Avoiding On-Orbit Control/Structure Interaction Problems with Space Station Freedom 922047

Space Station Freedom may be characterized as a large, flexible, free-floating structure with high modal density and low natural frequencies. These structural features allow for control/structure interaction problems to arise that may influence the SSF design or on-orbit operations. One such control/structure interaction problem is the effect of the Reaction Control System thrusters acting on the central truss structure. The central truss structure is the skeleton of SSF and supports all other hardware items on-orbit. The RCS thrusters are used for station reboost and attitude control maneuvers. Both the RCS and the central truss structure are the responsibility of the McDonnell Douglas Space Systems Company under contract to NASA through the Johnson Space Center.

The Permanently Manned Configuration of SSF may easily be excited to resonance by on-off pulsing of the RCS thrusters. It is the purpose of this paper to examine the control/structure interaction problems that exist when the SSF RCS is used and to establish control system constraints to avoid interaction with the structure and catastrophic structural failure. Control system constraints are established by restricting the frequency of the on-off thruster pulsings so that the excitation of the RCS thrusters will not create excessive loads in the SSF structure. A finite element model of the SSF PMC is used to analytically determine the modal content of the structure, those modes that may be excited by thruster firing events, and the necessary thruster pulsing frequency limitations needed to avoid excessive structural loads.