The Last Missing Piece of Hydrogen Aviation

The first jet airliners did not create the Jet Age by themselves. They required stronger runways, new maintenance regimes, high-volume fuel storage, airport hydrants and an international supply of tightly specified kerosene. Later, long-range twin-engined aircraft did not conquer the Atlantic merely because their engines improved. ETOPS turned reliability data, maintenance, diversion planning and regulation into a system airlines could trust.

Aviation history is full of machines that were ready before the world around them.

LNG became a global industry when engineers mastered liquefaction trains, loading arms, insulated ships and terminal operations, not when somebody established that methane could be made cold. Offshore wind scaled when installation vessels, ports and maintenance logistics caught up with turbines. Semiconductor leadership rests as much on process recipes, yield and supplier discipline as on chip architecture.

Every important technology eventually reaches the same unphotogenic stage. The prototype works. The constraint moves elsewhere.

Hydrogen aviation is approaching that point. ZeroAvia flew a 19-seat Dornier 228 testbed in 2023 using compressed gaseous hydrogen. H2FLY subsequently completed four flights of its HY4 demonstrator using liquid hydrogen, including one lasting more than three hours. Rolls-Royce and easyJet have run a converted AE 2100-A regional engine on hydrogen. Airbus has revised ZEROe’s timing and configuration, sensibly, but is still developing a liquid-hydrogen fuel-cell concept.

None of this means that a commercially useful hydrogen airliner is finished. Certification, cost, low-carbon fuel supply and non-CO2 climate effects remain difficult. It does mean that another flight proving hydrogen can propel an aircraft tells us progressively less.

The more useful question is whether an airport can refuel one.

Not once, behind barriers, with the project team watching every valve, but several times a day in rain and crosswinds, while baggage is loaded, catering trucks are moving and an airline operations controller is asking why Gate 12 is six minutes late. Aviation does not commercialise when a machine flies. It commercialises when the abnormal becomes routine.

A small project points to a larger change

The Department for Transport’s HyPRIME announcement on 23 July is interesting for precisely this reason. The project is not large by aerospace standards. Its maximum indicative cost is £3,417,518, of which the department will provide up to £2,124,651. ZeroAvia leads a consortium including ULEMCo, GeoPura, Bristol Airport and Birmingham Airport.

The consortium will demonstrate a mobile liquid-hydrogen refueller in a live airport environment. It will test a normal commercial turnaround and use boil-off hydrogen in an aircraft tug and power unit. More importantly, it will document procedures, costs and operational trade-offs.

That sounds less exciting than a new aircraft. It is also closer to the real bottleneck.

HyPRIME sits alongside two other awards in the same competition. CHOSAN, with a project cost of £1,722,191 and DfT support of £1,153,790, plans liquid-hydrogen flight operations on regional routes of up to 500 kilometres while developing the operational and regulatory framework around them. A separate £135,237 study led by Equilibrion, supported by £93,164 from DfT, will develop a liquid-hydrogen supply and infrastructure architecture for Bristol Airport.

These are not isolated demonstrations. They are early pieces of an operating system.

Why make hydrogen so inconvenient?

Liquid hydrogen exists in aircraft design because gaseous hydrogen eventually becomes too bulky and too heavy to contain.

Hydrogen’s attraction is gravimetric energy density. On a lower-heating-value basis it contains about 120 megajoules per kilogram, compared with roughly 44 for petrol and a similar figure for jet fuel. An aircraft is acutely sensitive to mass, so three times the energy per kilogram is a serious advantage.

Volume tells the opposite story. Liquid hydrogen contains only about 8 megajoules per litre, against roughly 32 for petrol or kerosene. Even after cooling to its normal boiling point of 20 kelvin, or -252.8°C, hydrogen needs about four times the fuel volume for the same energy. Its density is approximately 71 kilograms per cubic metre; hydrogen gas at 700 bar and room temperature reaches about 40 before the tank is counted. A complete 700-bar storage system performs much worse because carbon-fibre pressure vessels are not noted for weightlessness.

