Restoring Redundancy to the Wilkinson Microwave Anisotropy Probe Propulsion System

The Wilkinson Microwave Anisotropy Probe is a follow-on to the differential microwave radiometer instrument on the Cosmic Background Explorer. Attitude control system engineers discovered 16 months before launch that configuration changes after the critical design review had resulted in a significant migration of the spacecraft’s center of mass. As a result, the spacecraft no longer had a viable backup control mode in the event of a failure of the negative pitch-axis thruster. A tiger team was formed and identified potential solutions to this problem, such as adding thruster-plume shields to redirect thruster torque, adding or removing mass from the spacecraft, adding an additional thruster, moving thrusters, bending thruster nozzles or propellant tubing, or accepting the loss of redundancy. The project considered the impacts on mass, cost, fuel budget, and schedule for each solution and it was decided to bend the propellant tubing of the two roll-control thrusters to allow the pair to be used for backup control in the negative pitch axis. The problem and the potential solutions are discussed, and the hardware and software changes and performed verification are documented. Flight data are presented to show the on-orbit performance of the propulsion system and lessons learned are described.