Hydrogen propulsion and the race to replace jet fuel

Hydrogen is emerging as the most promising long-term replacement for jet fuel in aviation. Two distinct engineering approaches — fuel cells and direct combustion — are being flight-tested today, with initial certifications for regional aircraft expected by the late 2020s. The technology offers nearly three times the energy density of kerosene by weight, but significant challenges in storage, infrastructure, and green hydrogen production remain before widespread adoption.

Why Not Just Use Batteries?

The case for hydrogen starts with a brutal math problem. Lithium-ion batteries deliver roughly 250 watt-hours per kilogram. Jet A delivers about 12,000 watt-hours per kilogram — a factor of nearly fifty. Even a tripling of battery energy density over the next decade, which would be a massive breakthrough, still leaves a fifteen-to-one gap. Battery-electric works for short hops under 200 miles, but it falls apart for regional or cross-country flying.

Hydrogen sits in a completely different position on the energy density chart. By weight, hydrogen carries about 33,000 watt-hours per kilogram — nearly three times the energy density of jet fuel by mass.

The caveat is volume. Even as a liquid, cooled to minus 253 degrees Celsius, hydrogen takes up about four times the volume of jet fuel for the same energy. Keeping it that cold requires insulated tanks that are heavy and complex. The engineering challenge isn’t whether hydrogen has enough energy — it does. The challenge is storage, transport, and conversion to thrust.

How Do Hydrogen Fuel Cells Work in Aircraft?

The first major approach is hydrogen fuel cells, championed by ZeroAvia, based in Hollister, California. The concept: feed hydrogen and oxygen into a fuel cell stack, produce electricity through an electrochemical reaction, drive an electric motor, turn a propeller. The only byproduct is water. No CO₂, no nitrogen oxides, no particulates.

ZeroAvia completed a critical milestone in early 2025, running their 600-kilowatt powertrain on a testbed for extended durations — enough power to drive a regional turboprop. They’ve been flight-testing modified Dornier 228s with hydrogen fuel cell powertrains, targeting a certification-ready system for 9- to 19-seat aircraft by approximately 2028 or 2029.

The efficiency advantage is often overlooked. A modern proton exchange membrane fuel cell achieves 55 to 60 percent efficiency. A gas turbine, even a good one, manages 35 to 40 percent thermal efficiency. So even with bulky tanks, the fuel cell extracts more useful work from every kilogram of hydrogen carried.

What About Burning Hydrogen Directly?

The second approach is hydrogen combustion, the path Airbus is pursuing with their ZEROe program. Instead of converting hydrogen to electricity, you burn it directly in a modified gas turbine. This leverages decades of existing gas turbine engineering — you’re changing the fuel, not reinventing the propulsion architecture.

Rolls-Royce and EasyJet partnered to ground-test a modified AE 2100 turboshaft engine on green hydrogen in late 2023, with continued work since. The engine ran and produced thrust. The fundamental combustion physics work.

The problems are specific. Hydrogen burns much hotter than kerosene and across a wider range of fuel-to-air ratios. Combustion chambers must be redesigned to handle different flame characteristics and manage NOx emissions, since burning anything in air at high temperatures produces nitrogen oxides from atmospheric nitrogen. Hydrogen combustion is much cleaner than jet fuel, but it is not zero-emission like a fuel cell.

Airbus targets a hydrogen-powered commercial aircraft in service by 2035 — a timeline that is ambitious, not because the propulsion is impossible, but because of everything else that must change alongside it.

How Does Hydrogen Change Aircraft Design?

A hydrogen-powered airplane cannot simply bolt different tanks onto an existing airframe. Liquid hydrogen tanks must be cylindrical or spherical to handle cryogenic pressures and insulation. They don’t fit into conventional wing structures the way jet fuel does today.

The most promising configurations place hydrogen tanks in the fuselage, either behind the cabin or in a redesigned fuselage cross-section. Airbus has shown concepts using a blended wing body, which provides more internal volume for tanks without the drag penalty of a wider conventional fuselage.

