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starship-design: Scramjet/rocket launchers?

Just read NASA going to spend some money developing anmd testing
scramjet/rockets.  (Ever notice an external burning scramjet looks about the
same as an aerospike rocket?  ;)  )  The idea is the craft would boost as a
rocket up to mach 3, then cut back the oxegen flow and work as a scramjet up
to Mach 8, then switch back to a rocket and boost out of the atmosphere.  So
you don't have the big problem of carrying the weight of an extra set of
engines to orbit.

For refernce a SSTO at launch would be.

20 tons cargo
80 tons ship
100 tons liguid hydrogen fuel
800 tons of Oxegen to burn the fuel!

By geting about Mach 5 worth of boost without onboard air you could save a
lot of launch weight.  According to some charts in Zubrins Black Horse
artical, that might be good enough to let a propane or kerosine fueled
version of the ship single stage to orbit.  

Problem I worry about is that you might lose enough delta-V plowing through
the air to overcome the savings from the smaller lighter ship.

Sounds interesting though!