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.

Electrodynamic Tether Applications and Constraints

The current I to a cylindrical Langmuir probe with a bias Φp satisfying β≡eΦp/mec2∼O(1) is discussed. The probe is considered at rest in an unmagnetized plasma composed of electrons and ions with temperatures kTe∼kTi≪mec2. For small enough radius, the probe collects the relativistic orbital-motion-limited (OML) current IOML, which is shown to be larger than the non-relativistic result; the OML current is proportional to β1/2 and β3/2 in the limits β≪1 and β≫1, respectively. Unlike the non-relativistic case, the electron density can exceed the unperturbed density value. An asymptotic theory allowed to compute the maximum radius of the probe to collect OML current, the sheath radius for probe radius well below maximum and how the ratio I/IOML drops below unity when the maximum radius is exceeded. A numerical algorithm that solves the Vlasov-Poisson system was implemented and density and potential profiles presented. The results and their implications in a possible mission to Jupiter with electrodynamic bare tethers are discussed.

/pdf/relativistic-current-collection-by-a-cylindrical-langmuir-3w12r0yqka.pdf

Relativistic current collection by a cylindrical Langmuir probe

With a thin coating of low-work-function material, thermionic emission in the cathodic segment of bare tethers might be much greater than orbital-motion-limited (OML) ion collection current. The space charge of the emitted electrons decreases the electric field that accelerates them outwards, and could even reverse it for high enough emission, producing a potential hollow. In this work, at the conditions of high bias and relatively low emission that make the potential monotonic, an asymptotic analysis is carried out, extending the OML ion-collection analysis to investigate the probe response due to electrons emitted by the negatively biased cylindrical probe. At given emission, the space charge effect from emitted electrons increases with decreasing magnitude of negative probe bias. Although emitted electrons present negligible space charge far away from the probe, their effect cannot be neglected in the global analysis for the sheath structure and two thin layers in between sheath and the quasineutral region. The space-charge-limited condition is located. It is found that thermionic emission increases the range of probe radius for OML validity and is greatly more effective than ion collection for cathodic contact of tethers.

/pdf/low-work-function-thermionic-emission-and-orbital-motion-1fcdc3wcef.pdf

Low Work-Function Thermionic Emission and Orbital-Motion-Limited Ion Collection at Bare-Tether Cathodic Contact

The application of a recent concept, a thermionic bare tether (that is, a long conductor coated with a thermionic material), to a practical engineering problem (deorbit space debris from geostationary transfer orbit) is presented. Lorentz drag on a thermionic bare tether, during each pass through an arc close to the perigee, lowers the apogee progressively and produces the object reentry. The performance of a spacecraft equipped with a thermionic bare tether is studied at two different levels, using models that couple thermal and electrical effects. In first place, a simple formula for the eccentricity decrement produced during each perigee pass as a function of TBT properties is derived and used to select TBT optimal dimensions. For a given tether mass, the formula shows that long tethers with small cross-section areas, but large enough to accomplish mechanical constraints, yield the best performance. Second, full numerical simulations of the deorbit maneuver including Lorentz force, air drag, and J2 per...

/pdf/optimal-design-and-deorbiting-performance-of-thermionic-f68zvl173j.pdf

Optimal Design and Deorbiting Performance of Thermionic Tethers in Geostationary Transfer Orbits

The use of electrodynamic tethers for propulsion and power generation is attractive for missions to the outer planets, which are traditionally handicapped by large propellant requirements, large times of flight, and a scarcity of power available. In this work, the orbital dynamics of a spacecraft using electrodynamic tether propulsion during the mission phases of capture, apojove pump down and perijove pump up, in the Jovian system are investigated. The main result is the mapped design space involving mission duration, tether length, and minimum perijove radius. Phase-free flyby sequences are also included, which provide performance upper bounds for a given mission architecture. It is found to be advantageous to use inbound-only flybys of the Galilean moons during capture. Flybys during the apojove pump down phase are only useful in conjunction with a perijove-raising mechanism at apoapse such as solar perturbations or a small propulsive maneuver. The electrodynamic tether system is also shown to be capab...

/pdf/jovian-orbit-capture-and-eccentricity-reduction-using-sp27s9xtn3.pdf

Jovian Orbit Capture and Eccentricity Reduction Using Electrodynamic Tether Propulsion

A two-stage mission to place a spacecraft (SC) below the Jovian radiation belts, using a spinning bare tether with plasma contactors at both ends to provide propulsion and power, is proposed. Capture by Lorentz drag on the tether, at the periapsis of a barely hyperbolic equatorial orbit, is followed by a sequence of orbits at near-constant periapsis, drag finally bringing the SC down to a circular orbit below the halo ring. Although increasing both tether heating and bowing, retrograde motion can substantially reduce accumulated dose as compared with prograde motion, at equal tether-to-SC mass ratio. In the second stage, the tether is cut to a segment one order of magnitude smaller, with a single plasma contactor, making the SC to slowly spiral inward over several months while generating large onboard power, which would allow multiple scientific applications, including in situ study of Jovian grains, auroral sounding of upper atmosphere, and space- and time-resolved observations of surface and subsurface.

/pdf/a-proposed-two-stage-two-tether-scientific-mission-at-wq1soy4chy.pdf

R. W. Evans

Papers

A Proposed Two-Stage Two-Tether Scientific Mission at Jupiter