dan wrote:
The liquid air cycle (i.e. hotol) is less speed sensitive but
there's no easy way to carry enough cooling capacity to actually
liquify all the air you need. Best bet might essentially be a
cooled-inlet turbojet.

Alternately, there is SABRE.
http://en.wikipedia.org/wiki/SABRE

Which might be that "cooled inlet turbojet" you mentioned.

After attacking the SSTO problem
for awhile, at some point a two-stage solution begins to look more
practical.

Yes, though sometimes the SSTO looks tempting, like when you find a
very lightweight dense fuel design.

Mike Miller