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| Hydrogen aircraft concept (Airbus); engine testing (Rolls-Royce); and flight tests (multiple) |
There is a small cadre of very vocal hydrogen critics with an outsized presence on social media, and media coverage in general. I've written about some of the most common archetypes in a previous hydrogen myth busting post [1]. Yet in a Google Scholar search on "hydrogen aviation", these self-proclaimed experts are nowhere to be found in the ranking of published sources on the topic [2].
Unfortunately, peer-reviewed articles are also not a guarantee of unbiased accurate information as addressed in my recent LinkedIn post on the topic [3]. A contemporary example is an article co-authored by techno-economic and energy analysis experts, but without any apparent input from experts with relevant hydrogen systems experience [4]. As a result, it contains some errors, omissions, and assumptions untethered to real world data.*
In the real world, the increased use of hydrogen in new aircraft development and demonstrations requires a paradigm shift in conventional aviation systems engineering and integration. Unlike sustainable aviation fuel (SAF) options, hydrogen is not a drop-in replacement for legacy aircraft. However, hydrogen is an important long-term solution in terms of performance, environmental impact, many safety aspects, and full lifecycle cost.
SAFs provide a near-term solution for reducing CO2 emissions in existing aircraft as a drop-in fuel. However, the impact on overall CO2 emissions depends on the degree of uptake and it continues to grow over time for most scenarios.^ So, SAFs can help blunt the environmental effects of continued legacy fuels, but they can't fully decarbonize aviation. In addition, SAFs have some challenges at scale related to sustainable feedstocks and supply chain and infrastructure build out. Incidentally, some of the most promising formulations use hydrogen as a feedstock.
This leads to a few key questions. Why is hydrogen being used for aviation? What are the fundamental design drivers for successful implementation? How can lessons already learned from the aerospace sector be incorporated? This blog post is a short introduction to these topics in the hopes of dispelling some of the persistent misinformation that continues to be posted and published on the subject.
Options for Decarbonizing Aviation
The table below summarizes key characteristics of the primary atmospheric greenhouse gases (GHGs), with carbon dioxide (CO2) being the most prevalent in terms of concentration and with an atmospheric lifetime of thousands of years. The global warming potential of these GHGs in the last column has been normalized to CO2. Each of these characteristics are important to keep in mind when assessing the impact of any systems or processes that use or emit one of these GHGs.**
Aviation accounts for roughly 2.5% of all global CO2 emissions and 4% of the global warming to date [5], with CO2 emissions expected to double in 25 to 30 years based on current legacy technologies and predicted growth in air travel according to the FAA.^ These may seem like small percentages in the big picture, but aviation is a difficult industry to decarbonize directly with renewables-based electrification along with other large transportation and maritime applications. There are a few methods for potentially reducing GHGs that are applicable to aviation: SAF, batteries, hydrogen, methane, and nuclear.
Batteries are feasible for some electric aviation applications where the impacts to range, payload, and operations due to their low specific energy relative to chemical fuels and their charging requirements are acceptable. Most of these applications are short haul small aircraft flying at low to moderate altitudes using electric motor driven propellers or fans. Another fundamental drawback is that batteries weigh the same whether fully charged or fully discharged, so there is no reduction in aircraft mass (and propulsion requirements) common with chemical fuels. And landing is one of the highest energy consumption flight phases, resulting in a hot discharged battery that must be conditioned and recharged before the next flight which impacts operational turn around time and infrastructure requirements.
Hybrid electric aircraft concepts can potentially extend the suitable aviation applications using various fuels (including hydrogen) integrated with batteries. Depending on the fuel choice, there are tradeoffs related to technology development required, degree of decarbonization, and other parameters. Advanced hybrid electric propulsion systems are under development by NASA and other organizations that may be promising to commercialize as they reach sufficiently mature technology readiness levels.
Methane, which is the primary constituent of natural gas, and liquefied natural gas (LNG) have been considered and tested for aircraft. But neither are not drop-in fuels. In the case of LNG, cryogenic systems similar to liquid hydrogen (LH2) must be developed, albeit at a somewhat higher operating temperature as shown below. However, similar to SAF, combustion of methane still produces CO2 emissions, although at a lower rate than legacy fuels. More concerning is un-combusted methane that has 25 times the 100-year global warming potential of CO2. So any leaks, venting, dumping or other releases of methane anywhere in its journey from extraction or production to consumption represents a powerful GHG gas being added to the atmosphere.
