Podcast: Supersonic From Bongo To Boom

Aviation Week editors go back through the archives to the origins of supersonic flight, revisiting key programs and exploring the issues that are still challenging high-speed aircraft designers today.

Check 6 Revisits delves into Aviation Week's more than 100-year archive. Subscribers can explore our archive here and read key Aviation Week articles relating to this podcast here: 

• Sonic Boom: A Potential Weapon? (AW&ST, March 8, 1954, p. 12)
• Letter from Oklahoma City resident on Project Bongo (AW&ST, March 2, 1964, p. 98)
• Sonic Boom Report Expected in October (AW&ST, July 13, 1964, p. 28)
• Boom Restrictions May Swell SST Costs (AW&ST, Aug. 3, 1964, p. 38)
• Concorde is Coming (AW&ST, Feb. 8, 1971, p. 11) 

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Audio Clips

March 2, 1969: Concorde's first Toulouse test flight (BBC)

June 5, 1963: President John F. Kennedy spoke at U.S. Air Force Academy (JFK Library)

June 4, 1963: JFK phone call to Treasury Secretary C. Douglas Dillon (JFK Library)

Transcript

Christine Boynton:

Welcome to Check 6 Revisits, where we comb through more than a century of Aviation Week & Space Technology archives. On this podcast, our editors explore pivotal industry moments and achievements of the past, while considering how they might relate to the events of today. I'm your host, Christine Boynton, Aviation Week senior editor for air transport, and today I'm joined by Executive Editor Graham Warwick, Defense Editor Steve Trimble, and Senior Editor Guy Norris. Our subject today, supersonic flight.

Audio:

Still the volume of power increases something like 28,000 pounds of thrust from each of the four engines. [inaudible 00:01:05]. She rose 45 knots. A lot of dust showing on the runway. 90. 135. Rotate any second, nose, wheel well up, smooth rotation continuing. Nose come up to 20 degrees. She's airborne in 26 seconds. She flies. Concorde flies at last.

Christine Boynton:

You've just briefly heard BBC coverage of the first Concorde test flight in Toulouse on March 2nd, 1969. It would go supersonic for the first time several months later in October. We'll come back to the Concorde, but to kick off this episode, we're digging back a bit further, 5,000 miles away and five years before that flight. The place, Oklahoma City, and the year, 1964. This was the setting for something called Operation Bongo.

In January of that year, Aviation Week & Space Technology reported that studies of public acceptability of sonic boom over pressures were set to start by operating a series of 1,212 supersonic flights over Oklahoma City for a period of 26 weeks. Conducted by the FAA, in cooperation with the Air Force and NASA, they were planned to happen over both commercial and residential areas. FAA actually leased several houses in different locations to study their effects, and in the first 21 weeks, military fighters produced an average of eight booms daily, varying in intensity.

So, Steve, I know you've done a lot of research into this particular event. Can you kick us off here? I know these weren't the first tests of their kind, but what was the thinking, and how was this received?

Steve Trimble:

Right. Thanks, and great to hear that Concorde clip again too. So this is 1963, 1964, we've just celebrated the 60th anniversary of the invention of powered fixed-wing flight by the Wright brothers, of course, in 1903. And since that moment, the world has only known the steady progression of commercial airline travel to the point where within the previous decade, moving across continents on jets at high subsonic speeds, 0.7, 0.8 Mach, had become something routine for the traveling public.

So it's no surprise, really, for people in that time period to be thinking that, really, within the next decade, in the next two decades, there is going to be a proliferation of supersonic commercial flight. And this meant that communities like Oklahoma City, one of the four sites of Operation Bongo, were going to be subjected to multiple daily sonic booms by very large aircraft flying, albeit at fairly high altitudes above 50,000 feet, still producing large noises and overpressures that people at the time had recognized were starting to feel uncomfortable.

I noticed an article in Aviation Weekly in 1954 where [inaudible 00:04:18] aircraft engineers had actually done a study looking at using sonic booms as a weapon against enemy cities, just shattering all their windows at the same time. So they were aware of this issue, and they were aware of community complaints about the sonic boom over the period of 10 years by military aircraft, 10 or 15 years at that point.

And in January of 1961, a US government-funded study, it was called the SST Blue Book Report, came to the conclusion that supersonic commercial travel would be technically and economically feasible, but there were still going to be some issues that they were going to have to figure out before it became commonplace. And the big one, the number one problem they identified was going to be managing that sonic boom, and figuring out a way to introduce it without terribly annoying the traveling public. So thus, Operation Bongo was born.

And in hindsight, and 60 years later, it does seem rather extraordinary that a government would essentially select civilian cities, in this case starting with St. Louis, then Oklahoma City, then Edwards Air Force Base and the community around it, and then Chicago to perform a sort of loosely-controlled experiment of, "How much can we annoy these people? And what is going to happen by subjecting these people to sonic booms for this long every day?" Because They wanted the data, they wanted to know... They knew it was going to annoy people. What they didn't know was how much it was going to annoy people, and that's what they wanted to find out. And was it the overpressure of 1.0 pounds per square foot, if that was going to be acceptable? Or was it going to be two pounds per square foot?

That was the language they used at the time. We've since moved on to EPNdB, and Graham, and my colleague, Guy, will surely be explaining all that. But at the time, that was the measure, and they were trying to inform the policy on overland supersonic flight, and create a restriction of how much of this overpressure, this pounds per square foot, that was going to be necessary to allow public acceptance.

But even at the time that this was happening, there were already people in the industry, very prominent people, who understood that they were fighting a losing game. One of my favorite examples of this comes from an Aviation Week story in August of 1964, where they're quoting a friend of the podcast, Clarence Leonard Johnson, also known as Kelly Johnson, who was the founder of the Lockheed Skunk Works, and the designer of supersonic aircraft like the F-104 and the SR-71, which had actually just been revealed... Its existence had just been revealed just a couple of weeks before this article was published by the White House.

