METS

Mir Electrodynamic Tether System

METS was a privately-funded experiment TAI developed to test propellant-less reboost of the Russian space station Mir. It used a hollow cathode to emit electrons, like PMG, but a 40mm x 1km aluminum strip to collect electrons. Like TSS, it used a rotating spool, but with passive eddy-current braking rather than a motor/generator. TAI designed METS for easy mounting inside Progress, and easy crew installation outside a nadir-facing airlock on Mir. A surplus "manned maneuvering unit" docked there was available to serve as a 200kg ballast mass. Overall METS length was 7.5 km: 6 km of insulated wire attached to Mir, then 1 km of electron collector, and 0.5 km of "ballast tether."

Controls work on METS resulted in a very general control law for electrodynamic tethers, which we have patented. It infers the system state from recent measured voltage and tension data, and actively drives control currents that would be driven by computed "error EMFs" that result from differences between inferred and desired motions. This provides robust long-term damping of all differences between those motions. Simulations suggested that this control law would allow good enough control of Mir attitude for its CMGs to be turned off. This would free up more of Mir's limited power for reboost by METS.

About the time hardware export was approved, the Russian government committed to deorbiting Mir and focusing on the ISS. METS hardware still exists. It could be adapted to provide most future reboost of the ISS, at the cost of ~5kW of ISS power. If deployed down from a point forward of the station CG, METS could also improve ISS attitude and micro-gravity environments, by slightly shifting the station CG down and causing a nose-up shift in the unstable torque equilibrium angle.

Mir's manned maneuvering unit.

Mir with METS.