Airline transportation after the epidemic. The problem is being solved, but not the way you would have imagined

That's how a hydrogen-powered plane could look like. Honestly, it is not very impressive: it looks more like an Elvis sighting and one may even reasonably doubt that this thing could fly. If it were easy to power planes using hydrogen, someone would be working at real prototypes rather than just these drawings. 

Airlines have been on the edge of collapse for many years and for many good reasons, but the real problem, at the basis of all the others, is the need for fossil fuels. Airplanes are voracious beasts: they consume around 7% of the world's production of fuels and fueling planes represents about 20%-30% of the costs of operating an airline. So far, most companies have survived the vagaries of the oil market, but it should be obvious that crude oil cannot last forever. In addition, there is the problem of the pollution generated by fossil fuels, especially in terms of contributing to global warming. One result is the phenomenon called "flight shame," another headache for airline managers.

So, if you want to keep planes in the air, you need to get rid of the need for fossil fuels. And that turns out to be nearly impossible, at least for the current generation of planes. I already discussed the technical and financial aspects of alternatives to fossil fuels. Hydrogen is a good fuel for rockets, but not for civilian planes, it is a technological and financial nightmare, Biofuels can't help: there is no way that you can step production to levels sufficient to feed the current fleet of civilian planes. Electric planes are a nice idea for small planes, but there is no way that you can build an electric equivalent of a Boeing 747. Wide-body planes are optimized for the fuel they use, kerosene, produced by refining crude oil. And there is just so much that you can do to improve technologies that have already been optimized nearly to death. Spending a lot of money, you can refine and retouch this and that, and make the plane gain maybe a fraction of one percent in performance, but that doesn't eliminate the need for fossil fuels.

So, airlines reacted to the problem by cutting costs as much as possible. That was obtained by having passengers undergo all sorts of humiliations: crowded planes, long waiting times, unreliable flights, impossible schedules, non-existing ground services, horrible on-board food, and more. To that, add the ubiquitous harassment suffered by travelers at the entrance gates of airports. The whole looks more like the treatment inflicted on war prisoners en route for a concentration camp. But passengers seemed to be happy to accept being mistreated in exchange for low price tickets. 

But, eventually, the solution chosen by airlines led them to a no-way-out street: there is a limit to how much you can cut corners. Ryanair even proposed to carry standing passengers on board, but it was nothing more than an advertising stunt. And then there came the coronavirus epidemic dealing a terrible blow to the already strained airlines. Right now, it is hard to see how passenger service can be restarted: people are so scared that they would accept to fly on a plane only if everyone were wearing scuba diving suits equipped with autonomous oxygen tanks. Probably the scare will abate in the coming months, and it is probably possible to introduce some kind of a "health passport" that would allow passengers to sit near each other without fearing to be infected. But, in the meantime, with the industry deeply in the red, you can kiss the rotors of the turbines goodbye.

Nevertheless, as I tend to say, for everything that happens there is a reason for it to happen, and the destiny of the airlines was already written on the wings of their planes. Last year, the demise of the Airbus A380, the flower of the European Aerospace industry, was a death knell for the very concept of wide-body planes. It was known, and the effort of the aerospace industry was already moving in a different direction.

It is said that when the Titanic sank, the third class passengers remained locked in the lower decks and drowned. These passengers, evidently, were considered as a nuisance. Today, I think that the elites have already understood that there really is no reason why planes should have an "economy class." The result is a new generation of planes that only the rich are supposed to use. By getting rid of the commoners on board we can have less pollution, faster planes, lower use of fuel, and more comfortable flights. And here an example of the result: the Aerion AS2, a supersonic jet for just 8-12 passengers.

The Aerion AS2, an example of the new generation of passenger planes: supersonic business jets (SSBJs)

That is just an example, but the buzzword with the airlines right now is "bizliners," planes (not necessarily supersonic) designed and operated to cater to special groups of people who can afford them: politicians, business people, sports teams, etc. Just not the kind of planes that we, humble commoners, will ever be able to use. But so is life: flying was a privilege that many of our generation enjoyed, but nothing lasts forever. At least, we won't feel guilty about flying anymore!



A comment from Ugo Bardi's personal troll, Mr. Kunning Druger

So, Mr. Bardi -- I can imagine you gloating while you were writing this post, weren't you? Exactly what you and your friends always wanted, especially the braided witch from Sweden. You always hated the idea of ordinary people enjoying the benefits that fossil fuels bring to humankind. Flying, for instance. And now you think you are winning, right? I am sure that your friends in high places promised you a seat in one of those shiny new bizliners, I can see you gloating, indeed. Well, think it over. You were one of the crowd that was claiming "peak oil," right? It was another scam directed to enslave us, the people, to the powers that be. And now, with oil prices at historical lows, your scam has been exposed: there is no such thing as "peak oil." Oil is abundant as ever and planes will keep flying, no matter what you write in your ridiculous rants.