And in the process of doing that, C.L. Johnson had met with reporters and met with public audiences, and this issue of commercial supersonic flight had come up. And so Aviation Week quotes him in this article in August of 1964 saying, "Lockheed's C.L. Johnson still feels the sonic boom problem is a long way from solution. In a recent speech here, Johnson urged consideration of more powerful engines to give the aircraft a higher cruise capability to minimize boom effect. Johnson believes the boom level of 2.0 pounds per square foot during climb and acceleration must be cut in half, and the cruise boom climb eliminated altogether to make the aircraft acceptable. He admitted that this would require a major, but in his opinion, attainable design technology breakthrough."

And so now six years later, we're still looking for that technology breakthrough, and we're getting a little bit closer. But I'll let my colleagues start talking about that next.

Graham Warwick:

So yeah, so I want to go back to basics here, and just explain my understanding anyway of sonic boom, and how it's a problem, and how we might be able to deal with it. So as the aircraft flies through the air, it creates disturbances, and like dropping a pebble in a pond, those disturbances move away from the aircraft like ripples on the surface of the pond, and they move away at the speed of sound. So as the aircraft gets to the speed of sound, those ripples can't get away from the airplane because they're traveling at the same speed as the airplane is traveling at.

So they all pile up into, in effect, a bow wave at the front of the airplane. That's a pressure wave, a peak of pressure that sits on the nose of the airplane like a cone of this pressure peak that sits on the airplane. And then there's another one at the tail of the airplane for various reasons. So those two peaks of pressure propagate down through the atmosphere and reach the ground. Now they're moving across the ground at the speed at which the airplane is moving at. So an observer standing on the ground will hear that first pressure wave as it crosses them, and then the second pressure wave as it crosses them.

Now they're only fractions of a second apart. They're far enough apart, but what you really normally hear is what's called a double bang, which is an N-wave, a pressure peak, a decline, and a pressure peak, and it sounds like a bang-bang, as you hear it. And what annoys people is not just how big that pressure peak is, it's how quickly it rises. That's what the ear is really sensitive, is that the speed at which that pressure rises as the shockwave goes across the top of you.

So that's what Project Bongo they were hearing over Oklahoma City, it was that bang-bang of an N-wave shock wave flying across the city. It's what Concorde produced. And as Steve says, military airplanes at the time was somewhere in the one to three pounds per square foot, which isn't really very much because as we're walking around, we've got more than 2,100 pounds per square foot of pressure on us as we're walking around outside. But it's that tiny extra pressure and the speed with which that extra pressure came, that created this problem.

But if you look closer, and this is what they did around the time that Steve's talking about, they started to look really closely at what did this actually look like. So they started flying airplanes side by side at supersonic speeds, and they probed the pressures around the airplane, and they found that every time the air had to change direction on an airplane, it created a shockwave. So you'd get a shockwave at the nose, at the cockpit, at the inlet, at the wing leading edge, at the tail, you'd get this sort of system of shocks. And as they left the airplane, they're all separate. There's separate shocks coming off the airplane.

And you can see these, what they call Schlieren images, where you can actually see these shockwaves coming off the airplane. But as they propagate down through the atmosphere, they all catch up, they all bunch up, and you end up just with these two shockwaves, front and rear. But that structure to the boom is the key to how we might be able to reduce the boom because if we can take those two big booms and break them up into smaller booms, it's smaller shockwaves. And then if we can stop those shockwaves coalescing as they come down through the atmosphere back into a single big bow wave and a single big aftershock, instead of that N-wave, we might actually get a sine wave, an S-shaped sonic boom, which means that as the aircraft flies across the observer, instead of that sharp peak, you would get more of a ramping pressure.

And that ramping pressure stops sounding like that hard bang-bang, and starts to sound more like a thump, or even in some cases if you get low enough, like a whoosh, almost, because you've taken that really, really sharp increase in pressure and you've kind of spread it over this shockwave. So that's really key, and we'll get to this later when we talk about X-59. We know that's how it works, and we've kind of half demonstrated it with an F-5 that was modified back in the early 2000s, where the front of the airplane was reshaped, now that the X-59 is going to do the whole thing.

But that's what we're trying to take, is you have to understand what creates that shock, and then, in effect, design the way that the shocks come off that airplane so that they do not coalesce. They're less strong, and then they do not coalesce into these big sharp peaks, four and a half.

Christine Boynton:

So what came next? Going back to Project Bongo, this was 1964, and it would be another five years before the Concorde began its test flights, and another 10-plus years before commercial flights would begin. So as efforts to bring that program to life we're progressing, the US was evaluating its own supersonic transport. And we've got here a clip, this is from June 5th, 1963, of President John F. Kennedy speaking at the U.S. Air Force Academy.

Audio:

I'm announcing today that the United States will commit itself to an important new program in civilian aviation. Civilian aviation, long both the beneficiary and the benefactor of military aviation is of necessity equally dynamic. Neither the economics nor the politics of international air competition permits us to stand still in this area. Today, the challenging new frontier in commercial aviation and in military aviation is a frontier already crossed by the military, supersonic flight.

Leading members of the administration, an open preliminary design competition will be initiated immediately among American airframe and power plant manufacturers with a more detailed design phase to follow. If these initial phases do not produce an aircraft capable of transporting people and goods safely, swiftly, and at prices the traveler can afford and the airlines find profitable, we shall not go further.

Christine Boynton:

That was President John F. Kennedy speaking at the U.S. Air Academy on June 5th, 1963. But there was some drama behind the scenes that preceded this speech, it had to do with Pan Am and a decision on the Concorde. And what we're going to play for you next is a phone call that happened the day before this speech.

Audio:

I talked with him since then.

I think you ought to call him up, Doug, and say that we're goddamn sure about this. He knew the United States... My god, I had it in my speech for tomorrow. And we're about to announce a program. Everybody worked this weekend. I only got the Vice President's report on Friday or Saturday, and I talked to McNamara on Sunday, they worked yesterday, we're putting it in a speech for tomorrow. And for him to go ahead on Tuesday afternoon, which involves hundreds of millions of dollars of balance of payments, which is going to sabotage a program to put the United States up in the lead in the '70s, it's very difficult for us to go ahead if he's buying. I think he ought to retract that thing until he sees what sort of an offer we've got.

Oh, he isn't going to buy these planes.

