The Race to Build the First Sustainable Supersonic Jet | Hacker News Reader

The Race to Build the First Sustainable Supersonic Jet

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45 pointsjstelz791 comments

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.

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.

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

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-...

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...

"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.

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

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.

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

People care. And even if you somehow think they should not you are in no position to decide what they care about.

The climate system itself doesn't, but the total carbon budget has to be allocated to and split between 7 billion people one way or another.

What is a fair split?

Of course, if you want to keep flying at 800 km/h around the globe AND get power for your home's computers and 4k TV, you will have to advocate for cutting electricity AND blocking air transport of some other people until their coal plants are powered with solar panels. It's your right to decide so, but you have to admit it that it's unethical.

> fair split

And what about the billions extra that the developing world is predicted to add to global population?

This is a reasonable question to ask, although it's also a terribly wrong assumption.

First the ship has already sailed for most of Asia, where the fertility rate is already below 2 or near 2, and the billions extra are just from people getting older. China's population for instance is already expected to decrease from 2030-2035. Asking from these countries to reduce their population is equivalent to asking them to reduce their life expectancy.

In central Africa, the fertility rate is still very high, indeed. But now, remember we are talking about splitting carbon emissions? They already don't emit any carbon. Reducing the fertility rate would still be a good idea, for other reasons (they will be the most impacted by climate change-related disasters for instance).

Of course it is the right question to ask.

If 100 million people fly and contribute 2.5% of emissions, and 7 billion people contribute 25% of the omissions are using coal generated electricity, which probably should you solve?

1) This is a false dichotomy. We need to reach net zero by 2050 to keep warming below 2 degrees (if we're lucky), therefore we need to solve both.

2) The 100 million people who fly have the most concentrated wealth and therefore have more leeway to implement alternatives to flying (high-speed rail for flights within the US for example).

3) You could also imagine the 100 million people investing to build new low-carbon electricity generation infrastructures in developing countries via a carbon tax on their flight ticket. Of course, they could donate directly as well, either solution is fine. That was actually the spirit of the Paris agreement, which Trump walked out of. Since the 100 million people who fly actively refuse to let go of their wealth, alternative options such as 2) are being considered, even though they might have lower impact (since they affect on the 2.5% of GHG emissions instead of the 25% of GHG emissions).

If we had addressed the coal problem 2 decades ago, our net zero goal would have been 2060 or 2070?

Ignoring the coal problem, and actually greatly increasing the coal usage in the past 20 years, has pushed us into a corner where we need miracles. Now "everything" must be addressed!

I think it's obviously absurd to expect people to agree on "fairness" and tying it to doing something about global warming seems like a malicious way to prevent doing anything. You're saying that all ethical problems have to be solved before saving the current imperfect world.

If you like capitalism and fear "socialism" (which I assume anyone needling people about "fairness" this way does) then that is what allocates resources given constraints of the real world. Accepting that one of the constraints is a limitation on burning fossil fuels for the entire globe does not require changes to society beyond normal responses to price signals. Nor decisions by some authority, other than the overall cap.

Figuring out who gives up what from a central command is both impossible and unnecessary, so it shouldn't be part of the discussion. What "socialists" even advocate Soviet style central planning anymore?

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.

> 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.

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...

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.

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.

“ 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.

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.

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...

> 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?

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.

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

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.

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.

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.

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...

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.

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.

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.

"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.

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)

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

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.

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.

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.

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.

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

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.

I was surprised the article didn't mention Boom, which is targeting the business-class airliner tier. Their value proposition is different, and closer to Concorde: no plans for 'boomless' supercruise, but also vastly higher supersonic cruise at Mach 2.2. They haven't started constructing their airliner yet (named Overture) but XB-1, their supersonic demonstrator and testbed, is in fabrication right now.

It's clearly a submarine (http://www.paulgraham.com/submarine.html) article. The article is so light, and so full of hot air, that it practically advocates for balloon travel, not supersonic flight.

Feels like a losing battle against physics, combined with it only being really valuable in a single direction.

Overnight flights from east coast to EU are late enough that they don't impact productive time, and largely get in early enough for a full day of work.

Making that flight shorter - as anyone who's done a DEN-NYC redeye will tell you - doesn't actually help productivity.

Sure, on the return flight, it would be great to shave a couple hours off, but on the way there, with the time changes required, faster doesn't so much help with productivity. Especially with inflight internet these days.

> they don't impact productive time

Even flying in first class I’m sure I’d get there much more refreshed for a short four hour flight than an eight hour one, improving my productivity when I get there.

And why are you only considering overnight flights?

