Episode 180: The X-59 Goes Supersonic: What’s Next

Host Andres Almeida: NASA's experimental X-59 airplane went supersonic for the first time on June 5, 2026. The milestone proved the aircraft is capable of breaking the sound barrier, but that's only the beginning.

Before NASA can take the aircraft over communities to demonstrate quiet supersonic flight, engineers and pilots have to answer hundreds of questions. Perhaps most importantly, will it produce the quiet sonic thump that could reopen the skies to commercial supersonic flight?

In episode 166 , we spoke with senior research pilot David "Nils" Larson. He gave us an overview of the mission. In this episode of Small Steps, Giant Leaps, we're joined by research test pilot Jim Less - callsign Clue. He's been part of the X-59 program since its earliest days, helping define requirements and design the aircraft.

This is Small Steps, Giant Leaps .

Welcome to Small Steps, Giant Leaps , the podcast from NASA's Academy of Program/Project & Engineering Leadership, or APPEL. I'm your host, Andres Almeida.

Let's settle in here and talk about the X-59.

Host: Hey Clue, thanks for being here.

Less: You're welcome. Good morning.

Host: So, talk to us a bit more about your role.

Less: So, at NASA Armstrong, I'm a research test pilot.

I've worked there coming up on 16 years now. I fly a variety of aircraft: the F-15, the F-18. I've flown the King Air, the T-34. We flew a 747 for a while. I got to fly that. Certainly, the most unique thing I get to fly is the X-59.

I've been a project pilot on the X-59 program since its inception. I was actually working supersonic research prior to the start of the X-59 program, back before we knew we were going to build a purpose-built airplane. I was involved in writing some of the requirements for the airplane that eventually became the X-59.

By my count, this is the 59th aircraft I've ever flown.

Host: Fitting. So, the aircraft went supersonic for the first time in June. What do we know right now?

Less: We've not really begun to test or measure the sonic thump, the shock waves that come off it. At this point, we're in envelope expansion, making sure that we can get to the conditions that we need, that the airplane is safe to fly and operate under those conditions.

I will say it's fast. It accelerated very quickly, a little bit better than our simulation even showed. On the first supersonic flight, I took it to Mach 1.10, just a little bit above the speed of sound. The flight after that, I got to 1.20, and then the last flight we flew, Nils, my colleague, took it up to 1.40. And that's the design condition.

We'll eventually go a little bit faster, but that's really what it was optimized for: 1.40 Mach.

The scientists that thought of this whole program at the very beginning felt that if we could show it was, it produced a quiet thump at 1.40, that would show that the technology worked.

Future airplanes could probably push it as much as about 1.7 and still have a quiet enough sonic thump. But we felt we had to show at least 1.4 in order to prove that the concept works.

Host: Yeah.

Less: So, with a new airplane, one of the big unknowns is just the airspeed, and the airspeed indicator and the altimeter, the indicator that shows what our altitude is.

We measure airspeed and altitude using what we call pitot-static instruments. They measure pressure. They measure the air pressure of the wind coming in, as well as just the static pressure of the air around the aircraft.

And because the airplane disturbs the air around it, these pitot-static measurements are not always exactly what you expect them to be, and so there needs to be calibration of those.

The reason we went to 1.10 Mach on the first supersonic flight was to make sure that we actually got through the speed of sound.

I went to 1.10 indicated - I think they later figured out that that was maybe about 1.07 in reality. So, it was clearly supersonic.

We had already found out that when we indicated 1.0, we were still slightly subsonic. So, calibrating all those indications is one of the things you do early on in a flight test program. It's a software update to just calibrate the readings and make sure that what we're seeing on our gages is actually correct.

Host: Now the aircraft doesn't have cockpit windows the way traditional aircraft do. Is that easy to adapt to?

Less: Yeah, we've trained for years. We've developed all our maneuvers. We've practiced. We've trained. We've rehearsed using the simulator.

Of course, in the simulator, everything you're looking at is a computer-generated image. When we got into the airplane, it actually was better because we're looking at a very hi-def real-world image. It's no longer simulated.

We have side windows, and we have a canopy over us, so we can look around to the side. It's just forward that we can't see through a windscreen.