Well, now wait a minute, here's the AP story. "Pan American World Airways announced today it has ordered six new Concorde Supersonic jet transport." Well, Christ he's buying them.

No, he isn't going to buy them. I saw the contract and-

Well, I think he ought to put out a statement that he's not buying them.

He has an option to buy, that-

Well, except you see, that isn't the way the announcement reads.

I see the announcement reads differently, but I read the contract which he showed to me a week ago, and he-

Why don't they... You better-

... he has the right to pull out for a million and a half dollars, so-

Well, they better put out a statement because otherwise for me to go now ahead and announce our program is going to look awfully foolish. He ought to make an announcement that they've made no decision on buying the plane, they merely purchased an option. Why they felt they had to purchase an option, I don't know at this point. Christ, if we're going to go ahead, the French and the British aren't in such a strong position. He threw a million and a half bucks down the drain because-

I think he has [inaudible 00:16:33].

Because when we do our thing, then what the hell does he need an option? Everybody's going to be around competing to sell them.

Christine Boynton:

So he was not happy, not happy with Pan Am, not happy with the timing. And we'll put a link to that full telephone conversation in the show notes. But that was between President John F. Kennedy and Secretary of the Treasury Douglas Dillon.

So I'm going to throw it back over to you, Steve, Graham, and Guy, do you want to get into what was happening here with the US's own ambitions to launch their own program at this time?

Guy Norris:

Thanks, Christine. What amazes me about that is the fact that it was not even a question about really technology at that point. It was all about, "Well, of course, we can do this, it's just like when and who?" There didn't seem to be any doubts in anybody's mind that all of this was completely possible. And what makes me... when you look back at those events, was the fact that it's all sort of a sequential series of events that, really, you can trace to 1956, just slightly over a decade after the end of World War II, and the jet age was literally just over a decade old, when BOAC, the British Overseas Airways Corporation, sort of progenitor of British Airways today, shocked the UK industry by ordering Boeing 707s.

And that devastated... It was the same sort of thing as JFK was sort of shocked that Pan Am should be ordering non-US equipment. It was the same shock in the UK. So, Stanley Hooker, who ultimately was the engineering director at Rolls-Royce, and a key person behind many of their famous engines, including the Olympus engines which powered Concorde, and the Merlin, of course, which as a younger man he helped perfect to power the Spitfire and the P-51, said, "What else do we have if the US is producing this new jet?" He already could see it was going to eclipse the Comet, which the UK was building. And he said, "We must go to the next step. We must leapfrog ahead with the world's first supersonic transport," and that was in 1956.

So as a result of that, in the UK, they set up a Supersonic Transport Advisory Committee, or the STAC, and that brought together, like they did over in the US, nine air framers, four engine makers, to consider everything, including the economic and social impacts and sonic boom and noise, things that were way, way ahead of anything else that had been thought of at that time. Even, and this is something which we can talk about later, but things like the flight control issues due to the center of pressure changes with supersonic flight. So they were even aware of those nuances.

So as a result of that, they began to home in on configurations, which we're now familiar with the Delta, but having got rid of things like strange M and W-shaped wings and that sort of thing. And they brought together their concepts with the French government, which were looking at concepts which are much shorter range for European and African routes based on the Caravelle. And when in 1961, both of these designs were peered side by side at the Paris Air Show, and everybody couldn't believe it, they looked almost exactly the same. And so you gradually begin to see this coming together of the French and British plans.

So in the US, seven years before JFK was brought into this public stance on this, NACA, which is the predecessor to NASA, of course, produced a report to say, "It looks like, basically, supersonic transports are feasible." And, Steve, you might know more of this history on this, but General Elwood Quesada?

Steve Trimble:

Quesada.

Guy Norris:

Quesada. Thank you. The FAA administrator formulated the SST program in the summer of 1960, and he recommended to JFK later that, "We should spend $500,000 per week for two years, that needs this." Because there was this fear of... obviously, Concorde was coming, the Soviets were developing... obviously, they had their eyes on supersonic transport too. And the FAA successor to Quesada, Najeeb Halaby, was also a strong believer.

But here's the weird thing, I think that at the time the airlines actually weren't that very enthusiastic about it. They were already paying huge amounts of money for their new fleets of DC-8s, 707s, and Convairs, and they weren't really very interested in having to spend even more money on something that they didn't even think would even happen. So that bombshell that JFK dropped at the Air Force Academy in June of '63 really set... The race was on at that point. So that was an interesting background, really.

Steve Trimble:

Well, yeah, I thought I'd also just bring us up to speed on the US Supersonic Transport program that JFK speech there at the Air Force Academy in 1963 initiated, albeit reluctantly on the administration's part at the time. But that produced also some additional drama because at the time, Boeing was the commercial airline leader in the industry. Lockheed was there too, they had a big commercial aircraft program at the time, but they were seen more as the high speed aficionados, especially with the existence of the Blackbird confirmed at that point.

But there was a competition. Boeing had... Their design was called the 2707, so twice the speed of a 707, in fact actually, or twice as large as a 707 because it was actually three times the speed of a 707, so going up to Mach 2.7 with the GE4 engine, which is a derivative of the YJ93s that were used on the YB-70 bomber, a Mach 3 prototype. And then for Lockheed, what they proposed was the L-2000, which was another aircraft Mach 2.7 to Mach 3. And both of these designs were designed to a spec that the FAA called for. Because they were American, because we were American, we had to do it better than the Concorde, we had to be bigger and we had to be faster. So it was going to be 250 seats, and instead of Mach 2.0, it was going to be Mach 2.7 and Mach 3.

Now, Boeing's design started out with a swing wing, and that's what was selected by the FAA in 1966 to proceed into development, much to the surprise of industry at the time, I think it's fair to say. Lockheed was fairly shocked. I think Lockheed was even more shocked that they won the C-5A competition to develop the next subsonic large airlifter because Boeing was expecting to win that, and then had to proceed into development on their own of the 747.

But then as the development of the 2707 went forward with Boeing, they realized the swing wing was going to be impractical, so they changed it to a delta wing, to make it look quite like the L-2000 that Lockheed had originally proposed and said was going to work, so that was also pretty awkward. But in the end, those noise issues, as well as economic issues, and the oil price at the time in 1971, led to the cancellation of the program, and with no response to both, not only the Concorde, but let's not forget the Soviet Tupolev Tu-144 supersonic transport that also was in development, and pretty far along at that time as well.

