Skunk Works, the semi-top secret military aircraft development arm of Lockheed Martin, is looking into a follow-up to its infamous SR-71 Blackbird. The super-fast Mach 3 spy plane was used to run high-altitude surveillance on the Soviet Union during the Cold War.
Concepts for the SR-72 were shown in Aviation Week's look at the new project. This next-gen plane would be designed to break Mach 6, or six times the speed of sound. Much of the project will rely on data collected from the HTV-2 and HTV-1 missions DARPA conducted back in 2010 and 2011. Those designs used scramjets which are specialized engines designed to operate up to Mach 24 (which isn't too far from the speeds many spacecraft hit).
Hypersonic flight, as one might imagine, is fraught with complications. The original SR-71 was engineered with flexible skin, because at Mach 3 its frame hit temperatures hot enough to melt lead. Creating materials that are strong enough to support the craft, light enough for flight, and heat-resistant enough that the whole thing doesn't burn up isn't easy.
During trans-sonic flight, typically between Mach .8 and 1.2, airflow near the plane is unstable, with some of it breaking the sound barrier and some of it remaining a fair bit below that. Past Mach 1.2, almost all of the air is compressed into a single shockwave surrounding the craft. As those speeds begin to tick up, however, critical parts that keep the plane in control need to be able to withstand extreme forces. Add some heat and the entire frame faces some tough engineering goals.
Despite the challenges, Lockheed is confident they'll have a finished plane soon, perhaps as early as 2018.