Mission Design for the CubeSat OUFTI-1

CHAPTER8POWER SYSTEMThe electrical power system provides, stores, distributes and controls spacecraftelectrical power.In this chapter, we will take care only of the power source: the hardware forpower control and distribution won’t be part of this work.The power source on an earth orbiting satellite is usually the sun power: throughsolar arrays we can in fact collect the sun rays and transform their energy intoelectrical power. As the performances of solar cells are subjected to degradationalong the mission, we speak about Beginning Of Life (BOL) and End of Life(EOL). As the radiation environment over the foreseen orbit is hard, the solarcells will be affected by an important degradation of their efficiency: all theanalysis will be carry out with the EOL parameters.Usually the first step is to identify the power needed in order to adapt the solararrays surface to the requirements. In the case of a CubeSat, the problem isdifferent as the surface if fixed: even if deployable orientable solar arrays areavailable, the constraints of mass and volume often hold the design back fromthese heavy and risky elements. Furthermore, OUFTI-1 need power only forthe communication system and for the on-board computer. We will thereforeproceed in the identification of the available power and then we will size theD-STAR system in order to work with it.Two scenarios are still open, depending on the final design of the communicationsystem: the payload can be on all the time or it can be switch off when it’s notused, for instance over the oceans. If the former option is the safer as the systemis never turn off and there isn’t any risk of problems in turning it on, the latterwould allow an important power saving. As we follow the KISS principle asfar as it’s possible, we would prefer to leave the payload active all the time in73