I think focusing on longer flights will have a bigger benefit. Most of the time in a short flight is already taken up by getting to, being processed through, and waiting in the airport.

I also think it is common to have most long haul flights 6hr+ overnight.

Regarding Concorde and from a customer's business perspective, going west (from Paris or London to NYC) made sense as you'd pretty much land before you took off (in local time). So you'd gain time.

For instance you'd leave Paris at 10:30am and you'd land at JFK at 8:25am or so. But on the way back, you'd take-off from JFK at 8am to land at 5:45pm... To/From London would be 10:30am/09:25am and 8:30am/17:25pm.

Bottom line, going west is sweet as you get a full day ahead of you. But going east doesn't get you anything but going there faster.

That's why Concorde was often carrying more passengers on the way west than on the way back: people would choose to get a first class overnight flight...

That said, if they could solve the cost issue, I'd rather fly for less time, even if I still lost a day of productivity going east.

as an aside, I don't understand how people can fly several hours on a commercial airline and then just start their day as if it didn't happen. I need at least a full 24hr cycle involving sleep in a real bed to get back to 100%.

> I don't understand how people can fly several hours on a commercial airline

Flying in first, not drinking a ton of free alcohol, and showering at the arrivals lounge.

Yup, sleep well and have a shower right before landing. Rather easy (depending on the route), but could be expensive depending on how valuable your time is.

Doesn’t even always have to be first, lots of airlines have excellent product in biz too.

Honestly I think a lot are struggling to stay awake and just use a ton of caffeine to mask it. Certainly I've seen some execs whose brains just weren't there thinking they were fine to do SFO->DUB and start meetings.

It's mainly about routine. Given time and repetition people will get used to almost anything.

The anxiety surrounding travel is what makes it exhausting. Once you are used to it, it fades into the background as a well optimized habit. It's like your work commute where you are just on autopilot and don't really think about it.

Ultimately it becomes a few hours to yourself to just read, sleep, or get caught up.

Red-eyes are tough though it's not too bad in international business. I'm usually operating at about half speed when I overnight to continental Europe from the US East Coast.

On the other hand, flying cross-country on an early morning flight to SFO, I can pretty much be ready for a regular day when I land.

> The anxiety surrounding travel is what makes it exhausting.

That is actually a really good insight. I mean reading it, it seems obvious, it's just that I never realised it.

> That's why Concorde was often carrying more passengers on the way west than on the way back: people would choose to get a first class overnight flight...

So in a fully realized 1960ies space age utopia you'd have a fleet of supersonics circling the planet hub to hub slightly ahead of the dawn line and a fleet of saloon cruisers going the opposite direction. Instead we got mobile internet.

That's another thing you're right about: decades a go, working remotely wasn't really a thing... So all the business value of being able to get somewhere quickly makes a lot less sense now as you can video chat with people anytime, anywhere pretty much.

Timezones. 6PM in NYC is midnight in Paris. Leave the office at 3PM and get to Paris at Midnight? Lost half a day leaving, still not getting into a hotel until 1AM best case.

Leave NYC supersonic at 9PM, get to paris at 6AM, great, I might've been able to sleep 1h.

On a normal plane I can leave NYC at 6PM, get to Paris at 8AM, and hopefully have gotten to sleep for 3-4h.

This also doesn't account for block padding - busy destinations and origins are busy. JFK-CDG is blocked at ~7.5h, at least 45m of that time is for padding departure/taxiing/landing slots in JFK and CDG.

Doubling the speed of the middle leg is not the same as halving trip length.

My dad once told me he got upgraded to the Concorde on NY-LHR once. Aside from having the experience, he said it wasn't really a win. As he put it, with the regular first class, he got a nice meal on the plane and got into London in time for bed. With the Concorde, he got in in time for rush hour and had to immediately go out to dinner.

Yeah, for sure. You still burned your whole day and nothing but pain to show for it.

Still wish I got to fly it though!

Sounds like you only fly first class. Have you ever flown 10+ hours in economy and felt ready for work (or even vacation) stepping off the plane?

This will be a private jet, and they are pretty expensive. Apparently flying in a Bombardier Learjet costs something like $7000/hour [1], while a first-class ticket is more like $1000/hour.

[1] https://www.aircharterserviceusa.com/about-us/news-features/...

> while a first-class ticket is more like $1000/hour.

I usually pay like 160eur/hour for top tier F products like the Emirates A380.

I just paid a little over 4000 euros for a 32h round trip with that particular offering. I don’t chase after deals, I just tend to depart from a city without many F pax.

Of course, YMMV.

This doesn't seem possible - I have never seen first tickets that cheap from anywhere to anywhere. Can you provide a few example routes?

