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.