Aircraft Fuel Cell - Most of these designs generate electricity by either using liquid hydrogen to power a fuel cell or to combust in an engine, or a combination of the two. With hydrogen comes the opportunity to rethink aircraft design, including the wings, because of the need to store liquid hydrogen in relatively heavy, insulated tanks.
That might make future aircraft look a lot different, because lighter kerosene can be stored in the wings. It is also a chance to rethink practices that in some cases date to the 1950s. For the Flying-V, hydrogen means trade-offs that Kelly Johnson would have recognized, and which the kerosene powered version doesn't need.
Aircraft Fuel Cell
"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.
The Journey To Decarbonize Air Travel
"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.
But hydrogen could become a key asset in commercial aviation even before entirely new aircraft based around it are developed. Days before Airbus' announcement, Rolls-Royce and budget airline EasyJet said they had successfully converted a regular airplane engine to run on liquid hydrogen fuel — a world first, they claim.
The second problem was even greater. While it proved possible to produce enough liquid hydrogen, the infrastructure needed to run a hydrogen-powered plane was a different matter. Kerosene was just too cheap and convenient compared to transporting volatile liquid hydrogen in huge amounts to air bases around the world, storing it, and safely refueling the aircraft.
One of the test pilots, Michael Bockler, told TechCrunch that the aircraft “flew like a normal Dash-8, with just a slight yaw.” He noted that at one point, in level flight, the plane was flying almost entirely on fuel cell power, with the turboprop engine throttled down.
A Long Runway
The Lockheed CL-400 Suntan was more like a space plane, or a Thunderbird, than a spy plane. Led by Lockheed's genius designer and secretive Skunk Works founder Kelly Johnson, the dartlike flying machine was intended to fly at Mach 2.5 at 30,000m (100,000ft) with a skin temperature of 177ºC (350ºF), have a range of 4,800km (3,000 miles
) and be powered by liquid hydrogen – that is, hydrogen cooled down to cryogenic temperatures of around -423ºF (-253C). The Skunk Works, based in Burbank California, was a business-within-a-business that was free of the usual corporate oversight.
Eremenko co-founded Universal Hydrogen in 2020, and the company raised $20.5 million in a 2021 Series A funding round led by Playground Global. Funding to date is approaching $100 million, including investments from Airbus, General Electric, American Airlines, JetBlue and Toyota.
The company is headquartered just up the road from SpaceX in Hawthorne, California, with an engineering facility in Toulouse, France. 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. Part of the problem with today's fuel cells is that they can be tricky to cool. Jet engines run much hotter, but expel most of that heat through their exhausts.
Because fuel cells use an electrochemical reaction rather than simply burning hydrogen, the waste heat has to be removed through a system of heat exchangers and vents. 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."
Universal Hydrogen hopes to start shipping fuel cell conversion kits for regional aircraft like the Dash-8 as soon as 2025. The company already has nearly 250 retrofit orders valued at more than $1 billion from 16 customers, including Air New Zealand.
John Thomas, CEO of Connect Airlines, which plans to be the first U.S. carrier to use Universal Hydrogen's technology, said the "partnership provides the fastest path to zero-emissions operation for the global airline industry." The 15-minute test flight of a modified Dash-8 aircraft was short, but it showed that hydrogen could be viable as a fuel for short-hop passenger aircraft.
That is, if Universal Hydrogen — and others in the emerging world of hydrogen flight — can make the technical and regulatory progress needed to make it a mainstream product. With his famous bluntness, in 1958 Johnson told his paymasters in Washington that he was "building them a dog", and repaid around $90m spent on the project.
The hydrogen-powered plane became one of the few failures of his long career. It was easy to think that if Johnson and his Skunkworks couldn't make the new fuel work, no one could. How the hydrogen is used also creates a different footprint.
Burning liquid hydrogen in the open-air releases a small amount of greenhouse gas nitrous oxide. Using green hydrogen to generate electricity in a fuel cell, however, emits only water and warm air. When the Lockheed team stored hundreds of gallons of liquid hydrogen at the Skunk Works a visiting scientist warned them "My God … you're going to blow up Burbank."
Later, they were reminded of this prophecy when a fire broke out and nearly caused a massive explosion that could have demolished the top-secret facility, the neighboring airport and Burbank itself. ZeroAvia, another startup developing hydrogen fuel cells for aviation, crashed its first flying prototype in 2021 after turning off its fuel cell mid-air to allow it to cool, and was then unable to restart it.
ZeroAvia has since taken to the air again with a hybrid hydrogen/fossil fuel set-up similar to Universal Hydrogen's, although on a smaller twin-engine aircraft. The stripped-down interior contained two racks of electronics and sensors, and two large hydrogen tanks with 30 kg of fuel.
