In May 2021 Clean Sky 2 published its first Technology Evaluator1. One of its main conclusions is that “the main focus on decarbonising aviation should be on short-range aircraft flying distances of less than 4,000km, however with much larger passenger capacity, well over 300, even over 400 passengers in the cabin. This type of aircraft does not exist today.” This article, however, will show that the ideal type of aircraft almost exists now if liquid hydrogen is the preferred fuel.
The need for large airliners optimised for flights of less than 4,000km was identified by the DLR (German Aerospace Centre) when it upgraded its air traffic model to include airport capacity constraints, etc. The chart below summarises the results, with the scope of ‘<4,000km’ overlaid. Note that the chart shows the percentages of CO2 produced if all flights still used kerosene. The bottom line is that almost 60% of the CO2 produced by aviation will be removed if the ‘<4,000km’ type of large airliner is powered by hydrogen.
The current Airbus A350-900ULR provides the ideal starting point for the development of an ‘A350-H’ to meet the ‘<4,000km’ requirement. The key reason is that its fuel tanks have already proved to be big enough to provide a range of over 16,000km when its engines are running on kerosene or SAFs (sustainable aviation fuels). The tanks are in the A350’s wings, and will be replaced by new insulated tanks designed to contain enough liquid hydrogen to fuel flights of up to 4,000km.
If the new DLR forecast prompts Airbus and Boeing to develop hydrogen-powered versions of the A350 and 787, the ’emissions stigma’ of flying in them will be removed and the cost of refuelling will be much lower than using SAFs or heavily-taxed kerosene. And this novel form of airliner will appear comfortingly conventional. So passenger demand in the core of the market may build even faster than DLR is forecasting, and higher.
The Clean Sky 2 report also says, “Like the famous London double-deckers, high passenger capacity will be the key to respond to air traffic demand in the future, especially on short-haul routes (<4,000km). As a result of airport capacity constraints, a veritable ‘aerial autobus’ of large capacity will be required to move passengers from city to city mostly on intra-continental flights.” So Airbus is appropriately named.
Consequently, Airbus should now be planning to build a new version of the A350, the A350-H, powered by Rolls-Royce Ultrafans running on liquid hydrogen, producing low levels of NOx. The A350-H will carry as many as 350 passengers up to 4,000km, and it will be operable from existing airports with existing runway limitations. This promises to be the fastest way to minimise the output of CO2 from aviation, and will help to accelerate progress towards global net-zero.
It may be that Airbus and Boeing (and CRAIC?) have also realised this opportunity, but the concept has not yet been externalised by them or any government. So this may be an opportunity for the British government to announce at COP26 that it has already started to fund the development of the A350-H. The COP26 delegates will form the world’s largest concentration of guilt-ridden frequent flyers, but they should feel much less guilty when they fly home!
Because the range needed is less than 4,000km, members of the ‘stick a battery in everything’ cult may be tempted to suggest a battery-only version. However, although small battery-only aircraft are practical for short-range applications, a large airliner would need to carry over 300 tonnes of today’s best lithium ion batteries to fly the 3,976km from Los Angeles to New York. While the maximum take-off weight of a A350-900ULR is 280 tonnes, its maximum landing weight is only 207 tonnes. But a battery-only aircraft would be just as heavy coming into land as it was taking off. Consequently, batteries would need to become many times lighter by 2040 to make a significant contribution to the 2050 global net-zero target, which is rather unlikely.
Sustainable aviation fuels
A more practical alternative to batteries is to use a SAF as a direct substitute for conventional kerosene. However, there is general agreement that all genuinely sustainable SAFs will cost a lot more than kerosene, and also more than hydrogen by 2032 – the earliest there will be substantial demand for hydrogen in aviation.
From an emissions perspective, the ideal form of SAF is made from green hydrogen combined with CO2 extracted from the atmosphere using energy from solar and wind, sometimes referred to as Power-to-Liquid, or PtL. PtL will inevitably remain more expensive than hydrogen, even when solar has reached one cent per kilowatt-hour, probably by a factor of at least two. And liquifying hydrogen is likely to add only 20% to its price. So there is a window of opportunity for SAFs, but this will begin to close when the first hydrogen airliners come into service in 2030, and close completely when hydrogen-powered airliners eventually take over long-haul flying.
Most current forecasts predict that SAFs will provide less than 15% of all aviation fuel by 2030. And hydrogen-powered long-haul airliners are still a generation away, so most airlines will still need to operate conventional aircraft on routes of over 4,000km in 2035, but they will be strongly motivated to run them on PtL because of heavy taxes on kerosene. However, by 2035 almost all new airliners on order will still look conventional but will run on liquid hydrogen. The price of hydrogen will have fallen to the point where there will be a strong financial incentive to replace SAFs on long-haul flights, so the development of large hybrid-blended-wing airliners should be well underway. However, it is unlikely substantial numbers of these will be in service before 2040 because of their novelty and the consequent length of the testing and certification processes.
Consequently, to achieve the most rapid reduction in aviation’s emissions, this will initially require a combination of SAFs for long-haul and hydrogen for everything else with more than 100 seats.
Large medium-range hydrogen-powered airliners almost exist, and can be developed rapidly and inexpensively. This can deliver a much more rapid reduction in aviation’s emissions than previously forecast, provided the A350-H project is initiated soon.
