Ugo Bardi's Latest Post on "The Seneca Effect". The Hydrogen Myth

 

Ugo Bardi's latest post on "The Seneca Effect"

The Hydrogen Myth: Technology and Religion in the Decline of Civilizations

 

I just started a new blog titled "The Hydrogen Skeptics." It is about the hydrogen economy and hydrogen as a fuel and it is a little technical as a subject. So I thought it was not appropriate to discuss it in a somewhat philosophical blog like "The Seneca Effect." Yet, there are points in common, as I am arguing in this post. Above: the nuclear-powered car "Ford Nucleon", unfortunate technological prodigy of the 1950s, that never was turned into anything practical.

The Hydrogen-Based Economy: Is it Enough to Paint Something Blue to Make it Green?

A hopeful image for a hopeful article by Bertrand Piccard. "Blue Hydrogen" seems to be popular, nowadays. But is it enough to paint something blue to make it green? It turns out that "green" hydrogen, assuming it exists, is too expensive for what we need to do now in order to move away from fossil fuels and stabilize Earth's climate.

Hydrogen has come a long way since the time when it was discovered by Henry Cavendish as a component of the water molecule in the 1700s and then given its name of “creator of water” by Henry Lavoisier in 1783. It was later discovered that hydrogen is the most abundant element in the universe and the main component of stars.

Using hydrogen as a fuel is an old idea. It was, again, Cavendish who discovered that it can burn. The idea that hydrogen could be cycled as an energy storage medium is probably as old as the “fuel cell,” developed by William Grove in the early 1800s. In the 1950s and 1960s, the dream of "energy too cheap to meter" associated with nuclear technologies made it possible to think of hydrogen as an energy vector able to carry energy to the points of use, even vehicles, from a limited number of large nuclear plants. The first explicit mention of the concept of “hydrogen economy” was made by John Bockris in 1970. The nuclear promise never materialized, but the concept of the hydrogen economy was later linked to renewable energy. 

The idea of the hydrogen economy gained a lot of traction with the 2002 book by Jeremy Rifkin, titled “The Hydrogen Economy.” Conferences were held, research contracts were awarded, and prototypes were built. Sometimes, we saw lavishly illustrated pamphlets of the hydrogen-based world of the future, often depicted as something reminding the science fiction of the 1950s, except that it was quieter and greener. Then, it waned again when it became clear that the promises of clean prosperity could not be maintained except at stellar prices that no one was willing to pay. Today, we may be seeing a "third wave" of interest in the hydrogen economy. But is it a real possibility, or does it still remain in the domain of dreams?

Today, 50 years after the first mention of the concept of the hydrogen economy, and 20 years after Rifkin’s book, not a single application of the concept of cycling hydrogen as a fuel is present in the world’s economy. The “Hydrogen Car,” the fulcrum of the idea, found a recent incarnation in the form of the Toyota Mirai, but that’s hardly the kind of car that will replace conventional or battery-operated cars. After 6 years after having been introduced in the world market, there are maybe ten thousand Mirais running today in the world against some 10 million electric cars. Not a good performance for something that was touted to change the way people move in the world.

Things are not better for other facets of the hydrogen economy. Of all the prototype buses that would have used hydrogen as fuel, most can be found today in museums or have been scrapped – just a few seems to be still operational. The idea of using hydrogen as a large-scale storage system for the intermittent energy generated by renewable technology is too expensive to make sense. It simply doesn’t exist at present. We lack the network of hydrogen distribution stations envisaged as a necessity: there are maybe a hundred of them in Japan, maybe 30 in California. In the rest of the world, owning a Mirai is not a good idea. And nothing has happened of Rifkin’s grand idea that people would exchange hydrogen with each other using pipelines in the same way as people are exchanging data with each other using fiber optics cables.

In short, the hydrogen economy turned out to be 20 years (or even 50 years) of hype, but nothing that helped us to solve the problems that we face in terms of the desperate need we have to decarbonize the economy. It was at best a naïve idea. The costs and the problems involved were evident to everyone who looked at the matter in some depth.

