It looks like DARPA is continuing to develop turbine/ramjet hypersonic propulsion systems, (paid subscription required) called the Turbine-based combined-cycle (TBCC), despite the cancellation of the Blackswift demonstrator last month.

The concept remains the same, a turbine to get the vehicle off the ground and through Mach 1, with some level of cocooning beyond that as a Ram/Scram jet takes over.

DARPA is continuing to test the Falcon combined-cycle engine technology (Facet) developed for Blackswift, along with the high-speed turbojet engine development (Histed).

Basically, they are looking at integrating previously pieces, inlet, combustor and nozzle primarily, into a complete engine system.

Following this, they intend to combine Facet and Histed.

Obviously it is a non trivial issue to test the engines throughout their range without a flight test.