Electrodynamic Tether System Research Articles

Model Development and Code Verification for Simulation of Electrodynamic Tether System

We develop a numerical model of an electrodynamic tether system composed of two finite end bodies and a flexible tether. The equations of motion of the system are presented along with two methods of discretizing the partial differential equations governing the tether vibrations. The first method is the assumed modes method in which the tether displacements are represented as series of generalized coordinates and assumed mode functions, and the second is a finite element method in which displacements and slopes at points along the tether are interpolated by shape functions. The method of manufactured solutions is used to verify that the computer codes written to simulate the system motion implement the discretization methods properly. Both the assumed modes method and the finite element method perform well for relatively low discretization levels; however, as the number of longitudinal assumed modes is increased, the mass matrix for the longitudinal vibrations becomes poorly conditioned, resulting in numerical errors that can cause the tether vibrations to improperly diverge. This behavior is not seen in the finite element method because the mass matrices for both the transverse and longitudinal vibrations are always well-conditioned.

Stability analysis of in-plane and out-of-plane periodic motions of electrodynamic tether system in inclined elliptic orbit

This paper analyzes the stability of in-plane/out-of-plane motion of electrodynamic tether (EDT) systems in elliptic and inclined orbits. The chaotic librational motion of an EDT system in inclined elliptic orbits has been studied extensively and it is believed that delayed feedback control cannot stabilize the librational motion of an EDT in an inclined and elliptic orbit with high eccentricity. Contrary to previously published results, the analytical results in this paper reveal that delayed feedback control can stabilize the in-plane/out-of-plane coupling librational motion of EDT systems in inclined elliptic orbits with high eccentricity to a periodic motion because the in-plane and out-of-plane coupling motions do not have the odd number condition if the electric current can flow on tether in both direction, and appropriate control gains are chosen. The validity of the stability analyses in this paper is confirmed through numerical simulations in which a delayed feedback control with appropriate control gains is applied to the in-plane/out-of-plane coupling periodic motion of an EDT system in inclined elliptic orbit with high eccentricity.

Investigation of Endurance Performance of Carbon Nanotube Cathodes

The Aerospace Research and Development Directorate of the Japan Aerospace Exploration Agency (JAXA) is considering a demonstration of electrodynamic tether (EDT) systems in low Earth orbit (LEO). Carbon nanotubes (CNTs) have some advantages as electron sources compared to conventional Spindt type emitters, and so are expected to be useful in EDT systems. Experiments to investigate the durability of CNT cathodes in a space environment had been conducted in a diode mode, but it was found that electron extraction tests, in which the cathode with a gate electrode is used, are necessary to evaluate the endurance of CNTs more accurately. In this paper, we conducted long duration operating tests of a cathode with a gate. It was found that there was almost no change in cathode performance at current densities below 100 A/m2 even after the cathode was operated for over 500 hours in the high vacuum environment.

Research and Development of Electrodynamic Tethers for Space Debris Mitigation

The current status of research and development of an electrodynamic tether (EDT) system for space debris mitigation is described. EDT is a kind of advanced propulsion technologies, which requires no propellant and electric power in theory. Since debris de-orbit systems need highly efficient propulsion systems, EDT can be one of the most attractive candidates. The first step to make the debris removal system practical in future is the demonstration of the EDT operation on orbit because no one has ever demonstrated the thrust generation by EDT. Therefore, we have been conducting research and development of an EDT system using a small satellite. In this paper, the concept of the EDT demonstration on orbit and the current status of the research and development are described.

