Hypersonic speed is reached when velocity is above Mach 5 (equivalent to 3,600 miles per hour). NASA’s SR-71 is currently the world’s fastest air-breathing aircraft, soaring just above Mach 3.
Now, a design for a hypersonic aircraft that would have twice the fuel efficiency of commercial airliners and be able to fly between any two points on the globe in less than two hours has been developed by a researcher at the US Department of Energy’s Lawrence Livermore National Laboratory.
According to aerospace engineer Preston Carter, his HyperSoar concept could enable aircraft to fly at approximately 6,700 mph (Mach 10), while carrying twice the payload of subsonic aircraft of the same takeoff weight.
The key to HyperSoar is the skipping motion of its flight along the edge of Earth’s atmosphere. Using air-breathing engines, the aircraft would ascend to the outer limits of the atmosphere, turn off its engines, and then coast back into denser air. At this point, it would be propelled back into space by the combined action of increased aerodynamic lift and its engines. Outside the atmosphere, the engines shut off again and the process repeats. In this way, HyperSoar would skip off the top layer of the atmosphere every couple of minutes.
Whilst it may sound like a bumpy ride, Carter estimates that the angles of descent and ascent during the skips would only be 5°. Passengers would feel 1.5 times the force of gravity at the bottom of each skip and weightlessness while in space. ‘The average passenger will probably put up with the slight roller coaster motion if it gets them from San Francisco to Tokyo in less than two hours’, he adds.
Although there have been numerous designs for Hypersonic craft over the years, they have all been hampered by the problem of heat build-up on the airframe. Carter believes that the skipping motion of his design offers a solution to the problem. A HyperSoar plane would experience less heating, claims Carter, because it would spend two thirds of its time in the cold of space, where heat could be dissipated.
Whilst most hypersonic designs rely on rocket motors; HyperSoar is able to take advantage of the relative effeciency of air-breathing engines thanks to the fact that it drops back into the atmosphere at the bottom of each ‘skip’.
With potential applications ranging from passenger aircraft to bombers capable of carrying out sorties anywhere in the world ‘an aircraft with the speed, range and lift of HyperSoar could revolutionise intercontinental flight’ says Carter.
Around $140 million would need to be spent over the next few years to advance technologies to the point where a $350 million small-scale flyable prototype could be built and tested. In a similar project, NASA has developed an unpiloted, experimental, hypersonic air-breathing vehicle. The X-43 will use the body of the aircraft to form critical elements of the engine with the forebody acting as the intake for the airflow and using the aft section as the nozzle