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Decarbonizing aircraft propulsion

Sebastien Kahn
7 June 2023
capgemini-engineering

The biggest and most important lever for decarbonizing aviation is finding green sources of propulsion. Burning aviation fuel – which currently is mostly oil-derived kerosene – represents an estimated 99% of aviation emissions – the so-called ‘Scope 3’ downstream emissions (ie. emissions from products in use).

Sustainable Aviation Fuel is one option, and has the benefit of working with most existing engine designs. But entirely new propulsion systems, eg hydrogen and electric, require whole new designs for the aircraft’s powertrain, from engines to fuel tanks and transport, and power transmission to propellers. And in some cases, it may require a wholesale redesign of the plane.

This will not be easy. The companies who have started on this path see many years of work before they can get green planes into regular flight. There is an engineering challenge ahead on a scale and urgency the likes of which aviation has never seen. Nonetheless, companies large and small are taking on the challenge.

Time is of the essence, not only because the clock is ticking on climate change, but also because the companies that get there first will have a significant advantage. This doesn’t just mean fielding new aircraft, but also retrofitting existing fleets for sustainability. For example, just a year’s jump on competitors could mean many orders, before others catch up.

So, how can companies accelerate this Engineering R&D process?

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The challenge ahead

Decarbonizing propulsion comes with a series of options, each with its own challenges. We will summarize the opportunities and challenges of each, before discussing solutions.

The easiest and most promising short-term solution is sustainable aviation fuel (SAF), a category of fuels derived from biomass or from carbon capture, which remove CO2 from the air or emissions and chemically process it into precursors of kerosene. According to the International Air Transport Association (IATA), SAF could contribute around 65% of the reduction in emissions needed by aviation to reach net-zero in 2050.

In terms of redesigns, SAF is the easy option. SAF can ‘drop in’ – which means that it can be blended with conventional jet fuel and, in some cases (…more in future), replace conventional jet fuel entirely. This means that SAF requires little to no redesign. Airbus already has commercial and military aircraft capable of flying with up to a 50% blend of SAF, and aims for 100% by 2030. The UK MOD has begun accepting up to 50% drop in from its fuel suppliers and has already demonstrated a 100% SAF flight.

SAF, it should also be noted, can be produced in a carbon neutral way, but take CO2 out at ground level and return it at altitude – so, whilst a good deal better than kerosene, and an excellent transition fuel, SAF is not a completely green solution.

It’s worth mentioning that the production pathway of SAF (and thus its scalability) is an important factor. For example, it’s important to ensure that SAF created from biological sources (like forestry residues) does not compete with other sectors that need to use those same resides, like the paper industry. The EU and US are pursuing different approaches to this challenge. You can read more about the importance of sustainable supply chains in Article 4.

As a fuel source, hydrogen can be directly combusted, or used in a hydrogen fuel cell to produce electricity. Due to a later start, hydrogen has a shorter timeline and is likely to start seeing major aviation deployment in the 2030s. When combusted, hydrogen reacts with oxygen to create energy and water vapour, and so has no carbon emissions. If the hydrogen is produced from green sources, flights could, in theory, be carbon neutral (though it is unlikely we will completely eliminate emissions from hydrogen’s production, storage and transport infrastructure).

The energy density of hydrogen, by mass, is three times greater than kerosene, which makes hydrogen very attractive as an energy carrier. However, it has less energy by volume than kerosene: six times less for gas at high pressure (700 bars) and three times less for liquid (which requires it to be cooled to -253°C). So liquid hydrogen is more viable but will still require more space for storage than fuel, which will challenge aircraft shape and architecture.
 
As such, it will likely require planes to be redesigned to accommodate larger fuel tanks.  This, for example, could create an opportunity to improve aircraft by moving the fuel storage – for example, taking it out of the wings. The wings could then be made thinner, generating less drag and increasing fuel efficiency. It also creates complex challenges around the design, engineering and materials choices for hydrogen storage tanks, fuel injection, and the engine itself – which would need to be modified to deal with this new fuel source.


H2 (whether combusted or used in fuel cells) also produces contrails/water vapour, the dispersion of such clouds (contrail cirrus clouds) can trap heat that radiates from the earth below, increasing warming. Combusting it also produces nitrogen oxides (‘NOx’) which could be a cause of smog, acid rain, and respiratory problems in humans, though it produces less of these than kerosene.


Hydrogen could also be used to power fuel cells, which drive an electric powertrain, and which have no waste emissions. A few recent test flights, including from startups ZeroAvia and Universal Hydrogen, are promising. Major primes are betting on the technology too; Airbus wants to deploy its hydrogen fuel cell-powered engine at a major scale by 2035. It is worth mentioning, however, that the weight of these fuel cells may limit them to single-aisle planes, and medium ranges.

As with electric vehicles (EVs), batteries could power engines and be charged at airports in-between operations. The main constraint is the batteries themselves, which are heavy. This decreases flight efficiency and – thanks to the laws of physics – places an upper limit on how much energy can be stored before any given plane is too heavy to fly.

