IN THE SWAMPS of 1950s Florida, a loud roaring occasionally disturbed the serenity of the local alligators. Under conditions of strictest secrecy, engineers from Pratt & Whitney, an aerospace company, were testing a new type of engine that was powered by a strange substance apparently piped in from a fertiliser plant in the nearby town of Apix. In reality, the town was just a name on a map and the fertiliser plant was a ruse. The disturbances were the result of Project Suntan, an attempt by America’s air force to build a plane fuelled with hydrogen. It nearly worked. The engines operated successfully, but storing and supplying the hydrogen itself proved too expensive for production to continue.
Suntan was just the first of a string of failed attempts to use hydrogen to power heavier-than-air flight. The allure is great. Hydrogen packs three times as much energy per kilogram as kerosene, the current standard aviation fuel, and lightness is at a premium aloft. Tupolev, in what was then the Soviet Union, tried in the 1980s. Boeing tried in the 2000s. A small demonstrator has flown in Germany. But nothing has, as it were, really taken off. Hydrogen, though light, is bulky, making it awkward to store on board. It must be either pressurised or liquefied, both of which bring complications of their own. On top of that, there is no established infrastructure for making and distributing it.
Now, though, circumstances have changed. Aviation is under pressure to curb carbon-dioxide emissions by burning less kerosene. And talk of building hydrogen-manufacturing-and-delivery infrastructure for other purposes, such as heating and ground transport, is now becoming serious, meaning that hydrogen might become available as a commodity, rather than having to be made specially. The balance of advantage may thus be shifting. So a few brave souls are looking once again at the idea of hydrogen-powered flight.
This time it’s different…
Project Suntan used the stuff in the way that kerosene is used—to create the heat needed to power a jet engine. That is one way forward. But many planes are driven by propellers, and this permits a second approach, for propellers can be turned by electric motors. Using fuel cells, a 19th-century technology that is now coming into its own, it is possible to generate the electricity needed to do so with hydrogen.
This is the tack taken by ZeroAvia, a firm in Cranfield, Britain. In September ZeroAvia’s engineers unveiled a six-seater fuel-cell-powered aircraft that could take off, complete two circuits of the airport, and land. The plane in question is a modified Piper M-class—a single-propeller aircraft normally driven by a piston engine. The engineers replaced this with an electric motor, and installed a bank of fuel cells to power that motor and a set of tanks to hold the hydrogen which runs the fuel cells.
Val Miftakhov, ZeroAvia’s boss, hopes to see this demonstrator take a 400km trip, tentatively scheduled for the week of December 21st, followed by a longer distance flight from Kirkwall airport in Orkney next spring. The firm also plans to have a 20-seat demonstrator ready for flight in 2021. Certification for commercial use might follow in 2023.
Hot on the heels of ZeroAvia is H2Fly, a spin-off from DLR, Germany’s aeronautic research centre. In 2016 this firm added fuel cells to a motorised Pipistrel glider and used them to keep it aloft for 15 minutes. The plan is to extend that approach to a production-version propeller-driven plane in tests to be conducted imminently. Meanwhile, in America, an electric-motor manufacturer called magniX has announced a partnership with Universal Hydrogen, a firm in Los Angeles, to convert a 40-seat de Havilland Canada Dash 8-300 to run on fuel cells. This, they hope, will be ready by 2025.
Such approaches seem likely to work in principle. They will, though, have to compete in practice with electric aircraft powered by batteries. In May, an American firm called AeroTEC flew a nine-seater Cessna Caravan that had been converted to battery power through the skies above Washington state. The previous December, magniX collaborated with Harbour Air, a Canadian company, to fly a converted de Havilland seaplane in British Columbia. The two firms are now busy preparing this aircraft for commercial certification. More ambitiously, several companies, such as Eviation, an Israeli outfit, are attempting to build battery-driven aircraft from scratch rather than converting existing airframes.
Proponents of fuel cells say, though, that these are better than batteries for powering flight because the cells plus their associated fuel store many times more energy per kilogram than batteries can manage. “Batteries really give you the acceleration. But they won’t give you the range,” says Robert Steinberger-Wilckens, a chemical engineer at the University of Birmingham, in Britain. Battery technology is improving, but big breakthroughs will be needed before longer journeys with passengers and freight on board become possible.
Sticking electric power sources in an existing aircraft, whether in the form of batteries or fuel cells, is a start. But such propulsion could lead to a significant redesign, such as the one Eviation is considering for its putative product, Alice. This has three propellers, all of which face backwards. Though once popular, backward-facing propellers have been out of fashion for decades. Electric vertical take-off and landing aircraft—people-carrying drones sometimes touted as the future of personal transport—are also often powered by multiple smaller electric motors, making them a good fit with fuel-cell-based hydrogen power.
Bigger machines have bigger problems. It requires much more power for a plane to take off and land than to cruise, and neither batteries nor fuel cells yet have the oomph to do this for other than small aircraft. If larger ones are to be hydrogen-powered, that will require at least part of the work to be done by returning to the Project Suntan route and employing turbine-driven jet engines that burn the stuff as a gas.
That approach is now being adopted by Airbus, a European firm that shares with Boeing of America a duopoly on large passenger planes. In September Airbus unveiled ZEROe, a project centred around three hydrogen-powered concept planes. Though these are single-aisle short-haul designs, they are a step up from from anything that might be powered solely by fuels cells.
All three are designed to yoke the two hydrogen-based technologies together, with hydrogen-burning turbine engines boosting take-off and fuel cells powering the cruise. One of the concepts is a turboprop that would carry up to 100 passengers for distances up to 2,000km. A larger turbofan design would take twice that load twice as far. The third approach is more experimental: a “blended wing” model, in which fuselage and aerofoils form part of the same triangular aerodynamic structure. The advantage of this is that it creates extra volume for hydrogen storage.
The challenges of using hydrogen go beyond body shape, though. Redesigning a modern jet engine to run on the stuff is likely to be a multi-billion-dollar endeavour. Hydrogen burns faster than kerosene, and also burns hotter. That means materials exposed to its combustion experience greater stresses. It also risks increasing the pollution generated in the form of oxides of nitrogen, which would partially negate the environmental benefits of burning hydrogen. And it would be useful to arrange matters so that some of the energy used to compress or liquefy the hydrogen for storage could be recovered and put to work.
For the next few years, Airbus will focus on developing the twin technologies of fuel-cells and hydrogen-powered turbines in parallel with the design of their future aircraft. If demonstrations on the ground succeed, the firm hopes to have airborne demonstrators—what Glenn Llewellyn, Airbus’s vice-president for zero-emission aircraft, calls flying testbeds—in the air by 2025. A full-scale prototype would follow by the end of the decade, with the first zero-emission commercial airliner entering service by 2035. Who would supply the engines for such an aircraft is not yet clear. Rolls Royce, for one, though, are looking into the question of hydrogen-powered turbines.
So far, Boeing has not followed suit. This geographical split may be no coincidence. Airbus, though now a public company, started as a consortium of firms from three countries in the European Union, and a quarter of its shares are government-owned. EU public policy is firmly anti-climate change, as is public policy in Britain, which joined the consortium later and which has now left the EU. That policy translates into actual money for relevant research via the union’s Clean Sky 2 programme.
No such support, either moral or financial, has been on offer in America over the past four years. Joe Biden’s incoming administration, however, seems of one mind with Europe on matters environmental. And this new direction is likely, as in Europe, to be accompanied by public money. Boeing, moreover, would be taking a gamble by leaving hydrogen-power to Airbus. If the technology succeeded, it would risk losing an important part of its market—and that is something it certainly cannot afford to do.