Christine Boynton:

Guy, did you want to jump in here?

Guy Norris:

Yeah, I was just thinking that just as Steve was mentioning, the variable geometry design that Boeing adopted for the 2707, one of the reasons that it really failed was the fact that the wings with the swing wing mechanism was so enormous and so huge by the end of it that it was up to 7% of the gross weight of the aircraft, and instead of the just bearable 4%, which the FAA had wanted. And in a review in early '68, the FAA said, "We had an aircraft that could cross the Atlantic empty or fly halfway across full." So it just wasn't going to work.

So I don't think anybody was particularly surprised when they finally went to a delta. And the other thing, of course, was by '68 when President Nixon was... well, when he won the election, the Concorde had managed to get 78 options, I think, versus 122 for the 2707. And 58 of those orders for the 2707 were from non-US airlines, so they could see where the market was going, it was for people who could fly overseas because it was a pretty good consideration or recognition then that flying overland even then was not going to be possible.

But the other point about it is that Concorde and the Tupolev 144 both flew within the next year. And there was a two-year gap, which the FAA had been trying to keep within between the launch of Concorde and the US competitor, had by then grown to six years. At that stage, everybody still thought Concorde would be certified in 1972, but the 2707 certification target had slipped to 1978, and that meant that to the US size, Concorde could clean up, even though it was a much smaller aircraft, they thought that they would have a six-year lead, and the market suggested that they would be up to 500 aircraft sold or ordered by 1990. So the reason when the US project was canceled by the government, it's no mistake, obviously they could see the writing on the wall.

Christine Boynton:

Speaking of seeing the writing on the wall, we had an editorial in Aviation Week & Space Technology, and this is from February 9th, 1971, again, we'll post a link to the full version in the show notes, but this was Bob Holts, who I believe we also quoted in our first episode about the birth of the Blackbird. So to quote him again, he says, "Concorde was the first supersonic transport program to take shape more than 10 years ago. It triggered both the Russian and American efforts to get into this market." But he notes, "Concorde's early start has given it a significant lead over its most potentially serious commercial competitor, the Boeing 2707. At the very time that US political resolve to back its SST program is wavering, the Anglo-French government support for Concorde appears to be gathering strength." And he ends by calling the Concorde, quote, "The strongest challenger to American domination of the international transport market in history."

Guy Norris:

Yeah. And that's something that Hallaby, the FAA administrator, that was the thing he was saying that losing the 2707 relinquishes world transport leadership, that was his warning. So even though everybody... the airlines weren't convinced, the governments weren't convinced, and yet there were certain elements of the industry which just thought, "If you throw away this supersonic leadership, that's it. You've thrown away the crown," sort of thing.

So it's remarkable, even though there were these massive technical hurdles and political hurdles, and of course, the sort of headwind that was being beginning to blow in terms of climate activism, and the impact on the... not only the environmental challenges in terms of noise around airports, but also the upper atmosphere. All of these red flags were beginning to be raised, and yet people still thought that you'd be losing the leadership if you didn't go ahead with this project.

Graham Warwick:

If I'm correct, and this is where... I'm not a history buff, but it was the Bongo 2 flights and all... that really built up the bow wave of opposition to supersonic flight within the US. There was the Concorde thing, but if I'm correct, it was the Bongo flights that really got this groundswell of opposition to see which played into the cancellation of the 2707. Is that correct?

Steve Trimble:

Yeah. So Aviation Week's coverage of it goes back and forth over the years about what the results of the Bongo study were. At the beginning, I think there was an expectation there was going to be a lot of complaints, and then there weren't as many as people thought there were going to be. But by the end of the studies, Aviation Week was quoting the FAA administrator at the time, who came after Quesada, that the impact of these studies showed that the annoyance level was a lot worse than they expected.

And so they had to take that into account of what their policy was going to be on restricting that overpressure at somewhere between one and a half to two pounds per square foot. That was the whole debate within the administration, regardless of the other environmental issues and the economic issues, and just even airline interest that they were going to face until the cancellation in 1971.

Graham Warwick:

And we shouldn't forget that, really, Concorde's problems were not just the... Well, in fact, to be honest, Concorde's problems in the US were not really related to the boom, were they? They were really related to the airport noise, that was where the opposition came. That's when the flights were stopped when they were going into New York, and that was to do with landing and takeoff noise because it was a very, very loud airplane on takeoff and landing.

Guy Norris:

Yeah, no, absolutely. And that was when you looked at the way that Concorde... I think we should just for a few seconds drill down into the design of Concorde and the challenges. And the reason that it was obviously noisy was particularly due to the Olympus turbojets that were used. There was just no other option, really, for the power that was required for supersonic flight. But also the fact that the wing design, this famous ogive delta, which was an absolutely brilliant design for what it achieved, but on the other hand, it meant that your takeoff speed was extremely high as your landing speed was also relatively high too because it produces lift by this vortex generated process whereby you have to approach at a very high angle of attack. It's a very power-hungry machine, and this produces noise.

There's so much we could go into here, but essentially it was a noisy neighbor. There was no way that you could really ever hope to meet any of the modern noise standards, and yet they did their best.

Steve Trimble:

It's an airliner with an afterburner on takeoff.

Guy Norris:

Yes.

Steve Trimble:

I mean, come on.

Guy Norris:

Right.

Graham Warwick:

We started out talking about Bongo and talking about sonic Boom, the interesting thing is that the US did try again. They did try to do another supersonic airplane in the '90s, and again, Boom was not a factor. Basically, they tried again with... they wanted to go faster. So I think they started out in the 2-plus speed range. They wanted to go for 300 seats because they were looking at flying across the Pacific, not the Atlantic, we're targeting the Pacific. This is the High Speed Civil Transport, HSCT.

So they wanted to do this big trans-Pacific airplane. They wanted, they believed, to get economy fares down to a point where people would use it like they use wide body jets today to go on vacation or something like that, whatever. They had this very aggressive economic goal. They had this very large airplane. They went through another design process, Boeing and McDonnell Douglas.

