Neil E. Goodzeit

High Efficiency Flight Control System for GEO Spacecraft Hall Current Thruster Orbit Transfer

To increase mass to orbit performance, a flight control system has been developed that provides orbit transfer capability using Hall Current Thrusters (HCTs). This system, activated following a standard high-thrust orbit transfer phase, facilitates robust and reliable HCT firing for a period of several months to complete the transfer to the mission GEO orbit. During the HCT firing the spacecraft autonomously tracks a commanded reference frame and controls the solar arrays to remain sun pointed. The reference frame is generated based on the results of ground-based numerical optimization that solves for the minimum-time orbit transfer thrust trajectory. During the orbit transfer the spacecraft inertial reference is maintained by propagating the three-axis gyro rate outputs of a precision inertial reference unit, with periodic attitude updates using earth and sun sensors. For fuel efficiency, reaction wheels (RWAs) are used for attitude control and gimbaled HCTs provide RWA momentum adjust. High-thrust hydrazine thrusters are used for rapid attitude re-orientations, attitude-update slews, and contingency operations. The paper provides an overview of the HCT orbit transfer concept of operations, and the attitude determination and attitude control strategies. Illustrative simulation results are presented for a Lockheed Martin A2100 spacecraft that will perform a partial orbit transfer using HCTs.

Predictive Momentum Control for High Fuel Efficiency GEO Spacecraft Stationkeeping

To reduce stationkeeping propellant and extend orbital maneuver life, a predictive momentum adjust system has been developed for the Lockheed Martin A2100 spacecraft. This system is designed for maneuvers where high-efficiency Arcjets (AJTs) fire continuously for Delta-V, Reaction Wheel Assemblies (RWAs) are used for attitude control, and hydrazine Rocket Engine Assemblies (REAs) are pulsed for momentum control. The predictive momentum control strategy improves fuel efficiency by optimizing the use of the REAs, which have a much lower specific impulse than the AJTs. To reduce the REA firing impulse, sequences of contiguous REA pulses, or pulse bursts, are executed to drive the RWA momentum error at the end of the maneuver to zero. The final maneuver momentum error is calculated based on an estimate of the AJT disturbance impulse as well as the commanded and measured RWA momentum. By using prediction, the system eliminates REA firing in response to that portion of the momentum error that would naturally be corrected by the AJT disturbance torque alone. Innovative control logic maximizes the REA duty cycles to reduce the total number of pulses and increase the specific impulse of the firings. The system executes pulse bursts interspersed with intervals of quiescent RWA control in a way that adjusts the RWA momentum and simultaneously maintains the RWA speeds within their allowable limits. The recursive implementation provides feedback to ensure the target momentum is achieved in the presence of REA torque uncertainties and the time-varying AJT disturbance torque. The paper provides an overview of the control system logic, and includes numerical simulation results and in-orbit A2100 spacecraft flight data that illustrate the benefits of the new approach.