Nonlinear Control of a Double Pendulum Electrodynamic Tether System

T HE electrodynamic tether (EDT) system uses the Lorentz force as a thruster generated by the interference between the Earth’s magnetic field and the electric current along the tether. The main advantage of this system is that its required electric current can be produced by solar power and electric particles such as Xenon, the mass of which is much smaller than those of traditional chemical propellants. As a result, this system is expected to be employed as a new space technology that will be useful in many types of space applications [1] such as reboosting of the international space station [2]; deorbiting of dysfunction satellites [3] (space debris removal system); the momentum exchange electrodynamic reboost (MXER) [4] system, which could be used to boost payloads from a low Earth orbit (LEO) to a geosynchronous transfer orbit (GTO); and scientific missions that could include the observation of meteors, the highaltitude atmosphere, and the aurora. The motion of the EDT system consists of librations that are similar to themotion of a pendulum and vibrations of the tether that are similar to the vibrations of a string. The librational motion of the tether system in an elliptic orbit is known to be chaotic [5,6]. The librational and vibrational motion of the tethered satellite system must be stabilized to successfully perform space operations such as observation of the atmospheric environment at high altitude. For the purpose of stabilization, a great number of control schemes have been proposed for various aspects of control [7], such as stationkeeping, deployment and retrieval control, generation of optimal trajectories [8], suppression of tether vibrations by electric current variation [9], periodic motion of the electrodynamic tether systemon inclined orbits [10], and effect of the electrodynamic force on orbit [11]. However, to the best knowledge of the authors, except delayed feedback controls, application of nonlinear control schemes to the multipendulum type of electrodynamic tether systems for achieving constellation missions has not yet been widely studied. The tether system treated in this Note consists of two subsatellites and a mother satellite, such as the space shuttle, connected in series via conductive massless tethers. Although this model seems simple, because the mass and flexibility of the tether are ignored, it can be used to investigate the behavior of the in-plane motion of an electrodynamic tether system in an elliptic orbit without significant computational effort and to investigate the applicability of the nonlinear controllers to the simple model. In this study, two nonlinear control methods to stabilize the inplane librational motion of an electrodynamic tether system in an elliptic orbit are investigated. The nonlinear controllers treated in this Note are a decoupling control method [6,12] and amodel-followingdecoupling control method [6,12]. The decoupling control can be used to control each tether attitude independently, and this independent motion is suitable for achieving satellite constellations. If this motion is realized for a three-mass tethered satellite system, various scientific missions will be possible, including the observation of planets with a magnetic field and the observation of the aurora from more than two directions simultaneously. The model-following-decoupling control method is introduced to achieve a periodic motion by which the Earth atmosphere at the specific altitude can be studied periodically. In this study, the periodic in-plane motion of a tethered satellite system in a circular orbit is employed as the reference trajectory for tracking by the actual tethered satellite system in an elliptic orbit. The results of numerical simulations show that the nonlinear control schemes considered herein can stabilize the in-plane librational motion of a double pendulum electrodynamic tether system in an elliptic orbit.

3211 エレクトロダイナミックテザー(EDT)システムのカオス秤動運動に対する遅延フィードバック制御(テザーシステム)

3211 エレクトロダイナミックテザー(EDT)システムのカオス秤動運動に対する遅延フィードバック制御(テザーシステム)

Study on electrodynamic tether system for space debris removal

As a countermeasure for suppressing space debris growth (P. Eighler, A. Bade, Chain Reaction of Debris Generation by Collisions in Space—A Final Threat to Spaceflight? in: 40th Congress of the International Astronautical Federation, IAA-89-628, October 1989), the National Aerospace Laboratory of Japan is investigating a satellite capture, repair and removal system for non-cooperative satellites, part of which involves assessing the viability of electrodynamic tether (EDT) technology as an orbital transfer system. In this paper, some results concerning the time required to remove existing satellites, the behavior of flexible tethers during the debris separation phase, and orbital transfer strategies of EDT systems during space debris removal operations are described. From numerical simulations, it is found that EDT systems can transfer satellites from LEO to orbits with a short lifetime within a realistic timeframe. It is also found that the stability of EDT systems is compromised when debris separation occurs both while a tether current is running and when the ratio of the end mass to that of the service satellite is high. To ensure stability, the end mass should be selected from the target debris group with due regard for the maximum possible mass that can be maneuvered safely. Moreover, it is also found that orbital elements (a, e, i) can be changed independently with an adequate current control strategy.

SPREE measurements of wave-particle interactions generated by the electron guns on TSS-1 and TSS-1R

The Shuttle Potential and Return Electron Experiment (SPREE) was flown in the orbiter bay during TSS-1 and TSS-1R flights. Measurements presented here correspond to times of DC electron firings of the Shuttle Electrodynamic Tether System (SETS) fast pulsed electron generator (FPEG). SPREE included particle correlators measuring particle modulations in the 0–10kHz and 0–10MHz frequency ranges. Wave Particle Interactions, WPI, were observed on TSS-1 & TSS-1R in both frequency ranges: around a few MHz whenever the emitted beams were close to 90° pitch angle; and at a few kHz at many other beam pitch angles. On both missions problems with the tether deployment lead to more of the mission timeline being dedicated to the SPREE/FPEG active plasma studies reported here. The strong MHz waves occur when the 90° FPEG beam has the same low parallel velocity component, V par values as the thermal electrons. Then both the beam source and the thermal sink can interact with the same waves. The strong kHz waves are generated directly by the passage of the FPEG beam through the ionospheric plasma at the orbiter velocity with a period determined by the geometry of the disturbance.

