Citation
Sanmartín Losada, Juan Ramón and Lorenzini, Enrico C.
(2006).
Spherical collectors versus bare tethers for drag,
thrust, and power generation.
"IEEE Transactions on plasma science", v. 34
(n. 5);
pp. 2133-2139.
ISSN 0093-3813.
Abstract
Deorbit, 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.