Biofuels: Can They Save the Airlines from the Seneca Collapse?

Painting planes green is much easier than making them run on biofuels.

"Can the airlines be run on biofuels?" As it often happens, this simple question doesn't have a simple answer. First of all, it is a question that makes sense only in terms of a "sustainable" plane, that is one that doesn't run on fossil fuels. That's a major technological problem. Whereas cars can be made to run on battery-powered electric motors, the power/weight ratio of the combination is simply unacceptable for a passenger plane that could provide a performance comparable to that of current jet planes. Hydrogen planes have been proposed, but they are a nightmare for several reasons and it is unlikely that they could become practical in the short and medium term future.

That would leave only biofuels as a "sustainable" fuel that could power the current fleet of jet planes. Indeed, a small number of tests have been carried out showing that it is possible to fly planes using biofuels. But can it be done on the large scale needed to get rid of fossil fuels?

The first problem is whether biofuels are truly carbon-free. Most likely, the current fuels made from crops are not; in the sense that they involve extensive use of fossil fuels for their manufacturing. In many cases, however, even the current generation ("1st generation") of biofuels can provide a significant saving in the use of fossil fuels for the same amount of energy produced. This is the case, in particular, for ethanol produced from sugarcane in Brazil. But there is a more fundamental question is: what would be the consequences of ramping up biofuel production to the levels needed to power the current airline fleet?

In a recent paper on Nature, Rulli et al. discuss the effect of the large scale cultivation of 1st generation biofuels on various parameters of the world's economy, including the global food supply. They don't specifically examine the needs of airlines, but we can use their results for analyzing this sector. First of all, the total amount of jet fuel consumed in the world is reported to be 6,000,000 barrels per day. It corresponds to about 7% of the total world combustible liquids production. We may consider this value as approximately the fraction of transportation energy used by airlines since crude oil represents 93% of the total energy used in transportation.

Rulli et al. estimate that if we were to arrive at a 10% reliance on biofuels for the world's transport, that would leave food for no more than 6.7 billion people and, since the current world population is about 7.6 billion people, almost one billion people would starve. Now, since the airlines consume about 7% of the world's transport energy, feeding the airlines with biofuels would move us dangerously close to the threshold that would lead to killing a large number of people for the purpose of keeping planes flying.

Of course, these data are for first-generation biofuels. There is much enthusiasm for second and third generation biofuels from cellulosic plant tissues or algae which, theoretically don't impact on the food supply. Sure, but today the production of these fuels is non-existent or at best negligible. How long will it take to ramp up their production to the levels we are discussing here? And are we sure that they will work as promised?

The problem, here, is not just a technological one. We are dealing with a complex system, the world's economy coupled with the planetary ecosystem. In these systems, you can't change just one thing and leave all the rest unchanged. Once we start to produce biofuels on a very large scale, it becomes extremely difficult to stop at a certain threshold. If we have a product and a market for it, both tend to expand and it is nearly impossible to stop the expansion of something that generates a profit.

That would bring big problems, to say the least. Rulli et al. estimate that arriving to supply 1st generation biofuels in an amount corresponding to 20% of the transport energy would leave no more than 4.4 billion people alive in the world. That is, it would kill some 3 billion people. Or, if dealing with 2nd or 3rd generation biofuels, it would lead to whatever disaster generated by the appropriation for humankind an even larger fraction of the planetary photosynthetic activity than it is done today. The ecosystem has limits, after all.

Unfortunately, it is unlikely that ethical considerations would affect decisions in this field. The system is made in such a way that if producing fuels for the rich is more profitable than producing food for the poor, the system will produce fuels, even though that implies killing billions of people. So, we can only hope that biofuels will turn out to be too expensive even for the rich; but that may not be the case. With so much research and development ongoing, production costs might be lowered enough to turn biofuel into an effective weapon of mass destruction (and I wouldn't be surprised to discover that this is one of the reasons why biofuels are promoted so aggressively in some quarters).

Or, more simply, we may hope that the Seneca Collapse of the world's economy will take care of the "airline problem" once and for all. As I said many times, the Seneca Cliff is not a problem, it is an opportunity. In this case, it could lead us to develop better transportation technologies; more efficient and more benign for the ecosystem - although probably slower. But that's not a problem, either. It is an opportunity to travel only when you need to, and to enjoy the trip, too!




 Some further data on the extent of land needed for the cultivation of biofuels for airlines. 