Compressed gas is a rational starting point for small aircraft. It avoids liquefaction and supports modest ranges. Scale the aircraft, however, and the cylinders consume payload and cabin volume. Designers cannot put conformal hydrogen tanks in the wings as they do with kerosene. Efficient cryogenic tanks prefer spheres or short, fat cylinders; passengers and airport gates have other preferences.

Liquid hydrogen is not a perfect answer. It is the compromise that leaves enough aircraft to be commercially useful.

That is why Airbus, ZeroAvia, GKN Aerospace and others continue to work on cryogenic systems despite the nuisance. H2FLY’s move from compressed to liquid hydrogen doubled the demonstrated maximum range of its HY4 platform from 750 to 1,500 kilometres. Airbus’s current notional ZEROe concept uses two liquid-hydrogen tanks feeding four two-megawatt fuel-cell propulsion units. The Aerospace Technology Institute’s FlyZero work reached the same broad conclusion: liquid hydrogen offers the combination of mass and range needed to take zero-carbon flight beyond the smallest aircraft, whether the hydrogen ultimately feeds a fuel cell, a gas turbine or a hybrid system.

Hydrogen is only low-carbon if its production is low-carbon, and liquefaction adds energy consumption and cost. Direct hydrogen does not make sustainable aviation fuel irrelevant; drop-in fuels will remain essential for the existing fleet and routes where new infrastructure is impractical. Liquid hydrogen will not replace kerosene everywhere. Where direct hydrogen flight makes sense, however, cryogenics is unlikely to be optional.

A refuelling operation is a thermodynamic event

Conventional aviation fuel has spent a century becoming boring. Jet A-1 can sit in a tank, pass through pipelines and hydrants, and be transferred through equipment that ground crews know intimately. The airport safety system, fuel-quality regime, maintenance schedule and commercial contract have co-evolved around it.

Liquid hydrogen arrives at the stand at -253°C and immediately starts negotiating with its surroundings.

Warm pipework must be chilled before useful transfer begins. Heat entering the system creates vapour; pressure and temperature determine whether the flow remains liquid or begins to flash. Pumps must avoid cavitation. Lines have to be purged so air and moisture cannot freeze inside them. Materials contract, seals face thermal cycling, and trapped liquid must never be allowed to warm without pressure relief. A cold release may initially hug the ground before the hydrogen warms and rises. Hydrogen is non-toxic and disperses quickly in open air, but it has a wide flammability range, low ignition energy and a flame that can be difficult to see. Cryogenic contact adds hazards that a compressed-gas operator never encounters.

None of these problems is mysterious. NASA, industrial-gas companies and launch operators manage them. The difficulty is doing so at airline tempo, near passengers, buildings, baggage equipment and other fuel operations.

Turnaround time is not merely a faster pump. Today, refuelling can run alongside boarding, catering and baggage handling under established procedures. Liquid hydrogen may initially require remote stands or larger exclusion zones. Operators must prove how hoses connect, leak detection triggers isolation, vented gas is managed and failed valves are made safe. Fire crews need different detection equipment and tactics; maintenance teams need intervals based on repeated thermal cycles.

Then there is the fuel bill. Boil-off, flash gas, chill-down losses, transfer time, unused heel, tanker scheduling and storage occupancy all affect the delivered cost per kilogram. NASA has reported that roughly half the liquid hydrogen it bought during the Space Shuttle era was lost through boil-off and other operating modes. Modern integrated refrigeration can reduce or eliminate storage boil-off, but that requires capital, power and control. HyPRIME’s proposal to use boil-off in ground equipment is therefore more than an environmental flourish. It is an attempt to turn a loss stream into a managed energy flow.

This is where the economically important intellectual property will emerge: not simply in a valve or pump, but in the control logic, sequencing, maintenance data and operating envelope that make the whole transfer repeatable.

Mobile first, fixed when the traffic arrives

Early hydrogen aviation has a familiar infrastructure problem. Airlines will not order aircraft without airports able to fuel them; airports cannot justify permanent infrastructure without aircraft demand. Building a liquid-hydrogen hydrant network beneath an apron before either exists would be a splendid way to produce a stranded asset.

