ATTITUDE AND TETHER VIBRATION CONTROL IN SPINNING TETHERED TRIANGLES FOR ORBITING INTERFEROMETRY

The dynamics and control problems associated with an Earth orbiting system consisting of three identical tethered masses at the corners of an equilateral triangle spinning around an arbitrary axis is studied. The objective is to demonstrate the feasibility of controlling an optical astronomical interferometer consisting of three telescopes at the corners and one at the center with fiber optics in the tethers to bring the radiation to a combining station. The control problems are divided into two parts. First, the system is analyzed as a rigid spinning triangle with gravity gradient disturbance torques. A control strategy is derived which will maintain the orientation of the triangle and/or slew the spin axis between arbitrary directions on the celestial sphere, using continuous thrusting at the corners provided by ion thrusters. Second, the accuracy of the rigid body assumption is estimated by determining the disturbing forces acting on the tethers (gravity gradient, inertial, and solar radiation pressure) and the resulting vibrations. A method of damping these vibrations is suggested and preliminary estimates of its effectiveness are carried out by computer modeling. I conclude that multi-year missions of this type are possible with available ion thrusters, but more work is needed on the tether damping.