Details released by Airbus about the goals of its conceptual studies on hybrid-electric and electric propulsion and a superconducting technology demonstrator signal the airframer is in full battle order for the service entry of a carbon-neutral aircraft by 2035.

The company presented three concept studies last September for liquid-hydrogen-powered aircraft (AW&ST Sept. 28-Oct. 11, 2020, p. 16). They included a regional turboprop, a narrowbody in the A320-family size category and a blended wing body. In December, Airbus unveiled another potential configuration for a hydrogen-powered aircraft integrating a tank, fuel cell, motor and propeller in each of the arrangement’s six underwing pods. 

• Fuel cells are involved in all Airbus concept studies
• Superconductivity holds out hope to solve key issues in electric architectures

The first three concepts are hybrid-electric. “So it means that in all three concepts we have hydrogen powering gas turbines,” Glenn Llewellyn, vice president of zero-emission aircraft at Airbus, said during a webinar organized by Eurocontrol, the organization in charge of air traffic management in Europe. “In parallel, we can provide the gas turbines with electrical power in what’s called a parallel hybrid configuration.” In such a layout, the gas turbines would probably be designed for the cruise phase, and electrical power would provide a boost for takeoff and climb. The bottom line is more efficient gas turbines.

The electrical power comes from fuel cells. “Once you have hydrogen on board, it makes sense to use fuel cells to create your electrical energy through an efficient process rather than to carry batteries,” Llewellyn said.

The fourth concept uses fuel cells only, without any gas turbine. “We are using this concept and some variants of this concept that we’ve not made public to really understand the scalability of fuel cells, both in terms of technical feasibility and commercial viability,” Llewellyn said.

“[In any case,] hydrogen is the solution that allows us to have the most significant reduction in climate impact. . . . Of course, we have to make sure that we increase the amount of renewable energy and the amount of renewable hydrogen supply available,” he said. CO₂ emissions are practically eliminated when using hydrogen instead of a conventional fuel.

Moreover, so-called non-CO₂ effects are being increasingly understood as being major contributors to aviation’s climate impact. “When we look at nitrogen oxides, when we look at persistent contrails, hydrogen looks like it has the most potential compared to every other option to significantly reduce and potentially eliminate those non-CO₂ effects,” Llewellyn noted. Airbus is counting on extensive demonstration and flight-test programs “to convince us that what we have out of the exhaust of these aircraft is in fact what the theory is telling us,” he said.

The airframer plans to make decisions in 2022-23 about which concept it will take to the next phase. “We are developing technologies, and we will have flight demonstrations up until 2025,” Llewellyn said. “Then, in 2025-26, we’ll be in a position to press the button on the aircraft development activities to achieve an entry into service by 2035.”

As all four concepts rely—at least for a significant portion—on electric propulsion, Airbus has launched an ambitious demonstration program for the use of superconducting technology. It is aiming at a major efficiency improvement.

The idea stems from both the difficulty of designing an electric-propulsion architecture with conventional wiring and the opportunity to use liquid hydrogen as a cold source. Superconducting materials require cryogenic temperatures.

Superconducting technologies have been in service for decades in other industries, such as medical imaging and fundamental physics. In aerospace, the need for superconducting wires as part of high-power electric systems has been suggested for at least a decade. This is the first time, however, that an OEM has embarked on a demonstration program.

Called the Advanced Superconducting and Cryogenic Experimental Power Train Demonstrator (Ascend), Airbus’ project aims to explore “the impact of superconducting materials and cryogenic temperatures on the performance of an aircraft’s electrical propulsion systems.” The ground demonstrator will be designed and built at the airframer’s E-Aircraft System House in Ottobrunn, Germany, under the umbrella of the company’s UpNext research and technology subsidiary. Various configurations—with different cold sources, cooling systems and power levels—will be evaluated by the end of 2023.

The Ascend program looks like a follow-on to the aborted E-Fan X, which had begun studying hybrid-electric propulsion. Led by Airbus and Rolls-Royce, it involved a 3,000-volt electric system. The ability to handle hundreds of kilowatts of heat waste was described as a key challenge.

Superconducting technology solves the heat management issue, thanks to the absence of any resistance and the related Joule effect. The expectation is that the technology will increase efficiency by 5-6% compared to conventional technologies, adds Ludovic Ybanez, head of the Ascend program. The weight of the power train’s components could at a minimum be halved.

It is expected that electrical losses will also be cut in two compared to conventional technologies. Voltage can be reduced below 500 volts, making it easier to manage problems such as arcing.

When using superconductivity, the otherwise-challenging 20K (-424F/-253C) temperature of liquid hydrogen becomes a resource. A secondary system could use a heat exchanger and a neutral, easier-to-handle fluid such as nitrogen, Ybanez suggests. It could cool the superconducting wiring to the required 70-80K.

The technology for the core material is well mastered, the superconducting wiring itself being measured in microns. Cuprate superconductors are a likely choice.

Developing a suitable insulation, likely to have a diameter of 4 in., for use on board an aircraft is a major objective of Ascend, says Ybanez. Composite materials will be considered.

As for the motor control unit, it will be very innovative, Ybanez says. While magnetic resonance imaging and particle accelerators use superconductivity to create strong magnetic fields, aviation will target an increase in power density. The weight of the wiring (which recently has used aluminum) will thus no longer be correlated to the power it carries.

The motor will be compatible with a propeller, a fan or an unspecified hybrid propeller, Airbus says. The first three pillars of Ascend will be working on the motor itself, the motor control unit and the cryogenic system. The fourth pillar will be network safety and protection, including fault current limiters and circuit breakers.

While hydrogen is the presumed cold source, the 15-engineer Ascend team also will consider creating a different one in an electrically powered aircraft that would not rely on hydrogen. Ascend will study power levels in the 1-4-megawatt range and evaluate the benefits of introducing superconductivity and the accompanying cryogenics. The applications would potentially cover a wide spectrum, from a small electric vertical-takeoff-and-landing vehicle to a long-range commercial aircraft.

At the end of the program, the Ascend team will issue recommendations so that Airbus can make decisions about electric propulsion architectures.

Safran soon will become a partner in Ascend, along with other companies and research laboratories.