Supersonic Air Travel
TLDR: Supersonic flight is expensive, hard, and produces noise. Taxing rather than banning sonic booms can foster innovation and allow for win-win redistribution of wealth.
Prerequisites: Congestion Pricing
In my essay on speed limits, I made the case for day-fines — if a rich person is caught speeding and pays a fine, the price should be adjusted to their wealth. The logic for this is simple: speeding is dangerous, and we actually want to discourage it, not just limit it to those who can afford to pay some flat fee. This is the dual of the argument in my essay on congestion pricing — if something is merely inconvenient, rather than dangerous, we can restrict it to those with a high willingness to pay by taxing it and giving the collected money to those who are inconvenienced. In general, when there’s a negative externality to some behavior, the right move is to tax it, rather than ban it, and thus produce a win-win.
Alas, our world seems more inclined towards banning than taxing. One of the more prominent victims of this is, I believe, supersonic air travel. In 1975, the first passengers flew between Moscow and Almaty (in Kazakhstan) at supersonic speeds aboard the Tupolev Tu-144 in about 2 hours. Nowadays it takes about 4.7 hours. Just one month later, Concorde aircraft (jointly designed by the British and French) took off simultaneously from London to Bahrain (~4 hours vs 6.5 today) and from Paris to Rio de Janeiro (~7.251 vs 11.5). While extremely expensive, both in design and operations costs, the Concorde managed to be semi-profitable, largely by focusing on high-speed flights between London and New York. But, given their high costs and other factors, both aircraft were eventually retired from service, never to grace the skies again.
The Sound Barrier
Before exploring the economics of supersonic travel, let’s explore the underlying physical hurdles. Traveling at 2500 km/h is not as simple as taking an aircraft flying at 1000 km/h and pushing the gas until it’s going 2.5x faster. In fact, many early pilots believed that there was a physical law that prevented flying faster than 1234 km/h. They’d push their planes harder and harder, and the air would start fighting back with exponential force as they approached that speed — the speed of sound.
Sound is formed from waves of pressure moving through a substance. When an object (such as an airplane) moves through the air, it pushes air out in front of it, which pushes on the air in front of that and so on in an expanding wavefront. At subsonic speeds, the higher-pressure wavefront has time to move out of the way of the object, but as an object approaches the speed of sound, the sound waves in front of the object begin to stack up, forming an extremely high pressure wavefront that pushes back, harder and harder, on the object.
At the same time, the space behind the object is now vacant, creating a natural low-pressure zone that pulls the air around it, and so on, in a second wavefront. This second stacked wavefront effectively pulls2 airplanes near the sound barrier backwards, making it even more of an obstacle. (Cool fact: Just as increasing air pressure causes a temperature increase, decreasing pressure causes things to get colder. Moisture in the cold air following a transonic aircraft can thus spontaneously condense into a cloud called a vapor cone.)
But, as it turns out, there isn’t any fundamental limit preventing flying faster than the speed of sound, and in fact, after the sound barrier is broken, flying becomes more efficient again (though never quite as efficient as subsonic), due to not having to pierce the dense pileup of high-pressure air3. Other things change about the properties of air when moving at supersonic speed, such as which engines and wings are most efficient. As a result, some designers tried making aircraft that transformed into a different shape to adapt to each regime, though this generally brought too much complexity and brittleness to be worth it.
But even at supersonic speeds, stacked wavefronts continue to ripple off the aircraft, much like the wake of a fast-moving boat.4 We perceive these wavefronts as a sonic boom — a shockwave capable of damaging hearing or even minor damage to structures, depending on the type of aircraft and how far away it is when it passes overhead.
How Bad are Booms?
Concorde was LOUD. Even before it began flying commercially, the USA, Canada, and much of Europe banned supersonic commercial flight because people considered the sonic booms to be a nuisance. But even while flying subsonic, the Concorde’s engines were significantly louder than other passenger aircraft, due largely to its use of afterburners. This lead to the plane being banned in the NYC area for over 16 months, shortly after it entered service.
