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Figure 8.27
Overview of passive and active thermal control techniques, which rely on the conduc-
tion and radiation of heat.
thermal design: passive thermal control and active thermal control. Passive thermal
control means that the desired temperature range is maintained using appropriate
thermal surface coatings (paint or deposited material such as aluminum or gold),
blankets, and mirrors. Those techniques are supported by simple conduction and
radiation.
Active thermal control depends on thermostatically controlled electrical heaters
or mechanical devices of one sort or another that can enhance or directly alter the
thermal properties of the spacecraft. The primary example is the heat pipe, which
uses a gas-liquid thermal cycle to move heat from a warm place (the interior) to
a cooler place (exterior of the spacecraft). High-power GEO three-axis spacecraft
in the 8-kW and above class rely on heat pipes embedded in the equipment-
mounting surfaces. The absorptance and emissivity of the outer surfaces of the
north and south panels of three-axis spacecraft are enhanced through the use of
optical solar reflectors, which are specially designed quartz mirrors. The east,
west, nadir (Earth-facing), and zenith surfaces are usually covered with multilayer
insulation blankets. Satellites designed for GEO service with more than 10 kW are
designed as closed cages of heat pipes.
Reviewing the elements of a passive design in Figure 8.27, the heat producing
components (TWTs and other electronics) typically are attached to a heat-conduct-
ing surface such as an aluminum honeycomb equipment shelf. The primary source
of external heat is the sun (shining on the spacecraft from the right), which must
be precluded from raising the temperature in the compartment above acceptable
limits. In addition, the heat generated in the compartment by the electronics must
be transmitted to space, or it, too, will cause the internal temperature to rise. Both
those necessities can be accomplished with a quartz mirror radiator located on the
side facing the sun. The mirror acts like a filter, reflecting visible and ultraviolet
radiation away and transmitting infrared radiation from inside to outer space.
Mirrors have to be placed carefully on the spacecraft because any contamination
or clouding of the surface during the life of the satellite reduces effectiveness.
288
Spacecraft Mission and Bus Subsystems
Because of unavoidable degradation of the mirror surface, the temperature in the
spacecraft gradually rises on a yearly basis, which must be taken into account in
the overall design of the thermal control system and other components.
If the spacecraft contains an apogee kick motor (AKM), it is necessary to
provide considerable thermal control measures for it. As shown at the left of Figure
8.27, the AKM is surrounded by a thermal blanket. Electrical heater wires can be
placed under the blanket if needed. The purpose of these measures is to keep the
motor and propellant temperatures above a prescribed minimum prior to firing.
To protect the rest of the spacecraft from the heat of the motor firing, an insulating
wall and thermal barriers are placed between the motor and the sensitive compo-
nents.
A similar design consideration exists for the batteries, which need to be kept
cooler and within a tighter temperature range than other equipment. On GEO
satellites, the prevailing design is to give the batteries their own isolated thermal
environment, usually at the aft end of the spacecraft.
In the thermally controlled compartment, the electronics receive and generate
heat during their normal operation. It is the purpose of the thermal control subsys-
tem to achieve thermal balance in the desired temperature range during seasonal
variations and when units are individually turned off and on. The TWT shown in
Figure 8.27 produces localized heat, which can be distributed more evenly by
metallic heat sinking using thermal doublers (that technique, unfortunately, adds
considerable weight to the spacecraft). The receiver mounted below it, therefore,
is heated by the TWT, aiding in its temperature control. Thermal conduction is
improved by installing heat pipes under the hot portion of the TWTA. If and when
the TWT is not operating, the heat necessary to warm the receiver is provided by a
replacement heater located in proximity to the units. Replacement heaters, operated
either automatically or by ground command, are necessary to maintain an overall
‘‘bulk’’ temperature for the spacecraft.
High-power DTH satellites can be designed to enhance heat rejection by
employing TWTs with direct radiating collectors. The collector end is exposed to
space by mounting of the TWT on an externally facing panel of the spacecraft. As
mentioned in Chapter 6, the collector develops about half the total dissipation of
the TWT, so direct radiation is beneficial in terms of reducing thermal control
system complexity and weight.
This discussion of the primary aspects of thermal design is intended only as
an introduction. An important aspect to such study is the creation of analytical
thermal models for use in predicting temperatures in the spacecraft at various
critical points and times in the mission. The models must be verified by ground
testing in a simulated space environment. Once in orbit, it is difficult to compensate
for errors in analysis or design, although it happens all too frequently. That is why
it is important to provide a thermal design with adequate margin and to use
telemetry to measure the temperature of critical elements after the satellite is
operating in orbit. Satellite operators need to keep a close eye on temperature
trends through alert monitoring and careful recordkeeping. The data should be
fed back to the spacecraft designer to help improve future models.
8.3
Spacecraft Bus Subsystems
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