Spherical Collectors Versus Bare Tethers for Drag, Thrust, and Power Generation

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