BETs: Propellant less de orbiting of space debris by bare electrodynamic tethers

As a fundamental contribution to limiting the increase of debris in the Space environment, a three-year project started on 1 November 2010 financed by the European Commission under the FP-7 Space Programme. It aims at developing a universal system to be carried on board future satellites launched into low Earth orbit (LEO), to allow de-orbiting at end of life. The operational system involves a conductive tape-tether left bare of insulation to establish anodic contact with the ambient plasma as a giant Langmuir probe. The project will size the three disparate dimensions of a tape for a selected de-orbit mission and determine scaling laws to allow system design for a general mission. It will implement control laws to restrain tether dynamics in/off the orbital plane; and will carry out plasma chamber measurements and numerical simulations of tether-plasma interaction. The project also involves the design and manufacturing of subsystems: electron-ejecting plasma contactors, an electric control and power module, interface elements, tether and deployment mechanisms, tether tape/end-mass as well as current collection plus free-fall, and hypervelocity impact tests.

Bare wire anodes for electrodynamic tethers

The collection of electrons from the ionosphere is the major problem facing high-power electrodynamic tethers. This article discusses a simple electron-collection concept which is free of most of the physical uncertainties associated with plasma contactors in the rarefied, magnetized environment of an orbiting tether. The idea is to leave exposed a fraction of the tether length near its anodic end, such that, when a positive bias develops locally with respect to the ambient plasma, and for a tether radius small compared with both thermal gyroradius and Debye length, electrons are collected in an orbital-motion-limited regime. It is shown that large currents can be drawn in this way with only moderate voltage drops. The concept is illustrated through a discussion of performance characteristics for generators and thrusters.

Propulsive small expendable deployer system experiment

Relatively short electrodynamic tethers can extract orbital energy to "push" against a planetary magnetic field to achieve propulsion without the expenditure of propellant. The Propulsive Small Expendable Deployer System experiment will use the flight-proven Small Expendable Deployer System to deploy a 5-km bare aluminum tether from a Delta II upper stage to achieve ~0.4-N drag thrust, thus lowering the altitude of the stage. The experiment will use a predominantly bare tether for current collection in lieu of the endmass collector and insulated tether used on previous missions. The flight experiment is a precursor to a more ambitious electrodynamic tether upper-stage demonstration mission that will be capable of orbit-raising,lowering, and inclination changes, all using electrodynamic thrust. The expected performance of the tether propulsion system during the experiment is described.

Bare Tethers for Electrodynamic Spacecraft Propulsion

Electrodynamic tether thrusters can use the power provided by solar panels to drive a current in the tether and then the Lorentz force to push against the Earth's magnetic field, thereby achieving propulsion without the expenditure of onboard energy sources or propellant. Practical tether propulsion depends critically on being able to extract multiamp electron currents from the ionosphere with relatively short tethers (10 km or less) and reasonably low power. We describe a new anodic design that uses an uninsulated portion of the metallic tether itself to collect electrons. Because of the efficient collection of this type of anode, electrodynamic thrusters for reboost of the International Space Station and for an upper stage capable of orbit raising, lowering, and inclination changes appear to be feasible. Specifically, a 10-km-long bare tether, utilizing 10 kW of the space station power could save most of the propellant required for the station reboost over its 10-year lifetime. The propulsive small expendable deployer system experiment is planned to test the bare-tether design in space in the year 2000 by deploying a 5-km bare aluminum tether from a Delta II upper stage to achieve up to 0.5-N drag thrust, thus deorbiting the stage.

Performance of bare-tether systems under varying magnetic and plasma conditions

Electrodynamic tethered systems, in which an exposed portion of the conducting tether itself collects electrons from the ionosphere, promise to attain currents of 10 A or more in low Earth orbit. For the first time, another desirable feature of such bare-tether systems is reported and analyzed in detail: Collection by a bare tether is relatively insensitive to variations in electron density that are regularly encountered on each revolution of an orbit. This self-adjusting property of bare-tether systems occurs because the electron-collecting area on the tether is not fixed, but extends along its positively biased portion, and because the current varies as collecting length to a power greater than unity. How this adjustment to density variations follows from the basic collection law of thin cylinders is shown. The effect of variations in the motionally induced tether voltage is also analyzed. Both power and thruster modes are considered. The performance of bare-tether systems to tethered systems is compared using passive spherical collectors of fixed area, taking into consideration recent experimental results. Calculations taking into account motional voltage and plasma density around a realistic orbit for bare-tether systems suitable for space station applications are also presented.

Analysis of bare-tether systems for deorbiting low-earth-orbit satellites

Performances, design criteria, and system mass of bare tethers for satellite deorbiting missions are analyzed. Orbital conditions and tether cross section define a tether length, such that 1) shorter tethers are electron collecting practically in their whole extension and 2) longer tethers collect practically the short-circuit current in a fixed segment length. Long tethers have a higher drag efficiency (defined as the drag force vs the tether mass) and are better adapted to adverse plasma densities. Dragging efficiency and mission-related costs are used to define design criteria for tether geometry. A comparative analysis with electric thrusters shows that bare tethers have much lower costs for low- and midinclination orbits and remain an attractive option up to 70 deg.

