Semi-analytical orbital parameters description for thermal fatigue analysis

Thermal management is the most significant topic for a thermo-mechanical satellite design. If the lift-off phase represents the major contribution to the mechanical solicitation field about the spacecraft structure, the thermal management topic is the all life time long enforcement. The temperature influences directly the performances of the satellite since the EEE electrical characteristics strongly depend on the temperature. On account of this the thermal ageing will be considered when the assemblies and sub-assemblies are performed. The temperature has a double effect on electronic devices; the former one is about the electrical characteristics and performances degradation and the latter one is relevant to the mounting sub-techniques (glueing, soldering, etc...) used to join each EEE (Electrical, Electronic and Electromechanical) component on the PCBs or MMICs. The paper focuses on this second topic by concentrating the attention on the fatigue cycles calculation strategy. When a thermal fatigue analysis is done all the life phases are considered (screening, tuning, testing, on-orbit life, operational mode) in order to have a complete description of the equipment ageing. Fatigue cycles are computed basing on rough assumptions that normally lead to positive margins of safety. Other cases need to be analysed more accurately by describing models and paying particular attention to the on-orbit cycles description since the operating life represents the 60-70% of the entire fatigue ageing. A particular investigation will be dedicated to the description of the thermal condition during the orbital period commonly described with a trapezoidal function that represents a drastic overestimation of the thermal condition. A semi-analytical shape function will be proposed in order to improve the description of the real thermal fatigue parameters. The proposed improvements could represent a step forward for more challenging solutions about the equipment thermal design and thermal management. In fact by doing a more representative description of the effective fatigue life it is possible to reduce safety factors and to enhance the reliability of the components obtaining also a more performing design.