Most Emirates long haul flights departing from AMS,BCN or MXP seem to be that cheap, I'm sure there are other cheap airports but I'm too lazy to look. This is probably the case with all the euro airports that don't charge super high fees and lack large quantities of F pax.

I'm seeing MXP-HKG at 4000eur for a round trip, MXP-BKK for 3800eur. There are probably even better deals available if departing from some third world countries.

I guess you're from the UK, I'm seeing BHX-HKG priced at around 151GBP/hr. Shockingly, LHR-HKG looks even cheaper at somewhere around 100GBP/hr.

I think you’re seeing corona virus at work. Those F tix are usually more from what I’ve seen.

I was paying those prices last year. Only the LHR rates there seem highly unusual.

It sucks, but I've done it and do it. Unfortunately sometimes you just have to.

But these are maybe around 10h max flights - given the range of ~4000-4500 miles. So you're still on a plane for ~5 hours.

But you still have the same issues with time zones. If you need a lot of time to adjust to a new time zone, you were always flying in way earlier - so whether your flight is 5 hours or 10 doesn't really matter much.

You were never going to red-eye in for a morning meeting in London or Paris either way.

I wouldn't be surprised if economy class on this future supersonic jet costs the same as first class (or at least business class) on a traditional jet.

Though either way, even after a 9 hour business class flight, I'm certainly not ready to go to work when I get there even though I had a lay-flat seat.

I think contrary to what people think, Supersonic jets is actually sustainable from a physics perspective. One of the things I'm most excited about is active flow controls on planes. The whole idea is to dynamically create laminar seperation at strategic locations. The effects of this is pretty astounding. It can drastically minimize the drag of the plane and can be also used as flight controls. I can't remember the exact year but NASA did a presentation about this and project the technology to become more prevalent by 2035. Another thing NASA is working on is to use the same jet technology to create perturbations in the wake of the plane to minimize the noise from the Sonic boom.

I flown many long haul flights. The first time I heard Musk propose using rockets for international travel I thought it was insane. I still do. But if they are accident free for the next 10 years; would I then? Probably - if they could manage to keep my trip under ~3Gs.

I think we have some data already on the probability of accidents - I believe I read that a SpaceX rocket failed to land properly after something like 50 successes...but it was a "soft" landing in the water?

NASA lost 2 shuttles out of about 135 flights...

So if I was going to make a ballpark estimate without further information, I'd say 98% of the time you should get there safely...and that's better than the coronavirus!

People will often tell you how irrational it is that we demand such safety from commercial airlines, so isn't 98% really good enough?

For reference, that would be about 10x as risky as wingsuit BASE jumping so I guess the addressable market would be rather limited.

Yeah, I wonder, would a suborbital ballistic trajectory save fuel compared to chugging along in the atmosphere?

If this were to become common, would need some thinking how to distinguish this from a ballistic missile attack, though.

Even in conventional aircraft a parabolic flight plan (the vertical path) is optimally efficient, all other things being equal.

In practice it'd be a nightmare to separate traffic. Additionally it'd increase the pilot workload significantly considering the cruise phase of flight is typically the "ahh, I'll order some coffee from back" time as the primary duties then are (a) monitoring aircraft state (i.e., scanning the gages) and, (b) changing the radio frequency each time the flight moves into a new sector and checking in with ATC.

[me: aircraft dispatcher for a US carrier in a previous life]

One thing they got wrong in the article was on profits - whilst the Air France operation struggled, the British Airways Concorde operation was profitable, especially when they did research that showed most people thought seats on Concorde were more expensive than they actually were.

The only way this thing could be honestly sustainable would be if it were hydrogen-powered.

That might also be a prerequisite for profitability. But the airframe would need to be quite different from current designs because hydrogen tankage needs a lot more room than kerosene fuel would. Extra space on an aircraft is fairly cheap to provide. What is difficult is more weight, including extra fuel needed to get enough range at higher speed. Hydrogen fuel fixes that by having 3x energy per kg.

I'm sceptical. You'd need a lot of insulation and possibly refrigeration to prevent excessive boil off on long flights, adding weight. Also you'd want the tanks to be, if not spherical, at least cylindrical. Where would you put them?

Rockets make do with minimal insulation since they can top off the tanks just before liftoff and the flight itself takes only minutes.

And then there's also the thing that the climate forcing of flights is roughly a factor of 2.6 worse than the CO2 emissions alone, largely due to high altitude water vapor emissions. Switching to hydrogen would obviously not fix this.

Practical aerogel production makes a huge difference in insulation effectiveness.

Lifting body designs provide plenty of interior room for tankage.