But just how loud was the Concorde? If you were directly beneath the Concorde’s flight path, a reasonable estimate would is around 110 dB — about as loud as thunderclap from a lightning strike less than a kilometer away or a balloon popping in the same room. Pretty loud! These booms would spread over a width of around 80 km, and still be comparably loud to a motorcycle (~90 dB), even dozens of km away from the flightpath. Given the obnoxiousness, it’s understandable that the Concorde was limited to supersonic flight basically only over the ocean.
But there’s reason to suspect that this level of noise pollution simply isn’t necessary.
The “loudness” of a sound isn’t a straightforward physical property. For example, how loud is a bat’s chirp? On one hand, we can measure the pressure’s involved and find that it’s ~110 dB if you’re 10 cm from the bat’s mouth. On the other hand, it’s (mostly) outside the realm of human hearing, and thus silent.
The characteristic loudness of the sonic boom from an aircraft like the Concorde comes from the way the pressure waves combine. But if those same wavefronts had been spread out, it would’ve been much quieter. Enter the Lockheed Martin X-59 Quesst, an experimental supersonic aircraft designed to reshape the shockwaves coming off the body, significantly reducing the perceived loudness of the aircraft, even while flying at supersonic speed.
While originally slated for 2022, the craft’s first flight has been pushed back, and as of writing is scheduled for early 2025. If successful, it will demonstrate the capability of flying at over 1700 km/h while producing a sonic boom comparable in perceived loudness to a neighbor slamming a car door, even while directly under the flightpath.
Will they succeed? My guess is yes, more or less. And if they succeed, can that design be adapted to large-scale jets? My guess here is only somewhat. The X-59 is a single-passenger craft specifically designed to be quiet. Commercial aircraft will need to handle a whole additional set of constraints, such as size and efficiency, that could make things harder. Still, it seems worth trying.
Supersonic Economics
Unfortunately, the noise produced by supersonic aircraft is only part of the problem. Supersonic pressures cause extreme temperature gradients, which can stretch metals and warp parts. (The Concorde was designed to expand and contract as it flew, which is quite clever, but also produced a lot of wear-and-tear, complexity, and weight.) Supersonic pressures can also directly bend and break the bodies of the aircraft. Supersonic military jets are often made of (heavy) titanium just to prevent dissolving from the stresses.
Perhaps worst of all, supersonic flight requires vastly more fuel per distance in order to both get up to speed and to cruise. Construction Physics says it takes about 6 times as much fuel to fly supersonic. This extra fuel doesn’t just mean higher prices, it also means more weight and less space to devote to passengers and cargo. The Concorde *barely* had enough range to cross the Atlantic with a maximum of only 128 passengers. (Compare with the contemporaneous Boeing 747, which had a longer range and could fit over 300 passengers.)
During the Concorde’s most profitable period, a round-trip ticket on the plane between NYC and London was priced about $15,000 (in Nov 2024 dollars). Today, a round-trip economy ticket costs only about $600, and a business ticket is $3000. The Concorde was louder, less comfortable, and more cramped compared to today’s jets, but if we set that aside and imagine the premium for flying at twice the speed of sound is $12,000 per person, those passengers would have to value their time at over $3000 per hour. (The Concorde made the trip in ~3 hours compared to today’s ~7.)
There are certainly some people who value their time that highly! New York alone has thousands of people with personal wealth above 100 million dollars. For the ultra-wealthy elite, $12,000 is basically a rounding error, especially if it counts as conspicuous consumption that lets them low-key brag to their social group. It was this class of people that supported Concorde during the 80s and 90s.
While the Concorde overall was a huge money sink — the meager profits from flying between America and Britain never came close to recouping the cost of designing and building the planes — I think it’s reasonable to expect that with increased numbers of wealthy travelers in an even more interconnected world, there might be some market for supersonic flight today. Or more specifically, there might be if those planes are allowed to fly more routes,5 they’re safe to travel on, and they’re quick to board.
At the turn of the century, Concorde was crippled by two major disasters. In July of 2000, a Concorde hit a piece of debris on its runway when taking off in Paris, causing its tire to explode and leading to the airplane crashing into a nearby hotel, killing 4 on the ground and all 109 people on board. Concordes were taken out of service for inspection, with a plan to return them to the skies… in September of 2001.
