Shear dampers for deflections of microinch magnitudes

Abstract

Planetary spacecraft used for outer planet missions within our solar system, like the Mariner Jupiter/Saturn '77 (MJS77) Spacecraft, characteristically utilize either extremely long boom appendages to support science instruments sensitive to close spacecraft proximity or shorter booms that support heavy loads such as Radioisotope Thermaelectric Generators (RTG's). Common to both configurations are the small magnitude inertia effects (perturbances) that extended length or heavy boom weight might have on resulting spacecraft settling times important to clear TV picture generation by the video subsystem. For the MJS77 Spacecraft the rotation of the TV camera into a given position generates long settling times caused by the undampened perturbances of the 13-m magnetometer and heavy RTG booms. A series of modal analyses provided the necessary frequency and damping constraints for each boom to obtain short settling times. To ensure clear TV pictures 4% damping was required at very small displacements (ranging from 0.001 to 0.00005-in. deflections) and at low frequencies (1.8–0.2 Hz). The solution incorporated elastomeric shear dampers which had high loss tangent values in the frequency and temperature range expected at the base of each boom. The shear dampers were sized to ensure adequate strain and provide the desired boom stiffnesses. Presented are the shear damper design, results of full scale horizontal boom tests, and data on the effects of damper sizing, operating temperature, life cycle (fatigue) on boom frequency and damping. Instrumentation and method of boom exitation used for the full scale tests are also presented.