How loud is NASA's Space Launch System?

The answer: As loud as 40 million bowls of Rice Krispies. In this episode, we talk with Kent Gee of Brigham Young University about his recent research trip to measure the launch acoustics of NASA's Artemis-I mission, why understanding launch noise is so important, common misconceptions about how loud rockets are, and more.

Associated paper: Kent L. Gee,   Grant W. Hart, Carson F. Cunningham, Mark C. Anderson, Michael S. Bassett,   Logan T. Mathews, J. Taggart Durrant, Levi T. Moats,   Whitney L. Coyle, Makayle S. Kellison, and Margaret J. Kuffskie. "Space Launch System acoustics: Far-field noise measurements of the Artemis-I launch." JASA Express Letters 3, 023601 (2023); https://doi.org/10.1121/10.0016878

Read more from JASA Express Letters. 

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 Music: Min 2019 by minwbu from Pixabay. https://pixabay.com/?utm_source=link-attribution&utm_medium=referral&utm_campaign=music&utm_content=1022

Transcript

Kat Setzer  00:06

Welcome to Across Acoustics, the official podcast of the Acoustical Society of America's publications office. On this podcast, we will highlight research from our four publications. I'm your host, Kat Setzer, editorial associate for the ASA.  If you've been paying attention to the news lately, you may have heard about today's guest and his research regarding NASA's Artemis-I launch. Kent Gee of Brigham Young University was able to record the launch noise of NASA's Space Launch System, the world's most powerful rocket that has been successfully launched. He published his findings with a host of coauthors in the March 2023 issue of JASA Express Letters in the article "Space Launch System acoustics: Farfield noise measurements of the Artemis-1 launch," which we'll be talking about with him today. Thanks for taking the time to speak with me today, Kent. How are you?

Kent Gee  00:53

I'm great. I'm really happy to be here. Love talking about rockets.

Kat Setzer  00:57

I'm excited to have you talk about rockets. So first, just tell us a bit about yourself and your research background.

Kent Gee  01:03

I am a Professor of Physics and I'm also the chair of the Department of Physics and Astronomy at BYU. We have a strong emphasis of acoustics for both graduates and undergraduate students. And I've been studying the acoustics of loud sound sources for like 20 years now, from military aircraft to explosions to gunshot to rockets and volcanoes. And so we study the sources, we study the characteristics of the noise, and we study, in some cases, the impacts of the noise. So when we were preparing for this episode, you gave some backstory about how the research project came about and why there are 11 authors on this paper. Can you tell us a bit about the development of this project? Absolutely. The project came about to make measurements, right, of the Artemis-1 launch of the of the Space Launch System... came about as sort of this wild idea: can we take a team of undergraduate students and give them a world-class, once-in-a-lifetime experiential learning opportunity? And we worked with the Utah NASA Space Grant Consortium that was excited to fund a trip down to Kennedy Space Center with our equipment. We talked to BYU, got Rollins College involved, Dr. Whitney Coyle at Rollins College, because they were down in Florida, so they were close to the action. And so we ended up with three faculty members and eight students that were involved in this, six undergraduates and two graduate students at BYU that were involved in the launch measurements. And so that's the reason for so many authors. And it really came out of the opportunity to provide students with just an incredible experience to go take these kinds of data.

Kat Setzer  02:55

That's such an awesome opportunity for the students. Okay, so probably not a simple question to answer, but can you give us a bit of background about rocket launch acoustics in general? What causes it, why do we care about it, that kind of thing?

Kent Gee  03:06

How long do you have? We can talk all day about this, but to be hopefully straightforward, right? So inside a rocket engine, right? It's the fuel propellant tanks, right? They have stored chemical potential energy, right? So you convert that to the kinetic energy of the plume, right? So now you have this high speed exhaust plume. It's coming out the back of the rocket, which is providing the thrust. Well, we talk about often the mechanical power provided by the rocket plume, that which is related to its thrust and the plume velocity. So when we talk about that kind of power, for the largest launches, that's like in the tens of gigawatts, 45 gigawatts for the Saturn-V. A gigawatt is, just for reference, that's a billion watts. Right? So you're talking about 5 billion watts of power that are being generated by this. 

