Page 196 - Intro to Space Sciences Spacecraft Applications

Spacecraft Systems
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RegulatordConverters Mass (kg) = 0.025 x (Day Power) x 2 (8-6)

The mass of the wiring is based on a percentage of the overall space-
craft mass.

Wiring Mass (kg) = 0.04 x (Estimated S/C Dry Mass) (8-7)

These values define the output of the power subsystem which consti-
tutes the largest mass subsystem and one of the more complex elements in
the satellite design.

Thermal Subsystem

The thermal control system keeps the temperature of the spacecraft
within specified ranges. Too much built-up temperature may affect elec-
tronic systems, causing failures, and low temperatures may freeze up
movable systems or cause fuel lines to freeze and burst. The control sys-
tem must be able to react throughout the expected environmental and
operational configurations experienced. Low-earth orbiting spacecraft
continuously cycle in and out of the sun’s radiations, absorbing and radi-
ating energy in phases. Geostationary satellites may be exposed to (or
eclipsed from) solar radiation continuously for weeks and months, and
sun-synchronous satellites may be continuously exposed. Internally, a
spacecraft may generate large quantities of heat during times of peak
operation, and little during periods of dormancy.
Thermal control devices fall into two categories: passive and active.
Passive devices simply shield, insulate, or change their thermal character-
istics depending on the existing temperature of the satellite. The external
coating of a spacecraft determines the craft’s absorptivity (how much
external energy is absorbed) and emissivity (how much internal thermal
energy is radiated into space) characteristics to control temperature. The
space shuttle rotates in a maneuver called rotisserie to alternately expose
the black underside and the silver-coated open cargo bay doors for the
purpose of thermal control. Many spacecraft are wrapped in thermal blan-
kets to retain internal heat, and some spacecraft are equipped with pas-
sively heat-activated louvers which open or close to expose different
external surfaces to radiate or retain internal heat.
Active devices usually involve some sort of working fluid to carry heat
from one location within a spacecraft to another to either increase or