Mission Design for the CubeSat OUFTI-1

CHAPTER 9.THERMAL-CONTROL SYSTEM9.3.3 Hot and cold caseWe are dealing with a simplified model: we are not expecting to have a detailedthermal description of our CubeSat. The goal of this preliminary study is toidentify the maximum and minimum temperatures reached during lifetime inorder to avoid the overpass of the imposed limits.We identify therefore three possible cases:• the hot case: the satellite is in sunlight and the solar arrays do not produceany power but only cumulate solar heat flux. The solar flux is injected ona face and the absorption coefficient is α 0 as the efficiency is null.• the operating case: the satellite is in sunlight and the solar arrays areproviding the necessary power. The payload is on and we need to radiate0.5 W corresponding to the losses in the communication system convertedinto heat. In this case, the absorption factor of solar cells needs to beupdated as a function of the solar cell temperature• the cold case: the satellite is in eclipse and the payload is off. In this casethe solution seems to be trivial: the equilibrium temperature is practicallythe T cold but it doesn’t respect the reality. Furthermore in this case thehypothesis of steady-state cannot be applied integrally. We added thereforea flux of 847W/m2, which is the solar flux weighed on the averagetime of eclipse.For the operating case, we needed to add a worksheet to Thermal-Excel inorder to update the solar cells properties as a function their temperatures.9.4 Thermal results for OUFTI-1Once decided the kind of model to study and identified the material properties,we passes to the implementation into the software Thermal-Excel.As above mentioned, a starting guess temperature is demanded and for eachcase we calculated it as explained in section 9.2 respecting the characteristicsof each case.At the beginning, we had some problems as the final results depended on thestarting temperature, which is impossible for the static case. With an accuratereflection we find out the reason: the standard algorithm used by the softwarehas a really slow convergence when the number of radiative exchange is importantrespect to the conductive. As the criterion of convergence is the differencebetween successive temperatures, the software thought to have converged evenif it was not true. We needed therefore to strongly reduce the convergence criterionin order to have the good solution.Galli Stefania 93 University of Liège