The short-term goal is to create a delivery service: air-drop to any co-ordinates on the planet within 90 minutes, or it’s free.
The longer-term goal is to develop a craft capable of orbital insertion with minimal expenditure of propellant.
Ramjets tend to have quite different design parameters at supersonic vs hypersonic speeds.
A cleverly designed hybrid engine could, maybe, involve shape changes, that also becomes a rocket at very high altitudes, and rescue otherwise dead-weight.
Fuel bulk is one of the problems with existing designs. With fusion, that problem goes away
http://en.wikipedia.org/wiki/X-43A
