Attitude control
The main propulsion system of the Transfer Orbit Stage was an Orbus 21 solid rocket motor. Attitude control was provided by hydrazine thrusters. The inertial guidance system used a ring laser gyroscope produced by Honeywell. Its control system employed a digital optimal, time-scheduled control algorithm to provide stability whilst the Orbus 21 was burning, while a phase-plane controller was used to manage its reaction-control system.