What went wrong, then? A lot of things. Perhaps the main one was a basic misunderstanding in the way the idea was presented to the public. Free hydrogen is not an energy source; it is an energy carrier. Free hydrogen does not exist on this planet, so to create free hydrogen we must break the hydrogen bond in water molecules. That can be done using a technology carried electrolysis. It works, but it is not very efficient, it will always involve an energy loss that depends on various factors, but that is typically around 30%. So, hydrogen is a fuel, but it doesn’t come for free. You must pay for it and not so little. In practice, all the commercial hydrogen you buy today comes from the decomposition of natural gas, another process of limited efficiency. And that can’t help us much to get rid of fossil fuels since you start with a fossil fuel!

Then, there are lots of problems relative to how to store hydrogen. It is possible but expensive. Conventional steel tanks in which you store gaseous hydrogen suffer from the problem of embrittlement. Hydrogen atoms are so small that they diffuse into the steel making it fragile. You need different materials, typically more expensive ones. But, in any case, high-pressure hydrogen is not a good idea in terms of storage, especially in a vehicle. The tank would be huge, expensive, and dangerous. So, you can use cryogenic liquefied hydrogen that would still require a fuel tank of four times the size of a gasoline tank. In other words, a 30-liter tank of gasoline would be equivalent to a 120-liter tank of hydrogen. And you need to consider the energy needed to compress and liquefy the hydrogen, to say nothing of the unavoidable gradual loss from the tank, and from the danger that it poses. Hydrogen can leak from any container, no matter how well sealed it is. And liquid hydrogen will evaporate at a rate of around 2% per day.

Finally, there is a problem with the opposite side of the cycle, where you turn hydrogen back into water and energy. You can do that by burning hydrogen in a conventional thermal engine, but that’s so inefficient that it would make no sense. Indeed, the idea was, from the beginning, to use “fuel cells” – electrochemical devices that turn fuels into electric power. Fuel cells are normally efficient than thermal engines, but their efficiency is still limited, much lower than that of batteries. And fuel cells are expensive, the standard model that works at room temperature (PEM) need platinum as a catalyst at the electrodes. Platinum is a rare element, not only expensive, but that would be impossible to produce in amounts sufficient to replace even a fraction of the current park of road vehicles.

All that doesn’t mean that there are no niche applications of hydrogen that could be profitably used in the future. Maybe hydrogen could be a good fuel for ships, which have no problems with the need for a large and heavy tank. Or, hydrogen may be used for planes, although it would be impossible to couple with the current generation of planes that would need to be completely redesigned – not a task for the near future. And perhaps hydrogen could be used for large-scale energy storage. But all this is far away from the dreams of a prosperous and non-polluting hydrogen-based economy that were proposed in the early 2000s. 

All this is – or should be – known. Already in 2004, Joe Romm published a book titled “The Hydrogen Hype” directly conceived as a rebuttal of Rifkin’s 2002 book. Indeed, by the end of the first decade of the 20th century, the hydrogen economy seemed to be a dead duck. The collapse of the oil prices of 2009 and the advent of the apparently limitless “shale oil” in the US had convinced everyone that there were no problems with the oil supply for the near- and medium-term future. The idea of the hydrogen economy didn’t really die but went dormant, disappearing from the horizon of the energy events.

But, today, the situation has changed again. Depletion is making the extraction of fossil fuels more and more expensive. At the same time, we see the pressing need of decarbonizing the economy before it is too late to avoid a disastrous climate change. The fossil fuel industry is under heavy stress and the former miracle of shale oil is turning to be a canard. These are the probable reasons for the evident return of the hydrogen idea that we are witnessing today. It is not because new technologies made possible things that were not possible 20 years ago. It is, mostly a last attempt of the oil industry to propose a pie in the sky to retard the unavoidable demise of the polluting and unsustainable fossil fuels. The fossil lobby hopes that hydrogen will provide a niche for their products, counting on the fact that hydrogen – if we want it in large amounts – will have to come from fossil fuels for a long time. 

In short, hydrogen is not a good idea for the world of today. We need first to build up a real renewable infrastructure to produce energy. Only after that's accomplished, we could think of the luxury of using hydrogen to power cars and planes. For the time being, limited numbers of battery-powered vehicles, the concept of “smart grid,” and higher efficiency in every field, are the best way to go. We must move in that direction as soon as possible, without waiting for a pie in the sky that might never be within our reach.

