The vision for a midsize, long-haul, hydrogen-based airliner was already shared last December, but last week, the Aerospace Technology Institute (ATI) delivered on its promise to present two more FlyZero concepts for regional and short-haul aircraft this year. They are part of a technology and research project from the UK government. FlyZero's three concepts for hydrogen-based aircraft. “Three next-generation aircraft concepts have been created as part of the FlyZero project to help understand and demonstrate the potential of zero-carbon emission technologies in aviation”, ATI says in a media statement released on March 11. “The concepts also highlight the crucial future technology opportunities for the UK and additionally, each has its own specific objectives. The FlyZero regional aircraft is designed to demonstrate the feasibility of a fuel cell-powered aircraft, while the narrowbody explores how hydrogen could replace carbon-based fuels in the largest and most competitive commercial aviation sector. Finally, the FlyZero midsize assesses the potential for hydrogen to cover long-haul routes, overturning the view that hydrogen aircraft would be limited to shorter routes.” After assessing all zero-carbon energy sources, in its report, the FlyZero project team has identified green liquid hydrogen as the most promising fuel for large commercial aircraft. But the team also says that further research and development is needed on six so-called hydrogen technology bricks: power systems (hydrogen gas turbines), hydrogen storage and fuel systems, fuel cells (to convert hydrogen and oxygen into electricity), thermal management, electric propulsion systems, and aerodynamics. Another seven technology bricks have been identified that impact the design and operations of a zero-carbon aircraft: aircraft systems, sustainable cabin design, materials, manufacturing, design and validation, lifecycle impact, and airports, airlines, and airspace consequences. Regional aircraft For its regional aircraft, the ATI project team took the in-service ATR 72-600 as a reference and at first sight, its concept seems very similar. It has a high wing a T-shaped tailplane. At 28 meters, the FZR-1E concept is just 80 centimeters longer than the ATR, but the fuselage is wider at 3.5 meters versus 2.7 meters for the French/Italian aircraft with the 75 seats in a 3x2 arrangement. Span is 31 meters compared to 27.1 meters for the ATR. Maximum Take-Off Weight (MTOW) is 28.8 tonnes for the concept and 22.8 for the ATR. The regional aircraft concept of FlyZero looks very similar to an ATR 72, but underneath the skin, you can see the differences. (FlyZero) But looking more closely, you see the differences. On the FZR-1E, the passenger cabin extends just aft of the high wing. That’s because, behind the bulkhead, there are two vacuum-insulated storage tanks for liquid hydrogen. Under the cabin floor are the fuel cells that produce electricity for the six electric motors on the wings. Originally, the fuel cells were to be positioned aft in the fuselage, but for space, weight, and weight distribution have been moved to the belly-center position. ATI says that the concept airplane is calculated to require ten percent more energy than the ATR, while the heavy fuel cells are also a negative factor. But it expects further improvements in fuel cell development that improve energy efficiency: “Up to a point, the weight penalty for over-sizing the fuel cell stack is offset by the reduced weight of the thermal management system and reduced fuel consumption. Therefore, the system take-off power to cruise power ratio will differ from a normal combustion aircraft and the key to this is understanding how to manage transient heat loads at the extremes of the operating envelope.” An interesting feature likely to be found on many future hydrogen airliners is a water tank to store water that is produced in the exhaust of the hydrogen fuel cells. Dripping the water onto taxiways and runways at airports was deemed not to be acceptable, as it could make them slippery and impact safety. That’s why a tank is required that store the water until it can be released during flight. The FZR-1E is designed for a maximum range of 800 nautical miles/1.481 kilometers and a speed of 325 knots/602 kilometers per hour, but a typical mission will be 375 nm/695km long. The higher speed of the concept will reduce its block time to 1.6 hours compared to 1.9 hours for the ATR. Block fuel energy will be higher at 56.4 megaJoules versus 52 mJ for the kerosene-powered turboprop. Narrowbody concept The narrowbody concept FZN-1E already looks much different from today’s airliners, with the Airbus A320neo used as a reference. The concept plane has a fuselage with a smaller cross-section at the front and a wider at the back that – again – reflects the packaging of the hydrogen storage tanks. The two engines are at the very back of the aircraft under a T-shaped tailplane. There are also canards on the nose to improve longitudinal trim and the center of gravity. The FZN-1E has a length of 44.8 meters compared to 37.6 meters for the A320neo, a fuselage diameter of a maximum of 5.0 meters versus 4.05 meters for the Airbus, a wingspan of 39.3 meters versus 35.8 meters, and an MTOW of 70.7 tons compared to 79 for the neo. The range is 2.400nm/4.444km but typically just 850nm/1.574km. Maximum seats will be 180. The FlyZero narrowbody concept: wider at the back than at the front, two engines at the very end of the fuselage. (FlyZero) What is striking about the design is that the wings are positioned well back on the fuselage. Like on the regional concept, the cabin runs just aft of where the wings are, with all the hydrogen tanks and systems in the aft section of the (wide) fuselage. The rear-mounted engines are the preferred option as it allows for compact packaging of all the systems compared to wing-mounted