A New Kind of Dynamic Instability in Electrodynamic Tethers

  • PDF / 4,330,926 Bytes
  • 28 Pages / 496.063 x 722.835 pts Page_size
  • 100 Downloads / 195 Views

DOWNLOAD

REPORT


A New Kind of Dynamic Instability in Electrodynamic Tethers 1 J. Pelaez," E. C. Lorenzini," O. L6pez-Rebollal,2 and M. Ruiz ' Abstract Simulation of the dynamics of an electrodynamic tether on a circular inclined orbit shows a very complex motion driven by the electrodynamic forces acting on the conductive tether. These forces depend on the current flowing in the wire, the Earth magnetic field, the orbital velocity and the tether position. In this paper we use a simple model to describe the dynamic effects of these forces. The tether is modeled as a rigid rod with point masses at the ends. We also adopt a non-tilted dipole model for the Earth magnetic field, and we assume that the tether current is constant. When the current is null, the system has a stable equilibrium position with the tether aligned along the local vertical. When the current is different from zero, a periodic motion appears. A nonlinear analysis of the motion shows that the periodic solutions are always unstable (within the limitation of the model considered in the paper). The physical reason for the instability is that the electrodynamic forces pump energy continually into the system. The net energy increase per orbit for the periodic solution (or state space trajectory) is zero. However, any nonperiodic trajectory in its neighborhood has a positive net energy flux per orbit so that after several orbits the in -plane libration becomes a rotation. The mechanism responsible for this instability depends on the orbital inclination. Unlike other destabilizing mechanisms found in electrodynamic tethers, this one is present in any kind of tether system with either a flexible or a rigid tether, operating in the generator or thruster mode and utilizing a bare tether or a large spherical termination to collect the ionospheric electrons. The instability described in this paper is independent of the presence of resonant force components that may be generated by the magnetic and plasma fields.

Introduction The electrodynamic tether is one of the leading applications of tethers for inspace transportation. Basically, there are two different ways in which an electrodynamic tether can be operated: in the thruster mode, the tether acts as a thruster that 'presented as Paper 00-190 at the 10th AAS/AIAA Space Flight Mechanics Meeting, Clearwater, Florida, January 23-26, 2000. 2E.T.S .I. Aeronauticos, Universidad Politecnica de Madrid, Pza. Cardenal Cisneros 3, Madrid 28040, Spain. E-mail address:[email protected]@[email protected] 3Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, Massachusetts 02138, U.S.A. E-mail address: [email protected]

449

450

Pelaez, Lorenzini, Lopez-Rebollal, and Ruiz

converts electrical energy into orbital energy; in the generator mode, the tether converts orbital energy into electrical energy hence deboosting the spacecraft to which the tether is attached. A thin bare tether can collect electrons from the ionosphere with much greater efficiency than the classical approach (e.g. TSS) of