When NASA’s Saturn V rocket launched men to the moon half a century ago, each blast stunned onlookers with its power. The flames from the launch blinded. His explosive rise was very loud. It captured the imagination of many around the world and still holds a place in spaceflight lore.
Several Saturn V power stories dramatize the acoustic power of that explosive moment. The sound of the launch reportedly melted concrete and set fire to nearby grass.
Aeroacousticians have new calculations that confirm that any such effect is certainly not caused by sound of launch, described in a new paper published on August 23 in Journal of the Acoustical Society of America. But, they say, the noises of really big rockets like the Saturn V are increasingly important to understand. NASA’s latest rocket — its largest ever, the Space Launch System (SLS) — will launch the Artemis I mission on Monday. Meanwhile, governmental and commercial spaceflight efforts are expanding rapidly around the globe.
“If you’ve been to a launch, the acoustics are huge,” says Kent Gee, a professor of physics at Brigham Young University and lead author on the new paper. But understanding sound intensity isn’t just about what you might hear near a missile launch. “If you don’t understand the acoustics produced by the missile, you can’t design payloads efficiently,” he says, because the acoustics of a missile launch can cause damage to everything on board. And that could make spaceflight unnecessarily challenging as humanity pushes deeper into the cosmos.
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The rocket launch noise comes from a complex combination of sources, says Caroline Lubert, a mathematics professor and aeroacoustician at James Madison University, who was not an author on the new paper but has previously collaborated with Gee to reassess the half-century the end of the rocket. noise, a task force that supported the new Saturn V paper. Aeroacoustics are most concerned about vibrations caused by rocket noise, which could damage the spacecraft, its payload, or even launch pad structures. These structures can also amplify the rocket’s acoustics by reflecting sound.
And with spaceports popping up in new locations around the globe, Lubert says, noise pollution from launches is a growing concern for surrounding communities and wildlife.
New calculations to predict how loud a launch will be are in order. Many of the ideas about the acoustic power of rocket launches are based on noise research that was done as far back as NASA’s Apollo program in the 1960s, Gee says. And some of that old information was based on observations rather than directly recorded data. Determining the actual impact of a Saturn V launch allows engineers to make direct comparisons between that lunar launch and future ones.
When Gee and colleagues were investigating historical records of rocket launch acoustics, he found that reports of Saturn V launch sound levels varied dramatically. Some reports suggested that the sound levels of a Saturn V launch were as high as 180 decibels, while others reported as high as 235 decibels. (For context, commercial jet engines range from about 120 to 160 decibels.) And, because this is a logarithmic measure, every 10 decibels is an order of magnitude increase.
“Putting that into the perspective of a light bulb, that’s like saying a 10-watt light bulb and a mega-watt light bulb are the same thing,” says Gee. “People really didn’t have a good understanding of what the levels were and what those levels were saying.”
Part of the challenge when evaluating sound, Gee explains, is that there are two different things measured in decibels: sound power and sound pressure. Sound power, he says, refers to the total amount of sound energy produced by the rocket. Sound pressure, on the other hand, is the amount of sound that reaches a certain distance. The further away from the noise source, the quieter it is and therefore the less sound pressure at that point.
It’s likely that some of the lower decibel reports emitted by a Saturn V launch come from sound pressure measurements rather than sound power, he says.
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When Gee and his team made a computer model of the sound power of a Saturn V launch based on the rocket’s thrust and other characteristics, they found that it would have produced about 203 decibels of sound power. This is really, really loud – but not loud enough to melt concrete or start a grass fire. “Mankind has not produced a sound source that would be capable of this, just from sound waves,” says Gee. By comparison, he says, the acoustics of a Saturn V launch would be the same amount of sound as about 700 military jets flying simultaneously.
Gee and his team expect the SLS launch of Artemis I to produce a similar amount of sound power to the Saturn V, perhaps a decibel higher. “There’s a little more thrust and a little more power produced by the rocket as it launches, we’ll use that and the modeling we’ve done before,” he says, “we’d take the same approach and that would suggest that the SLS it will be slightly louder than the Saturn V.
But it’s also possible that the SLS will end up being quieter, Lubert says. “We’ve done other things to compensate [in the half a decade since the Saturn V launched],” she said. “There is so much variability and so much uncertainty in predicting vibroacoustic loading.”
On Monday, when SLS is scheduled to launch Artemis I, Gee and his team will be nearby. They have placed sensors at strategic points chosen based on their Saturn V research, ready to check whether NASA’s most powerful rocket will also be the loudest.