A350-H: outline specs
The target maximum range is 4,000km, less than a quarter of the 18,000km maximum range of a conventional A350-900ULR running on kerosene or SAF. Consequently, because of the 165,000 litres of fuel tank space already available in the wings of a conventional A350, an A350-H’s wings may not need to be increased in volume to deliver the required range, despite hydrogen’s low energy density.
An A350-900ULR can hold up to 127 tonnes of jet fuel. Liquid hydrogen with the same heating value weighs only 41 tonnes. But the maximum range required from an A350-H is only 4,000km, not 18,000 km, so less than 10 tonnes of hydrogen may be sufficient, initially. Notice how attractive this will become to the airlines once they can buy liquid hydrogen for less than $2/kg, or only $20,000, tax-free, for a ‘full tank’. This should result in an average fuel cost of $80 per passenger for a flight from Los Angeles to New York, or slightly less from Beijing to Moscow.
A flight of similar length from London to Cairo already carries a ‘standard rate’ tax, Air Passenger Duty (APD), of £185 (~$256), with a reduced rate of £84 for the smallest seats. If the British government imposes APD of only £50 for standard rate seats on hydrogen-fuelled flights of over 2,000 miles and none for small seats, this will offer a major incentive to the airlines to buy hydrogen-powered airliners. And perhaps APD will soon be raised to £300 for kerosene flights of more than 5,000 miles, encouraging airlines to reserve scarce SAFs for long haul.
The maximum landing weight can remain below the 207 tonnes of the conventional A350, despite the fuel weight falling by less than ten tonnes during the flight, because an A350-H will be so much lighter on take-off. As a result, an A350-H will be able to climb faster, reducing noise around airports, and it will use less energy to reach cruising altitude and slightly less in level flight.
Consequently, initial calculations show that only nine tonnes of liquid hydrogen may be sufficient, requiring a tank capacity of only 127 cubic metres, 38 cubic metres less than currently available. This will allow another 30% for insulation, etc. So no changes may need to made to the external appearance of the A350-H. Tank design will focus on insulating the liquid hydrogen. A small onboard fuel cell stack will help power a refrigeration system which will keep the hydrogen liquid, in the air and on the ground.
The UK should fund the development of the A350-H
The wings of most Airbus models are designed in England and manufactured in Wales, providing thousands of jobs. Designing and building the new wings will be the key activities in the development of the A350-H. Airbus currently has 2035 as its target date for the first of its hydrogen-powered airliners to be in service, and has said that it will not make its final decision on which of its three ZEROe concepts it will develop until 2024. However, the latest rumours out of Airbus suggest that the turboprop concept shown in the image below is the current favourite. But this will provide only 100 seats and a maximum range of only 2,850km, and consequently will address less than 10% of the CO2 problem. This might encourage Airbus to make the A350-H the fourth ZEROe concept. And soon the leading one? Costing the least to develop?
Unlike Boeing, Airbus needs no convincing about the important role of hydrogen. In its ZEROe announcement in September 2020, Airbus said, “All of these concepts rely on hydrogen as a primary power source – an option which Airbus believes holds exceptional promise as a clean aviation fuel and is likely to be a solution for aerospace and many other industries to meet their climate-neutral targets.”
However, ZEROe is on an unnecessarily long time scale for the development of an A350-H prototype. The CEO of Airbus recently stated that it will not start to develop its choice of concept until 2027, yet the first A350-H prototype could be in the air by 2025. So the development of the A350-H should be an almost completely separate project, the only obvious overlap being tank insulation.
The UK Government has funded, with Airbus, the new AIRTeC centre near Bristol, which would seem to offer the ideal location for the headquarters of the A350-H project. Perhaps Rolls-Royce should lead the project, with an array of other partners including not just Airbus but British Airways and Ineos.
How will the airlines react to the A350-H?
The key reason airlines will want to buy the A350-H will be because the cost of using liquid hydrogen will be much lower per passenger mile than using heavily-taxed kerosene or expensive SAFs. Over the lifetime of a typical large airliner the cost of the fuel it uses far exceeds the cost of the plane themself. Once the airlines realise that development of the A350-H is going ahead and their models produce similar forecasts to those of Clean Sky 2, the pressure will build on Airbus and Boeing to begin deliveries as soon as practical.
The delegates at COP26 will form the largest concentration of people on the planet feeling really guilty about the amount of flying they do, not just because of the number of trips they take but also because of their full understanding of the damage they are causing. So they should be delighted to hear the details of how fast and effectively their guilt can be assuaged. Now imagine the pressure on Airbus and Boeing by January to get on with the job! And fixing aviation will give hope that the other difficult sectors are capable of reaching net-zero faster than currently forecast. UK Prime Minister Boris Johnson can be the bearer of good news at COP26. Does he want to be?
Source: H2 View.
About the authors
Chris Ellis is a systems engineer, mechanic and writer, who obtained his Private Pilot’s Licence 55 years ago. After 10 years working for IBM and another three working for the UK Government, Chris held various senior roles in telecoms until concentrating full-time on limiting climate change.
Aeronautical Engineer John Coplin was chief designer for Rolls-Royce RB211, the parent of the Trent family of engines powering the wide body jets. He was former director of technology and director of design for Rolls-Royce, former visiting professor at University of Oxford, and later at Imperial College London. He served as UK Advisor to the President of the Republic of Indonesia, a distinguished Engineer, Prof Dr Ing B J Habibie.