For general aviation, a different model has been explored. Companies like Universal Hydrogen worked on modular hydrogen capsule systems — standardized capsules filled at central facilities and trucked to airports, similar to propane tank distribution. Ground crew loads full capsules, the aircraft flies, and empties are swapped on landing. This approach sidesteps the single biggest obstacle: infrastructure.

There is currently essentially zero hydrogen refueling capability at airports worldwide. Building liquid hydrogen storage, transfer systems, and safety protocols at thousands of airports is a monumental undertaking, with some estimates putting the global infrastructure investment needed at hundreds of billions of dollars.

What Are Realistic Timelines for Hydrogen Aircraft?

Small aircraft (9–19 seats), regional routes under 500 miles: Hydrogen fuel cell propulsion is plausible within the next five to seven years for initial certifications. ZeroAvia and several other companies are making real progress, and both the FAA and EASA are actively developing certification frameworks for hydrogen systems.

Narrowbody commercial aircraft: Mid-2030s at the earliest, with a realistic possibility of slipping to 2040. Certifying a fundamentally new fuel system on a commercial transport aircraft will be extraordinarily thorough.

Widebody long-haul aircraft: The physics become very difficult. The volume penalty for hydrogen storage means either accepting significantly shorter range or completely redesigning the aircraft. This is likely a 2045-plus conversation.

Who Is Investing in Hydrogen Aviation?

The investment signals are significant. Airbus has committed over €2 billion to their ZEROe program. The European Union’s Clean Aviation Joint Undertaking is funding multiple hydrogen aviation research projects. In the United States, the Department of Energy has funded hydrogen production research, though less specifically targeted at aviation. Major airports including Changi (Singapore) and Schiphol (Amsterdam) have begun feasibility studies for hydrogen infrastructure.

What Is the Green Hydrogen Problem?

Hydrogen production today overwhelmingly relies on steam methane reforming of natural gas, a process that produces significant CO₂ emissions. For hydrogen aviation to deliver on its environmental promise, the industry needs green hydrogen — produced by splitting water using renewable electricity.

Green hydrogen currently costs roughly three to five times more than conventional hydrogen. Electrolyzer technology is improving and costs are declining, but the gap remains substantial.

What Does Hydrogen Mean for Pilots?

In the near term, nothing changes in the cockpit. But for pilots thinking about the next two decades, hydrogen is worth close attention. The transition will likely start with regional routes where limited range isn’t a penalty but a natural fit — Hawaiian inter-island flights, Nordic short-haul, Southeast Asian island hopping.

Operational considerations will include new fuel system management procedures, different weight and balance calculations (hydrogen is much lighter than jet fuel per unit of energy), and potentially different emergency procedures related to hydrogen venting or fuel cell malfunctions. None of this is insurmountable. Pilots adapted to glass cockpits and fly-by-wire. They will adapt to hydrogen if the technology delivers.

The meaningful distinction hydrogen offers: it doesn’t require flying less or accepting dramatically shorter ranges. It requires changing what gets burned — and eventually, with fuel cells, eliminating combustion entirely.

Key Takeaways

• Hydrogen carries roughly 33,000 Wh/kg by weight, nearly three times jet fuel’s energy density, but requires about four times the volume even as a cryogenic liquid
• Fuel cells (ZeroAvia’s approach) offer 55–60% efficiency and true zero emissions; direct combustion (Airbus’s approach) leverages existing turbine expertise but still produces NOx
• Regional 9–19 seat aircraft could see initial hydrogen certifications by the late 2020s; commercial narrowbodies are a mid-2030s prospect at earliest
• Airport infrastructure is the biggest non-technical barrier — essentially no hydrogen refueling capability exists at airports today, and buildout will cost hundreds of billions globally
• Green hydrogen production must scale dramatically and drop in cost for the environmental case to hold up