NASA's Quesst for Quieter Supersonic Flight with Peter Coen

0:00 Hi, and welcome to the Neil Ashton Podcast. In each episode, we explained some of the fascinating ways that science and engineering are changing the world around us. We talked to leading engineers from elite level sports like cycling and Formula One to some of the world's top academics to understand how fluid dynamics, machine learning and supercomputing are bringing in a new era of discovery. We also hear some of their life stories, their career advice. The lessons they've learned on the way that I hope will be helpful to you too. So sit back and enjoy this episode. Hi, and welcome back to the Neil Ashton Podcast. Today's episode is one that is

0:44 focusing on a topic that I think is interesting to so many people and that is on supersonic flight, supersonic travel. I'm speaking to Peter Coen, who is a legend within NASA for all that he's done for the supersonic program over the past three decades. And I wanted to speak to him firstly because when I spoke to some colleagues and people I work with at NASA, they all recommended I speak to Peter as he is a fountain of knowledge on this program. We didn't really get into actually a focus on him himself. This was more of a topic on the NASA program. Although people tell me that he is a private pilot and has some

1:34 amazing stories to tell. But in, in this particular episode, we focus more on the actual NASA programs and what they're doing. And, and that is absolutely fascinating. We start the conversation going back and looking at how, how did we get to Concorde, you know, that incredible aircraft that was the first to go supersonic for, you know, passenger travel. And then talking, you know, why, why did it stop? What were the commercial reasons? What were the technical reasons? And then we transition into what makes supersonic travel difficult? What are the engineering challenges from a noise perspective, from a regulatory perspective, from an

2:13 aerodynamics perspective, and going through step by step in that process. And, and for anybody here who is an engineer, I think you'll find that particularly interesting talking about the specifics of how they overcame some of the issues with the sonic boom. And then we start to go through and talk about the specific Quesst programme. So this is NASA's project to develop a prototype, the X-59 aircraft that is currently in the process of being built. That will be a test to essentially validate all the work that's been done and to give data to the aviation authorities to be able to set the regulatory environment for

2:54 commercial entities to go and build and hopefully release these aircraft. So we're at a really interesting time. This project has been going on for quite a while and it feels like we're getting very close now to some very key tests that will dictate this. And we finish off with some discussion around, of course, hypersonic travel. Of course, if you can do supersonic, the logical question is, couldn't you do hypersonic? And then at the end, as I ask most people on this podcast, some sort of words of wisdom, I guess, to people who are aspiring aeronautics professionals. So this for me was personally one of the really interesting

3:32 episodes. I love the idea of supersonic travel and Peter is such a great communicator and it was an honour to have him on the podcast. So sit back and enjoy this episode with Peter Coen on supersonic travel. So maybe we could begin. I mean, this is, I think for most people a fascinating topic. But probably the one thing that people think of when we talk about supersonic travel is Concorde. So maybe as a bit of a starter, could you, maybe, I know this is difficult, summarize the history of supersonic travel. How did we get to the Concorde program? Sure. It really is—it actually is a fascinating history. I mean, of course, in 1947, we had

4:16 the first supersonic flight, the Bell X-1 out at what was then Muroc or Edwards Air Force Base. And you know, somebody heard of sonic boom then maybe. So that's a little interesting history, but you know, from there very rapid development of supersonic military aircraft. You know, in the US we had the Century series fighters. But you know, in in Britain it was, they were, it was the Lightning and other supersonic aircraft. And so supersonic flight became pretty common pretty quickly in the 1950s. And, you know, pretty much as soon as the basics were understood, people started thinking about, well, we should

5:05 have, you know, the next step in, in supersonic travel, in travel should be supersonic travel. So there were studies done all over the world, really. But, you know, of course, there were English studies too, and British Aerospace began working on designs and, you know, eventually came to the, the common program, the, the English, you know, the British French program to build the Concorde. So, you know, along that way, along the way, as supersonic flight became more popular, you know, initially sonic booms were kind of a novelty. You know, they'd do them at air shows and people would get all excited about them. But you know, as the airplanes began to be used in training

5:52 more and more often, you started having complaints of damage and, you know, noise complaints. And you know when people, when some people really started to think about. You know, regular supersonic operations, I think they were thinking of, we really were thinking about, you know, the entire air transportation market would become supersonic and people would begin to think about the number of overflights that that would involve. The number of sonic boom exposures that that people would, you know, would hear would become would become a problem. So. It led to a number of studies. Which involves community tests of people's response to sonic boom.

6:35 And most notably, there was a, a very long test with many, many exposures done in Oklahoma City in the mid 1960s, but was actually sponsored by the, by the US FAA. And, you know, it became quite apparent that regular sonic boom exposure would not be, would not be, you know, you know, would not be tolerable. So as Concorde development continued, you know, we also had the US SST program and that's really what drove a lot of the the sonic boom testing in the US was trying to understand the impact of the US SST which was going to be much bigger and and potentially much louder in terms of sonic boom than the Concorde.

7:20 So eventually, you know, Concorde did fly, but in, and I think it was 1968, maybe 1969 was was first flight commercial service started in 76. But in between those two times the FAA put in a ban on supersonic flight over land. They couldn't they there was that idea that if you could quiet the boom enough, you could, you could fly over land supersonic, but they didn't know what how quiet it would need to be. So eventually they just said, you know, no, no sonic, no, no supersonic flight means no sonic booms means nobody's going to get annoyed. So that, that was in 1973 that that ban was put in place internationally. There are other countries that

8:08 ban supersonic flight, but pretty much everybody sticks to what the International Civil Aviation Organization agreed to was they didn't put in a law, but they or, or a standard, but they said nobody should experience a, a negative impact, an unacceptable situation as they called it, due to sonic boom noise. So, you know, so by the mid 1970s we had, we had Concorde operations on a very limited scale and we had all the rules put in place and that that, that that continued into the 2000s when Concorde operations ceased. And but the research and, and trying to understand how to affect the sonic boom, how to make it quieter.