The Shuttle Electrodynamic Tether System (SETS) on TSS-1

The Shuttle Electrodynamic Tether System (SETS) experiment formed part of the scientific experiments comprising the first flight of the NASA/ASI Tethered-Satellite System flown at an altitude of 300 km and an orbital inclination of 28.5 degrees in July–August 1992. The SETS experiment was designed to study electrodynamic behavior of the Orbiter-Tether-Satellite system as well as provide background measurements of the ionospheric environment near the Orbiter. The SETS experiment was able to operate continuously during the mission providing a large data set. Details of the SETS objectives, its instrumentation, and initial results from the mission highlighting voltage, current, and charging measurements are presented here.

Positive spacecraft charging as measured by the Shuttle potential and Return Electron experiment

The authors report on obsevations of positive charging of the Orbiter during the deployed phase of the TSS-1 (Tethered Satellite System 1). The charging was observed to occur when the Orbiter was in darkness, during periods of low ionospheric density and while the SETS FPEG (Shuttle Electrodynamic Tether System Fast Pulsed Electron Gun) was emitting a 1-keV, 100-mA electron beam. The charging occurred when the ambient plasma density was too low to provide a current to match the FPEG emission. In the cases where the ambient plasma temperature was approximately 0.1 eV, and assuming a conducting area of the Orbiter of approximately 25 m/sup 2/, the positive charging occurred for densities below 6*10/sup 5/ electrons/cm/sup 3/. The charging is seen as an acceleration of the relatively hot electron population in the energy range above 10 eV that had already been greatly enhanced by the operation of the FPEG. Intense fluxes of electrons were observed at low energy for both the charged and uncharged Orbiter. This low energy component tended to be isotropic with densities as high as 4.6*10/sup 3/ electrons/cm/sup 3/. The return flux at the charging peak was anisotropic, with the anisotropy varying with the level of charging and the pitch angle. >

Plasma response to high potential satellite in Electrodynamic Tether System

We study plasma responses to a high potential satellite in the electrodynamic tether system with the aid of two‐dimensional electromagnetic particle computer experiments. In the computer experiments, one conducting body representing the satellite is placed in the center of the simulation plane in the vehicle frame. The satellite potential ϕs is fixed to be higher than the space potential of the surrounding plasma. The transient process of the plasma response to the satellite and the spatial profiles of plasma density and of plasma current are examined. Due to the high potential, an electron sheath and an ion cavity are formed around the satellite. The ion cavity is extended downstream by the ambient plasma flow. In the direction perpendicular to B0, the density profile of the plasma flow shows asymmetric distortion caused by the deceleration of the ambient plasma owing to the intense E×B effect. An intense transient current is observed followed by an exponentially decreasing current. The current flows both along and across B0. The cross‐field current is mainly produced by the electron E×B drift motion around the satellite. The current‐voltage characteristics of the satellite in the steady state are examined by changing the satellite potential up to eϕs/kBTe≃10³ and comparing to the classical theories. The average current obtained by the computer experiments is between that predicted by the Langmuir theory and that predicted by the single particle theory. From eϕs/kBTe≃10¹ to 10³, the theory based on the constant density model provides a good estimate of the upper bound of the current flowing from the satellite.

Electrical behavior of a Shuttle Electrodynamic Tether System (SETS)

Many novel electrodynamic and space plasma physics experiments can be done from the space shuttle using a gravity gradient stabilized sub-satellite tethered 10–30 km above the Orbiter via a long, conducting wire which is insulated from the ionospheric plasma. This system, called the Shuttle Electrodynamic Tether System (SETS), is described in the present paper with emphasis upon the various electrical processes which determine its coupling to the ambient plasma. The three most important physical effects include sheath formation and electron collection by the sub-satellite, the generation of a large emf through the orbital motion of the tether across geomagnetic field lines, and the active ejection of electrons from the Orbiter into the surrounding ionosphere. An electrical circuit analogy is presented for SETS together with a brief outline of possible areas related to the artificial generation of magnetohydrodynamic waves.