Flying on Vapour: How the HYDRA-2 Drone Brings Liquid Hydrogen Aviation a Step Closer
Photo by NLR
On an August morning in the Netherlands, engineers at the Netherlands Aerospace Centre (NLR) gathered on the edge of a quiet airfield. Their mission was deceptively modest: launch a drone, watch it fly, bring it back down again. Yet the aircraft sitting on the tarmac was anything but ordinary. This was HYDRA-2, a one-of-a-kind research drone powered not by kerosene, batteries, or even compressed hydrogen, but by cryogenic liquid hydrogen cooled to nearly minus 253 degrees Celsius.
When it lifted off, HYDRA-2 wrote a small but significant line in aviation history: the first Dutch drone to fly solely on liquid hydrogen. To the untrained eye, it may have looked like a routine demonstration. To those watching closely, it signalled the start of a new chapter in the pursuit of clean, long-endurance flight.
Why Liquid Hydrogen Matters
Hydrogen has long been touted as aviation’s best shot at slashing emissions. Burned in turbines or used in fuel cells, it produces no carbon dioxide — only water vapour. But there’s a catch: how you store it makes all the difference.
Most existing hydrogen-powered drones or small aircraft rely on compressed hydrogen gas stored in heavy metal cylinders at 350 to 700 bar. That approach works, but the trade-off is endurance. Even at high pressures, compressed hydrogen simply cannot match the energy density of liquid hydrogen.
Enter HYDRA-2. By cooling the gas to cryogenic temperatures, NLR’s engineers condensed hydrogen into liquid form, achieving five times the specific energy of lithium batteries and significantly higher than compressed gas. That means more flight time, more payload, and more scope for real missions.
The Engineering Behind HYDRA-2
At the heart of HYDRA-2 lies its vacuum-jacketed composite tank — a feat of materials science in its own right. Built from carbon fibre-reinforced polymers and insulated with multiple layers, the tank can safely hold liquid hydrogen at cryogenic temperatures without excessive “boil-off”, the inevitable warming and evaporation that plagues hydrogen storage.
Compared with traditional metal cryogenic tanks, this design is roughly 30 percent lighter, a saving that translates directly into range and performance. The fuel system feeds hydrogen into a proton exchange membrane fuel cell (PEMFC), which in turn generates electricity for the drone’s propulsion system. The only emission: water vapour.
Yet what makes the system truly robust is the integration of sensors and thermal management. Temperature, pressure, and flow rates are monitored in real time. An intelligent control system adjusts output dynamically, balancing efficiency with stability. Waste heat from the fuel cell doesn’t go unused either — it is captured and repurposed to regulate the system’s thermal balance.
A Leap Beyond HYDRA-1
This achievement builds on earlier groundwork. Back in 2019, NLR and partners developed HYDRA-1, a compressed-gas hydrogen drone. While it demonstrated the potential of fuel cell propulsion, flight times were modest. HYDRA-2 changes the equation, unlocking longer missions and more demanding use-cases.
The difference is not incremental; it is transformative. Where HYDRA-1 might be compared to a proof-of-concept, HYDRA-2 is a practical research platform pointing toward the future of emission-free aviation.
Collaboration Across Dutch Science and Industry
HYDRA-2 did not emerge from a single lab. It is the product of a uniquely Dutch ecosystem of collaboration.
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NLR (Netherlands Aerospace Centre) led the project, applying decades of experience in flight testing, safety protocols, and systems integration.
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Cryoworld, a Dutch cryogenic engineering company, developed the specialised lightweight storage tank.
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TU Delft’s AeroDelft student team contributed knowledge from their ongoing efforts to build a hydrogen-powered aircraft, the Phoenix.
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NLR’s Quick Response Drone Facility provided the operational test environment, ensuring that the drone could be flown safely and that data could be collected in detail.
This combination of public research, private engineering expertise, and academic ingenuity is characteristic of the Netherlands’ innovation landscape — pragmatic, collaborative, and laser-focused on scalable outcomes.