Mobile refuelling is the sensible bridge. Liquid hydrogen can arrive by road tanker, enter modest storage and move to the aircraft in a purpose-built refueller. The equipment can serve several stands and generate utilisation data. Airports learn where to put storage and which turnaround activities conflict. Regulators acquire evidence; investors discover whether the business case survives reality.

Mobility is not the final architecture. Once traffic grows, fleets of bowsers introduce congestion, labour and scheduling limits. The 2026 Jet Zero Taskforce hydrogen report expects higher throughput eventually to force a move towards fixed distribution and liquid-hydrogen hydrants at major airports. It also says that such hydrant technologies remain at an early stage worldwide. The progression is therefore logical: mobile equipment buys flexibility and learning; fixed infrastructure buys throughput once demand is visible.

This is another reason to resist treating HyPRIME as a vehicle project. The refueller is an instrument for discovering the future airport.

Britain is not starting from zero

The UK already has more of the system than a casual reading of individual announcements suggests.

Project Acorn at Bristol Airport established a safety case for gaseous-hydrogen refuelling and ground equipment in a live airside environment, with the Civil Aviation Authority reviewing the case and the Health and Safety Executive involved. It taught ground handlers, airport operations staff and fire and rescue teams what hydrogen changes in practice. In 2025, Exeter Airport, Cranfield University, TUI and ULEMCo extended that learning through a hydrogen-powered live aircraft turnaround.

ULEMCo has also led work on hydrogen ground-support vehicles at Teesside International Airport and RAF Leeming. ZeroAvia’s earlier HyFlyer programmes created on-site hydrogen production and fuelling experience around flight testing. The Hydrogen in Aviation Alliance has brought Airbus, Bristol Airport, easyJet, GKN Aerospace, Rolls-Royce, Ørsted and ZeroAvia around the same infrastructure, skills and policy problem.

The cryogenic layer is thickening. The ATI portfolio includes ZeroAvia’s £10.8 million LH-SIFT flight-test programme and £35 million for a second phase of Airbus’s ZEROe Development Centre infrastructure. Rolls-Royce leads three programmes covering hydrogen combustion, powerplant architecture and liquid-hydrogen fuel delivery. Government has committed up to £2.3 billion to ATI through 2035.

This is what an industrial capability looks like before it becomes an industry: several incomplete programmes beginning to share facilities, suppliers, people, test data and regulatory language.

The weak point is clear. The Jet Zero Taskforce report says the UK has no liquid-hydrogen production and depends on imports. Britain has excellent aerospace engineering, but has not operated a domestic liquid-hydrogen economy or a launch complex with NASA’s cadence. Industrial pumps and valves may not be qualified for aerospace duty. Planning authorities have little experience of airport hydrogen storage. The workforce that has repeatedly purged, chilled, transferred and maintained aviation-scale systems is tiny because the job scarcely exists.

That is not national decline. It is a capability gap—and capability gaps can be closed if they are described honestly.

America has continuity; Europe has depth

It is tempting to say that Europe trails the United States by decades. That is too crude.

NASA’s advantage is operational continuity. Kennedy Space Center built 3,200-cubic-metre storage spheres in the 1960s, transferred fuel to Saturn V, carried the practice through 30 years of Shuttle operations and now applies integrated refrigeration to Artemis. Air Products built tonnage-scale liquefaction for the space programme and still holds major NASA supply contracts. Chart Industries and Linde Engineering add commercial capability in cryogenic equipment.

The useful lesson is not that NASA solved boil-off in 1965. It did not. Its own loss record proves otherwise. The advantage lies in thousands of cycles, anomalies, maintenance decisions and design changes accumulated over six decades.

Europe has a substantial inheritance. Ariane 1 used liquid oxygen and hydrogen in 1979; Ariane 5 and 6 continued that capability through ArianeGroup, ESA, CNES and their supply chain. Air Liquide has operated cryogenic ground systems at Kourou for decades. The EU-backed GOLIAT programme now brings Airbus, Chart, H2FLY and airports together to demonstrate high-flow ground operations and develop standards.