8:53 And you know, that continued all along through a number of programs. So was it the case that it was almost a shock to the Concorde developers to have this ban because from what I guess it was imposed after the aircraft had essentially been designed, were was the assumption that they would be allowed to fly anywhere? And then when they put that ban in, it was almost, yeah, too late to really change the fundamental design or principles. Yeah, there's a lot, There's a lot to that actually. You know, I think, I think everybody kind, you know, in the early development of supersonic aircraft, you know, there was kind of the there was very much

9:40 kind of this is in the US, but as globally as well. You know, people thought, you know, technology advancement, all technology advancement was good and that in the 60s that changed. And sonic the, the advent of sonic boom, you know, the possibility of, of regular exposure to sonic boom was a part of that, that change. So, you know, certainly the US manufacturers and British Aerospace and Aérospatiale, you know, they, they, they expected that they would fly over land supersonic and that the airplanes would be used as. As regular air transport so. I don't know if it was a. Shock, you know, but but I'm sure they began to realize that

10:26 sentiment was building against, you know, exposing people to this new sound and it definitely, you know, it definitely hurt the market, you know, for the aircraft. I yeah, I suppose what I'm getting to because my second question was really why, Why did Concorde ultimately stop and and fail in some extent? And I'm just wondering to that point, was it an economic reason? Was it a technology reason? And is there anything that is essentially because they couldn't fly over land, they just destroyed the economic model and what what caused Concorde essentially to? To, you know, not I've, I've studied, I haven't really studied the, the marketing of

11:11 it, but you know, there's a couple of things. You know, one of the things that that NASA really believes and, and kind of the supersonic community believes that if you're going to have regular supersonic air transportation, you have to the airlines have to be able to operate the airplane as they would any other airplane. So limiting it to just certain over water routes limits the market and limits the effectiveness of the airplane. So that was a definite strike against, you know, Concorde. It was, you know, it was a marvel, you know, to, you know, again, you know, essentially the design was pretty much in place. You know, a few years after the first supersonic flight, you

11:54 had a supersonic airliner. Fantastic. But you know, so the technology that was incorporated in it it was was somewhat immature for airliner technology. And then we had, you know, we had a couple of fuel crises and then, you know, after the, after the accident. There was a, there was a downturn, another economic downturn, which you know, kind of limited the operation. So eventually, you know, being able to continue an operation with, with such such a small number of aircraft serving such a small market, I think you know, the airlines, basically, it was not economically viable for them to continue so. There was regulation, there was

12:38 technology and there was economics. That all went into, I think, the end of the the Concorde's commercial operation. Yeah, it is. I've, I've been on Concorde, not when it was flying, but in the museum and I'm always amazed at how small it is, how tight the seats are compared now you know, to, to to an aircraft how very and obviously how expensive the, the tickets were. So it was, yeah, I guess as you say it was, it was a novelty for sure. And I'm sure that helped for a while. But I guess it would be interesting even if it didn't stop for that reason. Would, would there be the pressure because of the I guess now like first class travel is so much about luxury and, and

13:23 the size of the cabins and all this and you go into Concorde and you're in this tiny little. Seat well, that's an interesting again, NASA does studies and we're we're we talk to companies that do studies and that that's a factor. But you know, you look at, you know when given the opportunity to get there sooner, most people will choose sooner in in the long run, so. Yeah. I from NASA's perspective, we're, we're NASA's perspective, you know, from the big picture for NASA, we're trying to innovate and do things that improve air travel for, you know, for the, for the traveling public. And so our mantra is kind of, you know, yeah, getting there

14:09 faster, yeah, over long distances is, is a marked improvement in the experience of the, the, the air traveler. So, you know, that's if we can do that in a way that is affordable, sustainable, and, you know, acceptable, environmentally acceptable, then if if we can put those technologies in place, then you know, there's a market there that would definitely be of interest to manufacturers. I mean, it's for sure, I mean, I, I travel quite a lot to the US for conferences or for meetings. And there is no doubt that the length of time it takes to travel from Europe to the US, I think does limit how closely those countries can work together.

14:56 Whereas I know that my US colleagues who have to fly from, let's say, California to Washington, it's still a six hour flight. But anything I think less than five or six hours is a different psychological, different jet lag different, just everything compared to ten hours or, you know, eight or ten hours. So I can totally see why the increasing business or or closeness that I'm sure there's economic benefits from making air travel faster, you know, promote people to maybe, yeah, stay in touch more easily. Yeah, that's, that's, you know, again, that's kind of NASA's part of NASA's vision is, you know, it it, it makes a smaller

15:38 world if you can get there in in half the time. Yeah. So what what the high level as we go towards speaking about the, you know, the NASA programs, could you maybe just explain to people the high level factors that make supersonic passenger jets technologically challenging? What is it about the nature of supersonic travel? A challenge from a noise point of view, From a fuel efficiency compared to transonic aircraft? Well, I mean the the so yeah, so we, we look at we look at as there are barriers, you know, that need to be overcome to make supersonic travel, you know, as I said, you know, economically viable and and environmentally acceptable.

16:28 So we kind of lumped them into categories. One of them is sonic boom. You know, we have to be able to fly over land without disturbing people. So we can talk a little bit more about about how we do that. But, you know, just right now. So the airplane needs to be shaped to to reduce the sound of the sonic boom. And that that's not a whole lot different from, you know, the what a supersonic, what Concorde would look like. But there are some things, you know, to go faster obviously requires more energy. So, you know, there's no, there's no getting around the fact that it needs more fuel. So but balancing, you know, the fuel burn at high speed versus

17:14 the fuel burn at at low speed and getting that that that integrated design that. Is efficient as possible, burns as little fuel as possible across the speed regime. That's, that's a key technology challenge too. One of the things that that, yeah, is, is maybe people don't realize is, you know, current subsonic air airplanes have those great big turbofan engines. So they're accelerating a lot of air, just a small amount to create the thrust that they need. And that's, that's both efficient and quiet at subsonic speeds. As you get to higher speeds, you need your jet has to be faster, you need to accelerate less air

18:00 more, you know to create the thrust that you need to go supersonic speed. So you can't have a high bypass turbofan. So coming up with an engine design that is again matches the the requirements of low speed and high speed, but also, for take-off and landing, can effectively operate quietly so with the lowest jet velocity you know, the, the speed the, the jet coming out of the tailpipe as, as you can so that that requires some unique engine design emissions is a problem. You know that the, the, the Mach, the Mach number of the airplane, the higher the Mach number, the higher you fly. Eventually you're, you're up in the stratosphere where the

