The HYDRA II, an innovative hydrogen drone from Royal NLR, has made the first successful flights using liquid hydrogen as an energy carrier. This is an important step in the development of more sustainable aviation to make hydrogen technology truly suitable for commercial applications.
The HYDRA II, a drone flying on liquid hydrogen, recently made its first successful test flights over the NLR site in Marknesse. This is a first, as it is the first time a device has flown in Dutch airspace using liquid hydrogen as an energy carrier. The flight lasted several minutes and not only demonstrated the operation of the technology using liquid hydrogen as an energy carrier, but also shows that we know how to safely handle everything involved when working with liquid hydrogen.
The electric motors of the hybrid-powered drone were supplied with electricity via a fuel cell, with a battery also providing support to ensure the necessary thrust.
Unmanned & autonomous
Joost Vreeken, NLR programme leader 'Unmanned & Autonomous': "With drones, the use of hydrogen actually comes into its own best when flying long distances. A VTOL, which stands for Vertical Take-off and Landing, will take off vertically as in our trial. This may involve using the batteries first because that phase requires a relatively large amount of power. Then, like an aircraft, such a drone will fly a horizontal path. Thereby, the use of hydrogen is ideal. In this configuration, it was mainly about demonstrating how the technology works during a short hover flight."
Challenge to become more sustainable
Aircraft need to reduce their emissions to limit their contribution to climate change. In doing so, it is important to look at alternatives to using paraffin. Hydrogen is seen as one of the solution options in making aviation more sustainable. But its use is new in (commercial) aviation and requires modifications in aircraft as well as on the ground. Drones offer an ideal test platform in that energy transition in aviation. They are small, manageable in terms of complexity and relatively cheap to buy.
NLR's contribution
NLR flew the HYDRA I on gaseous hydrogen in 2019. A lot of research has been done in the meantime, and so following that flight, NLR has now flown its successor, which uses liquid hydrogen as its energy carrier. For the HYDRA II, the hydrogen is stored in a vacuum-insulated aluminium tank at the bottom of the drone. This tank was developed by NLR in collaboration with a partner. The tank is not actively cooled; due to the heating of the cooled hydrogen, hydrogen gas gradually escapes from the tank. This is eventually fed into the fuel cell where it is turned into electricity. Inside the tank are sensors and heat elements that allow the tank to deliver the right amount of gaseous hydrogen to the fuel cell.
Innovation through ecosystem
To reduce aviation emissions, NLR says 'green' hydrogen is a promising alternative to the current use of paraffin. Tineke van der Veen, CEO of NLR: "Provided hydrogen is produced in a sustainable way, it can contribute to reducing the climate impact of fossil fuels. This is because water (vapour) is the only thing the vehicle, in this case the drone, emits. We see many parties at home and abroad working hard to make commercial applications of this a success."
Collaboration
"Parallel to this drone project, we ourselves are also working closely with Dutch parties such as Cryoworld and zepp.solutions. Here we are working on the integration of liquid hydrogen into our electrically powered aircraft, a Pipistrel Velis Electro."
Vreeken: "For NLR, at least, the milestone achieved with the HYDRA II is of great importance because it not only demonstrates that we have mastered the technology well, but also that we know how to safely go through the logistically complex processes. After all, there is a lot involved. Just think of the flight plan with safety procedures, the health and safety aspects, and of course the logistics chain from delivery, transport, storage to 'refuelling' the drone."
Parties outside the Netherlands have also shown that flying on liquid hydrogen is possible. Germany's H2fly, for example, conducted a test flight in 2023 that also involved Slovenian aircraft manufacturer Pipistrel.
Hydrogen as energy carrier
Hydrogen (H2) is the simplest, lightest and most abundant element known. At room temperature, hydrogen is a non-toxic colourless and odourless gas. At a temperature below -252.77 °C, hydrogen becomes liquid. This is close to absolute zero (0 kelvin). The drone uses hydrogen as an energy carrier (not an energy source): in a fuel cell, hydrogen gas is converted to water (H2O) along with oxygen from the outside air. That process releases energy in the form of electricity and waste heat.
Weight, or rather 'mass', obviously plays a big role in aviation.
The major advantage of hydrogen is its relatively high energy content per mass (kWh/kg). However, the mass density (kg/litre) and energy content (kWh/litre) of hydrogen at atmospheric pressure is very low. If you cool the hydrogen to around -253°C, however, the energy content is significantly higher, but still a factor of four lower than, say, diesel or paraffin.
Safety first
Energy sources - or in the case of hydrogen 'energy carrier' - always involve safety risks. Provided it is used and stored safely, hydrogen is not necessarily more dangerous than other fuels in this regard. It does require specific safety measures and procedures because it ignites relatively easily and requires less ignition energy than other fuels, such as petrol or natural gas.
The HYDRA II hydrogen drone is experimentally approved and NLR pilots are allowed to fly it without a 'Special Certificate of Airworthiness' (S-BvL). In addition, airspace above NLR may be closed to other air traffic.
Opening photo: Maiden flight in preparation HYDRA II In NOP, Marknesse
Read also Drones transport blood samples between London hospitals
