The SR-72, the proposed successor to the SR-71 Blackbird retired in 1998 is expected to fill what is considered a coverage gap between surveillance satellites, manned aircraft, and unmanned aerial vehicles for intelligence, surveillance and reconnaissance (ISR) and strike missions. With the growth of anti-satellite weapons, anti-access/area denial tactics, and counter-stealth technologies, a high-speed aircraft could penetrate protected airspace and observe or strike a target before enemies could detect or intercept it. The proposed reliance on extremely high speed to penetrate defended airspace is considered a significant conceptual departure from the emphasis on stealth in fifth-generation jet fighter programs and projected drone developments. There were unconfirmed reports about the SR-72 dating back to 2007, when various sources disclosed that Lockheed Martin was developing a Mach 6 (4,567 mph; 7,350 km/h; 1 mi/s) plane for the United States Air Force. Skunk Works' development work on the SR-72 was first published by Aviation Week & Space Technology on 1 November 2013.
The SR-72’s design incorporates lessons learned from the HTV-2, which flew to a top speed of Mach 20, or 13,000 mph, with a surface temperature of 3500°F.
HOW RAMJETS WORK
Ramjets forgo the big rotary compressors needed on turbojets and instead rely on their own forward motion to compress air. First, air is scooped into an inlet and compressed as it funnels into a diffuser. The diffuser also slows the air to subsonic speeds for easier combustion. From there, air and fuel are fed into a combustion chamber and ignited. Finally, an exhaust nozzle accelerates the resulting burst of hot, expanding air, producing massive thrust.
Turbojet engines can take a plane from runway launch to about Mach 3; speeds faster than that require an air-breathing ramjet, which compresses high-speed air for combustion, but which typically begins operating at about Mach 4. To bridge the gap, engineers are developing a hybrid engine that can operate in three modes. The aircraft will accelerate to about Mach 3 under turbojet power, switch to ramjet power to take it to about Mach 5, and then switch again to scramjet mode, which uses supersonic air for combustion.
It could reach any location on any continent in an hour—not that you’ll see it coming.
Aerodynamic friction at speeds exceeding Mach 5 will heat an aircraft’s exterior to 2,000 degrees. At that point, conventional steel airframes will melt. So engineers are looking at composites—the same kinds of high-performance carbon, ceramic, and metal mixes used for the noses of intercontinental ballistic missiles and space shuttles. Every joint and seam must be sealed: Any air leak at hypersonic speed, and the in-rushing heat would cause the aircraft to collapse. (That’s what doomed the space shuttle Columbia).
The stresses on a plane shift as it travels through subsonic, supersonic, and hypersonic speeds. For instance, when a jet is accelerating through subsonic flight, the center of lift moves toward the back of the aircraft. But once the craft hits hypersonic speeds, drag on the plane’s leading egdes cause the center of lift to move forward again. If the center of lift gets too close to the center of gravity it can cause dangerous instability. The plane’s shape must tolerate these changes, and more, to keep the craft from tearing apart.
Lockheed describes the SR-72 as an intelligence, surveillance, reconnaissance, and strike platform, but its exact payload is secret. Most likely, it hasn’t yet been invented. Taking spy photos or dropping bombs at Mach 6 will require extraordinary engineering. It will require hundreds of miles to make a turn. It will need powerful guidance computers to line up targets, 80,000 feet below. Also, you can’t just open a bomb bay at 4,000 miles per hour. The SR-72 will need new sensors and weapons to operate at such high speeds.