18:48 emissions of the engine can have a magnified effect on, on the ozone layer and the, you know, the, the, the, the quality of the atmosphere. So paying attention to the, the, how the fuel is burned in the engine, the combustion process and making a, a burner, a combustor in the airplane that creates as little emissions as possible. Is, is, is a major challenge for supersonic aircraft. If we go back into the 1990s when there was the High-Speed Research Program, emissions was one of the big focuses. Of that effort and the technology that kind of found its start in that program is now in a lot of the engines that you

19:40 see on subsonic. Airplanes. So it's kind of come full circle. And now we're. Looking at it again, can we, can we improve it even more to get a big, a bigger impact if we start using those on the, on supersonic aircraft again? So there's that and there's operations, you know, how do you, you operate the airplane? How do you mix the, the fleet together so that supersonic aircraft can fly as fast as it can for as long as it can without having an impact on the subsonic traffic. So there's, there's a lot of, there's a lot of technology factors. You know that that that that need to be addressed airport noise. sonic boom and emissions are kind of the three real barriers

20:29 to actually opening the market. I think you know to to even a supersonic business aircraft. Well, I'm glad you mentioned that because that was one of the things I was to ask you about from a, you know, conceptual design point of view. When you do the numbers and you figure out the maths on, you know, what's the, the best size or how many passengers can you put on this thing? What do the, what do the sums ultimately drive you to? I, I'm guessing not a 300-person aircraft like you would see now, but much smaller. So could you maybe talk about how the aircraft design maths work out in that point? Of view so you know we're we're

21:13 doing the X-59 now we didn't start out to do a demonstrator. We we started out to in the in the early 2000s, we had just flown the Shaped Sonic Boom Demonstrator and kind of proved that this approach for quieting sonic boom works in a real atmosphere. And there was a lot of kind of innovative thinking going on then about, you know, how we would, you know, how, how we could advance from where our our state of knowledge on, on quieting sonic boom to really getting to something that not only worked well, but was what was practical in a design. So we had some ideas. We had new CFD, we had faster computers, we had design systems

21:58 which enable us to do better optimisation. And we really thought we could make a major change in the in the loudness of the supersonic aircraft. So what we did is we worked with NASA, worked with Boeing and Lockheed in two separate studies to say, OK, if we have this technology and we incorporate it in a future design, you know, what, what, what's, what's practical. And what we wound up with was about 100-passenger airplanes, so something about the same size as as Concorde that flies at about Mach 1.8, two to three engines. And, you know, 4,500 or so miles range, so you know, trans- Pacific, but but barely that all

22:51 kind of fit and enabled us to make an airplane that met the boom requirements, met the landing and take-off noise requirements and the emissions requirements and was much more efficient at cruise than Concorde. So the boom-shaping technology, wanting to meet the noise requirements and trying to make the airplane out of materials that that don't have to be, you know, we don't have to use special materials for high heat. We have a propulsion system that's balanced for take-off or landing and supersonic flight, say about Mach 1.8 is the range that is the speed that you want to go for. So it's a little slower than

23:41 Concorde, but it's still twice as fast as current subsonic airliners. And is that where still you think the sweet spot is in terms of the aircraft, the the most practical aircraft, that's what you started off with it is that still the case that around 100 passengers, Mach 1.8, is that, is that really what all the sort of analysis studies converge to? There have been more studies, more market studies since then and, and well, just recently we've actually, you know, so there's actually a kind of another community of researchers and, and companies that are pushing for faster travel.

24:30 So Mach 4 or Mach 5. And so NASA commissioned some, some more recent studies that, you know, since this is kind of being driven by the high speed, the higher speed community, we set the Mach range at Mach 2 to 4.5. You know what, what's the best answer in that speed range? Well, all the best designs came out to be Mach 2. You know, So what they're kind of saying is, you know, and they're, what we really were seeing was those, those designs were, were somewhat impractical because they were constrained to be Mach 2. So we felt confident that, you know, if we had stretched that envelope down to Mach 1.7 or so, we would have found even, you know, we would kind of confirmed

25:20 our, our previous results. So the answer is yes. I think generally studies still seem to point to that, yeah, kind of given what we what we know in, in, in terms of technology and and given the the challenges of higher speed flight that for the first products that that bring kind of bring back supersonic flight that 1.7–1.8 Mach range is, is kind of the sweet spot. OK. And just to maybe, Oh no, please go. ahead. Well, take Boom. You know, Boom Supersonic is is trying to build essentially a successor to Concorde smaller, but you know, it's kind of that same mission supersonic overwater flight only they started out, you know, talking

26:03 about Mach 2.4 and their current design is Mach 1.7. So. That also kind of confirms where where the technology will take you. Yeah. And just maybe for people's sense, is this actually let let's maybe go through. Could you just briefly introduce the NASA program? You know, there is a specific NASA program with a certain name And so could you maybe just briefly introduce, you know, what is that? And the X-59 I know it there's like a specific project, isn't there? Yes. So again, after we, well, we, we, we kind of, we proved this technology to us at least. So in, in our Boeing and Lockheed studies, we, we did designs, you know, conceptual designs, we did a lot of CFD

26:51 analysis and then eventually we tested those designs in wind tunnels. So small scale models, really trying to test what we did. We did a number of tests, but the focus of the test was, did we get the quiet boom design right? And we, and lo and behold, we, our CFD predictions matched our experiment almost perfectly. So we knew that technology could work. And so there was, there was a lot of excitement about taking the next step. Well, let's, let's try and fly it. But you know, we were very confident in the CFD. So, you know, we weren't going to actually prove the technology in flight. We, you know, we could demonstrate it in flight, but

27:33 you know, what we learned from flight would be, you know, how robust is it? You know, does it change in different weather conditions? You know, what speed ranges work best? You know, kind of that information like that, which was, you know, exciting science to do, but was not a program that was getting much traction with, with, with NASA or with, you know, our, our, our budget people in the, in the government. In parallel to all this work, we were doing research on how people respond to the sound from supersonic aircraft. So we had done laboratory studies where we built these simulators which essentially create a sonic boom or a quiet sonic thump, you know, in a lab

