H AS IN JET:

In July 2020 a team from Delft University drove across the border from the Netherlands to Fassberg air base in Lower Saxony, Germany, on a mission to test a radical new design of hydrogen-powered commercial aircraft called the Flying-V. In the back of their van was a 3m-wide (10ft) scale model in the distinctive blue-and-white KLM livery. The team - including researchers, engineers, and a drone pilot - had a week to prove that their one-and-a-half years of hard work in the university's aerospace laboratory had not been a waste of time.

Delft is one of the top technical universities in the world and has one of the largest aerospace engineering faculties in northern Europe. The Flying-V was conceived by TU Berlin student Justus Benad, supported by KLM and Airbus. It is a radical new design that is 20% more efficient than a conventional aircraft, with the passenger cabin, cargo hold and fuel tanks integrated into the two arms of its V-shaped structure. Full-sized, the Flying-V would be around the same size as an Airbus A350, carry a similar number of passengers (more than 300) and could use the same departure gates.

The Flying-V is a type of aircraft called a "blended wing" because the wings and fuselage are smoothly blended, with no clear dividing line. Often called flying wings, they are seen as a natural fit for hydrogen-powered aircraft because they are more efficient than traditional tube-and-wing aircraft and have plenty of space for the hydrogen tanks.

Airbus itself has unveiled three Zeroe concepts for liquid hydrogen-powered aircraft, one of which could enter service by 2035. They are a rather conventional-looking short-haul turboprop and an intercontinental jet airliner, as well as a more radical blended wing that looks more like a space plane.

FlyZero, a British project aiming to realise zero-emissions commercial aviation, assessed 27 different configurations for hydrogen-powered airliners before producing its own. These included planes with two fuselages, one for hydrogen and one for passengers, through to gondola designs, with the tanks above the passengers, and a flying wing. Its own, recently unveiled concept, is for a mid-sized aircraft flying non-stop to San Francisco or Delhi which looks like a bloated version of a conventional airliner with ultra-thin wings.

There are many other designs for future hydrogen-powered commercial aircraft. "It is a question of where you can site these hydrogen tanks in an aircraft for the minimum penalty," says David Debney, a chief engineer at FlyZero. "We looked at wacky ideas, for example, where you could put a giant hydrogen tank between the wings and have two cabins, one at the back, one at the front, but they'd be separate. And you couldn't get from one to the other. That's not allowed under the regulations.

"Aircraft design is a compromise between many things, and you can get into get into a spiral when designing a plane. If you make it heavier, then you need more lift, and that means a bigger wing; a bigger wing means more weight, so you need even more lift but a bigger wing weighs more, and so on."

For the Flying-V, hydrogen means trade-offs that Kelly Johnson would have recognised, and which the kerosene powered version doesn't need. "We sacrificed two things: the first is about two-thirds of the cargo volume [which will hit profitability]," says Roelof Vos, an assistant professor at the Aerospace Engineering Faculty of Delft University of Technology. He is also technical lead on the project. "We will have sufficient volume for the passengers' luggage, but nothing more. The second is the amount of volume we have available for hydrogen, and how far we can fly on that." While a hydrogen-powered Flying-V could fly from London to Cape Town non-stop, a kerosene-powered version could reach as far as Sydney.

On 16 July 2020 the Delft team's hard work paid off. The scale model of the Flying-V was carried through the doors of the old wartime hangar onto the concrete apron at Fassberg. A little after 3.30pm, with a whine of its two electric motors, it rose sharply into the air for its successful five-minute-long maiden flight. "The flight of the scale model demonstrates that the Flying-V can be flown controllably with good handling qualities without any problems," says Vos.

"Hydrogen aircraft have flown now, so we know the fundamentals of the fuel, and we know the fundamentals of the aircraft," says Mark Bentall, head of operations for technology at Airbus, "and like we do with a traditionally fuelled aircraft... we will always take the benefit of the latest technologies."

Thanks to computer modelling our level of understanding of combustion is way, way more advanced than in Kelly Johnson's day - David Debney

Carbon fibre allows engineers to build lighter, stronger structures. Easily overlooked new manufacturing techniques such as friction stir welding (FSW) deliver more accurate high-quality joins. It uses the heat generated by friction from a rotating tool to fuse two different materials together. The Skunk Works team used wooden models and wind tunnels to design Suntan; today computer design and simulation tools help engineers to produce highly accurate designs, quickly and cheaply.

"Thanks to computer modelling our level of understanding of combustion is way, way more advanced than in Kelly Johnson's day, and this has helped kerosene engines, but it will help hydrogen-powered aircraft more," says David Debney. "Greater efficiency of aircraft helps massively with the volume of hydrogen fuel that you need to accommodate, and that's the big thing that's changed.

"If you were using 1950s aerodynamics and engine technology, for the same missions you'd need a lot more hydrogen, and that's hugely penalising from the volume perspective."

The innovation continues. Ultima Forma is a British technology company based south of London. Fuel tanks are heavy. Hydrogen causes corrosion embrittlement in metals such as steel, but less so in copper. Ultima Forma is developing ultra-thin liners made from copper for the inside of lightweight carbon-fibre fuel tanks. The same technology could be used in the transportation of hydrogen.

It is in everyone's interest, as well as the planet's, if the lessons learned by different teams could be shared. "I know for sure that the best design cannot come from one party," says Arlette van der Veer. "What would be really disruptive is an open knowledge-sharing economy to combine the knowledge of different manufacturers to produce the best designs."

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