Joby Beats Range Target For Hydrogen-Electric Air Taxi Demonstrator

While it pursues certification of its battery-powered air taxi and prepares to launch commercial service, Joby Aviation already is taking the next step: flying its electric vertical-takeoff-and-landing aircraft on liquid hydrogen and fuel cells.

The startup has taken its pre-production prototype S4, which completed battery-electric flight testing in May, and modified it into a technology demonstrator for hydrogen-electric propulsion. The U.S. Air Force’s Agility Prime program is supporting the effort.

• Prototype modified to hybrid battery/hydrogen-electric propulsion
• Liquid hydrogen and fuel cell extend range beyond 520 mi.

In flight testing conducted in June, the remotely piloted aircraft completed a 523-mi. flight over Marina, California, including a vertical takeoff and landing and landing with 10% of its liquid hydrogen (LH₂) fuel load remaining. This compares with the 155 mi. flown by the battery-electric S4 in 2021.

The demonstrator is fitted with a series-hybrid battery/hydrogen-electric propulsion system. The baseline S4’s propulsion units are retained, the six tilting propellers and their electric motors powered by the battery system, which the fuel cell system recharges in flight.

“At a high level, 90% of the systems on the aircraft stay the same,” says Joby founder and CEO JoeBen Bevirt. “We add the fuel cell, the liquid hydrogen system, modify the batteries, and we get an aircraft with dramatically more range and endurance.”

A vacuum-jacketed, 40-kg (88-lb.) LH₂ tank is installed in the fuselage of the demonstrator along with a fuel cell developed by Joby’s Stuttgart, Germany-based subsidiary H2Fly. A heat exchanger to cool the fuel cell is mounted under the nose of the aircraft.

Joby developed the insulated tank and heat exchanger internally. The 175-kW H2F175 low-temperature proton-exchange membrane fuel cell developed by H2Fly was used. The battery has the same architecture as in the S4 but with a higher specific-energy cell to reduce weight.

“Vertical integration paid off in the way we constructed the system,” says Didier Papadopoulos, aircraft OEM president at Joby. “This is really a game of optimization. Being vertically integrated allowed us to make the right optimization in terms of the dewar, the fuel cell, the compressors, the battery, where we put the sensing. Without that, this would have been a much more complex, lengthy project.”

Hydrogen-electric specialist H2Fly was acquired quietly by Joby in 2021, and in September 2023 the German startup performed the first crewed flights of an aircraft powered by liquid hydrogen and fuel cells, using its HY4 fixed-wing testbed aircraft.

“One of the unique things we were able to achieve is to draw on the existing pool of technology and pull out the battery-electric energy source and introduce this hybrid battery/hydrogen-electric solution. Everything else—the electronics, propulsion systems, airframe—remains the same,” Papadopoulos says. “The key takeaway is the power of electric propulsion to allow you to think in a very flexible way about how you develop new aircraft architectures.”

The hydrogen-electric demonstrator has been developed by a small team within Joby, Bevirt says, without drawing heavily on the financial and technical resources required to certify the battery-electric aircraft and prepare for Part 135 commercial air taxi services.

“This aircraft and the commonality leverage 90% of the work we’re doing with certification of the battery-electric aircraft, and the real strength of our vertically integrated approach is we’re then able, with small incremental investment, to create a compounded impact,” Bevirt says.

“We’re making investments in our Part 135 operations, in takeoff and landing locations, in our ElevateOS operating system, and the value of all those investments and infrastructure gets compounded, where for 10% incremental investment we get a huge expansion in the capabilities. You build a vertiport; it doesn’t just get you to another spot in the metropolitan area but gets you to anywhere in a 500-mi. radius.”

Joby sees the hydrogen-electric aircraft as complementary to the battery-electric S4. “We think it is very synergistic, where you have battery-electric aircraft serving short-distance trips within a metropolitan area and hydrogen-electric aircraft working side by side with them but also serving regional journeys.”

Airports ideally are positioned to become hydrogen distribution centers, Bevirt says, and the fuel cell aircraft’s greater range capability means it is not necessary for every vertiport in a network to be equipped with hydrogen refueling infrastructure.

“Battery-electric is the most efficient as long as you are doing a short-distance trip where you have a lightweight battery pack,” he notes. “If you try to go for a longer trip, the aircraft gets heavier, and soon you’re just flying around a big battery.

“But with hydrogen, because it’s 100 times lighter, we can make an aircraft that is good for medium- and long-distance flights,” Bevirt continues. “That cutover point moves as the specific energy of batteries improves. But today, with batteries around 300 Wh/kg, we think that cutover threshold is around 100 mi.”

With a range capability of 100 mi. plus reserves, the battery-powered S4 is designed to make several 15-25-mi. trips before having to recharge. Joby sees similar value in the operational efficiency that comes with hydrogen-electric, as the aircraft can make multiple, longer, back-to-back flights before refueling.

“Hydrogen is one of the best energy carriers in the world because it’s three times lighter than jet fuel and because we can convert the chemical energy that it contains into propulsion twice as efficiently as a small turbine can convert jet fuel into propulsion,” Bevirt says.

Joby is not giving a timescale for development and fielding of a hydrogen-electric air taxi but notes that the timeline is similar to that for the battery-electric aircraft. The company was founded in 2009 and began working with the FAA in 2015 on the regulations for battery-electric aircraft.

“We began our formal certification in 2018. We now have all of our area-specific certification plans agreed to and are well underway on testing,” Bevirt says.

FAA certification of the S4 is expected in 2025. “We anticipate that the relationship we’ve built bringing battery-electric to market . . . will be a valuable springboard as we begin having those conversations on the hydrogen-electric front,” he says.

“H2Fly started [working on hydrogen-electric propulsion] about 10 years ago, and now we are at a point where we’re able to integrate with the airframe that we have today and demonstrate,” Papadopoulos says. “That sets the stage for the next steps, moving into the certification framework and what the aircraft is going to look like.

“The FAA usually wants to engage when you’ve demonstrated the technologies,” he adds. “That’s important not only in terms of understanding what the technology can do but also understanding where the weaknesses are in the technology and how we need to mitigate those in order to introduce a safe and reliable airplane.”

Bevirt sees hydrogen-electric air taxis as providing a way to jump-start demand that justifies investment in liquid hydrogen infrastructure at airports.

“I think the aviation world writ large doesn’t realize what a game changer hydrogen will become for aviation, from a sustainability standpoint but also the operating economics,” he says. “Hydrogen-electric, in my view, is going to be one of the greatest disruptions in aviation in multiple generations.”