 

There is only one culture: bringing back science into the fold of humanism

Yesterday, I was invited to give a talk at a public meeting on the usual themes: climate change, resources, pollution, and the like. This time, a question I received from the audience caused me a small enlightenment that I am describing here as I remember it (h/t Lorenzo Citti for having organized this interesting meeting) (image source)

Thanks for this question - it is a very interesting question: "are we teaching enough science to our children?" And I can tell you that it is much more than an interesting question, it caused some small earthquake in my mind. Truly, I had a flash of understanding that I had never had before and right now I completely changed my view of the world. It happens to me: the world changes so fast and I do my best to follow it.

Your question is so interesting because it has to do with the idea that there are two cultures: a scientific one and a literary one. As a consequence, some of us think that instruction is unbalanced in one or the other direction: maybe we teach too little science to our children, maybe too much. The whole idea goes back to someone named Snow who proposed it in the 1950s. He was not wrong, I think, but there were problems with the idea. The concept of the two cultures can be intended as meaning that we need somehow to bridge the gap that exists in between. Or, and I think that's what happens most often, it can be interpreted as meaning that one of the two cultures is superior to the other. That can generate a competition between the two and divide people into two different tribes: literates and scientists.  We are very good, as human beings, at dividing ourselves into separate tribes fighting each other. And that's bad, as you can imagine. Actually, it is a disaster. Snow was a scientist and he decried the scientific ignorance of literates. On this, he was right but in the long run the result was that literates despise scientists as illiterate boors and scientists despise literates as feebleminded ignorants.

Now, I had been thinking about all this and, as I said, today I had this flash that focused my mind on a concept. I think we have to say this clearly: this story of the "two cultures" is an idiocy. It must end. There is only ONE culture, and that's what we may call "humanism," if nothing else because we are all humans. That is, unless someone in the audience today is an alien or a droid. In such case, would you please stand up? No......? Apparently, we are all humans in this room and so we call our culture "humanism" (or, sometimes, "arts and humanities")  How else would you call it?

So, there is really no reason for considering modern science a separate culture rather than part of the human culture that we call humanism. I am saying this as a scientist: science is part of what I would like to call human "sapience", what the ancient called "sophos"; that we translate as "wisdom" "sapience," or "knowledge." The term philosopher just means someone who loves sapience. And that's what we are; scientists or non-scientists, the very fact that we are here today, engaged in this discussion. means that we love knowledge: we are all philosophers. And that's a good thing to be; sapience is what makes us human and that's why we speak of humanism.

So, why do science and scientists sometimes pretend to be a separate branch of knowledge? Well, it has to do with another concept that comes to us from the Greek philosophy. It goes under the name of techné that we may translate as "craftsmanship" and that originates the modern term "technology". Here lies the problem.

Five minutes ago, someone asked me about hydrogen powered cars. I answered that they have been a complete failure and that was it. But I ask you to go a little more in depth with this question. Why do many of us think these things are important: hydrogen cars, a hydrogen powered economy, and lots of strange things we hear as proposed by scientists and that are said to be able to "solve our problems." Why is that? There is a reason and it goes back to a period in history when scientists found that they were able to devise some clever gadgets: you remember the "atomic age", right? It started more or less from there. Then there was the space age, the information age, and so on. There was this great wave of optimism when we really thought that science would bring us a new age of happiness and prosperity - it was the triumph of technology over everything else. The triumph of techné over sophos.

That period of optimism is still with us: anything that you say that disputes the sacred cow of economic growth is answered with "the scientists will think of something." Climate change? Resource Depletion? Pollution? Not really problems if you have the right gadget to solve them. And this brings, sometimes, the question "do we teach enough science to our children?" It is a result of the opinion that, in order to solve our problems, we need more gadgets and that, in order to have more gadgets, we need more science and that, in order to have more science, we need to teach more of it to our children. I think this is not a good idea. I think we have too many gadgets, not too few. And all these gadgets either don't work or cause more problems than those they are supposed to solve. Think about that: we wanted flying cars and we got killer drones, we wanted freedom and we got body scanners, we wanted cheap energy and we got Fukushima, we wanted knowledge and we got 140 characters, we wanted a long life and we got Alzheimer. The more gadgets we have, the worse the situation becomes.

Don't get me wrong: I am not saying that technology is bad in itself. We all live in heated spaces, we use electricity, when we have a headache we take an aspirin, and we use a lot of useful devices in our everyday life. I am not telling you that we should run to the woods and live as our stone-age ancestors - not at all. Being good craftsmen is part of being human. It is just that this fascination with gadgetry is generating multiple disasters, as we have been discussing today: from climate change to all the rest. One of these disasters is the decline of science, with scientists often turned into those raucous boors who feel they have to send out a press release every month or so to describe how their new gadget will save the world.

