Hydrogen-powered aircraft

Energy density of fuels: horizontal per mass, vertical per volume.

Hydrogen has a specific energy (120 MJ/kg)[2] that is 2.8 times higher than traditional jet fuel (43 MJ/kg).[3] Cryogenic liquid hydrogen has a 4.1 times lower energy density (8.5 MJ/L compared to 35 MJ/L), but needs to be cooled at 20 K (−253 °C) and kept in insulated but low-pressure tanks; while gaseous hydrogen has a lower energy density, needs high pressure tanks reaching up to 700 bar, often in carbon fiber, but can be kept at usual temperatures.

If hydrogen is available in quantity from low-carbon power such as wind or nuclear, its use in aircraft will produce fewer greenhouse gases (water vapor and a small amount of nitrogen oxide) than current aircraft. Currently very little hydrogen is produced using low-carbon energy sources, and there are several serious obstacles to the use of hydrogen in aircraft and other vehicles.[4] Due to the way it is produced, and the relative inefficiencies of its production given current technology, hydrogen is more expensive than fossil fuels.

Liquid hydrogen is one of the best coolants used in engineering, and precooled jet engines have been proposed to use this property for cooling the intake air of hypersonic aircraft, or even for cooling the aircraft's skin itself, particularly for scramjet-powered aircraft.[5]

Liquid hydrogen is 4.1 times less energy dense than kerosene based jet-fuel; and storing the liquid hydrogen needs a pressurised tank with a minimal surface for minimal thermal insulation weight, leading towards cylindrical tanks in the fuselage rather than wet wings in conventional aircraft. This leads to a longer, wider fuselage; adding more skin friction drag and wave drag due to the extra wetted area; extra tank weight; and weight and balance variations during flight.

As hydrogen specific energy is 2.8 times higher than jet-fuel, an hydrogen aircraft would need 2.8 times less fuel weight for the same range, ignoring the repercussion of added volume and tank weight. As airliners have a fuel fraction of the MTOW between 26% for medium-haul to 45% for long-haul, maximum fuel weight could be reduced to 9% to 16% of the MTOW, but this would be the case only for the unusual maximum fuel case of the payload-range tradeoff. The efficiency of a hydrogen-fueled aircraft is a trade-off of the larger wetted area, lower fuel weight and added tank weight, varying with the aircraft size.

Hydrogen aircraft using a fuel cell design are zero emission in operation, whereas aircraft using hydrogen as a fuel for a jet engine or an internal combustion engine are zero emission for CO2 but not for NOx: the burning of Hydrogen in air leads to the production of NOx. I.e. the H2 + O ->H20 reaction in a nitrogen-rich environment also causes the production of NOx.[6]

In February 1957, a Martin B-57B of the NACA flew on hydrogen for 20 min for one of its two Wright J65 engines rather than jet fuel.[7] On 15 April 1988, the Tu-155 first flew as the first hydrogen-powered experimental aircraft,[8] an adapted Tu-154 airliner.

The European Union's research project in cooperation with Airbus and 34 other partner companies dubbed CRYOPLANE assessed the technical feasibility, safety, environmental compatibility and economic viability of using liquid hydrogen as an aviation fuel. This was concluded in 2002 (with the final report published in 2003).[9]

In 2008, the Boeing Fuel Cell Demonstrator achieved straight-level flight on a manned mission powered by a hydrogen fuel cell.[10]

Boeing converted a 2-seat Diamond DA20 to run on a fuel cell.[11] It first flew on April 3, 2008.[12] The Antares DLR-H2 is a hydrogen-powered aeroplane from Lange Aviation and the German aerospace center.[13] In July 2010, Boeing unveiled its hydrogen powered Phantom Eye UAV, that uses two converted Ford Motor Company piston engines.[14]

In 2010, the Rapid 200FC concluded six flight tests fueled by gaseous hydrogen. The aircraft and the electric and energy system was developed within the European Union's ENFICA-FC project coordinated by the Politecnico di Torino.[15] Hydrogen gas is stored at 350 bar, feeding a 20 kW (27 hp) fuel cell powering a 40 kW (54 hp) electric motor along a 20 kW (27 hp) lithium polymer battery pack.

