Loss of rotation of a rotating two-body tethered spacecraft: Analytical and experimental techniques

An ionospheric space science mission called BOLAS (Bi-static Observations with Low-Altitude Satellites) is currently under development. The BOLAS spacecraft comprises two tethered sub-satellites that rotate end-over-end in the orbit plane to maintain a nominal 100 metre separation of the sub-satellites. Mission success depends on maintaining a sufficient in-orbit-plane rotation rate. Gravity gradient forces excite longitudinal cyclical stretching of the tether. The rotation rate is potentially reduced by energy dissipation from material damping of the longitudinal stretching motion and by aerodynamic forces acting on the rotating configuration.; This thesis first develops non-linear equations of orbital motion. Simulation results of the equations show the dependence of rotation rate on tether material stiffness and damping. Also, the thermal environment is modelled to establish the range of temperatures of the tether in orbit. Large-scale experimental tests designed to emulate the in-orbit dynamic state are done in which the tether (SPECIRA-1000RTM) is suspended horizontally and attached to a pendulum supported sub-satellite mass. The stiffness and structural damping properties are deduced from a model of the test configuration that accounts for tether tension variation due to gravitational cable sag effects. Small-scale laboratory tests of tether samples at cryogenic temperatures are also conducted to establish the temperature dependence of tether material properties. Tether stiffness and damping properties at orbital temperatures are then estimated from the experimental data from both tests and applied to the equations of orbital motion. The effect of aerodynamic forces on BOLAS rotation rate is modelled. The loss of rotation from both gravity gradient and aerodynamic sources is combined to estimate a total BOLAS loss of rotation.