A Dynamic Study of an Earth Orbiting Tether Propulsion System

Commercial expansion beyond Earth orbit demands efficient, low cost and regular access to space that is not given by current launch systems. An alternative to rocketry has been proposed in the use of tethers as a method of in-space propulsion. One possible implementation of tether propulsion involves the use of a long, vertically oriented tether orbiting the Earth. A suborbital launch vehicle will deliver a payload to the tether’s lower tip, which will then be carried up its length by an elevator car to the upper tip, where the payload is released on a transfer orbit. The orbiting space elevator represents a reusable second stage of a launch system designed to place payloads in high Earth orbit or trans-lunar trajectories. This study investigates several dynamics problems encountered in an Earth orbiting tether propulsion system. In addition to calculating the structural requirements for the tether to safely bear the payload mass, several analytical estimation methods of the tether’s orbital response to loading have been developed and compared to previous studies. A detailed mathematical simulation of the tether’s orbital stability has been created, accounting for natural perturbations to the tether’s orbit. With the dynamic simulation of the elevator’s orbit, predictions of the total tether mass required to handle a payload with out degrading its orbit have been quantified. The performance required by the suborbital launch vehicle’s operation has also been examined. Minimum propellant trajectories to the elevator’s lower tip are found using a Hamiltonian based trajectory optimization routine. The launch vehicle maneuvering requirements needed for rendezvous with the orbiting elevator have also been explored. The margins of performance needed for a launch vehicle to deliver a payload to the elevator lower tip have been calculated to be roughly equivalent to a single stage to orbit mission profile. The usefulness of the tether within the context of a trans-lunar transportation system has also been investigated. It has been shown that elevator-based transit to the Moon offers significant savings in Δv over a traditional rocket-based transportation scheme