Flying, but greener – that’s what the future of aviation has to look like. There’s no other option considering the current footprint of air travel on the global climate. Airline CEOs and policymakers agree on this point. The aviation industry has committed to cutting its net CO2 emissions in half compared to 2005 by 2050. Scientists say that 2050 is too late and are pressing for change by 2030. Air travel accounts for 3.5 percent of human-induced global warming, according to an international study led by Manchester Metropolitan University with participation of the German Aerospace Center (DLR) and others that was published in September 2020 and provides the most comprehensive insight into the climatic impact of aviation to date.

CO2 emissions, which have been the key metric for the environmental impact of aviation, account for only one third of it, whereas other effects account for two thirds of the climatic impact of aircraft. Nitrogen oxide (NOX) emissions at altitudes below 12,000 meters (39,000 feet) promote the undesirable formation of ozone. Undesirable because ozone does not block UV rays in these layers of the atmosphere but, like CO2, has the effect of a greenhouse gas. On the other hand, depending on weather conditions, soot particle emissions as cloud condensation nuclei contribute to cloud formation (in the form of the generally known contrails, among other things). These clouds reflect thermal radiation from the Earth and therefore contribute to global warming as well.

In this article, we present some interesting ideas of how aircraft and their propulsion systems might become more environmentally friendly going forward and which sustainable energy sources could be tapped for this purpose.

Traveling more efficiently in a flying wing

To gain efficiency in flying, new aircraft designs have to be studied. Flying V (pictured below) is a V-shaped concept consisting basically only of a wing, put forward by the Technical University of Delft in the Netherlands, supported by KLM and Airbus. “If this configuration indeed becomes the next long-haul aircraft, it would be the most revolutionary change we have seen in aviation since the introduction of jet engines,” says ­project ­leader Roelof Vos. In 2014, a first model of the ­flying wing took to the air for tests. Since then, the Technical University of Delft has further refined the aircraft concept and in July 2020, a 1:5 scale model of the Flying V flew with a wing span of over three meters (10 ft) and a weight of 22 kilos (49 lbs). The design itself is fairly conventional on purpose and the original is supposed to be slightly smaller than today’s Airbus A350, currently one of the most efficient airliners. Flying V can use existing airport facilities and haul the same number of passengers and cargo volume as the A350 – while burning 20 percent less fuel. “And this just comes from the fuselage shape, maybe we will be able to gain even more efficiency by improving propulsion technology,” Vos hopes. Potentially, the aircraft could be powered by hydrogen in the form of fuel cells or ­e-fuel. Hydrogen needs huge tanks that conventional jets could hardly accommodate. “This would be much easier in the outer wings of the Flying V than in the thin wings of current aircraft,” says the professor from Delft.

Efficiency through open fan-blade architecture

A new generation of aircraft has to be at least 20 percent more efficient than the preceding one, and that’s only possible with radical changes to propulsion such as open rotor engines that were first unveiled as far back as in the mid-1980s. Two openly exposed, counter-rotating fans with sword-like blades looked spectacular, but caused many problems – from noise to vibrations. Almost three and a half decades later, the successor model could turn into a big leap forward by being available as propulsion for an anticipated replacement of the Airbus A320neo family around 2030. Called RISE (Revolutionary Innovation for Sustainable Engines), manufacturer CFM in mid-2021 unveiled a new open rotor concept using just one unducted fan with carbon blades. This open architecture eliminates the whole structure around the fan and therefore a lot of weight and drag, so enabling ultimate propulsive efficiency. The diameter of the unducted engine shrunk to nearly four meters (13 ft), similar to the dimensions of current turbofan engines including their casing, for example on the A320neo or the Boeing 737 MAX. The key advantage is that RISE can run on all kinds of fuel, be it up to 100 percent SAF (sustainable aviation fuel) or even hydrogen. The RISE engine is supposed to reduce fuel burn by 20 percent and that, says the manufacturer, is only one component of the overall efficiency of the concept. With further improvements to the aircraft itself, the total effect could be increased to 30 percent more efficiency versus the status quo by 2035.

© Schaeffler

Plug-in engine power

Electric propulsion could be ready for use in hybrid regional aircraft of up to 50 seats on short routes in the next few years. The Swedish startup Heart Aerospace received a huge boost in July 2021 by US giant United Airlines ordering up to 200 units of the biggest fully electric aircraft under development so far, the ES-19. Finnair also cooperates with Heart and has secured up to 20 of the 19-seaters. It’s supposed to fly as far as 400 kilometers (250 miles) before the batteries have to be recharged, which will take a 1-MW charger on the ground about 40 minutes. After about 1,000 cycles, the batteries have to be replaced. The very first electrical aircraft for passenger flights could be in service in Scandinavia by the middle of the decade. In Norway, regional airline Widerøe, engine maker Rolls-Royce, and Italian aircraft manufacturer Tecnam have teamed up. The trio is aiming to put the nine-seat P-Volt into service in about five years as the first passenger aircraft solely powered electrically.

