Aviation has yet to end its 70-year dependence on the highly polluting cocktail of hydrocarbons known as kerosene, but several initiatives are propelling the industry towards a cleaner future. These are focused on sustainable aviation fuel (SAF), liquid hydrogen and electric power, both pure and hybrid.
SAF can be obtained from a range of renewable sources – for instance, forestry waste, used cooking oil, food packaging and even disposable coffee cups that would otherwise be destined for landfill. Since much of the renewable biomass that’s used in its production absorbs carbon from the atmosphere, SAF has “the potential to reduce greenhouse gas emissions by up to 80% compared with conventional jet fuel”, according to Shell.
Many airlines have pledged to adopt SAF. For instance, BA’s owner, the International Airlines Group, has committed to powering 10% of its flights with the fuel by 2030. In May it took delivery of a batch of SAF produced in the Lincolnshire refinery of US oil company Phillips 66. This will be added to the pipeline infrastructure that feeds the UK’s airports.
Similarly, United Airlines has agreed to buy up to 52.5 million gallons of SAF from Finnish refiner Neste over three years to fuel its services from Amsterdam’s Schiphol airport.
United Airlines’ chief sustainability officer, Lauren Riley, reports that “demand from customers to limit their flying emissions is growing exponentially. This agreement means that customers taking flights from Amsterdam and, potentially, other airports will be partners in our sustainability efforts.”
The search for a sustainable alternative
Despite these leading examples, more concerted action is still required to ensure industry-wide uptake, according to Simon Burr, director of engineering and technology in Rolls-Royce’s civil aerospace arm. Barriers to the large-scale use of SAF remain, he says. Chief among them are its current lack of “availability, scalability and affordability. We therefore need action on a global scale in forums such as the general assembly of the International Civil Aviation Organization. It is vital that governments agree on a long-term decarbonisation target.”
Promoting the use of SAF is a key part of Rolls-Royce’s sustainability strategy. The company has successfully tested several of its engines to verify their compatibility with the fuel. Its next-generation engine demonstrator, UltraFan, is set to undergo similar testing.
“Thanks to rigorous testing, we know our engines can operate on SAF,” says Burr, who adds that, if production can be scaled up sufficiently, “uptake will improve affordability and create an environment where more airlines can adopt SAF as part of their own flight to net zero.”
Liquid hydrogen is another cleaner alternative to kerosene that’s come on to the industry’s radar. The British government is pumping £27.2m into a scheme led by GKN Aerospace to develop a hydrogen propulsion system for smaller aircraft that could be scaled up. The H2Gear project is aiming to enable flights by 2026, using a system that converts hydrogen into electricity using a fuel-cell system. The process emits no CO2.
Partners in the initiative include Aeristech, Intelligent Energy and the universities of Birmingham, Manchester and Newcastle, while easyJet will provide insights into operational requirements and options for flight demonstrations.
“Technology is a key driver to achieve our decarbonisation targets, with hydrogen propulsion a front-runner for short-haul airlines like us,” says easyJet’s director of flight operations, David Morgan. His airline is optimistic that “it could begin flying customers on planes powered by hydrogen combustion, hydrogen-electric power or a hybrid of both by the mid to late-2030s”.
Making old machines green
While the arrival of new hydrogen-powered aircraft will create a new category of cleaner air travel, another approach is to retrofit existing planes with the technology.
ZeroAvia’s HyFlyer II project is developing hydrogen engines for demonstration on a 19-seater aircraft that it hopes will fly next year. The Anglo-American company will work with Canada’s MHIRJ Aviation Group to design and equip regional jets (aircraft designed to carry fewer than 100 people on short-haul flights) with a 600kW powertrain.
At the annual summit of the World Economic Forum at Davos in May, the founder and CEO of ZeroAvia, Val Miftakhov, declared that the hundreds of regional flights taking place daily across North America “can and should be zero-emission well before the end of this decade. This agreement is a giant step forward in delivering hydrogen-electric engines to the regional jet segment.”
