Mission Design for the CubeSat OUFTI-1

CHAPTER 7I x = I x,cube + I x,M + I x,ant = 2.91 · 10−3 Kgm 2I y = I y,cube + I y,M + I y,ant = 2.23 · 10−3 Kgm 2(7.7)I z = I z,cube + I z,M + I z,ant = 2.88 · 10−3 Kgm 27.2 Disturbing torquesThe first step to identify the most appropriate ACS is to quantify the disturbancetorques acting on the satellite. They are affected by spacecraft orientation,mass properties and design symmetry. In a preliminary design phase istherefore impossible to have a precise estimation of these torques because someparameters, as the inertia moments or the center of mass position, aren’t wellknown yet . The only thing we can do is to quantify the maximal disturbancetorque that we expect to have.Once the inertia properties are known, we can proceed with the quantificationof the disturbance torques.We identify four main sources of disturbance:• The gravity gradient: generated by the fact that the mass is not uniformlydistributed and therefore the gravity force vary depending on the position,it forces the axis of minimum inertia moment to align themself with thenadir direction:T gg = 3µ (R × IR) (7.8)r3 where R is the nadir direction and I the inertia matrix. If we pose θ =45 ◦ (angular displacement of the minimum inertia axis from the nadirdirection) and r =apogee radius, we are in the worst case:T GG,max = 3µ2r 3 |I max − I min | sin (2θ) = 1.33 · 10 −9 Nm (7.9)For an inertial spacecraft it is cyclic and it depends mainly from the inertiaproperties of the vehicle and from its altitude.• Solar radiation: generated by the solar pressure whose resultant acts overthe surface on a specific point, not coinciding with the center of mass.T SP = C sc (1 − R) ∑ r sk × ( n T k S ) SA k (7.10)Galli Stefania 66 University of Liège