28:20 situation. We had, we had found that, you know, the numbers that we were getting in our designs, we were able to get to in our designs were kind of at the threshold where people really stopped, almost stopped noticing the sound. So what, what, what really sold us was we really wanted to understand how, how people in a community, you know, going about their daily lives would respond to the sound, what level, what sound level would be acceptable. And that's where we got the, the traction. So that's what became the Quesst mission. That's the current big NASA activity. So what we're doing is, and the best way to generate that sound

29:01 in a community is to use an aircraft that creates a representative sound. So the Quesst mission kind of has three phases. The first phase, well, all phases are active now. The focus right now is building the X-59 aircraft. It's a unique. Technology. It's a demonstrator aircraft. It's a research aircraft. It's not a prototype for, for a supersonic airliner, but it does create a sound that's similar to what we think the sounds of the future supersonic aircraft will be. So we're designing that airplane. We've, we've contracted with Lockheed Martin to build the airplane for us. Their contract runs through getting the airplane in the air,

29:46 doing all the envelope expansion, as we call it, you know, the safety of flight checks. And then they'll essentially hand us the keys and the airplane will become NASA's property in the second phase of the of the effort. We're going to take that airplane, we're going to try, we'll make sure that the sound that we're getting is what we expected. So we call that acoustic validation. Is it loud enough? Can we vary it enough? You know, is it affected by the atmosphere? Things like that. We really want to understand that sound that we make because eventually we want people to hear that sound. And that's phase three. So phase three, we'll take the X- 59 and we'll go to different

30:27 communities. Right now that we've got five or six tests planned. It's something in different places in the United States. We haven't picked those places yet, but we're we're, we're narrowing it down if you wish. But so the idea is to is to expose a community to the sounds of quiet supersonic flight at different noise levels, have surveys, have microphones in the community to measure the sound, have people taking surveys to see how they respond to the sound. So we can create a dataset that describes how you know what, at what sound level it is, is the sound acceptable and what sound level is it annoying. In parallel to that, we're

31:10 working with the International Civil Aviation Organization, particularly their Committee on Aviation Environmental Protection, because those are the people that write the rules for noise from, from aircraft. The current regulations for take- off and landing noise all come from ICAO. They're implemented by, you know, in the UK it's CAA and the US it's FAA. But ICAO, all those groups participate in ICAO. They set the standard. So the Quesst mission, the main goal of the Quesst mission is to give ICAO the data that they can use to set the limit of sound for future supersonic aircraft. So to to create that standard that that manufacturers could then design airplanes to.

32:02 OK, I I see that's, that's really interesting. So what you're basically saying is, you know, they have to be conservative in nature when it comes to rules, the rules today that they need, they need as realistic data as possible to make an informed decision. And So what your project will do is help set the foundations, which I then guess when commercial companies like Boom or others want to certify their aircraft, they will be certifying to those noise levels and I guess a certain distance away and a certain height. And it's all the parameters I guess of how they measure. So the standard has, you know, kind of three elements. It's a metric.

32:49 So what are you going to measure? What, what, what, what, what's the best way of measuring the, the, the people's response? What, what number? What process? There's a procedure. You know, so everybody who applies for a type certificate, you know, has to go through the same procedure. So it's consistent and then there's a limit, you know what, what's the value of the metric that that you can choose. So the Quesst mission really helps to inform all three of those elements. You know, we'll be getting a lot of data. So we we've got some candidate metrics, you know, we'll be able to test which metric works best when we're doing that acoustic

33:30 validation phase that's kind of that kind of mimics what the certification process would look like. So we'll get it. We'll be getting data that can help define that certification process. And then when we do the community testing, we'll be actually getting the data that will help support defining the limit. You know, because NASA, NASA can give, you know, we've got a good reputation as being honest scientific, an honest scientific community. We can give that data. We're, you know, we're pulling in as much of the international community as we can so that they understand what we're doing. But, you know, it's, it's essentially creating that data. But then again, it's up to ICAO

34:10 and and the international community to define what the the actual limits and the actual procedure are going to be. So I'm sure the most difficult question is the timeline of of this. So when is the when is the aircraft expected to be handed over to you from Lockheed to be able to start doing some of that validation? Yes, yeah. So yeah, we've, we've experienced delays, very exciting though we just had our first engine runs. So we've started the engine on the airplane for the first time. So that's kind of a a big step forward in, in kind of the last phase before we fly. We have, we have to do all this, this really integrated testing

34:59 where we're essentially the airplane is completely alive and we're running all the systems and, and making sure everything's talking to each other. So once we got that done, then we, you know, of course you, you do taxi tests first and you drive around, you know, gradually increasing speed and then you have first flight. So originally we had planned to hopefully do first flight this year, but we've had some delays in getting, getting to the point where we could do the, the engine run. So now we're looking at sometime next year for first flight the phase one, this envelope expansion phase is planned for about a year. So, you know, things go well

35:40 sometime early in 2026, maybe we'll get the we'll get the keys to the airplane and. We'll be able to start our start our acoustic, you know, our acoustic testing. So would it be fair to say that, you know, realistically assuming various factors, 2030 could be when the international community would have, could set new rules, they've done all the tests, you've given them all the data, they've debated it and they've set and written these new rules. Is that approximately or you think it? Could be faster than that. It's interesting: CAEP is the Committee on Aviation Environmental Protection, they, they have a three-year cycle.

36:21 So they have a meeting and at the meeting all the decisions based on the work that's gone on in the previous three years are are finalized and accepted and then given to the ICAO Council for final approval. So actually the the 13th CAEP cycle is just coming to an end now. So in February of 2025 will be the CAEP/13 meeting. So. 2028 is the next one, 2031 is CAEP/15 and that is our target for, well that is that is ICAO's target now for bringing the standard forward to the council for approval.

37:10 So we are on a timeline to deliver that data. To give in, in, in, in in a timely enough manner, to give them plenty of time to review it, debate it. And then finish writing the standard to bring forward at the CAEP/15 meeting in 2031. OK, Yeah. So maybe before we go any further, I guess we should actually talk about what visually is probably the most distinguishing feature the the sort of long nose, which if I understand correctly was one of the technologies that has been able to reduce the the sonic boom. Could you maybe talk a little bit about the physics and how this helps to reduce the the noise? Yeah, sure. Yeah.