The TV display that we have is an ultra hi-def camera. It's color. It's a really great display, and it's almost like looking through a window.

We still find we're trying to look around the TV display and see if we can see what's really there. But you trust the system; you know that it's showing you what's out there. And we train that way, and we're flying that way, and it's working out great.

Host: And you have built-in redundancies.

Less: There are redundancies. There's actually two cameras. There's one on top, one on the bottom.

The one on the bottom, because it extends down into the wind, we can't go real fast with that. So, that's for takeoff and landing. We have to retract that up into the airplane (into the belly of the airplane when we're going to go fast), but it's there for takeoff and landing. The one on top gives us a much better view of the outside world.

Host: Was there a comparable, specific aircraft design you referenced during development?

Less: There aren't really designs like the X-59. There's a lot of unique things about it.

But as an X-plane, as a one-of-a-kind demonstrator, we did look at the X-31 program, the X-29 program. Many of the engineers from Lockheed that have done the nitty gritty designing and building of this airplane worked on other airplane aircraft projects with Lockheed, particularly F-35. That's the biggest, most recent program they've got going.

So, they bring lessons learned from multiple different aircraft over many years or even decades.

While the X-59 is a unique design in terms of the shape and what it's designed to do, underneath it has landing gear. It has an engine. It has fuel system, and all those things are generally out of other airplanes, not uniquely built or designed just for the X-59.

One of the main things that we're trying to learn with the X-59 is: Can we make a quiet supersonic airplane?

And certainly, all that information that we learn is going to be very relevant and available to any U.S. manufacturer that wants to try to build a quiet supersonic airplane.

Part of what we're calling Phase Two, where we go out and measure all aspects of the sonic thump, probe the shock waves coming off the airplane - all that data will show how closely what we built actually performs compared to what we predicted.

And that will help the engineers refine the software tools that they use to design it and make those tools better, more accurate for the next team that goes and builds a quiet supersonic airplane. So, those are the lessons that we will learn from this and will help future designs.

Host: Is there a formal database for you to look through?

Less: Well, we've looked back through reports and documents from, again, other programs. The Society of Experimental Test Pilots has a database of lessons learned from decades of flight test history. We've looked through that.

We've talked to people involved in previous programs. The Shaped Sonic Boom Demonstrator was a program over 20 years ago. They took an F5 and modified the nose of the aircraft to try to quiet part of the sonic boom, not the entire thing. We talked to one of the project pilots on that extensively, just to hear his lessons learned from that project, from other first flights.

There's a lot of emphasis on the first flight of this airplane. The first time you take any airplane airborne, you don't know exactly what's going to be what's going to happen. That's probably the biggest leap in the whole flight test program: to go from not flying on the ground to airborne.

Once we got airborne, everything after that is just an incremental step. Make sure everything's working right. Make sure it's matching predictions and go a little bit faster and a little bit higher.

Obviously, that first takeoff was the big one, and we always said we're going to know in the first three seconds how this airplane is going to fly.

And I was chasing. I saw him rotate, lift off. Nothing bad happened. I said, "Okay, this thing flies," and then we just went on and knocked through the test points that we had planned.

Host: Good, and I'm sure everybody was all ears and pretty much all hands on deck, right?

Less: There was a lot of people watching the first flight. Very excited. But we also had a disciplined team that knew just because it got airborne, there's a lot more that has to happen before we can relax.

I think the spectators were cheering, but the test team was doing their job. And we had practiced for so long, so many simulator sessions that everybody just settled in and did their job the way they had trained.

Host: So, what's next?

Less: Right now, we're still squarely in Phase One of the project, where we're continuing to expand the envelope. We've shown that we could get to our endpoint, but we need to fill in a lot more of the envelope in terms of airspeed and altitude. We need to go faster. We need to go slower at different altitudes. We'll take it a little bit higher.

Ultimately, we'll go a little bit higher and faster than we have now. But to make sure that it's a robust design and not just a single point design, we have to expand that envelope.

Host: Right. You're making sure the aircraft performs well before you fly it over communities.

Less: You're right.

Host: Yeah.

