Investigation of the effect of dynamic unbalance on the attitude maneuver of spinning spacecrafts

Spacecraft unbalancing could materialize during the spacecraft's mission, because of its change of mass properties. In a rigid unbalanced spacecraft without nutation control (torque-free), the nutation angle resulting from the unbalance has fluctuating value, and the behavior of the spin axis and the angular momentum of the spacecraft in the inertial space vary depending on the body nutation frequency or the inertial nutationfrequency. But, in an unbalanced spacecraft with active or passive control of nutation, the nutation angle arising from the unbalance doesn't have the fluctuating value of the rigid state without control, and it reaches the average value. This constant nutation, or wobble is never eliminated by any active or passive damping method and estimation of this angle has been described. This angle doesn't depend on the spacecraft's spin rate, but only depends on its degree of unbalance. Also, based on the performed simulations of the unbalanced spacecraft, and assuming the product of inertia and the spin rate to be constant, the higher is the ratio of the moment of inertia of the longitudinal axis to that of the transverse axis, i.e. the higher is the gyroscopic coefficient (in other words, when the spacecraft is oblate), the higher will be its stability against disturbance, and the lower will be its sensitivity to the product of inertia or the state of unbalance. While, in the balanced state in order to sustain the spin stabilization, it is only sufficient that the value of this coefficient be greater than one. Also it is proved that the moment of inertia of the plane normal to the spin axis has little effect on the amount of nutation and wobble that results from dynamic unbalance; and this is for the case where the spin rate of the transverse axis is low relative to that of the longitudinal axis and in the state of impulse torques because of transfer axis's spin rates this in not common.

Keywords:

Unbalanced dynamics , Spin stabilization , Nutation , Wobble , Unbalanced spacecraft