On June 7, Lockheed Martin's Advanced Development Programs (ADP), also known as Skunk Works, revealed new information about their hypersonic technologies. Specifically, they discussed the successor to the SR-71 Blackbird reconnaissance aircraft: the SR-72, a strike and reconnaissance aircraft. The SR-72 tops Mach 6, and is currently in development within the advanced aircraft-development division. According to Lockheed Martin's statements to Aviation Week, hypersonic technologies, including a combined cycle propulsion system that blends a rocket engine and a supersonic jet engine, are sufficiently advanced to allow the planned SR-72 project to begin.
A hypersonic successor to the SR-71 has been the ultimate goal for DARPA and the Air Force (USAF) since the early 2000s. In 2013, the USAF, at last, announced that it had begun to design a scaled SR-72 demonstrator. Since that time, however, details about the program have been scarce, if the information was released at all. Here's what we know now.
In 2006, Lockheed sought out Aerojet Rocketdyne for a partnership; it was this alliance that led to the creation of the combined cycle engine, modified from an off-the-shelf turbine. Ground tests on a combined cycle engine built with elements of both a rocket engine and a scramjet were conducted between 2013 to 2017.