Sonic booms are always pretty impressive, but they’re not always desired. That’s why NASA is working on supersonic aircraft that don’t make quite so much noise, and you’ll be able to see – well, hear – them conduct some test flights this November if you’re based in Galveston, Texas.
Sometime then, an F/A-18 fighter jet will conduct a series of maneuvers, those designed to create both very loud sonic booms and somewhat more muted ones. This jet isn’t designed to be muffled, but the loudness of the boom can be adjusted depending on how you fly.
The agency will then use feedback from audio sensors and 500 local volunteers to find out how much they heard. By using this data, they’ll then feed it into their sonic boom-killing research program, one that hopes to design supersonic-but-silent planes.
In the following footage, the jet produces both a normal sonic boom, then a "low boom":
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A sonic boom is more than just a loud bang, and it certainly isn’t an explosion in the way you may be thinking. It’s a fluid dynamics thing: When an aircraft traveling through the air – the fluid – moves at increasingly fast speeds, the molecules of air at its nose get increasingly compressed. This is represented as an uptick in localized air pressure.
When traveling at supersonic speeds, those faster than the speed of sound in air, these air molecules are pushed aside with such considerable force that the individual pressure waves merge and form a shockwave. This shockwave moves outward in all directions, and when it reaches your ears, it sounds like a “boom”.
It’s not one discrete noise though, but a continuous boom noise that you only hear once as the shockwave passes you by – at least while the speed of sound continues to be exceeded.
The associated noise pollution remains unwelcome, however, and supersonic flight over land in the US has been banned partly because of this since the early 1970s. In order to circumvent this problem, engineers are looking to create not a boom, but a barely perceptible “sonic thump”, a shockwave whose sharpness has been largely reduced by the time it reaches you on the ground.
That's why NASA is undertaking this project: to produce super-speedy aircraft that don’t make quite the same sonic entrance as others. They already have a concept for the plane, dubbed X-59 QueSST, something that’s been contracted to be built by the Lockheed Martin Aeronautics Company to the tune of $247.5 million.
The X-59 is shaped in a way that’s not conducive to the buildup of shockwaves. You’ll still get shockwaves, sure, but they won’t be able to coalesce and form a sonic boom.
The F/A-18 is no X-59 – the best it can do is to focus its shockwaves in one direction by performing a specific type of dive. This will create a boom near the dive point, but it should barely register in the other directions.
The X-59 doesn’t physically exist yet, though, and before it does, more data on how to disrupt and determine when a sonic boom can be heard is required. That’s what the upcoming flights over Galveston are designed to ascertain.
It’s not entirely clear when, but the Low-Boom Flight Demonstration project looks increasingly certain to end in the appearance of the X-59. If so, then supersonic travel over the US may once again be legal.