Attitude control of prolate satellites in transfer orbit

The stabilization is considered of spinning prolate satellites during a geosynchronous transfer orbit and in particular for satellites carrying a liquid fuel apogee boost motor. Two fundamentally different approaches were taken. The first concept is one which employs spin rates of less than 1 rpm with the aim of reducing the magnitude of the well known dynamic instability of a prolate body in the presence of internal energy dissipation. The second concept is the use of a conventional active nutation control system on such a satellite, the aim being to assess the performance and limits of acceptability of such a control system. Analysis of all phases of the transfer orbit, from separation of the launch vehicle to geosynchronous injection, was undertaken in a parametric manner, the magnitude of energy dissipation within the satellite being the main variable. Results indicate that in all cases the apogee motor firing is the critical phase of the transfer orbit since for a liquid fuel apogee motor the jet damping is very low whilst parasitic torque due to thrust vector misalignment can cause large injection errors at low satellite spin rates. As a result of this it is concluded that a spin rate of at least 5 rpm should be adopted and that a nutation control system should be employed using thrusters for nutation control and a gyroscope to sense the nutational motions. This system must also be in operation during the apogee motor firing.