Kat Setzer  04:08

Oh, wow. Yeah, okay, 

Kent Gee  04:09

Now, only a tiny fraction of that, less than 1%, on the order of a half a percent, but less than 1% of that power turns into sound. With that much mechanical power being generated, even a tiny fraction of it turned, being turned into sound turns into a lot of sound. 

Kat Setzer  04:26

Yeah, right.

Kent Gee  04:27

Right. That the Saturn-V produced on the order between 200 and 250 megawatts of sound power. Now why do we care we care about the sound generated by rockets? Because of their potential impacts. The sound from a rocket is so extreme that it can damage the payload. It can damage the rocket itself. It can damage critical launch structures surrounding the rocket pad. As you move farther out, it can have impacts on wildlife; rocket launch complexes often double as wildlife preserves. And then, of course, as you move even farther out, then you have the communities. What are the noise impacts at communities? What are the rumble, the crackle, the different sound qualities associated with rocket launchers, how do they impact sleep? How do they impact people's annoyance? How do they disturb people's day-to-day activities? We don't have great answers to these questions. And so that's why we're trying to study them.

Kat Setzer  05:24

Okay. Yeah, that absolutely makes a lot of sense. So you talked a little bit about Saturn-V already. In this article, you referenced one of your past articles, where you talk about how there were all these misconceptions about the noise created by Saturn-V back in 1969. Some people claim that it could melt concrete or set grass on fire over a mile away. And your article talked about how that's not true. Where do these misconceptions come from?

Kent Gee  05:45

Where do any internet misconceptions come from? I think is the is the question here. But we're not entirely certain. But I think, honestly, what happens is that one person makes a claim, right? Someonoe told a story. And then the next, you know, you have this game of internet telephone going on where one person says it, the next person says it, and then all of a sudden, people start citing that as fact. And so when we read this, we were just astounded that people all over the internet on these forums like Reddit and Quora were talking about these incredible claims about the Saturn-V. And so the misconceptions themselves got, we're not sure of the origin. But ultimately, they're founded in the misunderstanding of fundamental physics, they just don't understand that these kinds of sounds are not humanly possible. As sound that can cause fire, a sound that can that can melt concrete, you're talking about sound energy that's well beyond our ability to produce it.

Kat Setzer  06:52

Okay, got it. To get back to your recent article, how is your current research meant to prevent these types of misunderstandings?

Kent Gee  06:58

Our research is founded on the idea of taking high-quality measurements from different static rocket fairings and different space vehicle launches, and then using those measurements to develop physics-based models for how much sound is produced. What is the spectral or frequency content of the noise that's produced? What are its waveform or temporal characteristics of that noise? And then how does that noise travel out to the receiver? And finally, then, how does the receiver perceive, observe, you know, react to that noise? And so there's, there's these fundamental ideas of source propagation receiver in noise and noise control. And we are trying to tackle all aspects of that problem by starting with collecting high-quality data

Kat Setzer  07:52

Absolutely. Makes sense. Yeah. In this recent article, you are talking about the noise generated by NASA Space Launch System, which launched the Artemis-I mission, which is a super heavy lift vehicle. What are super heavy lift vehicles. Aren't all rocket super heavy?

Kent Gee  08:07

That is a great question. And you know, a few years ago, I honestly couldn't have answered that question because I was learning myself more about these rockets and different classifications of rockets. They classify rockets by their ability to lift different masses of payloads into orbit. And so a super heavy lift rocket is able to launch at least 50 metric tons, or a little about, about 110,000 pounds, 50 metric tons, into lower Earth orbit. And so the Space Launch System can actually launch almost double that, almost 100 metric tons, into lower Earth orbit. And believe it or not, the current version of SLS can launch something like 26 metric tons all the way to the moon. 

Kat Setzer  08:55

Oh, wow.

