Science: How Loud Are Rocket Launches?

If you watch a video of the launch site of the ‘Saturn V’, a large rocket developed by NASA in the 60’s and 70’s for the ‘Apollo’ project, there is an element that comes as a shock. The fashion and hairstyles of people at the time are also shocking, but above all, the spectators are much farther away from the rocket launch site than expected.

There are several reasons why spectators are so far away. And noise is one of them.

In fact, noise beyond a certain level can be life threatening, and the Saturn V rocket was one of the loudest artifacts of its time.

As the Apollo astronauts headed for the moon, the expectant spectators stood more than 5.1 kilometers away from them.

But even at this distance, the noise of a rocket launch was unbelievably loud. At the time, the sound waves generated by the engine of the Saturn V were so powerful that rumors circulated that the concrete of the launch pad had melted and that a lawn 1.6 km away had also caught fire. (Both rumors are false.)

The launch noise measured by NASA at the time was 204 decibels (dB). If you consider that the sound of a jet taking off is 120 to 160 dB, which is dangerous if a person listens for more than 30 seconds without any protective equipment, you can guess how amazing it is.

The launch noise of the Saturn V was measured at a whopping 120 dB even at a distance of 2.4 km from the launch site. This is equivalent to the sound of a rock concert or a car horn blaring very close.

Meanwhile, Anthony Lu, who runs a cafe in Florida where NASA’s ‘Kennedy Space Center’ is located and has taken pictures at rocket launch sites since the Saturn V, said, “Every time a rocket is launched, I am shocked by the feeling.” .

“There was a sound device called ‘Sensor Round’ in the 70’s. When shooting disaster films such as the movie ‘The Great Earthquake (1974)’, I used it to give the audience a feeling of ‘experience’ subsonic seismic waves directly in the theater.”

“The (rocket) launch is like a sensor round up close,” said Lu. The low frequency of subsonic speed briefly hits the ear and passes. A few seconds later, the sound of the shot merged into a roar, like the sound of a giant welding torch,” she explained.

Last year, researchers at Brigham Young University in Utah calculated how loud the Saturn V rocket must have been when it launched. And the result came out to be 203dB, very similar to NASA’s own record.

On the other hand, 160dB and 200dB may not seem like a big difference, but in reality the difference is huge.

“170dB is equivalent to the sound of 10 aircraft engines,” said Kent Gee, a physics professor at Brigham Young University at the time who led the research.

“A 10dB difference is a 10x difference.”

Then, was the Saturn V the loudest noise among the rockets launched so far? If thrust is taken as the criterion, probably not.

Saturn V produced 35 meganewtons (MN) of thrust at the time (the force pushing up when the rocket was launched), but the thrust of the unfortunate rocket ‘N1’, which the Soviet Union wanted to use for manned moon landings in the 1960s, was 45 MN, which is less than this. .

However, the Saturn V’s thrust was also formidable, so there were more factors to consider than just protecting the audience’s hearing.

First of all, a powerful rocket like the Saturn V can damage the spacecraft with just the sound waves generated by the launch noise.

NASA chief engineer John Blevins, who was responsible for the rocket’s Space Launch System (SLS), developed as part of Project Artemis, said preventing such damage had been a priority for NASA’s rocket engineers long before the Apollo program. .

One solution they came up with is to fill the launcher’s flame guideway with water to reduce the tremendous noise produced by rocket launch.

Researcher Blevins said, “During the Apollo program, NASA conducted several experiments on the ground, and now they are experimenting again for the SLS launch.” He also said that he studied whether this happens.

“You might not notice it when you sit and watch Banana Creek (a popular spot for watching rocket launches), but the peak noise from the rocket itself actually comes from about 45 meters off the ground. Instead of descending directly into a water-filled flame taxiway, a plume of smoke billows out, helping to dampen the noise.”

The launch of the ‘Space Shuttle’ also used a flame induction furnace filled with water. The daunting boiling cloud that erupts when NASA’s famous rocket launches is actually not smoke, but steam from water evaporated by the intense heat.

Meanwhile, the SLS is a new rocket used in the Artemis project, which NASA is promoting to send people farther into the solar system.

Researcher Blevins was at the site of ground tests of the SLS engine at the John C. Stennis Space Center in southern Mississippi.

These ground trials are an important step in the evolution of rocket design.

“I put my earplugs on about 800 meters (from the launch pad). In 600 seconds, there was an unimaginable amount of steam coming out of it. And when I got closer, I realized that one of the four engines was blowing out so much steam. You can see! But there are two on the side of the core stage[고체 로켓 부스터]has more thrust than these engines.”

“It’s part of how powerful this rocket is. It’s amazing.”

Blevins says the SLS is quieter than the Saturn V, but the noise level isn’t determined solely by the engine’s thrust.

“What people will hear will be different. For example, if it was a cloudy day and the sky was covered with clouds as high as 330 m, the (launch) noise would bounce back and forth across the state of Florida. The noise won’t die down easily, so on even the slightest overcast day you’ll hear rocket launches in the Tampa area.”

Tampa is on the other side of the Florida Peninsula, a three-hour drive from the Kennedy Space Center.

On the other hand, it seems possible that even louder rockets will be born.

The spaceship ‘Starship’ developed by ‘SpaceX’ for Mars exploration is scheduled to be launched on top of the super-large propellant ‘Super Heavy’. According to SpaceX, the super heavy’s thrust is expected to be 76 MN, more than twice that of the Saturn V.

So, if you plan to watch the launch of ‘Starship’ and ‘Super Heavy’ in person, it would be a good idea to bring earplugs.