Pratt & Whitney Rocketdyne has just tested a subscale combustor for a dual-mode ramjet/scramjet under the FACET programme to demonstrate a combined-cycle powerplant for DARPA's planned Falcon HTV-3X hypersonic test vehicle. The combustor was ground-tested from Mach 2.5 to Mach 6.0.
Lockheed Martin's Skunk Works is designing the HTV-3X. As the attached video shows, the unmanned vehicle would take off under turbojet power then accelerate to a speed where the ramjet when take over, then transition to scramjet mode to cruise hypersonically before returning to a runway landing.
Like most DARPA programmes it's a technological stretch - and Lockheed hasn't even built and flown the unpowered HTV-2 yet - although I've got my fingers crossed for this one. But, as Boeing's George Muellner jokes: "Hypersonics is the future...and always will be."