Kent Gee  08:55

It's a lot of power. A lot of power in this rocket.

Kat Setzer  08:59

Yeah. It's like, I can't even quite quantify how much that is. Like, how many elephants would that be?

Kent Gee  09:04

We'd have to look it up. I mean, I started thinking, how many, you know, pickup trucks, how many, you know, of this? It's amazing how much they're able to lift with this, right? And so, you know, more common rocket like the Falcon-9. It's a medium lift rocket in its ability to get payloads into orbit. So SLS is just... its ability to launch a payload into orbit, it's just absolutely incredible. But then when you get close to the rocket, and you look at how many stories, right, something like what is it, 312 feet or something like that, around 300 feet tall, you realize just how enormous this is. By itself, you have this incredible feeling of looking at this going. How is this you know, 28 story building or 26 story building able to go up into space, the rocket by itself! You just... it's hard  to believe that that is going to go up into space. Right? Yeah. How can that go up in the air and stay up in the air? Right? Yeah. But that gives you an appreciation for just how powerful these rockets are, right? That they are able to not only get themselves but a massive payload up into orbit.

Kat Setzer  10:19

So this is a good segue. How does the use of the Space Launch System compare to previous rocket launches?

Kent Gee  10:25

The Space Launch System is based around the Artemis program. The Artemis program is designed to take humankind back to the moon, right, to land the first woman and person of color on the lunar surface. And that will happen with the Artemis mission. The Artemis-1 was uncrewed. The Artemis-2 will be crewed around the moon, right, so launching astronauts all the way back to the moon, and it's with Artemis-3 will, should be the first mission that should land people back on the moon. It is absolutely exciting. And so the Artemis program, and then there are different versions of the rocket beyond that, that are being developed to eventually establish a permanent lunar colony.

Kat Setzer  10:56

That's exciting. Oh, neat! Just like in the sci-fi novels!

Kent Gee  11:10

We are absolutely in a sci-fi era. 

Kat Setzer  11:12

Yeah. 

Kent Gee  11:13

And it's amazing. It's incredible how little attention people are paying to these incredible things that are going on at NASA, at SpaceX, and just around the world, around the world in terms of the ability and the interest in creating a new space age that's just unlike anything the world's ever seen.

Kat Setzer  11:34

Yeah. So one of the cool aspects of this launch that you discuss in this article is how the atmosphere and timing allowed for visualization of the pressure waves during liftoff. Can you describe this? And how did the pressure waves relate to the noise created?

Kent Gee  11:48

Because the Artemis-1 launch was a nighttime launch, the launch pad was lit up by bright lights. And because of the intensity of the waves,  combined with the humidity of the atmosphere, as these intense pressure condensations-- but actually, more importantly, the rarefactions were produced, the rarefactions actually are the drops in the local pressure due to the sound wave, right? As the local pressure dropped, it dropped below the vapor pressure of the atmosphere and resulted in cloud formation. So you could see the pressure waves through the rarefactions, the decreases in local atmospheric pressure. You could see them through very rapid visible moving cloud formation. 

Kat Setzer  12:39

Oh, that's nifty. 

Kent Gee  12:40

And so those clouds that you're seeing there are in fact the pressure waves that are formed at the launch pad by the acoustics of the rocket.

Kat Setzer  12:47

That's so cool. So how did you measure the acoustics of the Artemis-1 launch?

Kent Gee  12:51

With great difficulty. No, we set up our equipment... We have equipment that we've developed that's weather robust, that can withstand rainstorms and wind and everything. And we have weather-robust, hardened equipment that we're able to set out there. And then it triggers either on time or at what the amplitude of the liftoff so it can all be done unmanned, right, autonomous recording systems that are able to make these measurements. And so we set out our equipment at 10 different locations around Kennedy Space Center. And we set up for the first launch attempts at the end of August. And of course, there were some challenges in it didn't launch the first time. And then of course, there were two hurricanes that went through. We did take our equipment down in between, but then we set back up for the November attempts and were able to collect our data at 10 different stations on center, as well as between BYU and Rollins College students, we were able to make recordings at manned stations outside of Kennedy Space Center at several different additional locations.

