The UK National Composites Centre (NCC) has developed a space liquid hydrogen storage tank demonstrator that is 750mm in length, 450mm in diameter, and holds over 96 liters of liquid hydrogen.

The tank is designed and manufactured with a nominal wall thickness of 4.0 to 5.5mm, allowing it to withstand a pressure of 85 bar. The carbon fiber composite body weighs only 8 kilograms, and further weight optimization is planned. NCC uses 300mm wide MTC510 epoxy carbon fiber prepreg. MTC510 is an epoxy resin system designed to cure between 80°C and 120°C and is toughened to improve damage tolerance. BINDATEX provided the prepreg tape, which was precisely slit to 6.35mm width and returned as 22,000 meters of material for use in Coriolis automated fiber placement (AFP) equipment. The Coriolis AFP device was used to wrap the 6.35mm prepreg tape around a washable mold, with the winding process controlled by specialized software to manage both helical and hoop winding. The winding process, with over 24 layers and a thickness of up to 5.5mm, can be adjusted to optimize the tank’s specific pressure or load requirements.

The core mold, with a wall thickness of 30mm, was cast in two parts and then bonded together. The tool includes three washable internal reinforcement rings designed to withstand the expected torsional loads during automatic composite layer placement and the pressure applied during autoclave curing. Metal fluid valve ports are integrated into the washable core mold, eliminating the need for secondary assembly and bonding operations on the final product. These ports are bonded with the carbon composite in the later stages of the manufacturing process. After winding, the tank is inspected for defects and thickness variations, cured in an autoclave at 100°C, and re-inspected. Post-cure non-destructive testing (NDT) using ultrasonic C-scan and thermography is compared to identify any defects like delaminations and porosities. Finally, the internal core mold is flushed with pressurized cold water to ensure the tank cavity is clear.

Why Use Liquid Hydrogen in Civil Aircraft?

Hydrogen has a weight energy density of 33.3kWh/kg compared to kerosene’s 12kWh/kg. Under normal pressure and temperature, hydrogen has a density of 0.090kg/m³. At 700 bar (700 times normal atmospheric pressure), hydrogen’s density is 42kg/m³, allowing a 125L tank to store 5kg of hydrogen. At -252.87°C and 1.013 bar, liquid hydrogen has a density close to 71kg/m³, enabling a 75L tank to store 5kg of hydrogen. Storing liquid hydrogen in low-temperature tanks helps further reduce volume.

• 3000 liters of gaseous hydrogen at normal temperature and pressure is equivalent in energy to 1 liter of aviation kerosene.

• 6 liters of gaseous hydrogen at 700 bar is equivalent in energy to 1 liter of aviation kerosene.

• 4 liters (1.05 gallons) of liquid hydrogen at -252.87°C and 1.013 bar provides the same energy as 1 liter of aviation kerosene.

From these data, it is clear that storing liquid hydrogen (-252.87°C) requires the smallest storage tank volume. Smaller tank volumes are easier to integrate within an aircraft’s aerodynamic shape.

Key Technical Issues of Low-Temperature (-252.87°C) Liquid Hydrogen Storage Tanks:

1. Maintaining the Tank’s Liquid Hydrogen Below -253°C: Currently, a vacuum-insulated structure is used between the inner and outer tanks. The inner tank is made of carbon fiber reinforced resin composites, while the outer tank contains multiple layers of special insulation.

2. Installing and Maintaining Internal Systems in the Tank: The challenge of installing and maintaining pipelines and system components inside the tank if using the current fiber winding process.

3. Material Selection for the Tank and Its Internal Components: The impact of the low-temperature environment (-252.87°C) on the materials used for the tank and its internal components.

4. Low-Temperature Testing Techniques and Fuel Slosh Management Technologies.

5. Enduring Frequent Take-offs and Landings: The hydrogen tank must withstand approximately 20,000 take-offs and landings.

