DARPA has released info in a briefing on how one might build a propulsion system that combines a Constant Volume Combustion (CVC) engine and a full-scale turbine engine to accelerate a hypersonic jet to multiple Mach. It's called the Vulcan, and it's a demo intended to power a full-scale reusable hypersonic cruise vehicle like the Falcon HTV-3X, and to do it by 2012. According to the DARPA briefing, the main point is to integrate a currently produced turbine engine like the F110-129 or the F119, with minimal modifications and a CVC.
The CVC will function only at supersonic speeds. It's essentially a slender tube with few or no moving parts through which air is compressed at high speeds. The fuel is combusted along the way. The exhaust comes out the nozzle faster than the air came in.
The turbojet engine is required for runway takeoff and to push the plane from zero to Mach 4. At this point the CVC would accelerate it to Mach 6 and beyond. Preferably, both would share a common inlet and nozzle.
The hypersonic jet capable of delivering 12,000 pounds of payload up to 9,000 nautical miles from the continental United States in less than two hours. It would also be used for reconnaissance, strike, and other critical national missions.