It can't work in this way. We need to take control of the technology we use, we need to stop being controlled by it. And I think the first step for retaking control is to bring science back into the fold of humanism. I am saying this as a scientist and as someone who loves science - I have been loving science from when I was a kid. Modern science is a beautiful thing; well worth being loved. It has been telling us so much that's worth knowing: the history of our planet, the origin and the fate of the universe, the thermodynamic engines that make everything move, and much more. We need to see science as part of the human treasure of knowledge and we need to love knowledge in all its forms. And, as I said at the beginning, someone who loves knowledge is a philosopher and that's what we can all be and we should be; because it is our call as human beings. If we want to save the world, we don't need gadgetry, we need to be what we are: human beings.


See also this comment on my "Chimeras" blog

Hydrogen powered planes: can they save the airlines?

Not exactly the same thing as the current generation of planes! To run on hydrogen, the airlines would require a completely new generation of planes. (source)

Years ago, a Ukrainian colleague told me about a plan that the Soviet Union had for their military presence in the Mediterranean Sea. Because of the long supply lines from the home bases, they were thinking of using their nuclear-powered battle cruisers to produce hydrogen in order to fuel their warplanes.

I have no way to verify whether this story is true or not; I couldn't find any trace of it on the Web. But it is not unreasonable that the idea of hydrogen fueled warplanes was seriously taken into consideration in the 1980s, when the Soviet Union still had dreams of being a superpower. In any case, nothing came out of it and there are good reasons for that: a hydrogen-powered plane is an engineering nightmare for several reasons that are well described in a post by S.H. Salter that Sam Carana published on his blog a few months ago. The full post is reproduced below.

If running a warplane on hydrogen is a nightmare, doing that with the civilian airlines is much worse. Salter makes it clear how complex and difficult the task is. Hydrogen was a good fuel for the Space Shuttle, but the shuttle was not a passenger plane and it carried a gigantic external tank full of liquid hydrogen. This is because hydrogen is a good fuel in terms of weight, but it is bulky. In a passenger plane, the fuel is carried mainly in the wings, but there is just no way to do that with compressed or liquid hydrogen without completely redesigning the whole plane. And that implies replacing the whole fleet of the civilian airlines.

In a little more than a century, we went from the flimsy planes of the Wright brothers to the current generation of wide-body aircraft. The lifetime of the present planes is supposed to be around 30 years or more and it took seven years to deliver the first Airbus A380 (in 2007) from when the decision was taken to design and produce it. And the A380 makes use of proven technologies - it is just one of a long line of aircraft that have been developed and tested over more than 50 years. How long would it take to rebuild the whole airline fleet? Can we afford to do it? Will we have to ground the airlines before it is too late to avoid the worst disasters of climate change?

So, it is easy to write books about the upcoming "hydrogen based economy," assuming that all technical problems can be solved by throwing a little money at them. It is not so easy. Then, of course, there are other renewable fuels that could be used instead of hydrogen, but I will discuss that in another post, but let me tell you that things are not much better. Making a "sustainable plane" is a technological nightmare, at least if we pretend from it the performance we pretend from the current generation of planes.

_____________________________________


From Sam Carana's blog

Can we Design Hydrogen-Fuelled Aircraft?

S H Salter, Engineering and Electronics, University of Edinburgh.EH9 3JL.

The collection of temperature measurements by David Travis following the 3-day grounding of all US civilian flights after 9/11 showed the astonishing effect of jet exhaust on the environment. If burning hydrocarbon fuel in the stratosphere ever becomes a criminal offence, the aviation industry will have an interesting problem. A possible solution is the use of hydrogen as a fuel. Is this technically possible?