Jet Competition
After chaos and delays, Concordes eventually began flying again. They were probably a bit more dangerous than other planes, due to their complexity, but still reasonably safe. But their demand was on the decline. There simply weren’t enough elite travelers willing to pay for the speed premium after 9/11. Some of this was due to fear, increasing preference for comfort over speed, or access to telecommunication on the burgeoning internet, but I don’t think that’s the real story.
I think the wealthy elite, after 9/11, shifted towards using private jets.
Unfortunately, I haven’t found a good way to test this with data.6 The General Aviation Manufactures Association’s archives indicate shipments of private jets declined 11.5% in 2002, likely due to the economic recession, but despite this, business jet flight activity increased 13% overall, and private aircraft have been on a generally positive trajectory. More broadly, the early 2000s represented an inflection point in private jet usage, as fractional-ownership companies like Flexjet saw a surge in business.
It also makes a kind of practical and intuitive sense. After 9/11, going through airports became slow and burdensome. If you’re a wealthy businessperson who wants a minimum of wasted time and hassle, chartering a private plane to take you directly to where you want to go seems potentially much better than having to go through JFK airport security and get a connecting flight out of Heathrow.
Utopian Supersonic Travel
I think it’s clear that the Concorde was a mis-step in the history of aviation, but I’m broadly glad that governments, companies, and engineers were willing to take risks in the 20th century. Designing aircraft is extremely hard and costly, and some mis-steps are bound to occur if people are pushing the limits of what’s possible. Research into projects like the Concorde benefitted the entire world, and I’m happy that we ran that experiment, even if the airplane itself wasn’t that beneficial. With additional rounds of investment and research, we can hope that next-generation aircraft can be made to be cheaper, stronger, quieter, and more efficient.
But will comparably ambitious projects ever re-emerge? While there are some startups in the field, the general ban on supersonic flight over land makes such research speculative and risky. And as long as boarding airplanes is slow and obnoxious, we should expect large-scale supersonic airlines to be particularly risky, economically. Thus, supersonic projects are in a bit of a double-bind: they’re illegal over land, but those that appeal to the typical (wealthy) customer will probably be too small to be able to cross oceans.
But things don’t have to be this way. In particular, we can tax the negative externality of noise pollution, and redistribute that money to those affected. Yes, getting hit by a sonic boom is obnoxious, but how would you feel about being given $100 every time you heard one? This kind of Pigouvian tax would likely restrict supersonic flight over land to the wealthy, and perhaps to particular flightpaths over rural areas, but this is precisely the kind of conspicuous consumption that is good to encourage from the elite. Instead of burning resources on wasteful yachts or whatever, those dollars would be fairly redistributed to ordinary folks who prefer money over quiet.7
Most importantly, in Utopia, the presence of taxes on airplane noise (of all kinds) leads to a kind of natural prize for innovators who can find ways to make aircraft quieter. My guess is that, as a result of this and less hassle around airports, Utopia has semi-quiet supersonic flights available to the elite in some parts of the world, and a broader sense that in the near future, ordinary travelers will be able to cross the world in a remarkably short amount of time — whether it’s supersonic… or hypersonic.
Due to range limitations, this flight had a stopover in Dakar.
Technically low-pressure zones don’t pull — they reduce a force that was pushing.
I’m less sure about this, but my physics intuition says that the low-pressure zone behind the aircraft stays approximately equally bad (because it’s near vacuum just behind the plane) regardless of flight speed. Don’t quote me.
A common misconception is that sonic booms only occur when an aircraft accelerates over the speed barrier. This would be convenient, since it would mean only one boom. But in fact there is more like a continual boom moving through space, which any stationary listener only hears once, as it passes over them. People on the aircraft, by the nature of flying faster than the speed of sound, can’t hear their own boom.
More routes means larger economies of scale and ability to spread out the fixed costs of R&D and building factories and maintenance hangars.
I looked high and low for a good database, but was unable to find anything solid. Let me know if you have a source!
Figuring out who’s affected is basically impossible if you need to be precise, but seems easy if you’re willing to be pragmatic. If each person has a registered address, simply mail checks (or allow something more clever with online banking) to every resident in the affected area a couple of times each year.