37:58 So, you know, of course, sonic boom is the result of of shock waves. Shock waves are instantaneous pressure changes that occur when an aircraft is flying faster than the speed of sound. Essentially, You know, for normal aircraft, the subsonic aircraft, it's creating a pressure change and essentially. That's being communicated in front of the airplane because. Pressure changes like a sound it, it all travels at the speed of sound, so that that's racing out in front of the airplane and kind of the air is starting to get out of the way. Well, if you're going faster than sound, you know the air doesn't know the plane is coming

38:33 essentially. So you get. An. Instantaneous pressure change called a shock wave. Whether it's created by an airplane or a rifle bullet, it doesn't matter. It's it's anything faster than sound creates an instantaneous pressure change. If you look at an airplane, anytime the flow is going over, it has to make a turn. As it goes over some part of the airplane, you create a shock wave. So there's a shock wave on the nose near the airplane. There's a shock wave on the nose; there's one on the canopy, the engine inlets. Anything that sticks out from the airplane creates a shock wave. So if you can visualize those shock waves, which we can do, we've developed some really

39:11 interesting technology. Because we've been able to do it in the wind tunnel for a while, but now we can do it in flight. You can actually see through what's called schlieren imaging. You can see the shock waves. And if you can see them, what you see is they're all different strengths and they're all kind of randomly spaced along the airplane wherever the components are because of that, because the shock waves, the different strengths as that signal. And again, that signal travels out in three dimensions. Some of it travels towards the ground and that's what we're most interested in. But as it's as that signal is traveling, because those waves

39:50 of different strength, they're actually starting to pile up on each other. So in a short distance from the airplane, all those shock waves have piled up into two: one, you know, nose shock—what becomes called the nose shock, the first leading one and then the tail shock. And that's a very stable acoustic system. Travels to the ground, it's getting attenuated, it's getting weaker and weaker as it reaches as as it goes towards the ground. But still when it reaches the ground, you hear two very rapid pressure changes which you hear as bang-bang. You know of, of, of a sonic boom. So what we found is the best approach, the the approach that seems to work and and can be

40:31 integrated into a practical airplane is to keep those shock waves from merging. So the whole shape of the X-59 that long nose is, is it's all designed to create a system of shock waves near the airplane where the shocks are relatively the same strength and relatively evenly spaced along along the length of the airplane. So this the initial system has all these shock waves in it. And because they're, they're similar in strength, they don't merge as the signal travels to the ground. They just get the each, each individual shock gets attenuated. And that's actually more of a benefit because you get down into the denser atmosphere, the

41:15 shocks start to get smeared a little bit more. So by the time you get to the ground, you're left with kind of this gradual pressure rise instead of instead of the sharp pressure increase. So you hear a thump or, or a thud-thud instead of that, that very disturbing bang-bang. So if you look at the the X-59, it has a lot of features that we learned about when we did those airliner studies that long. The long slender nose sets up most of the forward part of of the acoustic signature if you wish. And then at the back end it becomes much more complicated. So you're really trying to manage the shocks and there's a little bit more than just having

42:01 them be the same strength. You can kind of get them to cancel each other a little bit. So integrating the engine, so it's like the engine on the X- 59, it's mounted on top of the aircraft. So to prevent the shock wave from the engine inlet from kind of impinging on the bottom signature, the signature that travels to the ground, the the tail of the airplane is kind of unique on the X-59. We've got a little canard, we've got a conventional horizontal tail and then a little T-tail. Each of those features helps us generate the lift we need and helps us trim the airplane, but minimizes the impact of those features on the on the sonic boom.

42:47 They just look nice as well. I have to say the aircraft actually looks quite nice, which always helps, right I. Think yeah, exactly right. Well, this is an adage in aircraft design that if it looks good, it is good. Yeah, of course, Concorde famously had to, you know, tweak the nose for visibility. How about on on this type of design, what what's the visibility? And for the pilots, what's their opinion? It's bad. We, we, we have, we have a new approach that's it's and thank you for asking the question because it's really one of the one of the unique features that NASA has contributed to the design of the X-59 is our

43:26 external vision system. So rather than having a forward facing window, we have a camera that sticks out from the, from the nose. It's mounted on the forward fuselage and it creates a high-definition visual image for the pilot that's displayed on a screen in front of his seat. It's a really interesting technology. We've, we've, we've flown it in kind of demonstration flights on subsonic airplanes. And we found that it gives the, the cameras and the screens have reached the point where they can give the pilot the same visual acuity as they would have with normal, with their eyesight, their eyes looking through the,

44:14 looking through the window. We did a study where there was a pilot, you know, flying the airplane and then there was a pilot in the, in the back looking at the screen. And we flew against opposing traffic. And it turned out that the pilot in the back could actually, you know, would generally pick out the targets faster or earlier further away than the pilot in the front. But that's not the end of it. I mean, if you've got this screen in front of you, you can mix the radar, infrared, you know, other sensor features in to really improve situational awareness. You know, for people who fly, you might not see an airplane, but when when air traffic

44:58 controller says, you know, you've got traffic, twelve o'clock, two miles and you look in that direction. Your chances of. Seeing it are much better than if you just kind of, you know, generally looking well, just imagine if if instead of the controller saying it, you know, the radar system put a little circle on your screen and said, hey, there's an airplane out here. You, you, you would always be able to, to, to see the traffic. So it's a unique technology. It enables the supersonic aircraft to be designed without having a complicated heavy nose drooping system. But it's also something that you know could have applications in other in other types of aircraft

45:38 as well. How much? How much is the pilot or the flying experience influencing the design? You know, if you're a car, if you were, if you're designing a car, you actually think about driver comfort. You think about how easy it is to drive the car, to look out the window to the experience. I I always got the impression that planes were designed almost without any consideration for the pilot, but I guess that's maybe not true. I'm just wondering for this how much you factor in the pilot's feedback and their needs. So, you know, human factors is a big part of aircraft design now. And I think, you know, maybe, you know, when the first