But again, there's so many common threads here. The airlines really weren't that interested. And when Boeing acquired Douglas, and there was no longer two competitors to push the program along, it kind of just... Boeing wasn't interested in it, and the airlines weren't interested in it. So after spending, I think, about $2 billion of NASA money, the biggest NASA aeronautics program for a whole decade, basically, they canceled it in 1999, and then NASA's aeronautics budget just went into free fall after that because it was such a shock to the system.

But again, they were looking at... The boom figures I've got for that were like 2.5 to 2.7 or something like that for that airplane, which is up there. If you're above one, you're in trouble. If you're above two, you're in even more trouble. You're above three, you're in serious trouble. But this airplane was never intended to fly supersonic overland. The 2707 was a continental airplane, New York to Los Angeles type of airplane.

It really wasn't until NASA kind of picked itself up off the floor somewhere in the mid-2000s, and started to say, "If we're going to go back to supersonic, we've got to tackle the problem. And the problem is acceptance, and acceptance is sonic boom." So that's where they went, they said, "Before we can ever get a supersonic airplane certified, we've got to come up with, basically, a noise level we can put in a certification document." So the FAA can put into its certification document 75 DNLdB. Concorde was 110, and every three dB doubles the sound level, so if you go from 75 to 110, you can tell you're talking about a heck of a lot more noise for a Concorde versus what NASA's aiming for.

So that's where that came from. And then as Guy can talk to you about it, at the same time, the startups, we got into that whole thing where startups got interested in aviation, and supersonic was this gaping hole in the market, they thought, and therefore we saw the Booms, the Arions, and other thing start up. They really weren't focused on low boom, they were focused on trying to meet a business case. So Arion was a business jet. They're gone now, they failed. Boom is a transatlantic business class, first-class, business class, that's still around. But you see, it's got two things going on, even though all these years after Bongo we're really only getting to the crux of the matter now.

Steve Trimble:

Well, and it wasn't just those companies, companies like Gulfstream and Dassault in that same period, in the early 2000s, are thinking, "We've got to be ready for this," because they think NASA can actually come up with something. Gulfstream got started on a project called the X-54 Whisper Jet, which was going to be a Mach 1.5 business jet with a 75 EPNdB-

Graham Warwick:

PNLdB.

Steve Trimble:

PNLdb. Thank you. Signature, but then they got some DARPA money for it, they completed design studies. You could find those design studies online, but in the end, one, the commercial market is very uncertain about this, and the noise studies that NASA was doing were not conclusive, and certainly weren't enough to break the legislation either by ICAO, or the EU, or the US that bans either the effect of a sonic boom or a sonic boom outright like it does in the United States. So that project was shelved.

But I do think that if supersonic transports ever really become possible and feasible at this low boom level, what you're going to really need is those experienced, well-financed, deeply resourced business jet companies like a Dassault, like a Gulfstream, maybe even Bombardier that if you go way back in time, Canadair does have a supersonic legacy with some of their 1950s designs, which I won't go into. But that's where the best opportunity, I think, is for supersonic.

Graham Warwick:

So one of the vexing things about the supersonic market is it's actually relatively easy to show there is a business case for supersonic. You have to be a bit selective in how you do your market research. But so in the case of the business jets, NetJets, which is the world's largest fractional ownership company, back in the mid-2000s, basically NetJets said, "You bring me a supersonic airplane, I can sell that to my customers," because they could take the $120-million cost of a single airplane, and divide it over a dozen owners or something like that.

But what they'd seen was in their fleet, they had two key airplanes. They had the Cessna Citation X, and they had the Gulfstream V at that time. So the Gulf Stream V is big, big cabin, luxury Jet. Cessna Citation X, small airplane, really fast. The fastest civil airplane at that time after Concorde had been retired, Mach 9.98, really, really fast. And they were actually charging their customers more for the Citation X than they were for the Gulfstream V because speed sold into their marketplace. And this is transcontinental speed within the US, this is New York to LA, basically, and they could knock two hours off that timeframe from New York to LA because you could go supersonic.

So there really was a business case. Now, Gulfstream said, "We will do it, but we won't do it until there is a certification basis for a low boom airplane that can fly supersonic overland." Which is what they're still waiting for, which is what X-59 is supposed to enable is that certification basis.

Now on the commercial side, and again I'll have to defer to Guy here, Boom has a rationale for what it's doing and it's taking the business class passenger off the upper deck of a 747, and just transporting them into a supersonic airplane and flying them across the Atlantic, and saving them bags of money. It is or isn't a business model you believe in. But there's certainly people willing to pay for business class travel. And what Boom says, "We can do supersonic for about the same price, and therefore do get you there faster."

Steve Trimble:

The engine is so important. And when we talk about the example of Arion, the engine was the big issue. So when they get started around 2003 or 2004, by 2008, 2009, they have a pretty solid case going with a JT8D engine, which because of the financial crisis, they lost their resources in 2008, 2009, they weren't able to pursue that design to certification. By the time they were able to, once we get past the financial crisis, and the business jet market recovers, that engine, the JT8D is no longer acceptable for a new clean sheet design, you cannot certify it on an aircraft because the environmental standards have been raised and excluded that engine.

And ever since then we've seen the same thing with Arion as we saw with Boom, without an engine that's off the shelf that you don't have to certify that you can just put on your aircraft, and make a few tweaks and modifications, but not certify a brand new engine. What we saw with Arion is they went from two engines to three engines, back to two. Boom has been through the same progression, 2, 4, 3, I think they're back to four now. And in their case, their issue is they're having to create their own engine with their partner Kratos as well as possibly some involvement with GE. That's a totally different thing when you're designing an engine and an airframe at the same time.

The only company that I know that has pulled it off was HondaJet, and that's obviously for a subsonic light jet, and that was with GE's help. And you have Honda, where this whole project is essentially a rounding error on their balance sheet, it's so small to them compared to the size of the overall company. Whereas a startup faces a lot much tougher financial process of getting all that capital, billions of dollars that's required to get through engine certification, and then through aircraft certification for commercial transport. Very, very difficult.

