1,720,969 research outputs found
A "Free-Lunch" Tour of the Jovian System
Full text linkAn ED-tether mission to Jupiter is presented. A bare tether carrying cathodic devices at both ends but no power supply, and using no propellant, could move 'freely' among Jupiter's 4 great moons. The tour scheme would have current naturally driven throughout by the motional electric field, the Lorentz force switching direction with current around a 'drag' radius of 160,00 kms, where the speed of the jovian ionosphere equals the speed of a spacecraft in circular orbit. With plasma density and magnetic field decreasing rapidly with distance from Jupiter, drag/thrust would only be operated in the inner plasmasphere, current being near shut off conveniently in orbit by disconnecting cathodes or plugging in a very large resistance; the tether could serve as its own power supply by plugging in an electric load where convenient, with just some reduction in thrust or drag. The periapsis of the spacecraft in a heliocentric transfer orbit from Earth would lie inside the drag sphere; with tether deployed and current on around periapsis, magnetic drag allows Jupiter to capture the spacecraft into an elliptic orbit of high eccentricity. Current would be on at succesive perijove passes and off elsewhere, reducing the eccentricity by lowering the apoapsis progressively to allow visits of the giant moons. In a second phase, current is on around apoapsis outside the drag sphere, rising the periapsis until the full orbit lies outside that sphere. In a third phase, current is on at periapsis, increasing the eccentricity until a last push makes the orbit hyperbolic to escape Jupiter. Dynamical issues such as low gravity-gradient at Jupiter and tether orientation in elliptic orbits of high eccentricity are discussed
Spherical collectors versus bare tethers for drag, thrust, and power generation
Full text linkDeorbit, power generation, and thrusting performances of a bare thin-tape tether and an insulated tether with a spherical electron collector are compared for typical conditions in low-Earth orbit and common values of length L = 4−20 km and cross-sectional area of the tether A = 1−5 mm2. The relative performance of moderately large spheres, as compared with bare tapes, improves but still lags as one moves from deorbiting to power generation and to thrusting: Maximum drag in deorbiting requires maximum current and, thus, fully reflects on anodic collection capability, whereas extracting power at a load or using a supply to push current against the motional field requires reduced currents. The relative performance also improves as one moves to smaller A, which makes the sphere approach the limiting short-circuit current, and at greater L, with the higher bias only affecting moderately the already large bare-tape current. For a 4-m-diameter sphere, relative performances range from 0.09 sphere-to-bare tether drag ratio for L = 4 km and A = 5 mm2 to 0.82 thrust–efficiency ratio for L = 20 km and A = 1 mm2. Extremely large spheres collecting the short-circuit current at zero bias at daytime (diameters being about 14 m for A = 1 mm2 and 31 m for A = 5 mm2) barely outperform the bare tape for L = 4 km and are still outperformed by the bare tape for L = 20 km in both deorbiting and power generation; these large spheres perform like the bare tape in thrusting. In no case was sphere or sphere-related hardware taken into account in evaluating system mass, which would have reduced the sphere performances even further
Electrodynamic Tether Applications and Constraints
No full textAbstract. 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 andliquid 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 are over 200K(night) and380K(day) for the tape
and 320 and 415 K for that wire. Tether applications other than propulsion and power are briefly discussed
A Review of Electrodynamic Tethers for Space Applications
No full textAbstract - Space applications of electrodynamic tethers, and basic issues and constraints on their
operation are reviewed. The status of the bare-tether solution to the problem of effective
electron collection from a rarefied magnetized plasma is revisited. Basic modes of tether
operation are analyzed; design parameters and parametric domains where a bare
electrodynamic tether is most efficient in deorbiting, reboosting, or power generation, are
determined. Use of bare tethers for Radiation Belt Remediation and generation of electron
beams for ionospheric research is considered. Tether heating, arcing, and bowing or
breaking, as well deployment strategies are discussed
Saturn power generation with electrodynamic tethers in polar orbit
Full text linkA power generation scheme based on bare electrodynamic tethers (EDT), working in passive mode is investigated for the purpose of supplying power to scientific missions at Saturn. The system employs a spinning EDT on a lowaltitude
polar orbit which permits to efficiently convert plasmasphere energy into useful power. After optimizing the tether design for power generation we compute the supplied power along the orbit and the impact of the Lorentz force
on the orbital elements as function of the tether and orbit characteristics.
Although uncertainties in the current ionosphere density modeling strongly affect the performance of the system the peak power density of the EDT appears be greater than conventional power systems
Efficiency of Electrodynamic Tether Thrusters
No full textAbstract. 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
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