Why is there a renewed interest in flying at hypersonic speeds?

One of the most iconic examples of aircraft from the era is the Lockheed Martin SR-71 Blackbird, which has come to symbolise the espionage that went on during the Cold War. Designed as a recognisance aircraft, crews could fly from New York to London in less than two hours and reach speeds in excess of Mach 3. It continues to hold the title of the fastest air-breathing manned aircraft ever flown. By contrast, modern military aircraft seem to have accepted the speed limitation of the turbojet engine of around Mach 2. The extra effort, and expense, to go faster has these days been traded against stealth technology in a bid to continue to operate in airspace controlled by other countries, essentially to spy and gather intelligence. However, there does seem to be some renaissance from serious aerospace companies outlining plans to again push the boundaries of speed. There has been renewed interest by military forces, particularly in the US, to build another very high speed spy plane to fill what it considers to be a coverage gap between surveillance satellites, subsonic manned aircraft, and unmanned aerial vehicles for intelligence, surveillance and reconnaissance (ISR) and strike missions. Over reliance on satellite reconnaissance has spawned a growth in anti-satellite weapons, anti-access tactics, and counter-stealth technologies from opposition. In addition positioning satellites can day 24hours or more with movements able to be predicted and sometimes even tracked by a savvy enemy. A high-speed aircraft, however, could penetrate protected airspace in a matter of hours anywhere in the world, and observe or strike a target before enemies could even detect it presence. The US has historically felt most comfortable with technology that is a generation beyond its rivals. And by the mid-2020s it is believed that many countries will produce and export advanced aerial technologies that could effectively combat its current espionage tactic reliant on stealth and satellites. This is driving a rethink and the US Air Force is now seriously considering the further development of hypersonic aircraft to replace legacy systems that could soon become outclassed. Brad Leland, program manager for hypersonics at Lockheed Martin, says: "Speed is the next aviation advancement to counter emerging threats in the next several decades. The technology would be a game-changer in the theatre, similar to how stealth is changing the battlespace today." One of the most interesting examples with perhaps the most potential again comes from Lockheed Martin's Skunk Works. It is proposing a next generation hypersonic spy plane that could fly at twice the speed of Blackbird. Given its reputation and the fact it built the SR-71 as well as arguably the most advanced aircraft of the modern day in the F-35 Joint Strike Fighter and F-22 Raptor, it should be taken pretty seriously. Known simply by the designation SR-72, Lockheed Martin says its engineers are developing a hypersonic aircraft that could fly as fast as Mach 6 – that's London to New York in 45 minutes – to take up tactical roles for reconnaissance and combat missions. However, it is unlikely to beat all of Blackbird's records as disappointingly, but perhaps obviously, the SR-72 is expected to be unmanned. While the Blackbird was developed in the 1960s using a slide rule, paper and hand calculations, the SR-72 will be able to take advantage of the numerous cutting edge 3D CAD and simulation packages now available. In addition, while Blackbird relied on the pilot skill and analogue controls, the new generation of hypersonic aircraft will be a product of the digital age and need millions of lines of code, thousands of sensors and powerful computers to keep it aloft. The details of the SR-72 released so far focus a lot on the engine technology. The future aircraft will look to use an impressive and innovative concept on its proposed engine system. Initial reports so far say the engine will run an engine setup called Combined Cycle Propulsion. In a similar way to the Blackbird, it would have the ability to augment its inlet nozzles to allow the engine to switch modes, from turbojet to ramjet. Speculative reports look as if the SR-72 will use a flap at the leading edge of the air intake to direct the airflow between two separate channels for the turbine and ramjet engines. It is unclear, however, if these will be completely seperate engines, or just a single engine operating in different modes. For several years Lockheed Martin Skunk Works has been working with Aerojet Rocketdyne to develop a method to integrate a turbojet engine with the experiential supersonic combustion ramjet (scramjet) engine still being developed. The operation of a scramjet is likened to trying to light a match in a hurricane and relies on supersonic air being mixed with fuel and combusted. If successful, however, it would power the aircraft from standstill to Mach 6. Hypersonic speeds also place enormous thermal management issues on the engines and surrounding airframe. It created major problems when operating the Blackbird and lead to perhaps its most infamous design flaw. Due to thermal expansion, engineers could not get its fuel tanks to seal properly at ambient temperatures. On takeoff its widely acknowledged the aircraft leaked fairly considerable amounts of fuel over the runway. The 85% titanium structure is needed to heat up and expand to properly seal the tanks. In practical terms, this means that, once airborne the aircraft had to carry out a series of sprint flights to warm the airframe to such a degree that it would expand and seal the tanks. Then the Blackbird would need to carry out air-to-air refuelling before actually beginning its mission. During operations the airframe could reach over 300°C. Once landed, it required the pilots to sit in the cockpit for a cool down period to let the structure cool, and shrink, back to its rest state. Any future aircraft travelling at twice the speed of Blackbird will be subject to even greater heat pressures, and will need to overcome these shortcomings and design flaws. All of these factors support the widely-held belief that hypersonic flight is extremely expensive, technically near impossible, and will take decades to get right. However Lockheed disagrees and says future hypersonic aircraft do not have to be an expensive or a distant possibility. In fact it has been reported that Lockheed, 'have developed a way to work with an off-the-shelf fighter-class engine'. The SR-72 is now following the US Air Force's hypersonic road map for developing a hypersonic strike weapon by 2020, and a penetrating ISR aircraft by 2030. At present an optionally-piloted scaled demonstrator is planned to be built as soon as 2018. The demonstrator planned to be 18m long will be about the same size as the F-22 Raptor, and will be powered by one, full-scale engine capable of flying for several minutes at Mach 6. Flights of the demonstrator would be conducted starting in 2023 leading up to full scale testing and production in 2030. The SR-72 is to be similar a similar size to the SR-71 at about 30m long and is likely to have similar range. The SR-72 will face significant political challenges in being accepted by the US Air Force, which may well opt for lower cost, and much slower stealth UAV to perform future ISR missions. However, the SR-72 would promise to be a game-charger should it be built, smashing boundaries and breaking records. Its development could well usher in a new golden age of aviation.