Guy Norris:

I've been looking at the project since the USSD was terminated, and there's been approximately 20 various attempts either to do a follow-on to Concorde, a NASA-type study to revive interest in the long-range, supersonic transport over water, predominantly, as Graham mentioned, or the emergence of international teams like Jacks' studies, for example, again, looking at mostly trans-Pacific, or the business jet aircraft market, which as you guys have mentioned...

So what seems to be emerging now is the fact that the X-59 innovation, the idea of trying to legitimize a technical standard for overland supersonic flights and really prove that capability, has been a real theme since the '90s, and culminating... I'm thinking like the Quiet Spike project of 2006 with that great big extended 24-foot long barbers pole on the front of an F-15, a most incredible-looking thing. And the Northrop, the modified F-5 program as well just showed that you've got that bifurcation at that point of people saying, "If we're ever going to be supersonic, it's got to be viable overland. And the only way to do that is by getting rid of or reducing the impact of the boom, or by those people who've said, "No, we can go with modern technology," and still get a viable business case for a passenger aircraft that can travel overseas."

And I think that's why Boom's Overture experiment, really its concept, is worth giving some consideration to because their ethos is, "We've got 60 years now since Concorde technology brought this to the market. We've got so many changes out there. If we are reducing our target Mach cruise speed from Mach 2-plus to Mach 1.7, you reduce that stagnation temperature, which allows composite materials to be used throughout, and that does all sorts of great things for your takeoff weight, which there begins this virtual cycle in terms of what can we do with propulsion."

And yes, absolutely, propulsion is the key, always has been. And I'm amongst those that are willing to say, "I would love to see this work." They're sort of taking a SpaceX approach to this and saying, "Well, look, if SpaceX can do this for rocketry, why can't we come up with something like this for the commercial market?" So it's a massive challenge, and they know it. But the jury is still out, in my opinion, and I can't wait to see what they come up with.

Graham Warwick:

Can I just jump in on the engines there? Both Guy and Steve, you've talked about this. We tend to fixate when we talk about Concorde, we do tend to fixate on boom, but one of its biggest problems was its landing and takeoff and noise. It was right on the very limit. It was the loudest airplane flying. And subsonic airplanes were getting quieter just as Concorde was coming in. It would've been a sore thumb of enormous proportions had it stayed in service going forward. That issue is still there.

And if you look at Arion, as Steve said, all of the changes on propulsion was because one of the things they did because they couldn't use the original engine they wanted by the time they got to where they could move, they decided to go to meet stage five, which is the next noise iteration for subsonic airplanes. They ended up with two huge bypass engines on a supersonic airplane. Great big fat engines on a long thin airplane, and it really, really hampered their design. Speed came down, it started to not look as if it was going to be you'd get any benefit from going supersonic.

So you look at Boom, Boom is developing its own engine. But at the moment, the FAA official stance is... Because then here's the weird thing, there's never been a landing and takeoff noise standard for supersonic airplanes. They have just said they've just left it blank. Now a couple of years ago, the FAA said, "As far as we are concerned, you have to meet the same standard as subsonic airplane. When you're flying subsonic in a supersonic airplane, you have to meet the same noise standards as a subsonic airplane." The moment the FAA wrote that it was stage four, which is fine, that's what airplanes are kind of at the moment.

But stage five is going to change all that. You can't go into service with an airplane that meets stage four now because if you're going to be in service 20 years, you're going to be a problem 20 years down the way. So, Boom faces the same problem as Arian and everybody does, it's, "How do we produce an economically viable, low drag, high speed, swoopy supersonic design, but put engines on it that will produce noise as low as high bypass airplane of the same weight?"

And it's a huge challenge. It's probably more of a presenting challenge than the boom is because if you're avoiding the boom issue, as Boom is by flying over water, you can push that to the second generation of airplanes or the next generation of airplanes. If you're Gulfstream, you can wait for the certification, and then you can do it. But if you are Boom, and you're trying to get into market by sometime around the turn of the decade, landing and takeoff noise is going to be... and emissions at high altitude are going to be your two biggest issues that you're going to have to get through.

Guy Norris:

And Steve mentioned quite rightly, the fact that the redesign that Boom went through was really a recognition that they could not really achieve any of those noise targets with that configuration. They increased it to four engines. They also changed the plan form so that you could introduce much more in the way of high lift devices, and they reduced the target speed to 1.7, which really was achievable given all of those constraints.

And I think that's really the key, is they're trying to work into that niche where they still make it fast enough to make it an attractive proposition, but slow enough that they don't have to deal with the challenges of the propulsion airframe integration and the noises that you'd get out of that. There's just no way around the physics, otherwise.

Graham Warwick:

It's funny to think back in the days of Bongo, Convair actually had a supersonic transport version of the Hustler on the drawing board. Think how noisy that would've been.Guy Norris:

But it would have looked fantastic, wouldn't it?

Graham Warwick:

It did look fantastic.

Steve Trimble:

So enough talk about Boom for now. So the other track is that low boom design that muffles that sharp crack that can be so annoying and allows overland flight. But for that to happen if you work backwards, we're going to need the ICAO to relax the prohibition on either the effect in Europe of a supersonic boom, or the existence of a boom in the United States. To do that, you need to do a survey to produce data that shows that your low boom design is going to work, and that's what that X-59 program is all about. They're going to do that, and in their case, is the plan still to do over Corpus Christi?

Graham Warwick:

I don't think they've said where they're going to do the X-59. They did simulate it over Galveston. They used the F-18 flying the maneuver that produced the low boom. They did that, and that was a rehearsal for X-59. So the plan for X-59 is to, once they've done the envelope expansion at Edwards, is to do their initial survey flights over Edwards. But then you get the same issue as you did way back in the Bongo days where if you do your boom acceptability testing over a military base that has supersonic airplanes flying every day, nobody cares. In fact, they like the booms because it's what pays their salaries, so they don't care. But they do that to get their test experiment ready. That's what they did, the Galveston was more to do with, "How do we survey the population?" These a really interesting correlation back to Bongo.

