As the world moves towards clean energy technologies, aviation stands out as a particularly challenging application.

Human-made flying vehicles must generate incredible amounts of thrust to escape the clutches of gravity. They must also carry sufficient fuel to stay airborne for long durations across continents and oceans. By and large, electric batteries or solar power alone fail to supply both the power and longevity required for aerospace — hydrogen provides both.

Hydrogen fuel is readily available. It can be efficiently produced as a petroleum byproduct or through electrolysis. As a liquid or pressurized gas, it can be relatively easy to transport, and it’s quick to refuel, bypassing the long charge times required by today’s batteries.

Hydrogen burns cleanly, producing nothing but pure water as hydrogen atoms bond with oxygen. If the hydrogen itself can be produced using energy from renewable sources like solar or wind, it potentially represents a 100% clean energy cycle. Carbon dioxide, carbon monoxide, and other harmful exhaust vapors could be a thing of the past.

For decades, liquid hydrogen has served as a powerful rocket fuel, and more recently, aerospace applications of hydrogen have expanded to include both fuel cells and combustion fuel. Could hydrogen power be the future for both aviation and space flight?

Liquid Hydrogen (LH₂) Rocket Fuel.

Liquid hydrogen (LH₂) fuel has played an important role in space exploration since NASA’s Apollo program. The Saturn rockets used it for their secondary stage engines. Later the NASA space shuttle would use it to power its three main rocket engines.

Liquid hydrogen fuel has many benefits, including its low molecular weight and high energy output when burned together with liquid oxygen. Liquid fuels are often a popular choice for secondary/upper rocket stages after solid rocket fuels provide the extra thrust required for liftoff. Hydrogen also provides low-density liquid fuel for navigation thrusters in orbit.

The main engine of the space shuttle used liquid hydrogen fuel. Note how the hydrogen flames are almost invisible compared to the bright burn of the two solid rocket boosters on either side.

Today, hydrogen continues to show promise as a rocket propellant for both government and private industry launch systems and vehicles. The United Launch Alliance (ULA) Atlas Centaur stage rocket, Boeing’s Delta III and IV rockets, and Blue Origin’s BE-3 and BE-7 engines all use LH₂ rocket fuel.

WHA Industry Connection: Many of WHA’s founding engineers began their careers at NASA, and WHA Principal Chemist Dr. Harold Beeson served on the team that developed the NASA Standard for Hydrogen and Hydrogen Systems. This guide was later adapted into the AIAA Guide to Safety of Hydrogen and Hydrogen Systems.

Hydrogen Fuel Cells in Aerospace.

A little closer to the ground, commercial industry and NASA have partnered to explore the benefits of hydrogen, not as a rocket fuel, but in a fuel cell system. The Pathfinder and Helios projects were developed by AeroVironment, Inc. under NASA’s Environmental Research Aircraft and Sensor Technology (ERAST) program.

The Helios unmanned aircraft utilized a hydrogen fuel cell system regenerated by solar power.

These experimental long-range unmanned vehicles utilize a hybrid system in which hydrogen fuel cells are replenished by electrical power from solar arrays. During the day, solar cells produce electricity which separates water into hydrogen and oxygen through electrolysis. At night, the fuel cells generate electricity from the stored gases, and the cycle continues. This unique combination offers theoretically indefinite day and night continuous operation.

WHA Industry Connection: WHA engineers provided design support for both the Pathfinder and Helios projects. These projects leveraged WHA’s unique combination of expertise in both hydrogen and oxygen systems.

Hydrogen Combustion Engines in Aviation.

Fuel cells may be suitable for long-range light duty, but where do other aircraft fit in? Several major commercial airliners have their eyes on hydrogen as a clean alternative fuel for traditional turbojet and turbofan engines.

Recently on September 21, 2020, Airbus unveiled three concepts for hydrogen-fueled aircraft, all dubbed “ZEROe” for zero emission. They plan to launch the first craft by 2035, making it the world’s first zero emission commercial aircraft.

“I strongly believe that the use of hydrogen – both in synthetic fuels and as a primary power source for commercial aircraft – has the potential to significantly reduce aviation’s climate impact.”

Guillaume Faury, CEO of Airbus.

Airbus plans to use hydrogen as a combustion fuel for three new ZEROe concepts. Image courtesy of Airbus.

All three ZEROe concepts utilize liquid hydrogen fuel to power modified gas turbine engines. In the largest concept, hydrogen turbofans provide lift for up to 200 passengers with a range of 2,000+ miles. A smaller hydrogen turboprop design is also in the works, carrying up to 100 passengers with a range of 1,000+ miles. Finally, a bold blended-wing body design offers enhanced flexibility for hydrogen storage and distribution as well as cabin layout.

Challenges to using Hydrogen in Aerospace.

Before hydrogen can see widespread use as an alternative fuel, the aerospace industry must overcome several key obstacles to adoption.

• Extreme conditions: Hydrogen has a relatively low energy density, meaning that it must be stored in large quantities for any practical application as a fuel. To compensate, modern transportation applications are pushing the limits of technology with higher pressures and extreme cryogenic temperatures.
• Public perception: Hydrogen first saw action in aviation not as a fuel, but as a lift mechanism in Zeppelins and airships as early as the mid-1800s. Although hydrogen is no longer used commercially in this capacity, historic events like the Hindenburg incident have left a mark on the industry, even though hydrogen was not the main source of fuel for the event.
• Infrastructure: Airports will require significant infrastructure changes to accommodate hydrogen transportation and refueling. Handling of hydrogen on such a large scale represents additional logistical challenges and fire/explosion hazards.

WHA Industry Connection: WHA Mechanical and Forensic Engineer Dr. Dani Murphy brings a wealth of experience from NREL (National Renewable Energy Laboratory) where she was involved in research for hydrogen infrastructure, including filling station design and safety.

“The transition to hydrogen, as the primary power source for these concept planes, will require decisive action from the entire aviation ecosystem. Together with the support from government and industrial partners we can rise up to this challenge to scale-up renewable energy and hydrogen for the sustainable future of the aviation industry.”

Guillaume Faury, CEO of Airbus.

WHA Supports Hydrogen Technology in Aerospace.

For decades, WHA has worked with the aerospace industry to overcome the safety challenges associated with hydrogen.

Our scientists and engineers are intimately familiar with the unique risks of hydrogen and oxygen in aerospace, having been involved in the creation of multiple global standards including NASA’s Standard for Hydrogen and Hydrogen Systems.

We have partnered with both government and private organizations to provide failure analysis, hazard analysis and design support, custom testing, and technical training for hydrogen.

As the hydrogen economy grows, so do the risks. WHA is proud to stand with industry partners to help ensure a safer, cleaner future for everyone.