Supersonic flight
producing a boom that reaches the ground at all! The technology is named Mach Cut-off, with aerospace companies such as Aerion and Boom Supersonic putting much research and money into making the idea a reality. 9 Mach Cut-off uses the earth’s atmosphere to prevent a sonic boom from a Supersonic Transport (SST) from reaching ground level. It is a well-documented fact that the speed of sound in air increases with temperature, and thus decreases at higher altitudes. Thismeans that, when a sonic shock wave propagates off the wing of an SST, it does not actually advance in a straight line as one might imagine. Instead, warmer air closer to the ground speeds the shock waves up, causing them to refract forwards towards the vertical – just like a ray of light would as it exits a pane of glass into air, in which it can travel much faster. When sonic shock waves are produced, the ‘rays’ of sound they create follow perpendicular to the shock-front, until they reach ground level where they can be heard by observers as Sonic Booms (see the white line in Fig 2). 10 However, if the sonic shock-front is refracted towards the vertical on its way to earth, so that it reaches the ground at 90 ˚ , these ‘rays’ of sound it creates remain perpendicular to it, and thus curve to be parallel with the ground surface. As a result, the sonic boom is deflected away from the ground, and nothing is heard by the observer. 11 Of course, the earth’s atmosphere can only refract the sonic shock fronts of an SST by a small amount. This means the shock fronts propagating off a supersonic aircraft must already be at an angle close to the vertical as they form for this phenomenon to take place. Thus Mach Cut-off flight can only occur when an aircraft flies at a very low supersonic speed: around Mach 1.0 to 1.2, 12 as this is where sonic shock waves propagate at the largest angles relative to the earth, allowing for them to be distorted enough to mitigate the thunderous boom from being heard at ground level.
Achieving be revolutionary for the aviation industry, decreasing journey times by up to 35% 13 with no dire noise consequences, and very little in the way of aircraft modifications required from to day’s standards. However, with little testing having taken place up until now, there is still a long way to go before the technology is approved by the FAA, and a ban on overland supersonic flight is overruled. 14 What’s more, daily changes in atmospheric conditions could have frustrating consequences for the proposal: Mach Cut-off would
for example, a heavy prevailing tailwind from the jet steam when travelling east-to-west could speed up supersonic shock-fronts at higher altitudes - keeping them oblique, preventing the vertical curving effect from taking place 15 and causing loud booms to be heard on the ground. As such, supersonic cruise would not be applicable all of the time. In fact, on board computers would need to examine atmospheric 9 Cariosca, Locke, Boyd, Lewis and Hallion 2019: 20-23. 10 Taken fromWilson 1962: 41. 11 Wilson 1962: 41-42. 12 The term Mach 1 refers to the speed of sound. Mach 1.2 is equivalent to 1.2x the speed of sound. 13 On average, taking into account mean aircraft speeds today vs proposed aircraft speeds utilizing Mach Cut-off technology. 14 Cariosca, Locke, Boyd, Lewis and Hallion 2019: 23. 15 Wilson 1962: 42.
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