he views expressed in this commentary are solely those of the writer. CNN is showcasing the work of The Conversation, a collaboration between journalists and academics to provide news analysis and commentary. The content is produced solely by The Conversation.<\/p>\n
Last week, American company Boom Supersonic flew faster than the speed of sound with its XB-1 supersonic demonstrator aircraft. It\u2019s now the first piloted non-military aircraft to break the sound barrier since the Concorde was retired from service in 2003.<\/p>\n
It\u2019s the first step in Boom\u2019s ambitious goal to have supersonic airliners carry passengers by 2029.<\/p>\n
But what exactly is supersonic travel? There are good reasons why it\u2019s not more common, despite the hype.<\/p>\n
What is supersonic flight?
\nThe Mach number is defined as a plane\u2019s speed divided by the speed that sound waves move through the air. To \u201cbreak the sound barrier\u201d means to fly faster than the speed of sound, with Mach numbers greater than 1.<\/p>\n
The Mach number is an important ratio: as a plane flies, it disturbs the air in front of it. These disturbances move at the speed of sound. In supersonic flight these disturbances combine to form shock waves around the vehicle.<\/p>\n
When people say you can see a fighter jet before you hear it, they\u2019re referring to supersonic flight: fighter jets can travel at around Mach 2.<\/p>\n
The sound from the fighter jet is trapped inside its shock wave; until the shock wave moves to your position on the ground, you won\u2019t hear the plane.<\/p>\n
The allure of supersonic travel
\nFor efficiency reasons, most passenger jets cruise slightly slower than the speed of sound, at around Mach 0.8 (this is subsonic flight).<\/p>\n
Boom plans to build an airliner called Overture that can fly at Mach 1.7. Flying supersonically can drastically decrease flight times. The company claims a trip from New York to Rome on Overture could take just four hours and 40 minutes, instead of eight hours.<\/p>\n
Boom isn\u2019t the only company working on this lofty goal. American firm Spike Aerospace is also developing a supersonic business jet, with the tagline \u201cdelivering the world in half the time\u201d.<\/p>\n
This is the value proposition of supersonic passenger travel.<\/p>\n
In limited ways, it did already exist in the 20th century. However, due to timing, bad luck and the laws of physics, it didn\u2019t continue.<\/p>\n
Designs for supersonic airliners began in the mid-20th century, and by the 1970s we had supersonic passenger flight.<\/p>\n
There was the little-known\u00a0Russian Tupolev-144\u00a0and\u00a0Concorde<\/a>, a Franco-British supersonic airliner operated by British Airways and Air France from 1976 to 2003.<\/p>\n Concorde had a capacity of\u00a0up to 128 passengers<\/a>\u00a0and cruised at Mach 2. It regularly travelled from London to New York\u00a0in around three hours<\/a>. The flights were expensive, mainly shuttling business people and the rich and famous.<\/p>\n Concorde was designed in the 1960s when it seemed like supersonic passenger transport was going to be the next big thing.<\/p>\n Instead, the Boeing 747\u00a0entered commercial service in 1970<\/a>. Cheap, large and efficient airliners like it blew Concorde out of the water.<\/p>\n Designed to cruise efficiently at supersonic speeds, Concorde was extremely fuel inefficient when taking off and accelerating. Concorde\u2019s expensive, \u201cgas guzzling<\/a>\u201d nature was a complaint levelled against it for most of its lifetime.<\/p>\n A catastrophic\u00a01973 Paris air show crash<\/a>\u00a0of the competing Russian airliner, Tupolev Tu-144, also shifted public perception on supersonic flight safety at a time when many airlines were considering whether or not to purchase Concordes.<\/p>\n Only 20 Concordes were manufactured<\/a>\u00a0out of the\u00a0planned 100<\/a>. It is still disputed today whether Concorde\u00a0ever made money for the airlines who operated it<\/a>.<\/p>\n Remember the fighter jets? When a plane travels supersonically, its shock waves propagate to the ground,\u00a0causing loud disturbances called sonic booms<\/a>. In extreme cases they can shatter windows and damage buildings.<\/p>\nWhy supersonic passenger flight didn\u2019t take off<\/h2>\n
Noise is a real problem for supersonic flight<\/h2>\n