First of all, the total amount of jet fuel consumed in the world is reported to be 6,000,000 barrels per day . It corresponds to about 7% of the total world combustible liquids production. Now, we need to compare the values measured in barrels with the needs of the airlines, measured in liters. A barrel contains 159 liters, so 159*6=1000 makes about 1 billion liters/day, or 3.6x10^11 liters/year. Let's now consider the most efficient biofuel production: ethanol from Brazil's sugarcane. It can produce 6000 liters/ha per year  http://biotechnologyforbiofuels.biomedcentral.com/articles/10.1186/1754-6834-1-6  Note that ethanol is not as energy dense as jet fuel. It has only about 70% of the energy density of gasoline http://www.afdc.energy.gov/fuels/fuel_comparison_chart.pdf. Which means that the airlines would consume 3.6*10^11/0.7 = ca. 500 billion liters of ethanol/year. 

So, assuming that the whole production of Brazilian ethanol is dedicated to airplanes, we would need more than 80*10^6 hectares (eighty million hectares). The total arable land in Brazil is reported to be: 75 Million ha. http://www.tradingeconomics.com/brazil/arable-land-hectares-wb-data.html It means that the whole agriculture of Brazil should be dedicated only to produce fuel for the airlines. 

That is, of course, absurd, but it is also true that the world's total arable land is = 1,407 x10^6 ha (https://en.wikipedia.org/wiki/Arable_land), about 20 times the area available in Brazil. So, the airlines would need only about 5% of the total which is, by the way, just slightly larger than the global arable area used for biofuel production today  (about 4%)  http://www.nature.com/articles/srep22521. But note also that not all the arable land has the same good productivity as the land used for sugarcane production in Brazil, so the real fraction needed would have to be considerably larger than 5%, probably still less than 10%. How many people would starve if we were to arrive to that, it is impossible to say. 

The Train for Berlin: Can Railroads Replace Planes for Long Distance Travel?

This November, I went by train from Locarno, in Switzerland, all the way to Berlin, for the conference of ASPO Germany.  The trip lasted more than 11 hours and it involved four different trains, it was the longest daytime trip by train I ever took in my life. It was one of the several tests I have been performing during the past few years to see how and how much I could avoid using planes for traveling within Europe. (Above: crossing another train in Switzerland).

There doesn't exist a sustainable fuel that can power a passenger plane; at least not at the same price and for the same performance we can obtain from fuels derived from fossil hydrocarbons. While airlines dream of impossible "green planes," we need to find something that can take people from a place to another without emitting greenhouse gases, at least over medium-long distances. Maybe, one day, we'll develop a new generation of solar powered airships but, for the time being, the good, old trains look like the best option. Trains run on electricity, so they are directly compatible with solar and wind energy. They don't even need rubber tires or bitumen for roads - both produced from fossil fuels. So, I have been experimenting for quite a while with traveling by train in Europe and let me report to you about this experience.

First: the good news. During the past few years, the development of on-line services has made enormously easier to plan long distance train trips. The European railroads have also improved their ticket sales interface and you can now buy fully electronic tickets from one single national site for a multi-country trip. This is a big improvement. For instance, up to a few years ago, if you wanted to board a Swiss train, you had to have a physical ticket issued in a Swiss station or, if you didn't live in Switzerland, you had to have it shipped to you by mail, which was both slow and expensive.

Then, many railroad networks have now an on-board Wi-Fi system. That's a big plus because a long trip by train becomes actually a chance to do some work in holy peace - something that you can't do on a plane, where you can't even recharge your laptop (and not even open it, if you travel in economy class). In the image you see real-time travel information on a German intercity train.

Still, there is a lot of work to do to improve the service of railroads. For instance, in Switzerland, trains have no Wi-Fi (maybe because motion sickness is almost guaranteed if you travel in the Alpine region). Even in Germany, with all their hi-tech, the connection during my travel to Berlin worked only for the first half hour and then it died for the rest of the trip (and they wanted me to pay 6 euros for it!). In the picture, you can see that I was reading Epictetus on the train, a stoic philosopher who helped me survive the lack of an Internet connection! But, surely, that can be improved: in Italy, for instance, the Wi-Fi connection in the high-speed trains comes for free and it normally works very well.

Then, there remain two fundamental problems with long distance rail trips: one is that night trains are becoming an extinct breed in Europe, the other that the high-speed trains are not conceived for long distance travel.

First, sleeper trains. Theoretically, they are a very good idea: you travel overnight, while you sleep, and you arrive in the morning, ready for business or for sightseeing. This kind of trip may be considered also as something romantic if you can share the compartment with a significant other (assuming that neither of you suffers from motion sickness). Of course, sleeping in one of these trains is not the same thing as sleeping at home: the paradigm of the sleeper train is the 6-passenger compartment, hot and poorly ventilated, that can give you a feeling of what must have been like to be deported to a concentration camp during the second world war. But, even if you book a place in a single or a double compartment, the price is not unreasonably high if you think that you are saving the cost of one night at a hotel.