Nuclear aircraft were investigated by NACA and during the early days of NASA. And demonstrations of nuclear powered drones have been reported in recent years by multiple sources. In theory, the performance of these aircraft greatly exceeds any chemically fueled or battery powered vehicle. In practice, use in a non-military military application would run afoul of national security, regulatory, and public safety concerns. This is also the reason why nuclear powered naval ships and submarines have been successfully operating since the 1950s, but there is only one currently operating nuclear powered merchant ship.
And that leaves hydrogen as the final practical option, which can be used for an unlimited range of aircraft types and flight requirements. A multitude of hydrogen propulsion systems testing and flight tests have been successfully performed by combusting it in jet engines or feeding it to fuels cells for electric motor driven propulsion. Hydrogen is suitable for fixed wing or vertical-take-off-and-landing (VTOL) aircraft of any size currently used, at any currently feasible altitude and range, with no carbon emissions during operation.
Hydrogen Use in Aerospace
There are two primary reasons to use hydrogen for any aerospace application: superior performance and decarbonization.
Performance is the reason liquid hydrogen has been used for various launch vehicles and rocket stages continuously at large scale and full lifecycle for six decades. Historic examples include: Apollo's Saturn rocket upper stages, Centaur upper stage, and the Space Shuttle. Currently operational examples include: NASA's SLS (see design below); upper stages of the Atlas V, Delta IV, ULA Vulcan, and New Glenn; EU's Ariane 5 and 6; Japan's H-IIA; variants of China's Long March rockets; and India's LVM-3 upper stage.
The high specific energy (energy per unit mass) of hydrogen is three times greater than legacy fuels, and along with other energetic properties and characteristics enables its superior performance. This is offset by the low volumetric energy density which requires larger storage volumes compared to legacy fuels. However, fuel cell applications significantly reduce this requirement with more than double the efficiency of conventional combustion engines. Below is a plot illustrating some of these hydrogen properties.
For aircraft, decarbonization is the primary reason for using hydrogen, although performance gains are possible as designs evolve to optimize its usage. The first LH2 aircraft flight testing was done in the 1950s by NACA/NASA, and a dozen more hydrogen powered aircraft have flown as of 2024. Fuel cell power and propulsion systems only emit water vapor during operation and are currently limited to altitudes where propellers can be used.
Internal combustion engines (ICE) can cover the entire aircraft size range (small piston to jet engines) and can operate at any altitude currently feasible with legacy aircraft. In fact, hydrogen ramjets and scramjets can operate well above legacy aircraft altitudes. All combustion processes produce nitrous oxides (NOx) due the the disassociation of air at high temperatures. However, the wide oxygen-fuel (O/F) ratio possible with hydrogen permits very lean mixtures that can decrease NOx much more than legacy fuels or SAF.
There is much too be learned in implementing hydrogen in aviation from its adjacent space industry. Systems, integration, infrastructure, safety, components, supporting technologies, supply chains, and a host of other challenges have long ago been solved in the space industry. Adapting all of it to new industry sectors requires good systems engineering, sound business models, capital investments, infrastructure, scaling for cost reductions, and supportive policy and regulatory environments. All the enabling technology is already available.
When all lifecycle costs are included, hydrogen is an economically competitive investment in our future. As it scales, further cost reductions will occur as they do with any new technology shift. This has occurred with solar and wind renewables, just as it did with the shift to natural gas, and the shift from coal to oil before that, and the shift from wood to coal before that. It is the natural economic evolution of new energy paradigms [6].
Valid comparisons to legacy fuels must include their costs associated with: exploration of underground sources (including "dry well" and other abandoned costs); extraction; storage; delivery to refinery; refining; distribution; fueling depots and stations; legacy subsidies; and casualty losses and public health impacts from exposure to toxic fuels, fumes, smoke/soot, polluted air and water, accidents, and all the aggregated effects GHG emissions. Along with the outlays over the past 100+ years (private and public) associated with putting all the existing infrastructure in place for fossil fuels.
Hydrogen Systems Considerations
For aviation and most other industry sectors transitioning to hydrogen, there are a few key initial design drivers to consider, particularly for LH2: volumetric density, systems integration, cryogenic engineering, operations, and safety.
