Silencing the boom: is there a future for supersonic flight?
Gabriel de Almeida
On October 24 th 2003, the Concorde took off for the final time, destined for retirement. 1 Throughout its short lifetime, one particular issue had prevailed above all else, forcing it into the history books, and compelling the aviation industry to take a step back, and return to the current era of subsonic flight. The issue was Concorde’s notorious sonic boom, which followed it as it soared past the sound barrier towards its destination: breaking windows, triggering car alarms, and frightening residents in towns and cities across the world. As a result, since 1971, the FAA 2 has banned all supersonic flight over land, 3 meaning supersonic passenger operations are currently restricted to over oceans only: a factor that contributed greatly to Concorde’s ultimate failure, and will do so again to a new generation of supersonic airliners if the problem is not solved. The solution? A reduction, or complete elimination of the sonic boom: an area that is being heavily explored by engineers worldwide. In this essay, I will evaluate some of the innovative breakthroughs and proposals for amitigation of the sonic boomduring supersonic flight, to assess whether or not we will see the return of supersonic air travel in the near future – and what that may indeed look like. When an aircraft flies at subsonic speeds, the sound waves it radiates build up in front of it as it moves. 4 However, when an aircraft flies faster than the sound it creates on passing the ‘sound barrier,’ the air can no longer propagate ahead of it. As a result, these sound waves form highly pressurized,
compressed regions called shock waves which trail behind the plane in all directions. This effect leads to the creation of a sonic boom. A supersonic aircraft actually produces two distinct sonic booms as it flies: to put simply, one shockwave forms at its nose, which then claps together again after its tail (Fig. 1), 5 forming two pressure spikes which could be represented on a pressure-time graph as the letter N. 6 These booms are produced at all times whilst an aircraft flies past the speed of sound, 7 causing a loud bang to be heard on the ground below, which has been described as ‘ gunshot- like’ and ‘ l ike an explosion going off’ in the past. 8
Fig. 1: shock wave propagation from a supersonic aircraft
Whilst a sonic boom is a natural phenomenon, there are an array of ways to mitigate its strength using fascinating engineering principles. In fact, there is a way of surpassing the sound barrier without
1 Lawless, J. (2003) Final Concorde Flight Lands at Heathrow. Available at: https://www.washingtonpost.com/wp- dyn/articles/A11477-2003Oct24.html [Accessed 27/07/2020]. 2 Federal Aviation Administration. 3 Gonzalez, C. (2015) Supersonic Flight: Overcoming the Sonic Boom. Page 1. Available at: https://cdn.baseplatform.io/files/base/ebm/machinedesign/document/2019/03/machinedesign_3246_supers onicflightovercomingthesonicboom.pdf [Accessed 20/07/2020]. 4 This is known as the ‘Doppler Effect’ . 5 Taken fromGoldsmith 2018: 50. 6 Scott 2003: 35. 7 760 Miles per Hour, or 340 Meters per Second at 15 ˚ C at sea level. 8 Sonic BoomMyths (2016). Available at: http://www.supersonicmyths.com/supersonic-myths [Accessed 27/02/2020].
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