The Airbus 380 carries 250 tonnes of fuel with a total calorific value of about 1013 joules. Fuel is stowed in wing tanks but this would be a volume of about one eighth of the fuselage. The calorific value per unit mass of hydrogen is about 3.5 times that of jet fuel and so the weight of hydrogen for the same range would be only about 70 tonnes. Unfortunately the ratio of density of jet fuel to un-pressurized hydrogen is about 9000, so the design problem is how to reduce the volume ratio by about 2500. If we compress hydrogen to reduce its volume by a factor of, say, 100 we still have a fuel volume of 25 times the liquid fuel one or 3.2 times the fuselage volume. The cube root of 3.2 is 1.47 so by increasing all three fuselage dimensions by this factor we could have an aircraft with enough volume for all fuel in the fuselage but no passenger space. An increase by a factor of about 1.6 in both diameter and fuselage length would give enough volume for passengers provided they did not feel unhappy about being close to so much hydrogen.

The immediate reaction against the proposal will be triggered by embedded folk memories of the Hindenburg. Any use of hydrogen will need careful public relations. The Hindenburg survival rate was 64%, much better than crashes of modern conventional aircraft. Deaths were caused by jumping not burning. People who stayed aboard until the wreck reached the ground were unharmed. It is likely that the fire started in the fabric dope rather than the hydrogen. Because spilt hydrogen moves rapidly upwards there is much less risk than from a liquid fuel or heavier-than-air gases like butane or propane which regularly cause devastating explosions in boats and buildings. Furthermore the heat radiated by the invisible hydrogen flame is much lower than that from carbon particles in hydrocarbon flames. We can argue that hydrogen is actually safer than jet fuel, petrol and hydrocarbon gases.

We can spend the 180 tonne fuel weight-saving on gas storage bottles in the form of a low-permeability skin surrounded by wound carbon fibres. A helical winding of aluminium sheet with a low diffusion coefficient for hydrogen looks good. It can be made with the linear equivalent of spot welding. The axial stress in a thin-wall tube under pressure is only half the hoop stress, so we can use the gas tubes as fuselage strength-members. Once the fuselage bending moments are known, we can choose the wrap angle of the windings to give the right balance of directional strength. One structure might be a bundle of nine tubes in a hexagonal array with six full of hydrogen and three containing passengers. A cross section is sketched in the figure. Other configurations are being studied.

The smooth stress paths of the gas bottles would be badly disrupted by the conventional design of landing gear. Can we get rid of it? The requirements for processing the variable energy flows from renewable-energy sources have led to the development of new high-pressure oil machines using digital rather than analogue control of machine displacement. These machines have very high conversion efficiencies and very easy interfaces to computers (see http://www.artemisip.com/ ) . The extremely accurate control of very large energy flows allows many new applications. One of these involves replacing the landing gear of large passenger aircraft with a ground vehicle. Please suspend disbelief until you have considered the following facts:

1. The landing gear of the A380 weighs 20 tonnes, say, 200 passengers. This weight is carried round the world for many hours and then used for only a few minutes on each flight.
2. The landing gear occupies a substantial volume of the internal space. The volume restriction limits the travel of the landing gear and so increases acceleration forces.
3. The requirement for openings compromises the structural integrity of the fuselage and adds weight, even more passengers.
4. Landing gear must perform with very high reliability despite the weight penalty and extreme temperature cycling.
5. The full weight of the aircraft must be passed to the ground through highly stressed points.
6. Gas turbines are very inefficient for moving aircraft on the ground at slow speeds.
7. On the A380 the shape of the landing gear doors and opening spoils the aerodynamic fairness. 
8. There is a severe design conflict between tyre weight, tyre life and braking performance.

An alternative might be to provide the function of the landing gear by a special-purpose ground vehicle. It would of course have to have VERY reliable links to the aircraft ground approach electronics so as to be in exactly the right place and moving with the right velocity underneath an aircraft on final approach. However there would be no weight, volume or temperature compromises.

The contact between the landing vehicle and the aircraft would be provided by a nest of large air-filled tubes like very large, very soft V-block, running the full length of the fuselage. This would spread the weight evenly into the aircraft skin. The tube surfaces could have vacuum suckers, like an octopus, which could apply shear forces evenly to the aircraft skin. The bags could be on a frame which could have hydraulic actuators to give a much longer travel than the legs of the landing gear. Tilting this frame would remove the need for the angling of the rear underside of the fuselage required to prevent ground contact at V-Rotate. This would further reduce drag during flight. The absence of fuselage penetrations could allow safe water landings for emergency. Runways can have parallel lakes presenting a much lower fire hazard if fuel is spilt. The impact loading on the runway would be much reduced and it might even be possible to revert to grass runways with several parallel operations from any wind direction.

