> (But where does that mass go?) Spitballing here ... the mass is magnetic, and you shoot it down into a giant borehole that rapidly recaptures the momentum us

> (But where does that mass go?) Spitballing here ... the mass is magnetic, and you shoot it down into a giant borehole that rapidly recaptures the momentum using magnetic regenerative breaking. Of course you need to use all the energy you recaptured (and more) to bring the mass back up to the surface for your next launch ... Ok, maybe we are starting to get into sci-fi ...