of orbital translation and orbital maintenance, respectively.
The fuel mass is specific for the respective propulsion and orbital mainte-
nance categories where they accrue, and will be explained in that way in
the following paragraphs.
The propulsion and orbital maintenance calculations are both based on
the Av or change in velocity (vector) required to perform either an orbit
translation or orbital maintenance. Both calculations are based on the equa-
tions associated with orbital mechanics that determine the Av required for
making a change from one orbit to another. In the case of the propulsion
subsystem, the change is associated with going from the parking orbit to the
final orbit. In the case of the orbital maintenance system, the designer cal-