Alternative propulsion systems based on hydrogen and electricity will play an important role in aviation in the future, as will hybrid systems and synthetic fuels. DLR – the German Aerospace Center – has comprehensively described where the journey is headed in its white paper on zero-emission aviation published at the end of 2020. According to this paper, it would be foolhardy to try to transfer the technological strategies that have proven themselves in ground transportation to aviation. Unlike in the automotive industry, the sector has no blueprints on which new aircraft concepts might be built. In addition, the requirements for the power-to-weight ratio of a drive system and the energy density of the energy sources are many times greater than in automotive engineering. In any case, aeronautical engineers would be loathe to pack tons of batteries, tanks, and engines for an alternative propulsion system on board. Modern aircraft are perfectly balanced systems – if you make changes for new propulsion concepts, you have to accept tangible effects on design, safety, flight performance, and aerodynamics.
The aviation industry faces a Herculean task
But what does this mean for the development of emission-free aircraft? The technological maturity of alternative propulsion solutions for aviation is currently very low. The aviation industry is therefore faced with nothing less than the task of reinventing flight. However, due to the high safety requirements and associated hurdles for approval, new flight concepts need a lot of time – so it’s five minutes to midnight for setting a course for the future. We need radical and revolutionary aircraft and propulsion concepts. There is still a great deal of research and development work to be done before aircraft manufacturers can roll out the first alternative models on the runway.
Battery-electric concepts, for example, enable good power density and high efficiency – but the decisive disadvantage is the batteries’ low energy density. In this respect, hydrogen clearly holds the better cards. According to the hydrogen fuel cell drive specifications, the fuel cell’s power-to-weight ratio is high. And the storage of hydrogen also places high demands on the integration of voluminous tanks in the aircraft structure. In its assessment of the near future of aviation, DLR sees several development paths: Battery-electric regional aircraft will be used for travel within metropolitan areas. Short- and medium-haul routes will be the domain of aircraft based on fuel cells, while for the time being long-haul routes will be dominated by new gas turbine concepts in conjunction with sustainable fuels. In the long term, however, hydrogen and fuel cell propulsion systems are also conceivable.
Startups take off with bold and spectacular flight projects
The upheaval in aviation is taking place on a scale that can hardly be overestimated. At the same time, it is unleashing creative forces. In a market report published in September 2021, the international consultancy Roland Berger analyzes that more than 280 aviation projects with alternative propulsion systems were carried out around the globe at the beginning of this year. Around 85 percent of them happened in Europe and North America. Even today, the industry has a gold-rush atmosphere. In urban air mobility in particular, bold projects that would have delighted aviation pioneers like Otto Lilienthal or the Wright brothers are taking off. Like the Californian startup Archer, which unveiled a prototype of an electric vertical take-off aircraft with twelve propellers for lift and propulsion in mid-2021. On board the “Maker” are six batteries for a range of 100 kilometers. The Israeli aviation company Eviation Aircraft believes it has already found the magic elixir for regional aviation – they expect their battery-electric propulsion system to take to the air in the first half of this year. In the same segment, the startup Wright Electric from California is planning to develop an all-electric powertrain based on hydrogen fuel cells or aluminum fuel cells, which will be designed for a range of around 740 kilometers. The industry giant Airbus, in turn, is relying on hydrogen for long-distance travel, with direct combustion as a possibility, as well as the use in a fuel cell technology. The company will make the decision on the most suitable system by 2025.