So I've covered most of the general dynamics of the SR-71, from the engines and hybrid combustion systems to the air frame itself, and exterior structure. What more is there to cover? The potential development of an even more potent plane, the SR-72, announced as the latest project by Lockheed.
This conceptualized aircraft is an unmanned aircraft, capable of reaching Mach 6.0 speeds from standstill. The Airforce has envisioned using a similar air-breathing hypersonic propulsion system to the SR-71, with improvements to double it's speed. Aerojet Rocketdyne, the company merger between Aerojet and the SR-71 engine designer, Pratt & Whitney Rocketdyne, is working on developing the system from the scramjet powered HTV-3X, an aircraft concept that was cancelled in 2008.
The scramjet engine diagram (c.) in comparison to the ramjet (b.) and turbojet (a.) diagrams
Scramjets, much like the ramjet system I detailed in Part I, rely on a high initial speed to compress and slow down the incoming air before entering the fuel combustion chamber. Unlike the ramjet system, which maintains internal air speeds at subsonic velocities, the Scramjet operates with supersonic internal air speeds, significantly increasing the performance. Theoretically, scramjets can reach top speeds between Mach 12 and Mach 24.
A full-scale, optionally manned demonstrator is expected to be built in 2018. Tests for the design are expected to begin no later than 2023, with the final SR-72 model to arrive around the year 2030.