The comparatively low volumetric density of hydrogen is resolved in the systems engineering and integration of the aircraft. For legacy aircraft, innovative designs and packaging of hydrogen tanks is required. This can result in longer and/or wider fuselages, storage outside of the fuselage, or other methods to accommodate the volume required.
Larger tanks that can result in increased drag for legacy aircraft designs is a relevant consideration, although many aircraft have successfully flown with enormous outer mold line volumes in order to achieve overall vehicle and mission performance criteria [7]. New VTOL and fixed wing aircraft designs optimized around hydrogen can actually improve aerodynamic performance. Lifting bodies are an example where the volume requirements become much more easily integrated within the fuselage [8].
Therefore, the combination of higher specific energy but lower volumetric density of hydrogen relative to legacy fuels are primary parameters to optimize in aircraft design. Lower fuel mass requires less lift which can be traded against greater payload and/or range. Larger fuel storage volumes requires holistic integration with the aircraft to minimize the impact on drag.
Systems engineering and integration combined with cryogenic engineering best practices also plays an important role. A portion of the energy expended to liquefy hydrogen can be recovered for better overall system efficiency [9]. Examples include: pressure building systems for zero-power pressurization; heat rejection from other subsystems; thermal protection of structures exposed to high temperatures; cooling of superconducting components; Joule-Thomson cooling; vapor-cooled shields for reducing heat loads; para-to-ortho conversion cooling; and other methods.
Existing mature cryogenic engineering technologies also enable any LH2 storage and deliver system to have zero boil-off losses. Every LH2 storage system has a characteristic natural boil-off rate profile depending on the size, environment, operations, fill level, and other variables.^^ In all cases, boil-off gas losses can be driven to zero with a combination passive, hybrid, active, and operational techniques. Claims of unavoidable boil-off losses with LH2 systems are false. All boil-off losses are avoidable with proper system design [10].
Changes to aircraft operations, and required airport infrastructure, are also valid considerations [11]. Below is a notional example of how onboard LH2 storage is affected by the acceleration and thermal environments during key aircraft operations. In addition to onboard systems that properly manage LH2, ground support systems must also be in place to support fueling and other associated activities. Various organizations and standards working groups are actively addressing these aspects in preparation for the decades of development required to fully build out the infrastructure [12].
Safety is a paramount consideration with any fuel or energetic system, and hydrogen is no exception. Many codes, standards, training, and certifications are already in place; and many more are under development to address specific use cases and industries. Below is a comparison of key safety relevant properties for hydrogen and natural gas (methane). A few implications that may not be obvious on initial inspection:
- A given volume of methane has more stored energy for detonation than hydrogen under identical conditions; and the lower bound detonation limit in air is less than one-third that of hydrogen
- While both gases are lighter than air at ambient temperatures, hydrogen rises much faster (20 m/s or 45 mph) and diffuses much more rapidly in air
- The lower flammability limit in air (which any safe system is designed to stay well below) and the autoignition temperatures for the gases are not much different
- However, hydrogen has a much lower ignition energy and much higher flame speed
So which fuel is safer? It depends on the situation.# The same can be said for comparing aviation fuel with hydrogen, because each fuel behaves very differently. In a mid-air collision or an emergency landing where the fuel storage is breached, hydrogen (in liquid or gas phase) would be vaporized, gone/diffused, ignited upward, or some combination thereof within minutes. And with no smoke/soot, no persistent burning pools of spreading liquid, and no extended exposure to high temperatures. On the other hand, an undetected fuel leak that enters a sealed passenger cabin and ignites is a much lower risk for aviation fuel.
The Path Forward
Every new energy paradigm shift in history (and every other major technological disruption) has occurred due to market adoption within a supportive policy environment by innovating individuals and teams developing systems and products in the real world. Not on social media. Not in general media opinion pieces or news coverage. And not in peer reviewed articles that ignore information that doesn't fit the predetermined narrative of the authors.
If we had relied on the talkers during previous energy paradigm shifts, we would still be huddled around wood and brush fires. Afterall, a technoeconomic analysis before the industrial revolution could have easily showed that the complexities and costs of extracting fossil fuels and transitioning to their use made no sense compared to burning cheap available wood. And if we rely on today's talkers and delay addressing the existential threats of rising GHGs, public health impacts, and pollution, we will end up dealing with much larger fires and many more calamities at a scale unprecedented in human history.
