[jpeanuts]

In my opinion it is disingenuous to call any supersonic aircraft sustainable. Independently of any clever aerodynamic design to moderate the drag coefficient, aerodynamic drag always increases like the velocity squared. Doubling the speed will mean applying 4x the force for 1/2 the time, so all propulsive efficiencies being equal, you will expend twice the fuel to go the same distance - at an absolute minimum. In reality engine efficiencies decrease at high-speeds, and you won't be able to completely eliminate the Mach 1 "barrier" (drag peak), or design an aircraft that is equally efficient in takeoff/landing... so the factor will be bigger than that. But x2 fuel consumption is a hard lower bound.

I don't believe this can be called sustainable. Air transport is already a major contributor to CO2 emissions. We as a civilization badly need significantly more efficient ways to fly, not less efficient ways. To even slightly suggest that this might be helping the environment is deplorable.

Also, air transport is already miraculous. Going around the world in a day is not enough for you? Really? Buy a book.

[nbadg]

I don't personally want to make a value judgment about whether or not you could call supersonic air travel sustainable, particularly since it's fairly meaningless to remark on outside of a broader conversation (in much the same way that the "greenness" of electric cars is more complicated than saying they're zero-emissions, since you need to consider the power plants that are ultimately fueling them, plus the batteries, motors, manufacturing...).

But I do want to note that supersonic aerodynamics are extremely complicated, and I think it's just as disingenuous to say that drag always increases with the velocity squared in supersonic regimes. Strictly speaking you're right, but the drag coefficient itself is changing too, and in fact it converges to a value below that of its approximately-constant subsonic value. Also note that airlines are already traveling thoroughly within the transonic region.

I do agree that ultimately, in-atmosphere supersonic flight will consume more fuel than commercial airlines today. However, to say anything more specific than that, I think you really need to start doing a proper aero analysis; once you're transonic (or beyond), you've stepped outside the realm where napkin math is physically valid.

For a brief, okay discussion of drag coefficient changes wrt mach number, here's a link: https://onlinelibrary.wiley.com/doi/pdf/10.1002/978047011785...

[melling]

Nope, air transportation is not a major contributor to CO2.

There are over 100,000 flights daily. They account for about 2.5% of CO2 emissions yearly.

You know what is? Coal power plants that generate electricity.

Maybe we should focus on electricity generation from coal. I’m a little surprised that more hasn’t been done.

https://www.carbonbrief.org/mapped-worlds-coal-power-plants

13 years ago Google attempted their RE<C project.

https://www.forbes.com/sites/williampentland/2014/11/30/why-...

[mrpopo]

Air transportation is a major contributor to CO2, if you limit the scope to the people who use it. It is estimated that about 80% of the world's adults have never taken a flight, and 94% of the world's adults have not taken a flight in over a year.

So, the real picture is that this 2.5% of global CO2 emissions are caused by as few as 6% of the population, who also are incidentally the richest and bear a high personal carbon footprint in other domains than transportation.

EDIT: I'll add to this that the aviation sector is growing, and therefore its global contribution to GHG emissions will grow as well in the future. Discounting the carbon footprint of the aviation sector based on today's numbers is a mistake.

[cletus]

The real villain here is coal-powered planes!

Ok, in all seriousness, the efficiency story for air travel is... complicated. So you claim planes account for 2.5% of CO2 emissions but air travel and transport is still a relative luxury enjoyed by a privileged few so that's not necessary accurate.

I found this [1] that shows planes to be much worse than automobiles (per passenger per km) but again, it's complicated.

Personally I don't mind the investment in supersonic travel. After all, it's hard to know what fruit this will ultimately bear. Some people have asked what's the difference between crossing the Atlantic in 4 hours instead of 8? It's the difference between able to go for the day or not.

[1]: https://youmatter.world/en/plane-or-cars-which-means-of-tran...

[perl4ever]

"It's the difference between able to go for the day or not"

Which points to how increases in capability and efficiency can stimulate demand, which could be a bad thing from an environmental perspective.

[melling]

And in the meantime, enough emissions from fossil fuels to generate electricity is emitted every year to cover a couple decades of flying.

[mrpopo]

This comment makes no sense because it doesn't state how many people get electricity and how many people fly.

The carbon footprint of one transatlantic flight is on the same order of magnitude as one person's yearly electricity consumption.

People who fly every year can substantially reduce their carbon footprint by not flying, people who already don't fly every year (the majority of the population) can't.

[choeger]

The climate does not care how many people caused the co2 emissions.

[revax]

Almost the entirety of emissions are made up of 5% when you decompose by sector.

