DARPA has now revealed one of its hypersonic research programs,the Vulcan engine.

The test vehicle, see below, is about the size of the Have Blue stealth prototype (smaller than a F-5)

As shown in the slide, this would use conventional fuels.

There would be a common inlet and exhaust nozzle with the flow path varying depending on speed and altitude. A turbojet would be used for takeoff and low speed, and as speeds increased, the inlet air would be diverted to a pulsed combustion ram/scram jet type system.