Kat Setzer  13:54

Very cool. So how are far-field measurements used compared to near-field measurements?

Kent Gee  13:59

When we talk about far field, right, we're talking about being far enough away from the rocket that you almost consider it to be this point source, right? And you're far enough that it seems that it's small. Okay, near field though, right? When you're in the near field, you have this microphone and you have this enormous rocket and this enormous plume, and you're trying to understand what's happening right at the rocket, right at the launch pad. Our measurements, out of logistical necessity to be able to participate in the Artemis-1 launch, our measurements were required to be in the far field, or at least the closest where we were at was one-and-a-half kilometers away. So about a mile away, which it seems like it's really far, but with a rocket that tall, it turned out that it still felt pretty close. The idea is that if you can collect data in the far field, and if you understand the physics of what's going on, it helps you understand what's happening closer to the rocket. For example, one of the major analyses we're trying to complete is how much sound power does the Space Launch System generate? Now, that's not in the first article, and we still don't have that answer yet. We're working with NASA to get the required trajectory data to be able to produce that answer. But once we know how much sound power that SLS is producing, that's something that the people that care about what's happening at the launch pad, they can take that as a direct input to their model.

Kat Setzer  15:20

So how loud is the Artemis-1 launch?

Kent Gee  15:23

That is a nuanced question. How loud is it? Right, that's the million dollar question. Right? And in the sense of everyone wants to know how loud it is. But in acoustics, of course, it's a difficult thing to answer because loudness is not sound level, right? What we perceive as something is being a certain loudness depends on its frequency content and its amplitude. And so that is actually a question that we're not really able to answer directly is how loud it is. But I can tell you about the sound levels. At one-and-a-half kilometers away, it was approximately... the maximum level that we measured during liftoff was around 136 decibels, right? 136 dB. That's kind of like the sounds that you'd associate with a jet takeoff, right? But you're a mile away. 136 dB. In addition, right, so five kilometers away, it was round, 128-129 dB, so three miles, five kilometers away, right? We talk about it being in the high 120 dB range. But to put that in perspective, right? And I reused that five kilometers, because people were actually standing around those distances; the media were allowed to be at those kind of distances. So okay, so 128 dB was the maximum level, 128 or 129 dB at five kilometers.Well, at five kilometers, then you can say, well, let's at least do the human-weighted or the a-weighted sound level calculation. Well, we did that in our paper. And we said, well, even though we're not sure that that really corresponds to human perception, for a rocket, right, we calculated the a-weighted levels, and it was about 106 dB. Now, 106 dB is like the sound levels you would associate with you operating a chainsaw, right? And that's three miles away.  Yeah. And that's three miles away, right? So I'm trying to provide at least some sense of what you might have experienced. Or the average levels of a rock concert. But what makes it so different is that you're standing three miles away, right? Not... I wasn't that close. I was much farther away. But I know what the people heard and what they felt. They felt this enormous rumble, this roar that passes through their bodies, because the dominant frequencies present in that launch, we're somewhere around 20 hertz, which is the low end of the human threshold of hearing, right? Okay, you're feeling it, you're hearing this rumble, this roar, and it's going through you. But on top of that, it's this broadband signal that you also are hearing, this intense, popping and crackling and snapping. And that crackle, right, is the result of these pressure discontinuities or shocks that we talked about in the waveform. And those shocks present this enormous impact, this load on your ear. It's very intense, pop-pop-pop-pop that you're feeling and hearing all at the same time. So in the article we even made mention of you know, another thing that crackles, which is Rice Krispies. And you said that five-mile range, right, somebody... I didn't do it, but somebody measured the a-weighted sound level of a bowl of Rice Krispies. Love the description, right? "To which milk has been freshly poured," right? And most people have had a bowl of Rice Krispies and heard that. You know, five kilometers away, it's like you're experiencing surrounded by 40 million bowls of Rice Krispies just sitting there. People like, you know, online, of course, Reddit and Quora, people are like, notice this. "They're like, we should go measure a swimming pool full of Rice Krispies. Pour milk into it." And I invite those kinds of comparisons, right, because we're trying to

Kat Setzer  19:11

Yeah, right.

