Vehicles inundate the car parks at Dublin Airport as vacationers escape the Irish heat for summer stickiness elsewhere, and Ryanair places its biggest order yet with Boeing. It seems we are remaining compulsive flyers despite aviation carbon emissions.
In 2021, Willie Walsh, the director general of the International Air Transport Association, asserted that the global aviation industry would be carbon neutral by 2050. What are the innovations that could make this possible?
On the ground, we are transitioning to electric vehicles. However, in the air, electric aviation is likely to be a niche market. Batteries currently have far lower energy density than jet fuel. At best, electric aircraft may be restricted to air taxis and very short-range flights.
Instead, many in the aviation industry propose that sustainable aviation fuel (SAF) can fully replace aviation kerosene.
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Bio-fuel SAF results from refining hydrocarbon byproducts, waste oils, fats and biomass. SAF can be blended up to 50 per cent with aviation kerosene, with relatively minor impact on most current jet engines. Airbus has demonstrated both narrow and wide body flights with 100 per cent SAF. Nevertheless, the production of biofuel is not carbon neutral.
Hydrogen-based SAF is an alternative with potentially near-zero carbon emissions, if surplus green energy is used to extract hydrogen from hydrolysis of water. Several consortiums have declared their intentions to use offshore wind farms in Irish waters to produce hydrogen fuel. Nevertheless, for a given volume of fuel tank, hydrogen SAF is not as efficient as current jet fuels. It must be pressurised and cooled to reach even a quarter of the energy inherent in an equivalent tank of kerosene. Furthermore, to be cryogenically effective, a hydrogen fuel tank ideally should be cylindrical. But such tanks are challenging to fit into existing airframe designs.
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Boeing, Airbus and others are considering whether aircraft could be fundamentally redesigned to be significantly more aerodynamically efficient, and hence reduce fuel consumption, whether it be kerosene, bio or hydrogen SAF, or even electric.
Boeing recently confirmed it would evaluate a full-size prototype of a new airframe design by 2028 which, if successful, could lead to production by 2035. This would have a new wide but narrow wing, similar to the high aspect-ratio wings on extreme-performance gliders. It would be mounted on top of the fuselage, as a “transonic truss-braced” wing structure. A co-design with Nasa has predicted an 8-10 per cent reduction in fuel burn in a 130-210 seat aircraft, with a 52m wing span (as opposed to the 36m of a 737). The large increase in span would, however, require that the wing be foldable, like those often found on naval aircraft on aircraft carriers, to remain compatible with current airport infrastructures. This, in turn, would add weight to the wing, reducing its performance.
Airbus, Bombardier and JetZero (a Californian start-up) are considering even more radical designs for passenger aircraft, using blended wing bodies (BWBs). Lifting bodies, such as space shuttles, have small wings and a fuselage shaped to give lift. By contrast, a flying wing, such as the US B2 bomber, has a large wing area and small fuselage. A BWB differs from both lifting bodies and flying wings, in having no distinguishable transition from wing to fuselage.
In 2019, Airbus tested a small model of its blended wing concept, and followed in 2020 by a design of a 200-seat passenger aircraft. It anticipates that fuel consumption could be reduced by 20 per cent due to the extra lift and reduced drag. The engines would be mounted above the wing, resulting in both less ground noise and a quieter cabin. Because of the new airframe shape, in principle it should be easier to accommodate cylindrical fuel tanks, for liquid hydrogen, than current aircraft.
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Bombardier has recently announced successful flights of its second model BWB design for a business jet. It believes fuel consumption could be reduced by 50 per cent as a result of the novel airframe and propulsion systems.
JetZero intends to initiate commercial operations of its BWB design by 2030. The aircraft would accommodate up to 250 passengers, over a range of 5,000 nautical miles, also with a 50 per cent reduction in emissions. The design fits within the footprint of existing aircraft, thus avoiding substantial changes to airport infrastructures. The company recently previewed the spacious cabin layout, with both side and overhead windows. The cabin layout is essentially triangular with fewer seats per row at the front, increasing to more per row at the rear. Certification of course is yet to come, including how quickly a full cabin could be safely evacuated in an emergency.
Will radically new aircraft designs be successful? As ever in the history of aviation, innovation is frequently influenced by the military. Given the large internal volumes of BWB designs, the US Air Force is expressing strong interest in them for a possible new in-flight refuelling tanker.
