Propulsion and power from a rotating electrodynamic tether at Jupiter

Abstract

The Jovian system with its rapid planetary rotation and strong magnetic field presents exciting opportunities for the use of electrodynamic (ED) tethers. Previous analysis for a radial, 1Okm length tether demonstrated the possibility of propulsive forces as large as 50 Newtons near the planet. For orbital positions beyond about 2.5 Jovian radii, an ED tether can simultaneously be used for power and increases in the orbital altitude. Although the previous study demonstrated the physical feasibility of ED tether use at Jupiter, it did not address the issues of limited gravity gradient force for tether orientation and the practical use of this technology in an engineered system. This presentation will discuss these issues and current progress in an ongoing science/engineering transitional study. Introduction Due to cost and launch vehicle constraints, future exploration of the Jovian system will require efficient sources of power and propulsion to minimize spacecraft mass. ED tethers offer the potential of reducing propellant and power system mass by using the Jovian magnetic field to generate power and propulsive force. In previous science and engineering studies by Johnson, Gallagher, Moore, and Bagenal generation of power and propulsive force were demonstrated in the Jovian system using an orbital mechanics simulation coupled to a rotating Jovian magnetic field model and a Jovian plasma density model.’ The orbital mechanics simulation consisted of a 3 degree-of -freedom orbit propagator based on a 51h order Runge-Kutta-Fehlberg integrator using the Cash-Karp constants. The plasma physics model was based on the Jovian magnetic field model fi-om Khurana along with a simplified version of the plasma density model from Bagenal. In the follo&ng analysis the orbital dynamics of spinning ED tether equipped spacecraft were investigated using a modified version of this modeling tool. Primary changes made to the model were the removal of a factor from the tether current equation, which was originally included to account for the increased current collection of a bare wire tether as compared to a traditional insulated tether and collection sphere. Also, an inertial spacecraft spin axis and spin propagation routine were added to the model to allow the spacecraft o spin about an axis. The addition of spacecraft spin was necessary for tether stability due to the low gravity gradient forces at Jupiter. Both spacecraft propulsion and power generation using ED tethers were studied. Tether Performance Analvsis The main objective of this study was to revisit theJovian capture analysis of the previous study utilizing a rotating tether. In addition, power generation capabilities at Jovian orbits coinciding with the major Jovian satellites were investigated, along with the associated orbit disturbances due to the tether forces. Finally, orbital maneuvering capabilities were examined with respect to orbit element rate of change at various orbits in the Jovian system. These top-level simulations demonstrate the strengths and weaknesses of ED tethers and reveal key engineering challenges that must be overcome before Jovian tether missions will be practical. Jovian, Canture Analvsis One possible use of an ED tether is for capture into Jovian orbit. As a spacecraft approaches the Jovian system on a heliocentric transfer orbit, the spacecraft has a hyperbolic excess velocity of about 6.854 km/s. I-‘...American Institute of Aeronautics and Astronautics L (c)2600 American Institute of Aeronautics & Astronautics or published with permission of author(s) and/or author(s)’ sponsoring organization. To be captured into a Jovian orbit, the spacecraft must dissipate this excess velocity at its closest approach to Jupiter. This large velocity change is traditionally accomplished by burning a large quantity of propellant at perijove. Traditional chemical rockets are inefficient and significantly increase the spacecraft mass. This either reduces the available mass for the Jovian spacecraft and instruments, or requires additional throw-weight capacity from the launch vehicle. ED tethers of&r a potential mass saving solution by providing the propulsive force necessary for insertion into Jovian orbit without using expendable propellant mass. Additionally, either full or partial spacecraft power can be generated with the tether while in the Jovian system. For this study, several capture simulations were performed to determine the tether length necessary to capture into a Jovian orbit with a period of no more than 100 days. The spacecraft mass was assumed to be 340kg and the hyperbolic excess velocity of the spacecraft was 6854km/s. The spacecraft was placed at a distance of 0.32220 AU from Jupiter which is the Jovian gravitational activity radius.’ The spacecraft velocity vector was aligned along the hyperbolic approach asymptote for a perijove radius of 72,112km. The of&et distance d of the asymptote fi-om the center of Jupiter was calculated using the following equations: