Thermal
The purpose of the satellite
thermal subsystem is to control and maintain spacecraft component temperatures
within their specified limits throughout all mission phases. In larger
satellites, thermal control is done primarily through the use of active methods, such as heat pipes, thermostatically controlled heaters, and
louvers. Due to their small size, nanosatellites require more of a passive approach,
since the mass and energy budget of these items is significant.
To do an in-depth thermal analysis,
the mechanical design of all subsystems needs to be relatively complete.
However, the decision for the overall design of the UW Dawgstar is
still in progress. The focus of past studies has been on establishing the
groundwork for future, more detailed thermal design. This groundwork has
included the determination of thermal environments, component temperature
limits, average steady state and transient temperatures, significant thermal
control issues, and thermal control hardware.
A complete understanding of the
thermal environments that a spacecraft will
experience is critical for an effective thermal subsystem design. Due to the
nature of the UW Dawgstar project, the environments may change depending on
which launch vehicle is chosen.
In order to determine bulk satellite
temperatures, steady state and transient temperature analyses are performed.
Results of these analyses are shown below in their respective sections.
Nanosatellite thermal control is
difficult due to the restrictions that are placed on the system design.
Given the right circumstances the UW Dawgstar design is simple enough that
thermal control is possible with a minimum amount of hardware and software
applications. As a precursor to the final thermal design, two tasks that are
related to thermal control have been investigated: significant thermal
issues and thermal control hardware.
Two subsystem components that have
unique thermal control needs have been identified: the BEI GYROCHIP, and the
Sanyo CADNICA batteries. The BEI GYROCHIP has an operating
temperature range of –40o to 80oC. The Sanyo CADNICA batteries have an operating
temperature range of 0 to 40oC.
Two types of hardware that are used for
thermal control have been identified and are described in the following
sections.
Sensors. Most components within the UW Dawgstar
require some sort of temperature monitoring. To accomplish this, platinum thin
film resistance temperature detectors (RTDs) were selected because they provide
the most versatility and stability when compared to other temperature sensors.
Radiators. Thermal radiators are used to reject
waste heat from a satellite during times of excess heat loading and operational
inactivity. In order to assist with the surface area allocation on the UW
Dawgstar, an analysis has been completed to obtain an estimate of the radiator
size required to reject a possible 30 W of waste heat. The radiator is sized
following the procedure described by McMordie.
The analysis that has been completed so
far indicates that thermal control of the nanosatellite is relatively simple.
When comparing the predicted bulk transient temperatures to the component
temperature limits, there are only three projected temperature limit
violations. For worse-case-cold, approximately 9oC, 19oC, and 19oC
will exceed the low limits for the nanosatellite electronics, batteries, and
magnetometer respectively. Through the application of blankets, heaters, and
thermal isolators these components are thermally controlled very easily.
Two pieces of thermal control hardware
have been selected for use in the UW Dawgstar: platinum RTD temperature sensors,
and a 275cm2 thermal radiator. Platinum thin film RTDs provides the
capability for accurate temperature measurement with minimal hardware
requirements, and the thermal radiator allows the UW Dawgstar to radiate excess
heat.
The next step in the thermal design of
the nanosatellite is to build a more accurate thermal mathematical model using a
thermal analysis software package. Once the model has been constructed, a more
complete design of the thermal control system may be realized.
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