Electrical model and optimal design scheme for low work-function tethers in thrust mode

Abstract

A low work-function tether (LWT), a subclass of bare electrodynamic tether made of a conductor partially coated with a low work function material, can exchange momentum and energy with planetary magnetospheres without any consumable. If fed by an onboard power source to reverse the natural direction of the current given by the motional electric field, a LWT can produce an useful thrust in drag compensation and re-boost scenarios. In the considered scheme, the LWT has an anodic bare segment for passive electron collection, followed by an insulated segment, a power source, and a cathodic segment that emits electrons passively through thermionic and photoelectric effects. Current and voltage profiles along the LWT are obtained and used to compute the system efficiency, i.e. the electrical power to mechanical power conversion rate, and the minimum electrical power to avoid space-charge effects in the cathodic segment. The design conditions to reach a high-efficiency regime are presented. For a given orbit and required thrust, optimal values for the tether geometry, including the fractional lengths of all three tether segments, are found. The results are applied to the preliminary design of a LWT system that would compensate the aerodynamic drag on the International Space Station. The analysis shows that the mission can be performed by a LWT fed with a power source of 4.7 kW, length, width and thickness equal to 5.1 km, 2 cm and 30 um, work function 1.5 eV, and temperature 600 K.

BETsMA v2.0: A friendly software for the analysis of electrodynamic tether missions in Jupiter

“…BETsMA v2.0 does not only simulate deorbit maneuvers considering bare electrodynamic tethers, but also low work function tethers [10,11], which do not requires active emitters. These two types of tethers are handled by the software in both passive (power generation) and active [12,13] modes. Such a capability allows to analyze the performance of EDTs in a broad range of scenarios and looking for different missions goals like thrust and power generation.…”

A code for the analysis of missions with electrodynamic tethers

“…BETsMA v2 includes electric models for BETs [3] and LWTs [7,8,40] in both passive and active modes. The operation mode depends on the directions…”

“…Tether Type BET [3]/LWT [7,8,40] Tether Cross-section Round/Tape Tether Attitude Aligned with local vertical/spinning/ N-bar model [41] N-particle model [41] Thermal Model…”

“…As shown in previous works on BETs [3,45,46] and LWTs [7][43][8] [40], computing the current profile I(x) for each of the four configurations in Fig. 1 and 2 requires a specific analysis.…”

Trade-off analysis of C12A7:e− deposition techniques applied to Low Work Function Tethers

“…Its operation would be as simple as the one for drag augmentation devices, but the drag in an LWT is both magnetic (Lorentz drag) and aerodynamic, being the former the dominant mechanism for the orbits of interest in space debris deorbiting applications. Past studies on LWTs are restricted to theoretical analysis, and they included the operational regimes of their current and voltage profiles [7,8] and the performance of LWTs in Geostationary Transfer Orbit [9] and thruster mode [10]. These theoretical works created an envelope of performance as a function of the physical characteristics of the coating and the tether.…”