On January 11, 2011, an AeroVironment Global Observer unmanned aircraft completed its first flight powered by a hydrogen-fueled propulsion system.[16]

the hydrogen fuel cell-powered HY4 made its first flight in 2016

Developed by Germany’s DLR Institute of Engineering Thermodynamics, the DLR HY4 four-seater was powered by a hydrogen fuel cell, its first flight took place on September 29, 2016.[17] It has the possibility to store 9 kg (20 lb) of hydrogen, 4x11 kW fuel cells and 2x10 kWh batteries.[18]

In September 2020, Airbus presented three ZEROe hydrogen-fuelled concepts aiming for commercial service by 2035:[19] a 100-passenger turboprop, a 200-passenger turbofan, and a futuristic design based around a blended wing body.[20] The aircraft are powered by gas turbines rather than fuel cells.[21]

In March 2021, Cranfield Aerospace Solutions announced the Project Fresson switched from batteries to hydrogen for the nine-passenger Britten-Norman Islander retrofit for a September 2022 demonstration.[22] Project Fresson is supported by the Aerospace Technology Institute in partnership with the UK Department for Business, Energy & Industrial Strategy and Innovate UK.

As of 2021, ZeroAvia is actively developing aircraft with a fuel cell powertrain "capable of carrying up to 20 passengers about 350 nautical miles".[20]

In December 2021, the UK Aerospace Technology Institute (ATI) presented its FlyZero study of cryogenic liquid hydrogen used in gas turbines for a 279 passengers design with 5,250 nmi (9,720 km) of range.[23] ATI is supported by Airbus, Rolls-Royce, GKN, Spirit, General Electric, Reaction Engines, Easyjet, NATS, Belcan, Eaton, Mott MacDonald and the MTC.

In August 2021 the UK Government claimed it was the first to have a Hydrogen Strategy. This report included a suggested strategy for hydrogen powered aircraft along with other transport modes.[24]

Pratt & Whitney wants to associate its geared turbofan architecture with its Hydrogen Steam Injected, Inter‐Cooled Turbine Engine (HySIITE) project, to avoid carbon dioxide emissions, reduce NOx emissions by 80%, and reduce fuel consumption by 35% compared with the current jet-fuel PW1100G, for a service entry by 2035 with a compatible airframe.[25] On 21 February 2022, the US Department of Energy through the OPEN21 scheme run by its Advanced Research Projects Agency-Energy (ARPA-E) awarded P&W $3.8 million for a two-year early stage research, to develop the combustor and the heat exchanger used to recover water vapour in the exhaust stream, injected into the combustor to increase its power, and into the compressor as an intercooler, and into the turbine as a coolant.[25]

In February 2022, Airbus announced a demonstration of a liquid hydrogen-fueled Turbofan, with CFM International modifying the combustor, fuel system and control system of a GE Passport, for a first flight expected within five years, mounted on a fuselage pylon on an A380 prototype.[26]

In March 2022, FlyZero detailed its three concept aircraft:

• the 75-seat FZR-1E regional airliner has six electric propulsors powered by fuel cells, a size comparable to the ATR 72 with a larger fuselage diameter at 3.5 m (11 ft) compared to 2.8 m (9 ft 2 in) to accommodate hydrogen storage, for a 325 kn (601 km/h) cruise and a 800 nmi (1,480 km) range;
• its FZN-1E narrowbody has rear-mounted hydrogen-burning turbofans, a T-tail and nose-mounted canards, a 10 m (33 ft) longer fuselage than the Airbus A320neo becoming up to 1 m (3 ft 3 in) wider at the rear to accommodate two cryogenic fuel tanks, and a larger wingspan requiring folding wing-tips for a 2,400 nmi (4,400 km) range with a 450 kn (830 km/h) cruise;
• the small widebody FZM-1G is comparable to the Boeing 767-200ER, flying 279 passengers over 5,750 nmi (10,650 km), with a 6 m (20 ft) wide fuselage diameter closer to the A350 or 777X, a 52 m (171 ft) wingspan within airport gate limits, underwing engines and tanks in front of the wing.[27]

• Electric aircraft
• Emerging aviation fuels

• Bjorn Fehrm (February 7, 2020). "Bjorn's Corner: Why e in ePlane shall stand for environment, Part 8". Leeham News.
• Dan Thisdell (9 July 2020). "Forget batteries, is hydrogen the holy grail for carbon-free commercial aviation?". Flightglobal.