Humans have been fascinated with speed for ages. The history of human progress is one of ever-increasing velocity, and one of the most important achievements in this historical race was the breaking of the sound barrier.
Built by Bell Aircraft between 1946 and 1947, the Bell X-1 was the first plane to travel faster than sound at the impressive speed of about 330 m/s or 1127 km/h at Earth's sea level. The X-1 was the first of many supersonic aircrafts, with later vehicles reaching speeds over Mach 3 (1 Mach = speed of sound).
However, at supersonic speeds, flight doesn't work as we might imagine.
As a rocket moves through the air, the molecules around the rocket are disturbed and shift around it. If the rocket passes at a low speed, typically less than 250 mph, the density of the air remains constant. But at higher speeds, some of the rocket's energy goes into compressing the air and locally changing its density. This compressibility effect alters the amount of resulting force on the rocket. The effect becomes more important as speed increases and, near and beyond the speed of sound, small disturbances in the flow are transmitted to other locations with constant entropy. These disturbances generate shock waves with a thunder-like noise known as a sonic boom that can be as loud as 200 dB, way above the pain tolerance level of 140 dB. This deafening noise can cause distress to people and animals below or even damage buildings.
Recently, NASA's aeronautical innovators led a government-industry team that collected data to make supersonic flight over land possible, aiming to reduce the intensity of the sonic boom. The research resulted in the creation of the experimental aircraft Lockheed Martin X-59 QueSST.
Developed at Lockheed Advanced Development Projects Center in Palmdale, California, the X-59 is expected to cruise at Mach 1.42, being up to a thousandth quieter than the previous aircraft with a ground noise of around 60 dB while breaking the sound barrier, similar to the sound of a car door closing. All of this can be achieved thanks to special studies on the geometry of a vehicle that possesses a narrow airframe, keeping shock waves from merging (the technical term for the phenomenon is coalescence) during supersonic flight.
The first flight tests will start in 2022. Even if the cost, fuel consumption, and environmental impact of supersonic aircraft remain problematic, a new era is beginning for supersonic flight.
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