XMM-Newton’s operations preparation for the 4 wheel drive project

The Attitude and Orbit Control System of the 3 axis stabilised spacecraft (S/C) XMMNewton was designed to use three reaction wheels (RW) with a fourth wheel in cold redundancy. The 3 wheel configuration has been used for more than 13 years for the momentum management, to re-orientate the S/C with slews in order to achieve the required scientific pointing. With 3 wheels, frequent reaction wheel unloading is used both to compensate the external torque and to re-orientate the angular momentum to allow the slew execution, which results in fuel consumption of around 6 Kg per year. A fourth wheel in control gives the advantage of a higher momentum envelope and introduces the null space in the wheels matrix, allowing a wheel speed variation without changing the total angular momentum. With four wheels the reaction wheel bias will compensate only for the accumulation of angular momentum due to the external torques, resulting in saving a significant amount of fuel and extending the mission lifetime. The use of four reaction wheels implied the development of new attitude control algorithms and a software change on board. A design phase of this change was successfully completed and the project went through the implementation