Oh ye of little faith, behold: Pratt & Whitney Rocketdyne has successfully tested a sub-scale combustor for a dual-mode ramjet engine that could power the DARPA-financed Falcon HTV-3X hypersonic cruiser to Mach 6 and beyond. Dual-mode because the engine can function both at subsonic speeds and then kick up to supersonic scramjet speeds (greater than Mach 5).
By contrast, one of the fastest conventional Air Force jets--the SR-71 Blackbird--does a tortoise-paced Mach 3.4.
The engine technology was developed for Lockheed Martin's Falcon Combined-Cycle Engine Technology (FaCET) program. The goal is to develop hypersonic technologies that would enable "prompt global reach," which in this case means delivering 12 tons of payload up to 10,000 miles away in less than two hours.