Page 189 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecraft Applications
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nology which must be continually adjusted to reflect the latest available
components. The mass of the actuators that provide control torques to
counterbalance disturbances or perform maneuvers are a function of the
on-orbit mass of the satellite and include the following components:

thrusters
reaction and momentum wheels
magnetic torquers
The ARC design begins with a default/minimum value for the number of
components needed as a function of the stabilization method, as illustrated in
Table 8-2. There are two exceptions: Star sensors are only used in the default
result if the pointing accuracy requirement is c 0.1 degree, and magnetome-
ters are used only if the spacecraft operating altitude is < 2,000 km.
The designer can make decisions based on special knowledge about the
application. For example, the designer may add star mappers to a design
that does not require them for pointing to provide, after the fact, pointing
knowledge that improves data processing and geophysical application.
Another option is to increase or decrease the number of components.
Increasing the number of components would build in more redundancy,
perhaps extending lifetime.
The selection of an ARC design architecture impacts the power and
thermal subsystems. The most prevalent power subsystems make use of
solar cell power generation, and the ARC subsystem determines how the
solar cells view the sun. Spin-stabilized or gravity gradient ARC designs
do not continuously point the entire solar array at the sun as do the 3-axis
stabilized ARC designs. This is taken into account in the power system
calculation and determines the number of solar cells, of a particular type,
that must be used. Correspondingly, the thermal environment of a 3-axis
stabilized ARC design is more demanding than that created by spin-stabi-
lized ARC designs, and this will influence the thermal control system
mass. As indicated, a spin-stabilized satellite acts like a rotisserie, provid-
ing even exposure to hot and cold conditions. A 3-axis stabilized satellite
is subject to extreme thermal gradients because particular elements are
exposed to hot or cold conditions for extended periods of time.

Power Subsystem
These are the components which provide the power used by the space-
craft to support itself and perform its mission. Additionally, the hardware