46:28 airplanes came out, there was less attention to, you know, just give them the instruments. But now I think that's a big part of, you know, how do you design the, the, the aircraft instrument panel? How do you display information so the pilot can make maximum use of it? You know, in a safe manner? You know, one, one thing unique about the X-59 is it's, it's an X-plane. So you know, with the X-plane approach, the idea is we have. We have certain key things that we want to demonstrate and for an X-plane you really want to make that as small a set of things as as possible. So for us, it's. It's all about, it's all about

47:06 the sonic boom. So, you know, besides the shape of the airplane, the airplane is kind of a. You know, a mishmash of parts from other airplanes here. We've got F-16 landing gear, we've got an engine from an F-18, etcetera. So the X-plane you know, just try and make it work. So we did not spend a lot of a lot of energy on, on human factors, but we did, you know, look at it. So we got our pilots involved early. On, you know, we knew, you know, we wouldn't have the screen, but you know, we knew kind of beyond that kind of wanted to look pretty much like a, a modern fighter aircraft inside as a single-seat airplane. But you know, the pilots got in.

47:52 We have a, we have a really very sophisticated, we have actually have two, one at Lockheed, one at NASA, really very sophisticated simulators. So that we put them in the simulators, you know, with, with, with, you know, the cockpit design in there and they were able to interact with it, give feedback and then, you know, the, the design was updated. Yeah, as, as we went along. So it's it's, it's it's it's it's. It's not a huge factor for the X- 59. It's a big factor for, you know, actual commercial aircraft. So I was actually wondering as maybe a bit of a tangent, there's obviously a lot of discussion about computational

48:32 fluid dynamics reducing the reliance of wind tunnel technology. You know, if you can do more in CFD and I believe for this program, CFD was a big factor in being able to, you know, iterate quickly and, and visually see what what was going on. But I was actually wondering, you mentioned about simulators because obviously normally there's wind tunnel to CFD, but then you have to do a flight test, you know, you actually need to go out and do it. How much have simulator technology advanced that you can model some of the sort of flight test behaviour through a simulator? Is that getting also more and more accurate? Much, much more sophisticated, yeah.

49:18 And, you know, again, for X-59, it's, it's, it's, it's, you know, it's a unique airplane that, you know, the design will create some interesting, you know, some, some flight characteristics that are not typical of, of, of fighters or, or transports. So we really wanted to understand how, you know, how, how those things worked and, and how the pilot would interact with them. So getting a simulation. So there's, there's so much simulation that goes on now. There's, there's simulation without the pilot, there's simulation with the pilot, there's simulation without hardware, you know, like, like the control actuator and the simulation with hardware.

50:02 So, you know, the whole simulation process is, is a huge part of the, the aircraft design process right now. So you know, like for X-59, we, we did a lot of analysis, we did do some wind tunnel testing. You know, there's still, there's still design, there's still points in the flight envelope where the CFD is not all that great. So you like to augment your CFD analysis with with testing, but you know, compared to previous designs, we didn't do a lot of wind tunnel testing. As a matter of fact, we finalized the low boom design, the boom, you know, quiet-boom design of the X-59 without doing wind tunnel testing of the

50:47 geometry. So, you know, that's how far the CFD has come. We put all this stuff in the in the in the simulator. You know, it's been iterated on a few times. So, you know, we're, we're, we're, we're going into flight test with a very good idea of what, what the pilot will experience flying the airplane throughout the flight envelope. Yeah, it's, it is amazing how the technology is sort of, yeah, moved on from that point of view. Just maybe I'm trying to think of a few questions, more practical questions. So the noise, as you say, going over, the idea is it's a sort of soft thump, which is less. It's something like, you know,

51:33 we're used to everyday sounds like, you know, I guess drones—just distant thunder or, you know, somebody's car door closing. And we compare the the sound to to sounds like that. But what about take-off and landing? I mean the, you know, you can go on YouTube now and look at Concorde taking off and you know, it was an absolute racket, smoke billowing out of the engines. You know, like a real difference to, like a normal aircraft. What's the—not for the X-59 now? But I mean more just for like theoretical design concepts. How much is the the engine noise? Because it's, as you say, it's not a conventional engine. How much is that different or

52:17 limiting factor? It's, it's again, we talked about it earlier. It's another kind of barrier technology that you know that we, we made a lot of advances in. Again, it's not on the X-59. We're just using an existing engine, but as part of the, the bigger technology program in, in NASA aeronautics, called the Commercial Supersonic Technology Project. Airport noise is, is one of our, one of our, our big efforts. So, you know, we're looking at the, the, the engine cycle, you know, so we can't have a super high bypass engine, but at Mach

53:08 1.7 we can have some higher, we can have much higher bypass, you know, much more fan component than the Concorde did. So that's that's a major help. Our jet that you know, the jet coming out of the back of the engine at subsonic at take-off is actually subsonic. It's not like Concorde, where the jet was coming out faster than the speed of sound, which creates noise, you know, of its own. So our jet is subsonic. We've got nozzle technologies which mix faster flow with slower flow. So the aggregate result coming out the back end is slower. And there's a whole idea of, you know, the propulsion installation. Can you use that to, to help

53:50 reduce noise? So all of those things are being studied and you know, we, we think again, we've, we've just, we've done testing, but we think we've got a solution that can make supersonic aircraft as quiet as the current limits for noise for subsonic aircraft. So that's a, that's a big improvement relative to where Concorde was. So I guess the, the big question on most people's minds is what, what do you think is the likelihood of a commercial version of this? What is the, there's obviously the research going on you're doing, there's companies like Boom, but from what I could gather, Boom, that design looked more like a traditional or more

54:42 traditional design than than, than what you have, I mean. Yeah. So, so Boom's approach is kind of unique. They, they want it. They, they, they, they, they, you know, they see that there's a market now that they would like to capitalise. They've talked about their next generation of airplane incorporating, you know, boom shaping again. They, they don't want to commit to doing a design for a low boom aircraft without having a standard in place. As a matter of fact, if they don't want to commit really to, well, they're kind of committed, but they're working with ICAO and the FAA is working with ICAO to put a noise rule in place for supersonic aircraft—a landing-and-take-off

55:26 noise rule in place for supersonic aircraft. So that would give them that certainty. The work, the work with ICAO really started because there was a group of companies that really felt that they could bring a supersonic business jet product, business aircraft product to market if they had a regulation on how loud it needed to be when it flew over land or how quiet it needed to be when it flew over land. So there's, there's definitely that market out there in the near term for a smaller aircraft. NASA is still bullish: if we continue down this path, there are technology solutions out there which could make it

56:14 possible to, to design an airliner that could, you know, serve a supersonic market. So what's what's stopping it? So let's imagine the rules come out and they are essentially aligned to what you already know from your simulations. I, you know, because I presume you already have a, an estimate and all the flight test is doing really is proving it out. I guess you know, you have analytical methods. So let's just assume that they agree with your numbers. Is it is the technology ready? Is it a case that, you know, a Bombardier, a Boeing, or whoever the company is can just basically do it? I know, I know.