So I don't think they've said... They're talking about five cities or something like that, and it all depends on how much money they have, and when they fly. And they're already late in delivering the data to ICAO. They're going to miss their deadline, so it gets pushed back on a three-year cycle, so they'll have to go for the next cycle. If you go back to the Bongo, from what little I've read, is they hired the government public survey agency or something like that to survey the public and find out who the complaints, et cetera.

Well, NASA has now been working for 10 years on the experiment design on how to actually survey the population. And that's a whole science separate from testing airplanes. It's how do you get people to tell you what they really feel? If they know it's a supersonic flight and they're opposed to supersonic flight, they're going to say, "I hated it," it doesn't matter how quiet it is.

The other thing is with supersonic boom, and I guess goes back to Bongo in Oklahoma City, the biggest problem with boom is humans can tolerate a fairly high level of boom, buildings can't. Buildings will shake, rattle, and roll from booms, they'll break windows. There's actually a document here that says there was an actual number of broken windows per million panes that were set when they were doing those because broken windows are the biggest thing, and there's a lot of those. I think Steve has pointed out at the Air Force Academy not long after JFK gave his speech there, an F-105 Thunderchief flew over supersonic and blew the windows out of the academy. So it's a real thing.

And I've been in NASA's boom room in NASA Langley where they're working out how you do the psychoacoustic testing to make sure people are being honest with you. And one of the things they do is they put you in this room and it's very bare room with seats, and then they play a normal boom at you and you are indoors, so you're hearing it indoors. What happens is it's the low frequency noise that makes the building respond.

So they do the real boom, and they're really loud, and the building rumbles and all that sort of thing. And then they do the low boom, and all you hear is kind of like a stomp or something like that. But what they then do is they then come in with cups and saucers, knives and forks and spoons, and they hang pictures on the wall and they do the same thing again, and it's completely different because it's not the boom that you respond to, it's the rattles, it's the creeks, it's the other things like that because it's inside of it. So getting acceptable boom outdoors is one thing. Getting acceptable boom indoors is another thing.

Christine Boynton:

And to your point, Graham, we did have a letter that came in from an Oklahoma City resident during Project Bongo. We'll post the link to that because he raises some questions on the validity of the public feedback that FAA is gathering. So we'll make sure we post a link to that. But I think, Steve, you wanted to jump in and say something there.

Steve Trimble:

So now, it all comes down to that X-59 flight. They're laying out the experiment, but when is the X-59 actually going to fly? I was in the hangar in the Skunk Works and saw it nearly completed, I think they said it was like 93 or 95% completed, at the time. That was three years ago, and that was the year it was actually supposed to fly, but it hasn't flown yet. So, Guy, where are they with flying that aircraft?

Guy Norris:

Well, I'm glad you asked me that, Steve. They're at the point, as we record this, they're poised to begin engine runs. As you've mentioned, obviously, the program is significantly behind its original schedule. But yeah, they've got to do engine runs. The one thing that's different about X-59 is that because of NASA's budget, and Lockheed built it to NASA's contract, they haven't been able to build a classic iron bird development airframe. So they're having to use the actual aircraft as their iron bird effectively, they call it the aluminum bird.

And that means that it's already doing double duty as a test rig for its own program, and that's really hampered the progress in terms of the speed of getting things done. So I think the plan really is to get through engine testing, they have to do quite a lot of this sort of testing to evaluate the aeroelastics of the very unusual shape. We should remind listeners, of course, that almost half of the X-59 is made up of its nose. The other point about this long nose is that means that when it comes into land, it has to come into a very shallow angle because the long nose means that the gear is very strange, that the landing gear is at the back half. So your rotation angle is very shallow, and that means the approach angle is also shallow, and it means that therefore it has to come in at high speed.

It's got F-16 brakes. And it basically means that they wanted to be able to go into 8,000-foot runways, which would give them more access to runways that are nearby communities all over the Midwest or over the US. Now it looks like they've pretty much got to have a 10,000 foot or more runway, which immediately starts to limit the availability of where you can go and put it within range of a boom flight. So there's all these considerations that they're now having to look into. But it looks like first flight is going to be later this year, that's the best we can say. They're trying to say late summer, but my guess is it's going to be more like the fall. But anyway, who knows?

Graham Warwick:

It looks like a supersonic anteater really, doesn't it?

Guy Norris:

Yes, it does. It does. But anyway, remarkable looking airplane.

Graham Warwick:

And again, going way back to what I was talking about about shaping the boom, if you look at an X-59, you can trace every single one of the features on that airplane to where a shockwave initiates off the airframe and how strong it is. And so if you look at, again, those Schlieren images that you see, which actually visualize the shock waves, the shock waves coming off the top of the airplane are much stronger than the ones coming off the bottom of the airplane and going to the ground. So most of the energy in the shock waves goes up into space, or up into the upper atmosphere, and less goes to the ground.

And then you see that the shock waves that come off of the canards or the leading edge of the wing, that the inlets up on the top of the airplane, so you don't get one going downwards off the inlet, but you do get one off the engine plume and all that sort of stuff, they're going down and they're much weaker, which means that they do not tend to catch up with each other and coalesce into this very defined pressure waves fore and aft.

So it's really interesting. I've been covering the theory of this for a long time. And then you see a Schlieren image of the actual X-59 they've done in the wind tunnel, obviously, but they've also flown F-5s, and shown this. You literally can see how it's different, how those shockwaves are really different. And it's a application of shaping which we originally used to do for stealth, and now we've applied the idea of shaping to noise, and it's fascinating.

Christine Boynton:

If there had been more airline interest, Graham, in the HSCT, would we have gotten where we are now quicker, or what would've happened in that scenario?

Graham Warwick:

I think at any one of these points in time, if the airlines had shown greater interest, we might have had a different outcome. I think 2707 was basically a case of public opinion turning against it, and no real voices that were speaking on its behalf apart from industry. And therefore, had there been more airline or even public interest in supersonic travel, 2707 might not have disappeared. And HSCT, similarly, had Boeing who then became kind of the sole incumbent after they acquired McDonnell Douglas. Had Boeing been getting a strong signal from the airlines that people wanted to fly fast, then something would have happened.