Unfortunately, there are big problems with sleeper trains. One is that they are old, poorly kept, and don't smell so good. In my personal experience, they are also often delayed (two hours of delay the last time I went to Paris). Then, all the romanticism of the experience goes away when, in the morning, you are served a pure cardboard croissant and a cup of coffee that looks and smells like crude oil. Apparently, the fasts of the old "Orient Express" are past and forgotten. As a final outrage, I can report how, while traveling from Italy to Paris, I was awakened at 2 a.m. by the Swiss police who wanted to check my bags. Imagine that your plane from London to New York is stopped midway by the Icelandic police and made to stop in Rejkiawick so that they can check your bags!

But the main problem with sleeper trains is another one. When you arrive in the morning to your destination, you badly need a shower, but your hotel won't let you into your room before 1 p.m. (that is, if you are lucky, because some hotels won't let you in before 3 pm.). The problem is even worse with your trip back home. Your train leaves at, maybe, 11 p.m., but your hotel will unceremoniously kick you out of your room at 12 a.m. (and they can be quite nasty if you ask them for an extra half-hour). Then, maybe you have some business or sightseeing in the afternoon but then you are stranded in a foreign city with your bags and with nowhere to stay except in an unappealing waiting room in a train station. No wonder that these trains seem to be disappearing from the European railroad network.

Then, there are high-speed trains; wonderful machines that could compete with planes even for relatively long trips. At a speed typically over 200 km/hour, a train could cover the ca. 1500 km from Rome to Berlin in some 6-7 hours. Of course, you should add the time for a few stops along the way and the fact that not the whole network allows for high speed. Still, you could likely make it in less than 10 hours. That's reasonable for a comfortable daytime trip, where you can relax and work while you travel. But, in practice, there is no way to get to Berlin from Rome or Florence in a single day. My train trip to Berlin started from Switzerland; it was less than 1,000 km and it took more than 11 hours; an average speed of less than 100 km/h. The reason is that I had to change three times and that involved considerable idle time in stations (image: a coffee shop in Bellinzona, Switzerland. Nice place, and they had good Italian espresso coffee, but it was a lot of lost time)

So, despite the recent improvements, there still a lot of work to do before railways can become competitive with planes in Europe. Something that could make sleeper trains more practical would be the possibility of renting rooms in hotels for half a day at a reasonable price. That makes a big difference in comfort: I remember having done that in St. Petersburg, in Russia, while waiting for the night train for Moscow. But, in Western Europe, renting a room by half-day or by the hour remains something that hotels don't want to do, perhaps because they are afraid for their reputation. Things might be changing and some internet sites have appeared that offer this service for business travelers.

But the real problem with sleeper trains is that they are in direct competition with low-cost airlines and, as things stand today, trains can't just compete. Airlines offer a faster service for the same or lower prices. Only a serious carbon tax could change things and make sleeper trains competitive, but that doesn't seem to be coming fast.

The future looks more favorable for high-speed day trains. The main problem, here, seems to be related to planning. So far, national railway companies have been planning their schedules only at the national level, also because of the limited interoperability of the railway networks. In some ways, it seems that railroads are still operating as they did at the time of the first world war; when people thought that an enemy invasion could have been slowed down by making the national rail gauge different from that of neighboring countries. Different gauges in Europe still exist in Russia and in Spain and the railways operate different voltages AC and DC, varying from 750 to 25,000 volts. Also, the signaling systems vary from country to country. The result is that, for instance, high-speed Italian trains cannot run in Germany or in France, and the reverse is also true.

Nevertheless, progress is being made and the latest generation of high-speed trains is built with interoperability in mind. Soon, these trains should be able to roam the whole European network. What is lacking here, mainly, is a serious push from the European Government to convince national railroads that connecting the main European capitals by high-speed trains is important and useful. But the EU has done very little in this sense, so far. One more failure for them (they seem to collect failures as some people collect stamps or butterflies). There used to be a European Railway Agency, but something must have gone wrong with it because it was closed down and there is now a brand new European Union Agency for Railways. We can only hope they will do better than their predecessors.

So, is there hope that we'll be able to take again long travels by train in comfort and style in Europe, as it could be done in the 1930s? Could we revive the fasts of the old "Orient Express"? It is surely possible, but it will take some work and some strong political will. That will be absolutely necessary if we want to adapt European travel to the objectives of the 2015 Paris treaty. In the meantime, the most adventurous of us will still do their best to shun planes in favor of trains. (image below, the Italian FrecciaRossa high-speed train, photographed at the central station in Florence. Allow me a small display of national pride if I say that it is the best train I have ever traveled on - expensive, though!)

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?