57:03 I'm massively oversimplifying things here. Or is there still something missing? So I, I think that that for a, for a smaller aircraft, a business type aircraft, the technology is very close. The key challenge really for a near term product is the engine, because you need, you need something that, that gives you the efficiency that you need. but will meet that, that new landing and take-off noise rule. It's possible you could do it with a, with a modification to an existing engine, change the, change the front end, change the back end.

57:51 But more than likely you're going to need a, a new engine. And that's magnified a little bit more for the larger product. You really need kind of a, a, a bigger step in technology, in engine technology to be able to make the airplane meet noise and emissions limits. And by big and small are you talking small being the hundred person? No, I'm talking small being yeah, the next step for Gulfstream say, yeah, an airplane that's, you know, room for maybe eight or ten passengers or maybe up to 50, like Boom was imagining—or 40. I can't remember exactly where their target is, but so that's the smaller airplane. What we're thinking

58:43 about for the first generation of you know, airliner products would be around a 100-seat airplane. OK. So what I understand you're saying that it's, it's the engine technology in some ways that is the the more challenging technology and it obviously gets a more important or challenging, the bigger, the heavier that it becomes. Think of it as: a business jet can afford to be a little less efficient because there's people willing to pay for speed. But the airliner needs efficiency, quiet and low emissions, which takes a new set of technologies. There are ideas out there; there are technologies at kind of the infancy

59:26 level, but they need to mature. And is there any, you know, how the economics work when when something aligns to something else, there's double the reason to do it? Is there any similar reason, like a military reason or another reason that aligns this? Or is this quite unique in some ways for a passenger supersonic? The military is not really interested in quiet. They would love to have supersonic speeds. They don't, It doesn't seem to be a major requirement for military missions, you know, at the moment, because if you look what's out there now, the airplanes can go supersonic, but not very supersonic. They certainly don't

1:00:14 cruise supersonic. Yeah, if they could do it, would they? Yeah, maybe. But yeah, they obviously, you know, other tech, other requirements, you know, stealth and IR are kind of driving military designs, which you know, the intersection between that and low boom design is not there. So, so it really is, would you say companies are just hedging their bets or waiting because they're just they're just not sure when the rules will change. So it's almost hard for them to put the investment in to start designing something if they're unclear? Is that also one of the key barriers to it? Yeah, it comes and goes. I mean, there was, there was,

1:01:05 there was a lot of work going on a few years ago. Some of the companies have, have put those efforts on the back burner. Essentially. I, I, I would say yes, they're waiting for those regulatory guidance, you know, that, that, that, you know, understanding of what, of what, what their design requirements will need to be before they, before they plunge back into looking at products. So maybe, and again, this is always a common question, I must admit I'm guilty of sometimes thinking the same. And you, you partially answered it before, but maybe we could be more explicit if let's say the supersonic 1.8 Mach number is solved.

1:01:49 So, you know, the new rules come out, there is sufficient economic reason for the for the engine manufacturers to do it. And a supersonic business jet comes out. In your mind, what's the technical or economic potential of a hypersonic passenger travel? OK, not 100 people, but you know, 20–30 people. Mach 5, Mach 6. Is that science fiction? Is it? Is it? Is it possible? I wouldn't, I wouldn't put it in terms of science fiction, but the you know, the challenges are are are much, much greater than for for supersonic. You get to you get to very high altitudes and you get to very high temperatures when you get to Mach 4 and Mach 5.

1:02:39 So, you know, being able to design something that's reusable, you know that the materials that will last for, you know, for thousands of flights. And then so there, you know, then some of the safety considerations, you know, one of the things that airplanes are designed for the event of a depressurization, you know, the, the, you know, there's a way to keep people, you know, keep people alive until the airplane gets down. When you're up at 70,000–80,000 feet, that's a huge, huge challenge. So yeah, I know NASA is NASA is, we have a hypersonics project. They're looking at hypersonics technology more from the the point of view of, you know what,

1:03:26 what would it be for helping space launch? You know, what would its applications be in space launch? But again, we had this whole study looking at, you know, is, is high speed flight a possibility? Peter's opinion is it's going to be a very long time before the technology is ready to even even take, you know, eight or ten people into hypersonic flight. Who knows? Maybe, you know, the whole idea of orbital transport, you know, will come before hypersonic flight because, again, you know, getting, getting people on board and making it safe and economical is a, is a, is a big step.

1:04:15 Yeah, I think the overriding thing for most people is, and I'm sure you feel the same being so passionately involved in this project. There's always, normally we're used to technology adoption being a linear thing. You know, something gets faster, faster, faster. And we almost never go backwards, do we? You know, the internet's got faster, computers have got faster. It's always a sort of peculiarity that we did go supersonic and we were doing it and now we've sort of gone backwards. And I always, I guess that's why for most people, supersonic travel is such an intriguing thing because they know it used to be. And I guess some people

1:04:56 were lucky enough to fly in Concorde and maybe they even more so wish it were still here. But for people like me, it's, it's just a fascinating thing that we did it and now, you know, I guess it's like going to the moon. You know, we, we did it, but we haven't been back for quite a while. And but I guess that's also changing but. You know. It's it's. Yeah. I mean, you know, spaceflight did the same thing. It was, you know, it was we needed that. We needed that technology. There was a space, there was a space race. So, you know, it was a stretch. You know, the moon, the moon landing was a was a big stretch. Concorde was a big stretch. And you know, maybe we stretched