Beneath the plane's right wing, an electric motor from magniX was being driven by the new hydrogen fuel cell from Plug Power. This system turns hydrogen into electricity and water — an emission-free power plant that Eremenko believes represents the future of aviation.
Eremenko originally started Universal Hydrogen to design standardized hydrogen modules that could be hauled by standard semi-trucks and simply slotted into aircraft or other vehicles for immediate use. The current design can keep hydrogen liquid for up to 100 hours, and he has often compared them to the convenience of Nespresso units.
Universal Hydrogen says it has over $2 billion in fuel service orders for the decade ahead. Universal Hydrogen will now conduct further tests at Moses Lake. The company will work on additional software development, and eventually convert the plane to use liquid hydrogen.
Early next year, the aircraft will likely be retired — with the fuel cell heading to the Smithsonian Air and Space Museum in Washington, DC. "In terms of aerodynamics, the A380 is a very stable aircraft.
So the pod attached to the rear fuselage via the stub doesn't pose much of an issue," said Mathias Andriamisaina, head of ZEROe demonstrators and tests at Airbus, in a statement. As a Universal Hydrogen-branded plane, equipped with the largest hydrogen fuel cell ever to power an aircraft, made its maiden test flight in eastern Washington, co-founder and CEO Paul Eremenko declared the moment the dawn of a "new golden age of aviation
.” Even though an estimated $500bn (£370bn) is going to be spent globally on hydrogen infrastructure, not every problem that Johnson faced has been solved, and some – including whether the hydrogen is produced locally or centrally, how it is distributed and how it
is stored at the airport – are far too big for an aircraft manufacturer or airline to solve on its own. Almost all the hydrogen used today is produced at the point of consumption. That's not only because hydrogen leaks easily and can damage traditional steel containers, but mainly because in its most useful form — a compact liquid — it has to be kept at just 20 degrees above absolute zero, usually requiring expensive refrigeration.
Several other aircraft engineers disagreed. On 15 April 1988 the rather mundane-looking Soviet experimental aircraft the Tupolev Tu-155 flew using liquid hydrogen, and the modified airliner went on to fly around 100 flights. The fall of the Soviet Union curtailed the program, but a handful of hydrogen-powered small planes or UAVs (unmanned aerial vehicles) have flown since then.
The prototype of Boeing's Phantom Eye high-altitude, long-endurance, liquid hydrogen-powered drone flew for the first time on 1 June 2012. On the last of its nine flights the Phantom Eye flew for eight to nine hours at 16,500m (54,000
ft). A lack of funding eventually grounded the drone. Carbon fiber allows engineers to build lighter, stronger structures. Easily overlooked new manufacturing techniques such as friction stir welding (FSW) deliver more accurate high-quality joints. 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. 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. Liquid hydrogen has advantages over the alternative, pressurized hydrogen gas, which includes a higher energy density (vital for longer ranges) and not needing strong, heavy tanks.
Nevertheless, while Johnson's design for Project Suntan was as long as a B-52 bomber, it still could not achieve the range Johnson had promised the US Air Force. The French aviation giant revealed at Airbus Summit 2022 on November 30 that it will mount the engine between the wings and the tail of a modified A380 superjumbo.
Test flights are estimated for 2026, as part of the Airbus ZEROe initiative to launch a zero-emission aircraft by 2035. Crucially, commercial aviation will have to learn from other industries that work with hydrogen every day.
"One of the reasons why we brought Zeroe to public attention early was because we need to work as an ecosystem to make it happen," says Bentall. Conversations between airports, airlines and manufacturers have started. "Safety is the sole purpose of everything we do... but what I discovered in my research is that there are some mindsets and approaches from the 1960s or 1970s that still prevail today despite all the new testing methods," says van der Veer.
"If I designed the most perfect aircraft...but there is no fuselage, it's not cylindrical, it would be a case of the computer saying 'no'. The certification authorities need to develop certification methods for aircraft designs that they have never seen before.
" Hydrogen has long been touted as a sustainable alternative to traditional jet fuel, either as a combustible fuel or used to generate electricity via fuel cells. The aviation industry produces 2.8% of global CO2 emissions, but it faces harder challenges than other sectors in decarbonizing itself, and progress has been slow.
The engine uses fuel cells to convert the hydrogen into electricity, which then powers a propeller. "At scale, and if the technology targets were achieved, fuel cell engines may be able to power a 100-passenger aircraft with a range of approximately 1,000 nautical miles," he said.
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. "Thanks to computer modeling 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. Engineers believed they were in a "hydrogen race" against the Soviets after U-2 flights over the Soviet Union spotted the construction of liquid hydrogen plants.
The Americans became convinced that the Soviets were developing their own space plane/spy plane, or a high-flying, high-speed interceptor to shoot down the U-2. The true Soviet motivation became clear in 1957, when Sputnik was launched on top of a liquid hydrogen-powered rocket.
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