There is no silver bullet for reducing your carbon footprint. You should lower emissions for all industries at the same time.

[jpeanuts]

This is a very important idea. On a global scale this idea has been codified as "climate stabilzation wedges": https://johncarlosbaez.wordpress.com/2010/11/16/stabilizatio...

Each "wedge" is one method of reducing net CO2 output by 1 gigaton/year by 2054. Each wedge is a major economic and technological undertaking. Current estimates are that 13 wedges would be needed to limit CO2 to below 500 ppm. We have currently implemented 0 wedges.

[melling]

“ 4. Efficient coal plants: raise the efficiency of coal power plants to 60%. In 2004, when they wrote their paper, “coal plants, operating on average at 32% efficiency, produced about one fourth of all carbon emissions”

Coal is 25% of carbon emissions. We could have reduced this 2 decades ago. Instead we’re quibbling over 2.5% of emissions.

[pengaru]

> There are over 100,000 flights daily. They account for about 2.5% of CO2 emissions yearly.

The generally accepted emissions weighting factor for aviation co2 emissions is ~2.6. So it's more like 6.5%, and growing fast.

[tda]

Just to clarify: in the cases of high-altitude airliners which frequently fly near or in the stratosphere, non-CO2 altitude-sensitive effects may increase the total impact on anthropogenic (human-made) climate change significantly

https://en.wikipedia.org/wiki/Environmental_impact_of_aviati...

[lm28469]

Are there any studies comparing the impact of burning fuel at sea level VS at cruising altitude?

Also, it's not all about co2. I'm quite sure planes release a lot of other nasty things.

[taneq]

"Sustainable" doesn't mean "optimized for minimal energy usage", it means "we can keep doing it without running out of anything."

If we can make jet fuel out of atmospheric CO2 using solar power without crippling the rest of our industry, it's 'sustainable' even if we just use that jet fuel for Nitro Circus antics.

Also, 'efficient' doesn't mean anything without saying what you're optimizing for. Slow, lightly loaded aircraft might be 'more efficient' in the sense that they use less jet fuel, but if your business involves a lot of air travel, it's going to be 'more efficient' in the sense of time and money to use a faster aircraft.

[usrusr]

We didn't stop because we ran out of Concordes either and nobody would call them sustainable.

The article is truly disingenuous in this part because it starts with the sustainable claim, then goes on listing pollution as the biggest problem of the Concorde alongside two noise issues, as if building up to take them down one by one. Then it's taking down only the noise issues and calls it a day. In fact it even has to admit that the new plane will be just as range-limited as the Concorde was. This hints at the fuel issue definitely not being solved, given the 60 years (!) of engine advancements between them. (it isn't a sized issue, even business jets can reach quite impressive ranges)

[mrfusion]

You’re not factoring in the ability of supersonic planes to fly at higher altitudes lowering air resistance.

Also these planes could entice the rich to forgo flying on private jets. I’m sure 50 folks on a supersonic jet is still more efficient than 50 gulf streams.

[namirez]

This is a common misconception. Flying at higher altitudes doesn't mean just lower air resistance; it means lower lift as well. As a result you have to fly faster for compensating lower air density. If you do the math, you realize the overall effect would be higher energy consumption.

[freepor]

Come again? The drag is proportional to the lift, but if you’re flying higher, and thus faster, to get the same lift, you’re getting to your destination faster.

[namirez]

What you're missing is the effect of speed and altitude on engine performance. Please take a look at this (pdf):

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.167...

[lancesells]

Flying private is as much about comfort as it is about time efficiency. Maybe the speed increases will make up for time spent on security lines, ticketing, boarding and departures but I suspect you'll start to see private supersonic jets for those that can afford them.

[nradov]

In theory we could use renewable energy sources to manufacture synthetic jet fuel in a carbon neutral loop. It would be more expensive, but sustainable. Hopefully costs will come down enough to make this economically viable.

[CaptArmchair]

The problem is that we are fighting the laws of thermodynamics. Any jet engine is based on the rapid expansion of a gas. Any such expansion is based on generating heat.

We do know many chemical reactions that create heat. But none of them can do so as efficiently as high density carbon based fuels. And those that do come close still emit noxious molecules into the atmosphere.

At it's core, high density carbon - fossil - fuels have an incredibly high potential energy delta. A gallon of jet fuel packs a huge amount of punch. Per the law of conservation of energy: that punch must come from somewhere. It's literally sunlight which was converted into carbon form through biochemistry. Tectonic activity compressed those carbon molecules into an extremely dense form.