Footnotes
* For example: ignoring flight demonstrations and testing, alternative aircraft concepts, hydrogen systems state-of-the-art, cryogenic systems integration, propulsion dynamics, regulatory and certification pathways, infrastructure assessments, energy density trades, thermal management, cost projections, etc.
** Note: the so-called 'indirect greenhouse gas' effect of hydrogen is an unproven hypothesis based on unverified assumptions and no atmospheric data at scale. And initial studies into the contrails produced by hydrogen aircraft indicate they dissipate quicker than legacy aviation fuel contrails.
^ The website that documented this information appears to have been taken down by the current administration (https://www.faa.gov/sustainability).
^^ There is no standard value or accurate rule of thumb for estimating natural LH2 boil-off rates despite many sources that attempt to claim otherwise (which is a clear sign of someone lacking experience in this domain).
# Paradoxically, the most infamous historical hydrogen incident would have been much worse with any other fuel under similar conditions: store the fuel in an enormous fabric container with a highly flammable coating; suspend 97 people from it 200 feet (60 m) above the ground; have more people underneath it; allow a lightning strike to ignite the flammable coating; record it with a 1930s era black & white motion camera (that won't record hydrogen flames in daylight, and all the hydrogen is out of the frame within 2 seconds anyway). With hydrogen, it did not detonate nor apparently even ignite any significant amount since there were no water droplets. And 62 of the 97 people onboard survived, with one casualty on the ground. What would be the outcome of repeating that scenario with any other fuel? Or with batteries?
References
[1] "Myth Busting (Episode 4): Hydrogen Haters", LH2era.com, Oct 26, 2023.[2] "Hydrogen Aviation" Google Scholar search.[3] Misinformation Hydrogen Zombies, LinkedIn post, Jun 27, 2025.[4] "Realistic roles for hydrogen in the future energy transition", Johnson, Liebreich, et al., Nat. Rev. Clean Technol. 1, 351–371 (2025).[5] "What share of global CO₂ emissions come from aviation?", Ritchie, Apr 8, 2024.[6] "Why All Hydrogen Cost Projections Are Wrong", LH2era.com, Aug 5, 2023.[7] "List of large aircraft", Wikipedia[8] "Lifting body", Wikipedia[10] "Hydrogen Myth Busting (Episode 3)", LH2era.com, Apr 16, 2023.[11] "Decarbonizing Mobility with Liquid Hydrogen", SAE Research Report, 2024.[12] "Hydrogen Fueling Stations for Airports, in Both Gaseous and Liquid Form", SAE International, issued Nov 11, 2024.
[1] "Myth Busting (Episode 4): Hydrogen Haters", LH2era.com, Oct 26, 2023.
[2] "Hydrogen Aviation" Google Scholar search.
[3] Misinformation Hydrogen Zombies, LinkedIn post, Jun 27, 2025.
[4] "Realistic roles for hydrogen in the future energy transition", Johnson, Liebreich, et al., Nat. Rev. Clean Technol. 1, 351–371 (2025).
[5] "What share of global CO₂ emissions come from aviation?", Ritchie, Apr 8, 2024.
[6] "Why All Hydrogen Cost Projections Are Wrong", LH2era.com, Aug 5, 2023.
[7] "List of large aircraft", Wikipedia
[8] "Lifting body", Wikipedia
[10] "Hydrogen Myth Busting (Episode 3)", LH2era.com, Apr 16, 2023.
[11] "Decarbonizing Mobility with Liquid Hydrogen", SAE Research Report, 2024.
[12] "Hydrogen Fueling Stations for Airports, in Both Gaseous and Liquid Form", SAE International, issued Nov 11, 2024.
Author Bio
Matt Moran is the Managing Member at Moran Innovation LLC, and previous Managing Partner at Isotherm Energy. He's been developing power and propulsion systems for more than 40 years; and first-of-a-kind gas, slush, and liquid hydrogen systems since the mid-1980s. Matt was also the Sector Manager for Energy & Materials in his final position at NASA where he worked for 31 years. He's been a cofounder in seven technology-based startups; and provided R&D, engineering, and innovation consulting to several hundred organizations. Matt has three patents and more than 50 publications including his online Cryogenic Fluid Management guide and Decarbonizing Mobility with Liquid Hydrogen SAE report. He has created and taught liquid hydrogen courses, webinars, and workshops to global audiences.
