In order to ensure that the components of the University of Alberta Ex-Alta 1 cube satellite operate within safe temperature ranges, a series of increasingly detailed thermal models were developed to calculate the transient thermal behaviour of the satellite. The first approach was a lumped capacity analysis that treated the satellite as an effective homogeneous mass with a uniform temperature. This analysis offered an estimate of the effective temperature ranges and the response time of the satellite, allowing for an assessment about the need for active thermal management early on in the design process. The thermal model was then approximated in significant detail by using thermal circuit analysis. This approach offers significant insight into the temperature profile within the satellite and allows for analysis of a number of relevant thermal scenarios and possible design implementations. While the ultimate model is based on a detailed, realistic Finite Element Analysis (FEA), the computational cost of this approach only makes it feasible for relatively short transients. The thermal circuit approach offers the best balance between level of detail and simulation cost.
For the thermal circuit model, all components that are significant in generating thermal power and producing thermal resistance and capacitance for the Ex-Alta 1 cube satellite were represented using equivalent circuits and sub-circuits. These circuits were implemented in the circuit simulation program, LTSpice, to take advantage of its convenient graphical interface and extensive solver capabilities. In LTSpice, heat sources are represented as current sources, thermally resistive pathways as resistors, and thermal capacities are represented by capacitors. By analogy, the voltage measurement at a node in the circuit denotes the temperature of the corresponding component in degrees Kelvin. Individual components such as PCBs, solar panels, washers and spacers are modelled as sub-circuits and instantiated into the main circuit. The LTSpice model also accounts for the thermal power absorbed through the six faces of the cube satellite throughout an orbit, which allows for accurate transient analysis in a variety of relevant orbits.
The end goal of the thermal model is to obtain estimates of temperature ranges experienced by different satellite components (e.g. solar cells, battery, electronic components) and compare the results to design specifications. After supporting the design phase by ensuring that components are within an acceptable temperature range during expected scenarios, the model will also be used in support of operations, to assess the impact that thermal conditions have on satellite operations.