Spherical/cylindrical tankage is needed only to contain high pressure, but we don't need to contain pressure.

The greater energy density of hydrogen means much less fuel burnt at altitude, vs. diesel. Water vapor is a grrenhouse gas, but condensed into clouds they reflect sunlight before it gets converted to IR. Not so easy to know the net effect, but the lifetime of water vapor is much less than CO2. Visible contrails suggest that condensation is preferred.

> Spherical/cylindrical tankage is needed only to contain high pressure, but we don't need to contain pressure.

You want the minimum surface area to volume, to minimize heat losses (and minimize the surface area that has to be insulated when weight matters such as in an aircraft). Spherical being the optimum, but obviously unpractical. If you look at tanks in current aircraft, typically in wings, they have huge surface area vs volume.

Lifting body might be better, I'm not sure.

> Not so easy to know the net effect

Luckily we have researchers who have dedicated years, if not decades, to studying this exact problem. Still, not an easy problem, surely. Some quick searching turns up this overview: https://www.carbonbrief.org/explainer-challenge-tackling-avi... (and it seems I was wrong, it's not water vapor that is the major factor)

Thank you, this provides a lot of detail I was not aware of.

The NOx contribution argues for use of onboard LOX, which would then enable a much higher flight deck, increasing efficiency further on long-haul routes.

I wonder what mechanism transports water vapor out of the upper atmosphere.

Availability of aerogels radically alters the equations that have dictated tank shape for liquified gases.

Methane is a more than viable alternative and there are a few others. Of course for both we'd need infrastructure to generate it from renewable energy (as opposed to extracting using or extracting hydrogen from natural gas). In terms of energy density, methane is good enough for space flight. E.g. the spaceX raptor engines burn methane. The long term plan for SpaceX is to use renewable energy to generate methane; which as solar & wind cost drops, might actually become cheaper than using natural gas. On Mars this will actually be the only way to generate methane as there is no natural gas there.

Even just switching to natural gas would already be more sustainable as it burns a lot cleaner and there's less need for energy intensive processing of crude oil to produce kerosine (not to mention shipping oil around the globe using oil tankers & trucks powered by bunker & diesel fuel). Kerosine is nasty stuff in terms of sustainability.

So, there's a path to 0 carbon for jets (supersonic and otherwise). Even compressed natural/synthesized methane gas is viable for GA and there have been some prototype planes for that. For jets, you'd want the cryogenically cooled liquid variety because it's easier to store (lighter tanks).

Of course this would require the usual suspects to get off their ass and start innovating. E.g. Boeing had some plans for this https://www.wired.com/2012/03/boeing-freezes-design-with-liq... but not much seems to be happening. In light of Boeing's recent trouble with the 737 Max, this might be worth revisiting for them. Also the blended wing concept that Airbus demoed recently would apparently be quite good for holding liquid methane tanks. Also a 60% reduction in fuel cost sounds like it should be a nice incentive.

I wouldn't be surprised to see SpaceX pivot to producing jet engines at some point as it's an obvious thing to do and they seem to have a lot of competence around the logistics and technology involved with generating, handling, storing, and burning methane.

In any case, SpaceX is already doing supersonic flights powered by methane every time they launch a Falcon 9 and they have some wacky notion of actually using their spaceship to enable 90 minute hops to the other side of the planet.

Guilt free sustainable supersonic travel is possible but it requires people to start thinking out of the box. IMHO the first aviation company that nails this is going to do extremely well.

45-minute hops, I would hope.

But, yes, the upside is tremendous. Aviation has long been innovation-averse, but that could change any time.

> Methane is a more than viable alternative

Is it?

https://www.sightline.org/2019/02/12/methane-climate-change-...

https://www.sciencedaily.com/releases/2019/12/191216140715.h...

https://www.livescience.com/52715-natural-gas-not-as-clean-a...

We're already coming to the conclusion that electric cars aren't that clean so when I hear about carbon neutral supersonic planes I kind of feel like I'm in a bad dream.

You make a good argument why synthesized methane is a great alternative to natural gas.

Also, electric cars are way cleaner than ICE vehicles over their life-time. I don't think there's any dispute about that.

What about Boom Supersonic? They are partnering with Prometheus fuels that pulls carbon out of the air and turns it into fuel and also focusing on a commercial jet rather than private. There isn't even any mention of sustainability other than in the title of this article.

This definitely read to me like a paid (by Aerion) puff piece. No surprise there's no mention of competitors.

The CO2 emissions argument is easily resolved: just price tickets high enough that journeys can be made carbon negative.

The race to spend excess liquidity injected by central banks: a bubble story.

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