Lockheed's SR-71
The SR-71 is currently the fastest manned plane in the world. Wikipedia

A new type of jet that flies so fast across the Atlantic you could take off from London as Downton Abbey begins and reach New York before the final credits roll has been announced by defence contractor Lockheed Martin.

Engineers at the company's "Skunk Works" base in California say they will be able to construct the SR-72 jet after solving a problem which has long hindered attempts to build planes capable of flying at more than twice the speed of sound.

Although not absolute, the speed of sound - Mach 1 - is usually understood as 761 mph. Mach 6, therefore, would be a speed of 4,567.2 mph, which is three times faster than Concorde.

Currently the world's fastest manned plane is Lockheed's SR-71 Blackbird reconnaissance plane, which has reached speeds of Mach 3.2.

Designers have been trying to design planes capable of hypersonic (Mach 5+) speeds for decades, but the stumbling block has been the engine. Traditional airliners use conventional jet engines which struggle to fly faster than Mach 2.5. The so-called "scramjet" works best at speeds exceeding Mach 5 (and is thought capable of attaining speeds of up to Mach 10) but is less efficient at lower speeds, so the trick has been to bridge the gap of this "thrust chasm".

Lockheed engineers say the answer is to build a turbine/scramjet hybrid engine which shares one inlet and nozzle. The airflow between the two engines can be shifted by mechanical systems as the airspeed changes.

A combined team of developers at United States Air Force, DARPA, NASA, Boeing, and Pratt & Whitney Rocketdyne have already achieved hypersonic flight with their X-51 Waverider scramjet, which on 1 May 2013 reached speeds of Mach 5.1 (3,400 mph) for approximately four minutes.