I've been think about what is needed for a next generation shuttle that could begin operations around 2020.  My thought is that this is about the earliest this could happen.  In the interim we will be stuck will the CEV.  Basically we need a craft that could do what the current Shuttle couldn't, i.e. economical transport to LEO.  The engines will be capable of both ram and scram jet operation, i.e. combustion can either be sub or supersonic jet flows.  Plus it will be able to supplement the ram air with onboard liquid oxygen.

Because this vehicle needs to get up to speed before starting the engines I proposal that we launch it with an electromagnetic catapult.  I very high energy maglev type system that will push it up to Mach 1 at sea level in order to power up the engines.  It will accelerate uptil about Mach 8 on external air then gradually add LOX to the mix as it climbs into thinner air.  At the same time the specific impluse of the engines increase from around 200 sec to over 400 sec as the H2O portion of the thrust dominates over the N2.  Finally we reach full internal propellant at over 10,000 mph for the final push to orbital velocity. 

I think that this craft would rapidly climb into the stratosphere and would be much like a rocket in flight path.  We do have be ballance the fact the air scooped into the engine would inclease with speed linearly while heating would go up with v-squared at a given air density.  A compensating factor is that at very hight speeds we would fly higher than the linear need for air by adding LOX.  Also as the fuel is consumed we can let the thrust fall as the craft gets lighter.  It this way we could stay on a path to orbit that avoids to much v-squared heating.  This would be a SSTO vehicle and would carry more LH2 and far less heavy LOX then a typical rocket stage.  The air inlets would be above the wing in order to protect them on reetery.   It would return to earth much like the current Shuttle orbiter.