engines. This configuration also reduces aircraft noise. The wings, which have an aspect ratio of 13:1 compared to 10:1 on the Airbus, are ‘dry’ without any fuel. They could have folding wingtips, as the span slightly exceeds that of the ICAO category of narrowbody aircraft. Because of its shape and characteristics, the FZN-1E will require four percent less energy than the A320neo: 0.92mJ versus 1.17mJ. The engines, that will have direct hydrogen injection, have a width of 70.5 inches/1.79 meters versus 78 inches/1.98 meters for the Pratt & Whitney GTF or CFM LEAP. The by-pass ratio is 13:1 versus 11:1. The study notes: “On a hydrogen aircraft a lighter, smaller diameter engine reduces aircraft fuel burn even though the engine specific fuel consumption (SFC) is worse. This effect was first identified during a midsize concept trade study and therefore is covered in more detail in that section, but the principle reads across to the narrowbody concept. Smaller diameter engines are also helpful for the narrowbody concept engine position at the rear of the aircraft.” The ATI study says that although this market segment will account for 67 percent of all aircraft acquisitions between 2030 and 2050, the commercial risks of introducing a hydrogen airliner in this segment are too big to make it the entry point. Midsize concept ATI’s midsize widebody concept targets to long-haul segment, which it says could actually be the first for which a hydrogen aircraft could be developed. Although long-haul is the smallest market segment and development costs are higher, “it is possible that a hydrogen aircraft could enter this market segment first, despite relatively higher development costs than a smaller aircraft. This is because an initial hydrogen widebody aircraft route network would cover a relatively small number of major airports.” The study provides more details of the FZM-1G concept that was already unveiled last December. Its seats 279 passengers in a nine-abreast all-Economy configuration. Overall length is 59.6 meters compared to 48.5 meters of the reference plane, a Boeing 767-200ER. The fuselage diameter is 6.0 meters compared to 5.03 meters. The wingspan is 52 meters versus 47.6 meters. MTOW is 150.8 tonnes compared to 179.2 tonnes for the Boeing. The maximum range would be 5.750 nm/10.649km or 5.273nm/9.766km for the 767. A typical mission would be over 3.700nm/6.852km. The required energy would be 1.07 mJ for the hydrogen plane compared to 1.74 mJ for the 767. The FlyZero midsize widebody concept looks like an Airbus A350, but it too has striking differences to accommodate for the liquid hydrogen. (FlyeZero) The project team made a trade-off between fuselage length and diameter when positioning the hydrogen tank. It has opted for a single bigger tank in the aft fuselage and two smaller ‘delta’ tanks in an unpressurized area just in front of the low wings. The position of the delta tanks takes into account regulations, like hydrogen exclusion zones. “Hydrogen storage becomes more weight and volume efficient as the tank diameter increases, which then requires the fuselage diameter to increase. This has the benefit of reducing the overall aircraft length, at the expense of increased drag. The alternative is to increase the fuselage length at the same diameter, but in practice, there is a limit to the maximum aircraft length for design and operational reasons. Relative to kerosene, the net effect of this for a hydrogen aircraft is to reduce the aircraft size where the transition to a widebody configuration makes sense. As a result, the midsize concept has a fuselage diameter comparable to that of large twin aisle aircraft like the A350 or 777X, which is significantly larger than the reference and baseline aircraft.” The two directly-injected turbofans that are placed in a conventional position under the wings have a fan diameter of 101.9 inches/2.58 meters and a bypass ratio of 13:1, smaller than that of the Rolls-Royce Trent XWB-97 or General Electric GEnx but wider than the 93 inches/2.36 meters on the Boeing 767. The wings are dry without fuel. The report notes how hydrogen impacts engine size: “During the midsize concept analysis, it was noticed that for the same thrust requirement, a smaller diameter gas turbine led to a lower mission fuel burn, even though the specific fuel consumption (SFC) of the smaller gas turbine is worse. This is different to how engine size trades for a kerosene aircraft. The reason for this is that kerosene or SAF is relatively heavy compared to hydrogen as a fuel, and therefore an increase in engine efficiency or SFC gives a significant fuel mass reduction at the aircraft level, which outweighs any increase in engine weight from increasing the engine diameter. With hydrogen, the reduction in fuel mass is relatively small so a smaller, lighter engine is the better option overall.” More to do The three concepts within the ATI study have helped to identify the technical challenges at stake and resulted in solutions. But they aren’t projects ready for production. ATI says that until a hydrogen airliner can be certified, new regulations are to be defined and a better understanding is needed. Areas include the fundamental behavior of liquid hydrogen on materials, storage of LH2, cryogenic fuel pumps, hydrogen combustion systems, fuel cells and their thermal management, and aircraft integration. These same challenges have been identified by Airbus, Lufthansa Technik, German Aerospace Center (DLR), and others and will be addressed in upcoming tests. Airbus announced in early March that it will configure its A380 test aircraft into a flying testbed for the ZEROe engine demonstrator. Lufthansa Technik said last week that it will convert an A320 as part of a two-year program with DLR, MTU Aero Engines, and the ZAL institute to study hydrogen and its effects on various areas of aircraft operations.