© Tecnam

However, the principal problem of electric propulsion still lies in the energy density of today’s batteries. 200 to 250 Wh/kg are the maximum that can be achieved, while kerosene offers an energy density of 12,000 Wh/kg. The available efficiency of lithium batteries, though, is increasing by five to eight percent every year and NASA expects them to achieve an energy density of 350 Wh/kg by 2030, enabling a 30-seater electric aircraft for short-haul flying. The car manufacturing industry is now working on solid-state batteries of up to 400 Wh/kg and there are even plans for vehicle batteries with 1,000 Wh/kg. In aircraft, though, fully electric propulsion will only be viable for smaller propeller aircraft even in the mid-term future. For bigger regional jets with about one hundred seats, hybrid concepts, i.e., a combination of electric motors and conventional jet engines, are the most suitable approach. This would enable a much longer aircraft range, while an added electric power source, mainly during takeoff and landing, would lead to significant reductions of fuel burn, emissions and noise.

I can imagine a world in which short hauls in regional aircraft will be battery-electric. For distances of up to 2,000 kilometers (1,250 miles), hydrogen is the suitable energy source, either as carbon-neutral fuel burned directly or by powering electric propulsion systems via a fuel cell. For long-haul flights, I do not see any alternative to synthetic fuels produced from renewable energies in the next few decades. These developments result in outstanding opportunities for Schaeffler Aerospace going forward. Aviation has its own exacting standards and we see ourselves in a position to transfer our very good in-house development and production know-how in the area of electric drive systems to this sector

Armin Necker,
Managing Director Schaeffler Aerospace

Supersonic, but sustainable this time around?

Since the end of Concorde in 2003, there have been no more supersonic passenger flights. Due to their extremely high energy consumption, the inevitable sonic boom, and emissions at high altitudes, such flights would no longer be appropriate today. Even so, a new supersonic era might begin soon: Boom Supersonic from Denver is planning to reintroduce supersonic flights by the end of the decade with its Overture aircraft as a smaller successor to Concorde, in more sustainable ways and causing a less noisy boom. United Airlines, the world’s fourth-largest airline, in June 2021 announced its intention to purchase 15 Overture aircraft and Japan Airlines holds options as well. Starting in 2029, it’s supposed to fly passengers at 1.7 times the speed of sound (2,100 km/h (1,300 mph)), which is significantly slower than Concorde that achieved Mach 2.02 (2,494 km/h (1,548 mph)). Although accommodating only 50 to 60 passengers – fewer than Concorde used to with its one hundred seats – the new airliner would emit three to five times more CO2 per passenger than a subsonic aircraft flying the same route. Plus, according to estimates by independent scientists of the International Council on Clean Transportation (ICCT), it would burn five to seven times more fuel per passenger. Boom is planning to enable Overture to fly with 100 percent SAF as the first airliner ever. However, the supply of greener fuel is still far too scarce. The 15 United Overture aircraft alone might require twice the amount of SAF that will be available in all of the EU at the end of the decade, estimates the ICCT.

Does hydrogen work wonders?

By 2035 Airbus is planning to bring the first “zero-emissions” airliner to market. Independent scientists have dismissed this announcement. Dieter Scholz, a professor at HAW, for example, says that zero emissions will never be possible. The aircraft manufacturer from Toulouse has presented three different concepts, among them a conventional-looking jet and a blended-wing body, of which one is supposed to become reality, most likely the turboprop variant. The idea to use hydrogen instead of kerosene as an energy source for aircraft engines is difficult to realize. So far, there is a lack of hydrogen aircraft concepts that could cope with everyday flight operations and be economical at the same time. Establishing the infrastructure on the ground required for storing hydrogen and refueling aircraft is a complex proposition as well. Hydrogen is not easy to handle: It offers three times the energy density of kerosene, which is a major advantage over batteries, and only weighs a third, but requires up to four times more volume. And space on board of aircraft is notoriously scarce. Additionally complicating things is the fact that hydrogen is a so-called cryogenic fuel: a gas that only liquefies at minus 253 °C (–423 °F) and can be used for propulsion only after having been compressed under high pressure. Hydrogen can be used on aircraft in different ways: for direct burn in modified gas turbines, converted to electrical energy in fuel cells, or as synthetic kerosene produced in combination with CO2. The startup Universal Hydrogen has developed a simple capsule system for delivering hydrogen to turboprop aircraft. On the ground in between flights, hydrogen capsules measuring two meters (6 ft) in length would be replaced in the aft of the aircraft to produce in-flight energy in a fuel cell powering two electric motors. The first airlines have already ordered the technology and are planning to use it for nearly emission-free operation in existing aircraft such as the ATR-72 and Dash-8 starting in 2025.