What It Proves
HYDRA-2 demonstrates three critical things:
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Technical Feasibility: Liquid hydrogen can be stored, handled, and used safely in flight. This moves the concept from whiteboard diagrams to field-tested hardware.
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Endurance Potential: With higher energy density, drones can remain airborne longer, carry more payload, or both. For logistics, surveillance, and mapping, that’s a game-changer.
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Scalability: The tank and fuel cell system can be adapted for larger aircraft. Indeed, NLR envisions a two-seater demonstrator as the next logical step.
Towards Manned Flight
The vision doesn’t stop with drones. HYDRA-2 is part of a deliberate roadmap leading towards manned hydrogen-powered aviation. A two-seater experimental aircraft, already on the drawing board, would provide a platform to test not only endurance but also passenger safety, certification hurdles, and the realities of refuelling.
This effort dovetails with European initiatives such as Clean Aviation and the TULIPS project, which are developing hydrogen refuelling and storage infrastructure at airports including Rotterdam The Hague Airport. Without ground facilities, airborne progress cannot scale.
Regulatory and Policy Impact
Beyond engineering, HYDRA-2 generates something equally important: data for regulators. Certifying hydrogen systems for aviation is uncharted territory. International bodies such as ICAO, EASA, and national agencies need evidence of safety and performance to craft standards. Every flight test, every telemetry log, adds a piece to that puzzle.
Policy makers, too, are watching closely. With the European Union committed to climate neutrality by 2050, and aviation emissions a stubborn obstacle, hydrogen offers a tangible pathway. Demonstrators like HYDRA-2 serve as proof points in the political argument for investment, infrastructure, and regulation.
Global Context
The Netherlands is not alone in this race. Airbus is developing hydrogen concepts under its ZEROe programme, while start-ups in the United States and the UK are testing fuel cell systems for regional aircraft. Yet liquid hydrogen remains a challenging frontier, fraught with engineering, cost, and safety questions.
By flying HYDRA-2, the Dutch have carved out a position of leadership in this field. It is a reminder that disruptive aviation technology need not emerge solely from the aerospace giants. A small nation with a strong research culture can move the needle on global sustainability challenges.
Voices from the Team
Dr Roel Benthem, senior researcher at NLR, summed it up in a recent technical note:
“With HYDRA-2 we’ve shown that liquid hydrogen is not just a laboratory curiosity. It can be integrated into a flight-worthy system, operated safely, and controlled dynamically in real-world conditions.”
Cryoworld’s engineers emphasise the broader relevance. The tank technology developed for HYDRA-2 could find applications in other sectors where cryogenic storage is critical, from shipping to space exploration.
Challenges Ahead
Of course, challenges remain. Liquid hydrogen is difficult to produce sustainably and requires energy-intensive liquefaction. Unless the hydrogen itself is generated from renewable sources, the climate benefits are diluted. Handling cryogenic fluids also demands robust infrastructure and training, raising costs in the short term.
Yet as with electric vehicles, early demonstrators often look impractical until scale, policy, and market dynamics shift. HYDRA-2 is less about immediate commercial viability and more about proving that a different aviation future is technically achievable.
The Road Forward
The coming months will see extended flight testing of HYDRA-2 under varying payloads and mission profiles. Each sortie generates invaluable data for engineers and regulators alike. The next milestone will be the development of a two-seater manned demonstrator, bridging the gap between drone and commuter aircraft.
Parallel efforts are underway at Rotterdam The Hague Airport, where partners are trialling hydrogen refuelling and storage solutions as part of the TULIPS initiative. If infrastructure and aircraft development proceed hand-in-hand, hydrogen aviation could move from experimental to mainstream far faster than sceptics predict.
A Symbolic Flight
On that August morning, HYDRA-2’s take-off may have seemed routine. But symbolism matters. In a sector often criticised for moving too slowly on climate, here was a clear signal: alternatives are not only imaginable, they are taking to the skies.
If aviation is to meet its climate commitments, radical changes are required. HYDRA-2 doesn’t provide all the answers, but it proves one thing unequivocally: flying on vapour is no longer a dream- it is actually happening.