Actually... Sorry, it's a little bit of diversion here, but Sonic Cruiser kind of was the line under this because HSCT kind of folded because Boeing found that passengers weren't willing to pay to go faster. Non-economy passengers, so the guys up the front, yes, but the folks down the back, they weren't willing to pay to get to where they were going faster. And that, again, repeated when they did the whole Sonic Cruiser study that eventually led to the 787. They thought when they started 787, that they could charge a premium to get there faster. Now, that was only a small amount faster, just above Mach 1, basically. But again, the same results that they got from HSCT is that the traveling public, the ones that fill the seats, weren't willing to pay a premium.

Guy Norris:

And just to jump in there, of course Boeing said to the airlines, at that point, "Are you willing to use that 20% speed change really, or trade that for 20% efficiency gain?" Which is where the 787 came from because they traded all that technology, reinvented it really, repackaged it in the 7E7, which became the 787. So that 20% change, or roughly that sort of number, was the airlines embraced the efficiency gain rather than try and trade it for speed.

And I think that that's the difference between this approach and, say, the boom approach, which is very pointed in terms of its market. It knows it's a niche market, but it thinks it can make a business there because overseas long range travel is the sort of area where they still think they could make a difference. And certainly, you've got American, United, Japan Airlines, all the people that have traditionally said, "Yeah, we'd be interested." But who knows where it'll go? It's going to be worth watching.

And I think the one last thing I was going to mention is that if X-59 makes it into the air in the fall, my prediction, if it happens around November, then that would be exactly 21 years since the last Concorde was flown. So, 21 years. Wow.

Christine Boynton:

So best-case scenario, looking into the crystal ball. How long do I have to wait?

Guy Norris:

Well, Boom's going to be the one, obviously, if it's going to be anybody, and they're still targeting entering the service around the end of the decade. My guess is it's going to be a little bit longer than that, but yeah.

Graham Warwick:

I would say that you are going to get the same situation that guy just described with Sonic Cruiser versus 787. I think that the Deltas and the Americans of this world... The Americans and the Uniteds, the people who've signed for Boom, it's just sex appeal. At the moment, it's just a bit of a PR boost or something like that because they've not put any serious money down. Now, they may do eventually, but at the moment it's just...

The real issue is going to be looking ahead 10 years to where they need to put real money on the table to buy airplanes. You're going to have the same question, are you going to spend that money to go faster or are you going to spend that money to be more efficient and less emitting? And I'll tell you, I will put money on it, the airlines will vote for efficiency, emissions, and not speed.

Steve Trimble:

And if you look at the history of commercial aviation, I think the best model or the most tried and true way of bringing in a new innovation like this is find a bunch of rich people who want a toy or really want to pay to go faster, and that's your market to get in. And so I think it's going to be the private jet. I think it's going to be a Gulfstream. If Boom works out, I think it's going to be as a private jet, not as an airliner at first, at least. But I think that's the path that they have to go down.

But it's an expensive path, and you need a lot of those people to invest in this, and that's why you also need overland certification. So that's a tough road, and I'm not sure we're going to go down, especially as... even subsonic commercial travel is becoming an environmental issue, and that trend doesn't seem to be going in a different direction.

Guy Norris:

And, of course, they are pushing... they're trying to say, well, we have got our sustainable aviation fuel coming, we're going to be burning that. So they're looking at that angle to mitigate that. But then there's the military side too, they've always pushed this, a dual role. Will it be a supersonic Air Force One? Those are the sort of things which... One thing I was going to say is I never flew on Concorde, sadly, although a couple of us have managed to break the sound barrier in military aircraft since, luckily. But I did manage to clamber aboard one of the very last Concordes while it was still alive, if you like, with its APU running. It was still hot to the touch when the British Airways donated Alpha Golf to Seattle to Museum of Flight.

And I clambered up after the last of the rich passengers got off. That aircraft was delivered across the US in November 5th, 2003. And it flew from across Northern Canada, where it was allowed to fly supersonically because nobody cared. And they got special permission for that overflight. And it actually set a new record from east to west crossing in North America on its retirement flight to a museum, a bit like the SL-71, really. It's insane, isn't it?

But anyway, so I went aboard, and Les Brody was in the cockpit. And he said, "Gosh, when was the last time you flew on Concorde?" I said, "Never." He said, "Oh my gosh. Well, sorry about that." He said, "But do sit down." So I sat in the right-hand seat, and we talked about how wonderful it must have been to have a career flying this. And he said, "Oh," he said, "Before they turn the AP off, do you want to do the nose?" So I said, "Really?" He said, "Yeah." He said, "This will..."

Remember that one of them... We didn't really talk about the technical aspects which made Concorde amazing, but they had to have this nose-tilting mechanism to be able to see where they were going because this was before the advent of the external vision systems, which will feature on X-59, by the way, to allow the crew to see. So yes, I was able to lower the nose and then raise it. And this is the weird factoid from this, the little switch in the cockpit that you used. He told me that that was the only part of the Bristol Brabazon to be reused in a commercial cockpit. It was a little tidy switch in the cockpit. I challenge any Concorde aficionado to come back at me to say that's not true. But that's what [inaudible 01:07:25].

Graham Warwick:

Oh, fight of the British aircraft industry.

Steve Trimble:

You've been holding onto that scoop for a long time, Guy.

Guy Norris:

I know. I tell you, I've never published that.

Graham Warwick:

Look for next week's cover of Aviation Weekly.

Christine Boynton:

All right. Well, on that note, thanks, everyone, for being here. Fascinating to hear you guys talk about this stuff, and that's a wrap for this episode of Check 6 Revisits. So a special thanks to our podcast producer in London, Guy Ferneyhough, and to Graham, Steve, and Guy for being here today. Special thanks also to JFK Library and BBC for the archive audio. For links to the full recordings, please check the show notes on aviationweek.com, Apple Podcasts, Spotify, or wherever you get your podcasts. We'll also pull from the Aviation Week archive and post some of those articles as well. And if you'd like to delve into our archive for yourself, Aviation Week subscribers can head to archive.aviationweek.com to dive in. If you enjoyed the episode and want to help support the work that we do, please head to Apple Podcasts and leave us a star rating or write a review. Thank you for listening, and have a great week.