Less: Before we before we do that, after we've finished Phase One, the envelope expansion, we'll go into Phase Two. That's where the scientists really get to see just how well it's performing. That's where we're going to measure that sonic thump and see if it matches predictions. Right now, we don't have a whole lot of data on that.

The first time, the only time we went to our design conditions, Nils was in the cockpit. I was in an F-15 right next to him, so any sonic thump from the X-59 was drowned out by the sonic boom from the F-15. Scientists measured both and think they could pick out the difference, but we don't have any clear, pristine data yet to show how it's doing.

Host: When you're in the chase plane, are you communicating with both the ground team and the aircraft? What's it like in there?

Less: You're communicating. You're listening. You're mostly listening and trying to catch things that the test pilot in the test aircraft is missing because he's focused on flying the test points, getting everything just right, and the chase plane is just another set of eyes and ears providing mutual support.

It's a best practice of flight tests since the early days, '50s and '60s. Using a chase plane has proven over the years to be beneficial. It's usually hard to quantify.

But when something starts to go wrong - I've been chasing where I've seen a panel coming loose, or some wiring, some external sensors on the skin of the airplane start coming off, if there's a problem with the engine.

Certainly, when you put the landing gear down, if you get any kind of anomaly in the cockpit indications, it's really nice to have another pilot in an airplane right there to look at the gear.

Host: Yeah.

Less: There was one time one of our Global Hawks (an unmanned airplane) had a potential landing gear problem, and I launched up in an F-18, took a photographer who was able to, we were get able to get up close, get pictures of it, get back to the ground, and show the team on the ground what was going on. And they, they were able to decide that I was okay, safe enough to land, and they brought it in.

So, you can't predict what the chase airplane is going to end up doing, but it's a best practice throughout the flight test industry, the military, the civilian manufacturers, as well as NASA.

Host: On a career level, what was your giant leap?

Less: Oh, I don't know if there - not sure whether I believe in giant leaps. And to get anything significant done, it's a lot of small steps. It's years of hard work and staying focused. All your steps aren't necessarily in the right direction, but as long as you keep moving, you eventually get there.

But I guess to try to answer your question, as far as becoming a test pilot and getting to the point where I'm doing what I'm doing: When I was a kid, even up through high school, I had no idea that that was something that I could do. I didn't know how people got to be pilots. I thought that was special people and test pilots even more so.

So, I guess the big leap was about the time I was getting ready to go to college. I was looking into studying engineering. I was looking into Air Force ROTC to help pay for that.

The big leap was when I realized, hey, if I want to be a pilot, I can actually do that. There's a path, and anybody that wants to work hard at it can go ahead and do it. And then after that, it's been a lot more small steps to get where I am today.

Many of my colleagues wanted to be pilots from the time they were little kids. As I said, I didn't think about being a pilot. I didn't really know how that would be done.

My family did not travel by plane, so I was already off in college, already starting to think that I might want to make flying a career and be a pilot. I had never flown in an airplane.

So, it was through the Air Force ROTC program. They were able to get a bunch of us cadets the opportunity to go for the ride in the back of a C-130. That's a cargo plane, four-engine propeller-driven cargo plane at a local Air National Guard base. And that was the first time I got to ever fly in an airplane.

Host: A huge C-130.

Less: Yeah.

[Laughter]

Less: It wasn't all that exciting. It was nighttime. We sat in the back and flew around and trying to look out the window and see what's going on.

The second flight, I flew in a Cessna with a, a friend I met at college who had his private pilot's license, and then my third flight in an airplane was my first lesson.

Host: Your first lesson was…

Less: My first lesson. After my freshman year in college, I went home and said, "Guess what I'm doing this summer, Mom?" And went to the local airport and started taking lessons.

Host: Fascinating. Well, Clue, that's our time, but it's been great talking with you and learning more about the X-59.

Less: Oh, it was great talking to you.

Host: That's it for this episode of Small Steps, Giant Leaps. For a transcript and for all other episodes, visit nasa.gov/podcasts. And while you're there, check out our other podcasts like Houston, We Have a Podcast, Curious Universe, and Universo curioso de la NASA. As always, thanks for listening.

Outro: Three. Two. One. This is an official NASA podcast.