TOKYO -- Hopping on a supersonic plane for a 3 1/2 hour flight from Tokyo to Singapore? It sounds too good to be true. But such flights could become a reality in a decade or two, given the progress being made by engineers at the Japan Aerospace Exploration Agency (JAXA).
Planes that can fly at speeds above Mach 1 (1,224kph) are moving faster than sound waves can travel, so they are called supersonic aircraft. Almost all fighter jets are supersonic by this definition, so much of the technology is already at hand. But a passenger jet requires quieter flight, without the sonic booms that can accompany supersonic flight. The noise created controversy for the supersonic Concorde, which could reach maximum speeds of Mach 2; it was retired in 2003.
JAXA has been working to design just that kind of quiet supersonic passenger plane, and a recent drop test with an experimental model in Sweden suggests the agency is on the right track.
Conducted in late July at the Esrange Space Center in northern Sweden with the help of state-owned Swedish Space Corp., the D-SEND#2 drop test used a balloon to lift JAXA's Silent SuperSonic Concept Model (S3CM) unmanned supersonic glider to a height of 30,000 meters and drop it. Sensors measured the shock waves that reached the ground as the glider gained a top speed of Mach 1.39 in free fall.
"Attenuating the sonic boom is the biggest issue we face with supersonic aircraft, and our technologies have been proven," said D-SEND#2 project manager Kenji Yoshida.
An object moving faster than sound powerfully compresses the air in front of it, creating an area of high pressure that spreads out in all directions as a shock wave.
This kind of shock wave is what shattered the windows in thousands of buildings when a meteor streaked across the southern Russian city of Chelyabinsk in February 2013.
As a shock wave propagates through the air it begins to attenuate, weakening to become a sound wave that can result in what our ears perceive to be a loud explosion, as in a fireworks display. This is a sonic boom.
Back in 1976, Concorde became the first supersonic passenger plane to offer regular scheduled international flights, making the trip between Paris and New York in just 3 hours and 45 minutes.
It seemed at the time that supersonic planes were poised to take off, even in Japan, and Japan Airlines for a while considered introducing them.
But the boom almost spelled doom for Concorde. The engineers who designed the plane in the 1960s assumed that if it flew at high altitudes the shock waves would weaken enough before they neared the ground to eliminate the need to take sonic booms into consideration in their design plans.
However, test flights showed Concorde to be much noisier than expected, and that killed the idea of flying the plane on transcontinental routes. Restricted as it was to transoceanic routes, there was simply not enough demand to justify mass production of Concorde. Unable to benefit from economies of scale, the supersonic plane was expensive to make. And as the focus in the airline industry shifted from speed to passenger capacity, Concorde's terrible fuel efficiency led to its retirement.
What caused Concorde to make the characteristic double-boom sound that is made by any plane, including the Space Shuttle, flying at supersonic speeds?
The greater the air is compressed in front of a moving object, the more powerful the shock wave and the faster it travels. In the case of Concorde, the wings cause the greatest air compression, so the more powerful shock waves from the wings travel fast enough to catch up with the less powerful shock waves from the nose before they reach the ground, merging to create a larger pressure wave and a louder boom.
A similar but opposite process takes place at the tail end of the plane, where the air through which the plane has just passed is powerfully decompressed to create a shock wave. As air rushes back to fill the space, another shock wave is generated and these merge to produce what we hear as the second boom.
Any plane flying faster than the speed of sound generates shock waves. JAXA knows that is unavoidable, so it is working to design a supersonic passenger plane whose nose and tail create more powerful shock waves that travel fast enough to stay ahead of the shock waves from the wings and elsewhere.
For example, the nose of the plane has a flat, rounded shape like the beak of a duck-billed platypus, designed to compress the air more powerfully, resulting in shock waves that travel faster.
In addition, the tail is designed to powerfully decompress the air and create shock waves that can cancel out the waves generated at the front of the plane.
Since the shock waves generated by the different parts of the plane do not coalesce, the peak pressure created by the plane is lower, dampening the noise.
That is the idea, anyway, and the drop test in Sweden this July proved that the JAXA design is on the right track. The measurements recorded during the test showed that the 7.9 meter-long S3CM model built with the help of Fuji Heavy Industries created around 32% lower pressure than a Concorde model of the same size.
JAXA hopes to use this new design to develop a 70 ton-class, 50-seater supersonic passenger plane by 2030. Computer simulations suggest the design can reduce the shock waves by 75%. And since this should reduce the noise, it promises to free supersonic planes to fly overland on any routes, including the 210-minute flight from Tokyo to Singapore for a business daytrip to the city-state.
To get its bird in the sky, JAXA may even consider licensing the technology to a foreign entity.
While JAXA is busy in Japan, the U.S. space agency NASA is collaborating with Boeing and Lockheed Martin on a project of its own. Meanwhile, European aircraft maker Airbus Group is working with the U.S. startup Aerion to develop the Aerion AS2 12-seater supersonic business jet. The AS2 is designed to fly at Mach 1.5, but Airbus is developing supersonic jets meant to fly faster than Mach 4.
If the engineers can get it right, supersonic passenger jets could once again begin flying the skies as early as the first half of the 2020s, and this time over land as well as across the ocean.