ORBITAL LIFETIME OF TETHERED SATELLITES

Two mathematical models developed to study the orbital lifetime charac teristics of uncontrolled tethered systems are described. Systems of i nterest are free tethers, satellites trailing a tether, and tethered p airs (i.e. two subsatellites tethered together). The first model, a '' lumped-mass'' or ''multibody'' model, includes an extensible, non-cond uctive tether, and is used as a ''truth'' model. Derived Newtonian equ ations of motion are integrated numerically. The second model, utilizi ng a more efficient, dynamically simplified approach, is an orbital el ement propagation technique. In both models, the tethered system orbit s an oblate Earth, and transits an oblate, rotating, temporally and gl obally averaged reference atmosphere. End-body subsatellites are model ed as spheres, and tether segments are modeled as right circular cylin ders. Earth gravitational forces and aerodynamic drag, calculated usin g drag coefficients which vary as a function of Knudsen number and bod y shape, are assumed to be the only external forces acting on the syst em. The effects on calculated orbital lifetimes of Earth and atmospher e shape and state, aerodynamic drag coefficients, and tether diameter and length are demonstrated. Errors resulting from using single-mass o rbital lifetime prediction techniques with tethered systems are discus sed. The effects of initial orbit inclination and argument of latitude on calculated lifetimes are summarized. A graphical technique for pre dicting the orbital lifetimes of free tethers is described.