The venture has secured air-safety experimental certificates for its prototype aircraft from UK and US airworthiness authorities, passed significant flight tests, and established partnerships with original equipment manufacturers and global airlines. It’s on track to start commercial operations in 2024.
ZeroAvia is also developing a 5MW modular powertrain for use in 40- to 80-seat turboprop aircraft – a project with backers including Alaska Air and United Airlines.
As urban road networks struggle to accommodate commuter traffic, the only way is up. That’s the rationale behind the evolution of a new category in aviation: advanced air mobility – a mash-up of urban air mobility (flying taxis) and commuter aircraft.
“These aircraft will introduce all-electric flying, which is quieter and more efficient, enabling us to reduce travel times dramatically,” says Matheu Parr, customer business director at Rolls-Royce.
The company is planning to introduce all-electric and hybrid-electric engines for service by the mid-2020s. To this end, it’s using next-generation demonstrator aircraft, including the Spirit of Innovation. Powered by a 400kW unit, it can claim to be the world’s fastest all-electric vehicle, having topped 387mph during a test flight.
Rolls-Royce has also been working with manufacturers Tecnam and Rotax to complete flight-testing of a hybrid-electric plane powered by parallel-hybrid propulsion – tech that could be applied to larger aircraft.
Data gleaned from tests is already being applied in the urban air mobility market. A Rolls-Royce unit has been chosen by Vertical Aerospace for its all-electric VX4 aircraft, which is designed to transport a pilot and four passengers, emission-free, over 100 miles at 200mph. The Bristol-based firm is aiming to secure certification for VX4 in 2025 and has garnered conditional orders and pre-order options for up to 1,350 aircraft from players including American Airlines and Virgin Atlantic.
Electric aircraft versus sustainable aviation fuel
Whichever alternative energy source becomes the prevalent choice, one certainty is that tomorrow’s skies will feature a broader spectrum of vehicle types. This presents an opportunity for disruptors to shape the future of energy-saving propulsive technology.
Bedford-based SME Blue Bear Systems Research is leading a seven-member consortium that’s developing “a highly power-dense, quiet and efficient propulsion module with zero tailpipe emissions”. The design can be adjusted for general aviation aircraft, large cargo drones, air taxis and regional planes.
Co-funded by the government’s Aerospace Technology Institute programme, the Integrated Flight Control, Energy Storage and Propulsion Technologies for Electric Aircraft (Inception) project is focused on optimising the conversion of electrical energy into thrust.
The project started in January 2021 and production of the completed design is under way, with wind-tunnel testing due to start imminently. The plan is to install the module on an aircraft platform, aiming for airworthiness certification in 2026.
The industry needs to have more faith in disruptive new entrants if it’s to have a greener future, argues Blue Bear’s CEO, Dr Yoge Patel.
“The UK is one of the most inventive nations, attracting a lot of investment – and Blue Bear has had great government support,” she says. “But how do you get that brand credibility when you’re a startup?”
The next big challenge, once you’ve established a foothold in aviation, is staying in the game, Patel says. And the third is to grow your business, either organically or through investment, without losing the innovative culture that “keeps you sharp, stops you becoming complacent and prevents you from being process-driven”.
Her firm, for instance, “set out to become an agile systems integrator. It was also important to create an agile culture and not to be dependent on any of the large companies”.
Vertical integration has been crucial to the Inception project. As well as producing the engine, Blue Bear provides all the testing and evaluation infrastructure too.
“We aren’t dependent on a third party for that. We also do our own certification and assurance,” Patel says. “We have verticals that we’ve joined together to allow us to go from an idea to the delivery of flight-tested goods.”
That level of control is unusual for an SME. And, while this has given the firm freedom, there is a crucial factor over which it has no sway: the regulation of the next generation of energy-efficient engines that will serve many forms of aviation, potentially sharing the same airspace.
“The rules and procedures will change,” Patel says. “The way the industry supplies airworthiness evidence also needs to change if we’re going to do this more quickly and thoroughly. Along with funding, that is an absolute barrier.”