Efficiency of electrodynamic tether thrusters

The performance efficiency of electrodynamic bare tethers acting as thrusters in low Earth orbit, as gauged by the ratio of the system mass dedicated to thrust over mission impulse, is analyzed and compared to the performance efficiency of electrical thrusters. Tether systems are much lighter for times beyond six months in space-tug operations, where there is a dedicated solar array, and beyond one month for reboost of the International Space Station, where the solar array is already in place. Bare-tether propulsive efficiency itself, with the tether considered as part of the power plant, is higher for space tugs. Tether optimization shows that thin tapes have greater propulsive efficiency and are less sensitive to plasma density variations in orbit than cylindrical tethers. The efficiency increases with tape length if some segment next to the power supply at the top is insulated to make the tether potential bias vanish at the lower end; multitape tethers must be used to keep the efficiency high at high thrust levels. The efficiency has a maximum for tether-hardware mass equal to the fraction of power-subsystem mass going into ohmic power, though the maximum is very flat. For space tugs, effects of induced-bias changes in orbit might need to be reduced by choosing a moderately large power-subsystem to tether-hardware mass ratio or by tracking the current-voltage characteristic of the solar array.

Role of superconducting shields in electrodynamic propulsion

An electrodynamic tether can propel a spacecraft through a planetary magnetized plasma without using propellant. In the classical embodiment of an electrodynamic tether, the ambient magnetic fleld exerts a Lorentz force on the current along the tether, the ambient plasma providing circuit closure for the current A suggested propulsion scheme would hypothetically eliminate tether performance dependence on the plasma density by using a full wire loop to close the current circuit, and a superconductor to shield a loop segment from the external uniform magnetic fleld and cancel the Lorentz force on that segment. Here, we use basic electromagnetic laws to explain how such a scheme cannot produce a net force. Because there is no net current in the superconducting shield, the circulation of the magnetic field along a closed line outside the full cross section, in its plane, is just due to the current flowing in the loop segment. The presence of the superconducting shield simply moves the Lorentz force from the shielded loop segment to the shield itself and, as a result, the total magnetic force, acting on full loop plus shield, remains zero.

Jupiter power generation with electrodynamic tethers at constant orbital energy

An electrodynamic tether system for power generation at Jupiter is presented that allows extracting energy from Jupiter's corotating plasmasphere while leaving the system orbital energy unaltered to first order. The spacecraft is placed in a polar orbit with the tether spinning in the orbital plane so that the resulting Lorentz force, neglecting Jupiter's magnetic dipole tilt, is orthogonal to the instantaneous velocity vector and orbital radius, hence affecting orbital inclination rather than orbital energy. In addition, the electrodynamic tether subsystem, which consists of two radial tether arms deployed from the main central spacecraft, is designed in such a way as to extract maximum power while keeping the resulting Lorentz torque constantly null. The power-generation performance of the system and the effect on the orbit inclination is evaluated analytically for different orbital conditions and verified numerically. Finally, a thruster-based inclination-compensation maneuver at apoapsis is added, resulting in an efficient scheme to extract energy from the plasmasphere of the planet with minimum propellant consumption and no inclination change. A tradeoff analysis is conducted showing that, depending on tether size and orbit characteristics, the system performance can be considerably higher than conventional power-generation methods.

Electrodynamic tether applications and constraints

Propulsion and power generation by bare electrodynamic tethers are revisited in a unified way and issues and constraints are addressed. In comparing electrodynamic tethers, which do not use propellant, with other propellantconsuming systems, mission duration is a discriminator that defines crossover points for systems with equal initial masses. Bare tethers operating in low Earth orbit can be more competitive than optimum ion thrusters in missions exceeding two-three days for orbital deboost and three weeks for boosting operations. If the tether produces useful onboard power during deboost, the crossover point reaches to about 10 days. Power generation by means of a bare electrodynamic tether in combination with chemical propulsion to maintain orbital altitude of the system is more efficient than use of the same chemicals (liquid hydrogen and liquid oxygen) in a fuel cell to produce power for missions longer than one week. Issues associated with tether temperature, bowing, deployment, and arcing are also discussed. Heating/cooling rates reach about 4 K/s for a 0.05-mm-thick tape and a fraction of Kelvin/second for the ProSEDS (0.6-mm-radius) wire; under dominant ohmic effects, temperatures areover200K (night) and 380 K (day) for the tape and 320 and 415 K for that wire. Tether applications other than propulsion and power are briefly discussed.

Survival probability of round and tape tethers against debris impact

The current space environment, consisting of manmade debris and micrometeoroids, poses a risk to safe operations in space, and the situation is continuously deteriorating due to in-orbit debris collisions and to new satellite launches. Bare electrodynamic tethers can provide an efficient mechanism for rapid deorbiting of satellites from low Earth orbit at end of life. Because of its particular geometry (length very much larger than cross-sectional dimensions), a tether may have a relatively high risk of being severed by the single impact of small debris. The rates of fatal impact of orbital debris on round and tape tethers of equal length and mass, evaluated with an analytical approximation to debris flux modeled by NASA’s ORDEM2000, shows much higher survival probability for tapes. A comparative numerical analysis using debris flux model ORDEM2000 and ESA’s MASTER2005 validates the analytical result and shows that, for a given time in orbit, a tape has a probability of survival of about one and a half orders of magnitude higher than a round tether of equal mass and length. Because deorbiting from a given altitude is much faster for the tape due to its larger perimeter, its probability of survival in a practical sense is quite high.