Incredible amounts of energy have gone into that natural process which spans eons, and no synthetic process can hope to recreate that in an economically viable manner. The latter meaning: production here and now, in enough volume at an affordable price.

So, any alternative is competing with an energy store or sink (it's not really a source since it doesn't create anything!) which is extremely portable and yields a high potential.

Moreover, since the invention of flight, only 2 main modes of active propulsion have emerged: a gas-based exhaust (jet engine) or propeller based.

Going electric means ditching the jet engine. But then you end up with a less efficient propeller based plane. The main reason why commercial airlines ditched propeller based planes for jet engines is because the latter can efficiently keep a large load (people / cargo) in the air.

Batteries don't pack the amount of energy needed to go long-distance, plus their weight makes battery-based flight not really a viable option. Plus, that's just shifting the problem: the energy still needs to come from somewhere.

Solar array based flight has potential, but solar cells aren't anywhere near efficient enough yet. Current solutions combine solar-based propellers with gliding, which result in large span widths. Whereas they are nowhere close to being able to lift the load of a 767.

The U.S. even invested and developed nuclear jet engines. No joke! Look up Project Pluto: a nuclear based ram jet engine. The idea being that nuclear power would rapidly heat air. They even build a bomber (NB-36H) that carried a nuclear engine in hopes of one day hooking that up to a jet engine. Kennedy cancelled that program because he believed that it was all just too provocative in a Cold War era.

So, the strategy on the table is finding ways to make the combustion process itself (a) more efficient (b) less wasteful. It's a delicate balance and it's by definition a limited balance. Inevitably, the gains will get smaller and smaller until it's not economically viable anymore to keep researching them.

To my mind, I feel we're already past the largest gains that we could make in terms of fine-tuning the technology. And to add insult to injury, the gains that still can be made are always offset by the increasing demand of more/cheaper flights / destinations.

The hard reality is that flying should be far more expensive to even remotely approximate the impact on the atmosphere. The other option is that everyone should be able take a plane like they take a bus (democratization), and accepting the downsides, even when those contribute to our own demise.

[nradov]

None of that is relevant to my comment. Sustainable power sources can be used to manufacture carbon neutral jet fuel. This has already been tested.

[rayiner]

As someone who studied aerospace engineering hoping to see advanced like this, I find your attitude incredibly depressing. Cutting off our ambitions at the knees is not the path to the future.

[DuskStar]

> But x2 fuel consumption is a hard lower bound.

Can't you gain a bit of that back by flying higher than a subsonic design could?

[ddalex]

Lift and drag are proportional. You fly higher you need to go faster as to get the same lift, and you'll encounter the same drag.

[DuskStar]

That's only true for induced drag, though. Parasitic drag is independent of lift, and would be reduced by flying higher.

[namirez]

You are right but the reality is even worse. Fuel consumption is proportional to power (velocity * drag) rather than drag itself. When you go twice as fast, the travel time is halved, so the total fuel consumption quadruples.

And as you said, supersonic drag coefficients are typically larger than subsonic ones, which makes things even worse.

[jpeanuts]

Thanks for correcting my embarrassing error! It's too late to edit my comment, but indeed x4 is the lower bound.

[sliken]

You are making some pretty big assumptions. Maybe the first gen planes will create a market for fast travel, and then the 2nd gen will double the altitude and skip most of the atmospheric drag.

[KoftaBob]

Air transportation only makes up 10% of emissions caused by the transportation industry, and that industry as a whole makes up 26% of total global emissions. In other words, air transportation makes up 2.6% of global emissions, which is not even remotely close to being a "major contributor to CO2 emissions".

As for your last sentence, if we can find ways to travel the world faster, why the hell not? Once we start having this attitude of "good enough", we stagnate as a society.

The biggest sources of pollution are passenger cars, energy production, and agriculture. Rather than stagnate our aviation technology in order to tackle something that makes up 2.6% of pollution, let's make those other industries green.

[jabl]

As another poster pointed out, the climate forcing due to high altitude emissions is about a factor of 2.6 worse than the CO2 alone. So 2.6 becomes about 6.8%, and rising fast. And to make it even worse, we basically know how to decarbonize other forms of transportation and electricity production, heating, some major industrial processes etc., meaning that when those solutions start rolling out at scale, air travel will rise even faster as the proportion of the total.

By all means, let's continue developing aerospace technology, but I think it's a fantasy that we'd be able to have anywhere close to the same volume of flights as today in a decarbonized world.

[satya71]

IIRC from my course work drag is proportional to the 4th power of velocity, not squared. But that's for subsonic motion.

[growlist]

And if it could be made carbon negative with sufficient carbon fixing measures etc., would you still be against it?