Europe does not lack cryogenic expertise. It lacks its broad diffusion into commercial airports. Britain sits one step further away: connected to European aerospace knowledge and industrial-gas suppliers, but without routine domestic liquid-hydrogen production or operations. The strategic task is not to recreate NASA in Gloucestershire. It is to translate specialist cryogenic knowledge into a repeatable civil-aviation service.

Operational knowledge compounds

Patents are easy to count, which is why governments like them. Operational knowledge is harder to photograph and often more valuable.

A semiconductor fabrication process is protected partly by patents, but also by recipes, equipment settings, yield data and the judgement of people who know which small drift will become a ruined wafer two days later. LNG terminals share broadly understood thermodynamics, yet experienced operators command value because loading, boil-off, maintenance and emergency response have been refined over millions of tonnes. Offshore wind developers can buy a turbine; they cannot instantly buy an experienced installation supply chain or a decade of North Sea weather decisions.

Liquid-hydrogen refuelling will develop the same tacit layer. Which coupling design remains reliable after thousands of thermal cycles? How much chill-down loss occurs after a two-hour gap? Which sensor produces false alarms in rain? Can baggage loading continue on the opposite side of the aircraft? When does a mobile system become more expensive than fixed pipework? How should an airline price schedule risk when an airport has only one cryogenic refueller?

The answers become procedures, software, safety cases, training courses, component specifications, maintenance intervals and cost models. Some will be codified into public standards, as they should be. The advantage remains with the organisations that know why the standard says what it says and what to do when reality falls just outside it.

This is why ULEMCo may become more strategically important than its size suggests. Its value in HyPRIME is not simply the fabrication of another specialist truck. The company has already accumulated experience integrating mobile gaseous-hydrogen refuelling, ground-support equipment and live airport operations. Moving into cryogenic transfer will require new partners and new engineering; gaseous experience does not confer liquid-hydrogen mastery by magic. But the organisations that participate in the first hundred awkward operations will learn faster than those arriving for the thousandth routine one.

There is no experienced liquid-hydrogen airport workforce for a competitor to hire wholesale. It has to be created.

What government should buy

Government support for first-of-a-kind infrastructure is often criticised because the assets are small, expensive and commercially premature. That is exactly why public support can be justified—provided it buys reusable capability rather than theatre.

The test is straightforward. Does a project leave behind trained people, qualified components, regulatory evidence, open interfaces, cost data and equipment that can support the next programme? Or does it culminate in a photograph and a container that is quietly removed when the grant ends?

Britain’s industrial strategy has traditionally been more comfortable funding an advanced aircraft component than an airport procedure. The ATI portfolio and the DfT competition suggest a welcome broadening. Yet the pieces still need coordination: access to liquid hydrogen, shared test facilities, a CAA certification pathway, HSE and local-authority guidance, common refuelling interfaces, emergency-response training and procurement that gives pioneer airports a reason to invest before utilisation makes the spreadsheet respectable.

This is not an argument for selecting hydrogen as the universal future of aviation. Batteries will serve some short routes; sustainable aviation fuels will carry much of the existing fleet; improved aircraft and operations remain indispensable. Hydrogen must earn its place on aircraft economics and lifecycle emissions.

It is an argument for recognising where option value now lies. Britain does not need to own every future hydrogen aircraft programme to benefit. It could supply tanks, valves, refuellers, control systems, certification evidence, training and operating services to whichever aircraft succeed. That is a more resilient industrial position than attaching national ambition to a single demonstrator.

The aircraft attracts attention because it leaves the ground. The infrastructure creates value because everything depends on it staying there and working.

History rarely remembers the first prototype for long. It remembers the companies that built the system everybody else had to use.

HyPRIME may be a modest refuelling project. Or it may be the moment Britain began learning how to make hydrogen flight ordinary—and ordinary is where industries begin.