1:05:37 a little too far, you know, proved that it was practical. But supersonic flight, for example, revealed that there were some, some things which, you know, which you have to, you have to create a balance. You know, we like to go faster, but we can't disturb people's lives. You know, now we've got aircraft. Aircraft are going to have to be sustainable. They have to, you know, not, not contribute to, to, to climate change. So there's more things to weigh than just speed, which, you know, again, kind of puts barriers in the way of, of, of reaching that goal. One quick question before I maybe completely change topic: battery

1:06:24 technology, it's always been I remember for cars, there was a point when people said it was never going to be available for cars electric, you know, they have a little golf buggy and that's about it. There's no way it's ever going to be for a car. And yet suddenly the whole market started to transform and it really felt like there was a step change in the adoption of electric. I haven't been following it as closely, but where do you see battery? Battery technology for aircraft propulsion? I don't mean complete electric aircraft. I'm not a battery guy. But I will say that, you know, in in NASA aeronautics, unique new

1:07:13 sustainable propulsion systems are, you know, are are, are an area of intense interest. So we have a number of efforts looking at yeah, battery-powered electric flight for smaller aircraft. We have something called the Electrified Powertrain Flight Demonstration. It's an effort that's going on right now. There's two contractors, GE and magniX. magniX has built a number of electric-powered airplanes already, investigating, you know, what it would take to get there. Would it would it be battery? Would it be hybrid? Would it use fuel cells? You know, all all those kind of features towards, you know, making essentially non-

1:08:01 combustion propulsion a viable option. There's there's a lot of work that needs to be done. But again, you know, like you said, I mean, when people, you know, a few years ago, people couldn't imagine an electric car, people couldn't imagine a a rocket that would come back and land on the ground. So, so you know, never say, you know, it's you try to challenge yourself, you know, to be realistic and think about what you know we can do in the near term, but you really should never say never. Yeah, exactly. Yeah, being caught by some scissors that probably wasn't on people's minds a while ago. So maybe the maybe the final question for you, which is

1:08:44 you've worked for NASA for decades now, I guess, you know, maybe more than 30 years. What advice would you give to people who are looking to start a career in aeronautics? But I know it's probably a question you get a lot, but what would your advice be if you, if you're somebody maybe in the final years of school, thinking about university, thinking about career, what would be your advice? Well, I mean, one of the things I always tell people I know I got a degree in aerospace, aeronautical engineering, which you know, gave me a unique set of skills, obviously, you know which got me into the aerospace business. There are other degrees, you know, which you should, you

1:09:27 know, electrical engineering, you know, mechanical engineering, all, you know, all of those things can give you an opportunity to get into an aerospace field. You know, for people in school, you know, there's, there's so many internship opportunities. I, I, you know, I say, you know, take any, any opportunity that you have to do that, you know, take it because it, you know, gives you experience to, to help you, you figure out what you really want to do. It gives you exposure to how industry works and you meet people, you know, who could, you know, be a benefit to you when you start your career. You know, beyond that, when you get to work, be a team player.

1:10:18 Be willing to do something that puts yourself outside of your comfort zone. I was, you know, pretty much a technology guy, you know, very happy doing my little aircraft design work. My bosses kept pushing me to get involved in more of these multidisciplinary studies that were going on. And then, you know, it was a challenge for me, you know, to get out there and interact with people. But, you know, it definitely gave me a skill set which was, you know, really very important as I got into the position where I helped create the program that eventually led to creation of the Quesst mission. So, you know, by being able to work with people and, and, you

1:11:05 know, set common goals and, you know, set up an environment where you can achieve those goals that, that was all part of the background. You know, exposing myself to, to just to things outside of, you know, just the, the typical math and science helped me, you know, helped set me on that path. And is there any in in now, I guess as the aerospace industry is changing and the startups around and you know, do you think how valuable is it for people to, for example, do a PhD and go down the more academic becoming an expert or how much of it is valuable to also gain sort of industrial experience? You know, would you, if you're

1:11:53 looking at employing somebody, would you, would you like somebody who's, you know, worked at different smaller aircraft manufacturers or companies and bring diverse skills? Or would you tend to say to people, no, go down the more academic you know, doing a PhD or maybe becoming a research scientist? Is there anything you've observed? There's, there's benefits to, to everything. Again, I, I talked about being, being able to work as part of a team, you know, so to me, if I'm hiring people, someone who's had that kind of experience would be someone that I would look for, you know, to bring forward. You know, that said, you know, we've got, we've got people, you

1:12:40 know, that have come to NASA from, from, you know, from different higher education, you know, from PhDs, from, from master's programs. And yeah, they're, they're just, they're, they're brilliant, you know, so that puts them on the right path to begin with. They've kind of honed that knowledge. They've already maybe, you know, got this idea that they can do something new and unique. And you, you put them with a team that addresses all the other aspects of that, that new and unique idea. And that's how you get to breakthroughs. And that's kind of, that's a lot of what happened with the design that, you know, the design that led us to

1:13:21 X-59. Well, thank you so much for taking the time to speak. I mean, what I think I'll do is I'll put a bunch of links for people to follow because I know there's lots of, you know, videos and, and, and papers that have been published on these things that maybe people could dive into. And of course, you know, the NASA website, nasa.gov/quesst—two s's— there's all kinds of information about the, the project there. People can sign up for our boarding pass, where, you know, your name will be digitally carried aloft on the first flight of the X-59. And, you know, again, it's a great way to interact with, you know, get information

1:14:04 about what's going on at NASA. Great well, I, I certainly hope that I'll have the opportunity to fly on a supersonic aircraft and I, I, I'm sure that if it does happen it will most likely be thanks to the work that you and your team has done so anybody listening to this should be thankful of the work that you and your team are doing because it might help save us from eight-hour flights across to the US. Yeah, we're just hoping, you know, again, it's, it's important that yeah, whatever we do, the public buys into, you know, so getting their opinion is what Quesst is all about. But we do, you know, for NASA, you know, high-speed flight

1:14:46 is part of our, kind of, the foundational things that we're, we're working towards. Yeah. Well, thank you again. Really appreciate it. It was great to talk to you. And yeah, good luck for the rest of the programme. Pleasure talking to you, Neil.