Nonetheless, electrical propulsion has already demonstrated promise in smaller aircraft. Pipistrel claims to be the first company to get certification for an electric aircraft (the Velis Electro), back in 2020.
More recently, in September 2022, US-based Eviation Aircraft demonstrated what it claims to be ‘the world’s first all-electric passenger craft’, with a predicted service date of 2027, and a plan for commuter and cargo flights between 150-250 miles.
 
The primary engineering challenge then will be squeezing optimal efficiencies out of battery storage and efficiency, as well as making them lighter weight, to extend the range of electric planes. Progress may come from new battery chemistries that are lighter and more powerful, like lithium sulphur. There is also much improvement to be gained from better thermal management, which can also help to prolong battery life

The secondary challenge of electrification will be redesigning plane subsystems and control surfaces with electric motors and transmission lines to replace hydraulic ones. These have differing operating considerations to existing hydraulic controls and major work will need to be done to retrofit them to existing aircraft.

This may nonetheless be viable. ‘Electrification’ can be used on an aircraft with any kind of engine (eg. conventional, SAF, H2), provides potential weight savings compared with conventional hydraulics and potentially draws less energy from the aircraft’s power plant, as well as being simpler to install and maintain (due to fewer moving parts) whilst arguably offering more precise control.

Hybrid electric propulsion (in which a vehicle uses electric power combined with other propulsion sources) has already proven itself in the automotive sector. An aircraft could use an electric drive for better energy management, for example, during taxiing, or in conjunction with the aircraft’s other engines to provide assistance during take-off and ascent.

Airbus claims that hybrid electric propulsion could reduce fuel consumption by 5% per flight. It could also be invaluable when combined with other kinds of power sources that lack the peak power output of kerosene.

Digital enablers: getting there faster

The challenges above will clearly take energy and research. Given the safety-critical nature of aerospace, they will also take a lot of testing before passengers are allowed anywhere near them. Some of this just has to be done, but some elements can be sped up through new digital engineering approaches.

Digital design tools can help scope design and architecture, engineering, as well as the electrical, mechanical systems and physical domains, and how they should join up. Modelling – when designed by aerospace entering experts – can help optimize and define the most effective configuration for fuselages, tanks and wings, predict the best materials choices, and design the integration of electrical, electronic, and mechanical components. Even Artificial Intelligence (AI) can help propose optimal designs if provided with clear input criteria, reducing the number of false starts, and the need to produce early physical prototypes.

Simulations and physics-based systems modelling can be used for understanding important properties, like thermal management, which will be critical for the safety and efficiency of designs of battery packs, and engines using new fuel sources stored at different pressures and temperatures.

Software design will also be increasingly important for system management, as powertrains are electrified and systems need to be monitored and held at particular states throughout the flight.

Model-based system engineering (MBSE) – ‘the application of modeling to support system requirements, design, analysis, verification and validation (V&V) activities’ – allows designers to take a holistic view; analyzing the aircraft system as a whole throughout its entire lifecyle, and identify the interactions between its components. The digital approach can help to accelerate projects, hastening the Validation & Verification (V&V) certification process, for example, by allowing more of the test and evaluation work to be done digitally.

AI can also be used to translate flight data into scenarios for simulated testing from the component level up to virtual flight tests in varied and challenging conditions. This helps spot challenges early, reducing risk in costly real-life flight tests – though these, of course, must be done eventually. Once they are, detailed data collection, post-processing and visualization can help understand risks and improvements – which can be fed back into the digital model in order to improve the design.

Airbus’ Digital Design Manufacturing and Services (DDMS) program is a good example of ‘digital-first’ in action. It has been used to help develop the Future Combat Air System and the A321XLR, which Airbus claims burns 30% less fuel than previous generation aircraft.

No silver bullets, but many choices

The commercial aviation community agrees on a mixture of propulsion solutions, but there are no ‘silver bullets’.

In the near future, SAF-powered aircraft will likely predominate, as SAF requires no modification to airframes and is an improvement on conventional fuel from a sustainability perspective. SAF alone won’t get us to Net Zero, however.

This leaves us with hydrogen and electricity. Electricity is always likely to be limited to short distance flights due to the weight of batteries. Green hydrogen will likely be the eventual solution for commercial flights, due to its highly sustainable credentials and ability to be stored on board as a liquid fuel. Nonetheless, hydrogen will add storage and weight to current aircraft compared to kerosene, and so SAF is likely to be the best option for long-distance flights, at least in the medium term.

Neither electric nor hydrogen propulsion is anywhere near ready to meet the full needs of commercial aviation, but both are progressing rapidly. Both will require major redesigns of aircraft, followed by endless optimization and rigorous testing. Those that get through this process first will have a significant competitive advantage. Digital engineering will likely determine the winners.

Meet our expert

Sebastien Kahn

Vice President Sustainability & Industry, A&D Sustainability Lead, Capgemini
For the past 15 years, Sébastien Kahn has been supporting public and private players in their major ecological transition projects, in particular energy decarbonization strategies, hydrogen or electric ecosystems, and the associated financing and skills plans. A graduate of ESSEC and MIT, he teaches decarbonization policies at Sciences Po Paris and leads the Capgemini Group’s decarbonization activities in the Aerospace and Defence sector.