Kent Gee  19:12

 It's so hard to describe what you actually experienced with a rocket that we try to connect it to other things, but having been there, it is so hard to even express the feeling and the sound, because it's unlike anything else you've ever heard in your life.

Kat Setzer  19:30

Yeah, I was gonna say it sounds almost kind of surreal in a way. 

Kent Gee  19:32

It is absolutely surreal. And so... or cereal, ha ha!

Kat Setzer  19:38

Good pun. Well played. 

Kent Gee  19:41

It's what we do, right? People actually asked me why we included the comparison of the Rice Krispies there. And I said, Well, you know, the chainsaw and the rock concert is one thing, the Rice Krispies was to give a sense of that sound quality of the crackle because not all things crackle. But honestly, it was an effort to inject a little bit of humor, right? I believe in humor, right? Anytime we can inject a little humor into our science, it humanizes the experience, right, of our research. And I like doing that.

Kat Setzer  20:11

I appreciate that. Yeah. So what can you do with this information that you gained from your measurements?

Kent Gee  20:16

That's an excellent question. One that's very timely. Just before this podcast, I was just on the phone with NASA this morning. And we were talking about what we can do with our analyses, and how they hope to take the information that we're learning and use it to improve the models that they have for the Space Launch System rocket. So we hope to use our data to understand what's happening not only far away from the rocket, extrapolate outwards, to say, well, what's happening with the wildlife, what's happening with communities, what's happening at different structures that may be sensitive to vibrations? But we also want to extrapolate inward and see what's happening at the launchpad, what's happening near the rocket. And once we have this information, they can input that into models and say, well, how good are our models? And where can we improve them, because this is not the only SLS rocket, right? They call this SLS Block One, and then there's going to be Block Two and Block Two Cargo. The rockets are going to get progressively larger. And as they get larger, they will be more powerful. And as they're more powerful, they will be, they should be louder based on the physics. And so the information that we are gaining now is critical to the development of better models of the impacts of these larger SLS rockets.

Kat Setzer  21:38

What are the next steps in your research?

Kent Gee  21:40

We are working with NASA to get the rest of the required data that we need to finish our analyses. We eventually hope to answer the question that everyone has been asking me: How does SLS compare to the Saturn V? How will it compare to SpaceX's Starship? These are questions that we don't really have the answer to yet. But we, as we continue to analyze the data, we will be able to answer those kinds of questions, at least to the ability that we're able to understand what really happened with the Saturn V back in the 1960s. But beyond that, we are trying to understand the source. We're trying to understand aspects of the propagation. And then we are also trying to understand the impacts at the receivers. And so a source propagation receiver is all part of our next steps and plans for what we can do with the data.

Kat Setzer  22:29

Awesome. Do you have any closing thoughts?

Kent Gee  22:33

This was an amazing opportunity to be able to make these kinds of measurements  with NASA, and to be able to be at Kennedy Space Center, to be able to watch the largest rocket launch ever, right, successful rocket launch ever and to watch this go up into space. And it was such an experience for not only the students but for myself, and honestly one of the greatest acoustical research experiences, if not the greatest of my entire life and career. So I feel very fortunate to be able to participate in such historic event.

Kat Setzer  23:05

Yeah, that is so cool. And it sounds like it was such a great experience for you and, like you said, for your students. So thank you for taking the time to speak with me today about it and shed some light on rocket noise for our listeners. You know, as much as we can understand it without experiencing it. It sounds like there's still a lot to learn, and I look forward to seeing where your future research takes you. 

Kent Gee  23:25

Thanks Kat. 

Kat Setzer  23:27

Thank you for tuning into Across Acoustics. If you would like to hear more interviews from our authors about their research, please subscribe and find us on your preferred podcast platform.