In the 2nd gen version of this they use a maglev concept to keep the thing in the air. They base this on maintaining a current of many MA along the tube and on the ground. OK.
Where does the current come from/where does it go? On the Startram web page they have a textbook illustration of two parallel wires carrying currents in opposite directions, but those will always be part of some complete circuit. So how do you complete the circuit without messing everything up?
With the ground component of the system one could presumably route your cable such that the return path is very far from the launch tube (we're thinking big already, so what's a hundred km or so of extra superconducting cable?). But the part that's in the air needs to return current at the airborne end of the tube, and you can't just shunt it back down through the tube because that current would experience a downward force through its interaction with the same field you're using to keep the whole thing airborne. I suppose you might run it down the long tether(s) at the ends, but then you're adding some pretty big forces to those as well.
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