Effect of unbalanced attitude control burns on STARDUST trajectory design

Abstract

Introduction STARDUST (Discovery IV) is a comet, Wild-2, flyby sample return mission. Sample contamination concerns have resulted in a spacecraft design with an unbalanced thruster configuration that imparts translational forces during all attitude control (ACS) activities. Due to the long duration of the mission (7 years), it is desirable to determine the cumulative nature of these unbalanced ACS forces and their effect on the mission's delta-V (AV) budget. In addition, high precision Earth re-entry and comet delivery requirements require determining the effect of these unbalanced ACS forces on the ability to achieve the required navigation delivery accuracies. This paper describes the STARDUST spacecraft ACS modes, mechanisms, history, and corresponding mathematical models. Integration of these models into a standard Jet Propulsion Laboratory (JPL) trajectory propagator and optimizer allows trajectory design studies to characterize the effect of the ACS perturbations. It is shown that 1) ACS activities impart a cumulative impulse amounting to 16 m/s, 2) the AV budget for the mission could increase by as much as 43 m/s, post-launch, if modeling of ACS activity were not performed, and 3) the ACS perturbation contribution to navigation delivery errors are insignificant compared to expected navigation errors at comet encounter, but must be accounted for during Earth return. * The study described in this paper was performed by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. f Member of Technical Staff, Mission and Systems Architecture Section Copyright©1998 by the American Institute of Aeronautics and Astronautics, Inc. The U.S. Government has a royaltyfree license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. STARDUST is the fourth mission of NASA's Discovery program. Its primary science goal is to collect comet Wild-2 coma dust samples in an aerogel medium and return them to Earth. Bonus science is anticipated in the form of collection of interstellar particles (ISP), images of the comet coma and nucleus, and insitu comet and dust particle analysis and flux monitoring. Figure 1 illustrates the spacecraft trajectory for the first launch opportunity. The STARDUST spacecraft is shown in Figure 2 in its encounter configuration. The aerogel medium, when in use, is deployed above the spacecraft upper deck (+z-axis). The spacecraft is three-axis stabilized using active thruster control. To avoid contamination of the aerogel medium, all of the thrusters are mounted on the lower deck (-z-axis). This thruster configuration imparts an unbalanced force, i.e. translational thrust, to the spacecraft every time attitude control burns are executed. Over the seven-year mission, the cumulative ACS activity is estimated to amount to tens of meters per second. The sum of trajectory correction maneuvers required to compensate for the ACS burns could be intolerably large unless the ACS effects are accounted for in advance while designing the baseline trajectory. Four deterministic Deep Space Maneuvers (DSM) are used to shape the trajectory and 14 statistical maneuvers are planned to navigate the trajectory. The statistical maneuvers support correction of errors in Launch (L) injection, DSM execution, Earth flyby (EGA), Wild-2 flyby (E) and approach to Earth return (R). Table 1 provides a summary of the mission's maneuver profile. The ACS activity is also expected to affect navigation delivery accuracy, which is especially important during two events: comet encounter and Earth return. The bulk of the encounter sequence of activities transpires very rapidly within a few minutes of closest approach to the comet. Inaccuracies in the delivery of the 1 American Institute of Aeronautics and Astronautics Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc.