re: Regenerative closed cycle staged combustin engines.
Are you familiar with the IPD Program?
Integrateded Propulsion Demonstrator -
http://www.aero.org/publications/crosslink/winter2004/03.html
http://www1.msfc.nasa.gov/NEWSROOM/background/facts/powerhead.pdf
This is as close to a fast elevator that we're ever going to get, and this is the way to reduce the cost of space flight.
Now, after IPD, we would have to build an engine like this with the thrust level of the RS-68, and the chamber pressure, T/W and Isp of the SSME.
Surely we can. In that case, it would only be a matter of figuring out how to get the engines back to the launch site from an equatorial launch in a single orbit.