Impact on Aircraft Structure

The fuel tanks in an aircraft’s wing structure are cavities used to store fuel. An A320 wing tank can store approximately 20 tons of aviation kerosene (similar for Boeing 737 and COMAC C919). Replacing kerosene with liquid hydrogen, a 94m³ cylindrical liquid hydrogen tank could only be installed in the rear fuselage, requiring significant lengthening of the fuselage. The rear fuselage is a conical shape with a maximum diameter of less than 4m. Simply extending the fuselage to accommodate a 94m³ tank is impractical; therefore, the fuselage diameter must also be increased.

In the new A320 design, a round and a conical tank are installed in the rear fuselage. However, whether the fuselage diameter will be increased is still unclear, though it is likely. The UK has unveiled a liquid hydrogen-powered civil aircraft design, with the narrow-body “FZN-1E” to replace the current A320. This new design extends the fuselage by 10m, increases the diameter by 1m, has a dual-aisle cabin layout, redesigned wings, added "foreplanes" on the nose, and engines mounted on the tail.

Progress

Civil aircraft engines come in two types: turboprop engines and turbojet engines. For aircraft with turboprop engines, hydrogen generates electricity via fuel cells to power generators driving the propellers. This type of engine is mainly installed on regional aircraft with 10 to 70 seats and small general aviation aircraft. Initial hydrogen-fueled research began with these aircraft types. On April 12, a German 4-seater “HY-4” hydrogen-electric plane successfully flew from Stuttgart to Friedrichshafen. Later this year, we may see 19-seat “Dornier” and 75-seat “Q-400” and “ATR72-600” hydrogen-electric planes in the sky. In April 1988, the Soviet Union test-flew a modified Tu-155 with a liquid hydrogen turbojet engine. Following the Soviet Union’s dissolution, Russia did not continue this research.

Currently, only four companies globally produce and develop civil aircraft with over 100 seats: Boeing, Airbus, COMAC, and Russia. According to a recent foreign media report, only Boeing and Airbus are conducting actual liquid hydrogen civil aircraft application research. Boeing's project, conducted over a decade ago on a small "Dimona" propeller glider, was preliminary. Airbus is ahead, having begun high-altitude flight tests of liquid hydrogen-fueled turbofan engines. They have also provided preliminary designs for three types of aircraft: propeller aircraft, 150-seat aircraft, and wide-body aircraft. More information is available for the 150-seat aircraft, which is set to replace the single-aisle, 150-seat A320 that has been on the market for nearly 40 years. Airbus plans to launch a "new A320" between 2030 and 2035. The new aircraft will feature an "albatross" aerodynamic configuration with ultra-high aspect ratio, folding, flapping wingtips, and no fairing flaps. The materials used will be thermoset carbon fiber reinforced epoxy resin composites for the wings and high-performance thermoplastic carbon fiber composites for the fuselage. This new aircraft will use liquid hydrogen instead of aviation kerosene, with a design and manufacturing goal of producing 70-100 aircraft per month. Airbus is far ahead of Boeing in developing liquid hydrogen-fueled aircraft (no information about Boeing replacing the 737 with liquid hydrogen has been reported).

What Can We Do?

Using hydrogen instead of fossil fuels not only addresses carbon emissions but also has strategic importance for countries lacking oil resources. China is the world’s largest hydrogen producer, with an annual production of about 33 million tons. Several companies are involved in liquid hydrogen production, and China is the second-largest producer of carbon fiber globally. Thus, developing and producing composite hydrogen storage tanks have a solid material foundation.

The different aerospace and aviation liquid hydrogen storage tanks discussed in this article demonstrate that storage tanks are designed and manufactured to meet the specific needs and structural spaces of various products. Currently, many industrial products still use fossil fuels or grid electricity. These can consider switching to hydrogen power. There is a vast array of products to be developed in the hydrogen storage field, and many tasks await us.

Some data in this article, sourced from the internet, have been repeatedly verified for accuracy. These data can be used to estimate the initial design dimensions and capacity of hydrogen storage tanks.