The ground vehicles could use Diesel engines with much higher efficiency at taxi speed than gas turbines. They could have higher acceleration during take off and higher deceleration during landing. The hydraulic transmission would also allow regenerative braking, so the kinetic energy from one landing could be used for the next take-off. All-wheel steering and the option of direct side movement would allow much better use of ground space. The ground vehicle could have many more tyres, which need have no weight or volume compromise to achieve high braking. It could have an air-knife to dry runway surfaces and remove snow. There would be plenty of time to inspect and exchange landing vehicles and they would be in use for a much higher fraction of the time. The landing vehicles could gently lower aircraft on to passive supports at each loading pier and be used for other movements while aircraft were being boarded or serviced.

Images by S H Salter, University of Edinburgh.

The volume of most aircraft wings is much below that of the fuselage and so there is not a strong reason to use gas tubes as structural wing members. However they would offer a way to offset the extra drag of the larger frontal cross-section. From the original work by Prandtl, it has long been known that sucking air from the upper surface of an aerofoil section will reduce the drag by an amount which far offsets the power needed for a suction pump. Schlichting in figure 14.9 of Boundary Layer Theory gives a graph showing a factor of more than two. An objection to suction on wings, where the outer skin is a structural member, is that perforations and slits cause stress concentrations. This should not apply to wing spars made as gas tubes supporting an unstressed skin.

It is important that using fuel does not move the centre of gravity of the aircraft. This happens automatically with fuel stowed in wing tanks. If large quantities of fuel are to be stored in the fuselage it will be necessary to have the centre of pressure of the wings close to the centre of gravity of the fuselage-engine combination. The choice of a ground-based landing vehicle suggests high wings and engine placement above the wing. In theory at least, this will give some advantage from higher air-velocity over the upper wing surface and lower noise transmission to ground level. It is much easier to service and inspect equipment if you do not have to reach above your head. Cranes lifting an engine upwards are much more convenient than forklift trucks working from below. While some change in the architecture of maintenance hangers would be required, high engines accessed from above would by no means be unwelcome to ground crew.

Gas tubes may not be ideal for connections to a low-chord wing and so the longer attachment line of a delta wing, such as used in the Vulcan and Concord and many fighter designs, should be investigated. A flat underside will relax the requirement for precision in yaw during landing. Suction may be able to offset some of the disadvantages of the delta wing as applied to civilian aircraft provided always that they can land safely after a failure of the suction system. A delta wing with a deep thickness and a leading edge made from very strong but transparent material, perhaps poly carbonate, might even allow passengers to sit in the wing enjoying a splendid view if their vertigo allows.

The range of the A 380 is 15,000 kilometres. While this may have been chosen for passenger convenience with the properties of present fuels, it is larger than necessary for trans-Atlantic flights and could allow a further volume reduction. The San Francisco to Sydney distance is only 12000 km and stops in mid Pacific could be very attractive.

Before we waste time on radical new aircraft designs and ground-based landing systems, it is necessary to confirm that burning hydrogen in gas turbines at high altitudes will be a chemically appropriate solution. If we burn hydrogen in ambient air there will be no release of carbon dioxide but there will still be the formation of nitrogen–oxygen compounds collectively known as NOXes. If these are cooled very rapidly, as in the adiabatic expansion of an internal combustion engine, they can be ‘frozen’ at the high-temperature equilibrium state with lots of very nasty acids. The lower combustion pressure and slightly slower cooling of a jet exhaust should be less severe but we want to quantify the severity of the problem. There may even be problems from ice crystals formed from the exhaust. I have asked colleagues at the National Centre for Atmospheric Research at Boulder Colorado for an opinion.

There is one engine design in which the combustion products cool slowly enough for almost all the NOX production to revert to ambient values. This is the Stirling engine originating from 1815 but abandoned because of the absence of materials with good thermal conductivity and high hot strength. Much better materials are now available. By an extraordinary coincidence, the digital hydraulic systems needed for the speed and accuracy of the ground-based landing gear can also radically change the design of Stirling engines by using hydraulics to replace the crank and connecting rods of the conventional Stirling engine. A Stirling-engined aircraft would probably have to use a ducted fan or propeller propulsion but these could still allow civilian aviation to continue in a NOX-sensitive world.

The best way to do experiments on high-altitude engine-chemistry might be from a balloon. Do we know anyone with an interest in this area?