The HYDRA II, a drone that flies on liquid hydrogen, has made its first successful test flights over the NLR site in Marknesse. This is a Dutch first, as it is the first time an aircraft has flown in Dutch airspace using liquid hydrogen as a fuel source. The flight lasted several minutes and not only demonstrated the functionality of the technology using liquid hydrogen as a fuel carrier but also showed that it can be handled safely.
The electric motors of the hybrid-powered drone were powered by electricity generated by a fuel cell, with a battery providing additional support to ensure the required thrust.
Joost Vreeken, NLR programme leader for ‘Unmanned & Autonomous‘: “For drones, the use of hydrogen comes into its own when flying long distances. A VTOL (Vertical Take-off and Landing) initially takes off vertically, using batteries as that phase requires a relatively high power output. Once airborne, it transitions to horizontal flight, similar to a conventional aircraft, and that’s where hydrogen comes in. With the current configuration, we specifically focused on demonstrating the functionality of the technology during a short hover flight.”
Sustainability challenge
Aircraft need to reduce their emissions to minimise their contribution to climate change. To achieve this, it’s essential to explore alternatives to kerosene. Hydrogen is seen as one of the solutions for making aviation more sustainable. However, its use in (commercial) aviation is new and requires adjustments to aircraft and ground infrastructure. Drones provide an ideal test platform for this energy transition in aviation, as they are small, not too complex, and relatively inexpensive to purchase.
NLR’s contribution
NLR flew the HYDRA I on gaseous hydrogen in 2019. Since then, much research has been conducted, and NLR has now also flown the successor, the HYDRA II, which uses liquid hydrogen as a fuel carrier. For the HYDRA II, the hydrogen is stored in a vacuum-insulated aluminium tank under the drone, which was developed in collaboration with a partner. The tank is not actively cooled; instead, heated hydrogen slowly escapes from the tank and is eventually fed into the fuel cell, where it’s converted into electricity.
Innovation through ecosystem
To reduce aviation’s emissions, ‘green’ hydrogen is a promising alternative to kerosene, according to NLR. Tineke van der Veen, CEO of NLR: “If hydrogen is produced sustainably, it can contribute to reducing the impact of fossil fuels on the climate, as water (vapour) is the only emission from the vehicle, in this case, the drone. We see that many organisations, both domestic and internationally, are working hard to make commercial applications a success.”
“In parallel with this drone project, we are also working closely with Dutch companies such as Cryoworld and zepp.solutions to integrate liquid hydrogen into our electrically powered Pipistrel Velis Electro.”
Vreeken: “For NLR, the milestone achieved with the HYDRA II is of great importance because it not only shows that we have mastered the technology but also that we can safely manage the complex logistical processes involved. This includes the flight plan with safety procedures, technical aspects related to the Working Conditions Act, and the logistical chain from delivery to storage and ‘refuelling’ of the drone.”
Outside of the Netherlands, other parties have also demonstrated that flying on liquid hydrogen is possible. For example, the German company H2fly conducted a test flight in 2023, in which the Slovenian aircraft manufacturer Pipistrel was involved as well[1].
Hydrogen as an 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 combined with oxygen from the air to produce water (H2O), releasing energy in the form of electricity and residual heat.
Weight, or rather ‘mass’, plays a crucial role in aviation. The significant advantage of hydrogen is its relatively high energy content per unit of mass (kWh/kg). However, the mass density (kg/litre) and energy content (kWh/litre) of hydrogen at atmospheric pressure are relatively low. If you cool hydrogen to around -253 °C, its energy content increases significantly but is still about four times lower than that of diesel[2] or kerosene.
Safety first
Energy sources – or, in the case of hydrogen, ‘energy carriers’ – always involve safety risks. As long as it’s used and stored safely, hydrogen is not inherently more hazardous than other fuels. However, it requires specific safety measures and procedures because it can ignite relatively easily and requires less energy to ignite than other fuels like petrol or natural gas. The HYDRA II hydrogen drone has been experimentally certified, and NLR pilots can fly it without a ‘Special Airworthiness Certificate’ (S-BvL). Additionally, the airspace above NLR can be closed to other air traffic.
[1] https://www.youtube.com/watch?v=lFJWRXqB89Q
[2] https://www.waterstofnet.eu/nl/waterstof/wat-is-waterstof#default (in Dutch)
