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Tuesday, July 22, 2025

Thermal Stratification, Cavitation, and Sloshing with LH2

Source: Liquid Hydrogen Systems Course, Moran Innovation LLC, 2025.

I had two very interesting conversations recently that touched on aspects of thermodynamic behavior in a liquid hydrogen (LH2) system. The first was with a US organization involved with LH2 fueling stations looking at pump cavitation. The second was a company in the EU that is retrofitting and recertifying a commercial aircraft for LH2. In both cases, thermal stratification is an important design and operational consideration.


Steam Analogies


Working with LH2 and other cryogenic systems requires developing an accurate mental model and intuition about the associated thermodynamic behaviors. Many systems do not operate with a fluid near its saturation conditions as is routinely done with LH2. Notable exceptions are refrigerants and steam, with steam engines and power cycles being perhaps the closest analogous systems.

Steam engines were the original workhorses of the Industrial Revolution, and associated boiler explosions were the catalyst for developing the first ASME Boiler and Pressure Vessel Code (B&PVC). Not many are in use any more beyond a few steam engine trains, tractors, and other historical machines. Unfortunately, there are still occasional accidents with them that provide lessons to learn. 

A number of years ago, a steam engine tractor heading to a county fair was pulled over by the local police because its steel treads were tearing up the asphalt road it was on. When the tractor stopped, its boiler exploded. The operator had allowed the water in the boiler to get too low and the boiler had either a malfunctioning or improperly sized pressure relief valve. Thermal stratification in the boiler wall above the low water level caused rapid vaporization and pressure increase when the tractor sloshed the puddle of water onto the much hotter portion of the boiler wall as the tractor came to a stop.

Steam power plants like the one I started my engineering career at in the early 1980s provide additional insights for LH2 systems. Below is a representative schematic showing the boiler, pressure stages, heat exchangers, and other subsystems used to maximize overall plant efficiency. At its core, it's all about managing temperature differentials and heat exchange to drive the turbines which are shaft-connected to the generator. Drop the temperatures by a several hundred degrees C, and some aspects of this system and its operation share similarities with LH2 systems.

Stationary LH2 Systems


Cavitation is a potential issue in all cryogenic liquid systems, and thermal stratification in the liquid of a storage tank can play a role. Cavitation occurs when the local pressure in some part of the flow system drops below the saturation pressure of the liquid. This causes vapor to form that can damage components and diminish, stall, or even reverse flow in the system.

There are two primary operational parameters that affect cavitation: liquid temperature and system pressures. The lower the liquid temperature, the lower the saturation pressure required for cavitation to occur. Thermal stratification near the interface of a stationary storage tank is important to consider in this regard since draining a tank can introduce warmer liquid into the system as the tank level reaches the liquid thermal boundary.

For local system pressures, the pressure drops through all components, pipe lengths, and elevation changes must be addressed along with the source pressure and downstream exit pressure. In addition, pumps have a net positive suction head (pressure) (NPSH or NPSP) that must be maintained during operation.

Below is an example analysis I did for a liquid nitrogen system modification designed to feed a vacuum chamber coldwall during planned static testing of a J-2X rocket engine. This was a pressure fed system with no pumps, but avoiding cavitation was still a critical concern.






Mobile LH2 Systems


In cryogenic liquid systems subject to acceleration forces due to vehicle motion, all of the above issues with thermal stratification and cavitation apply. However, another issue must also be considered: liquid sloshing. There are dynamic load effects on a vehicle during slosh that are well known for any fuel, but thermodynamic effects (pressure and temperature) must also be considered for cryogenic systems just like the old steam engines previously mentioned.
 
When sloshing occurs in a tank that has thermal stratification near the liquid interface, a drop in pressure can occur as the cooler bulk liquid circulates to the interface causing increased condensation. This pressure drop (or ullage collapse) impacts the mass flow out of the tank during fueling, engine feed, or fuel cell feed operations. Below is a slide showing some of the published test results we did at NASA in the early 1990s on LH2 sloshing in a spherical tank.

Source: Liquid Hydrogen Systems Course, Moran Innovation LLC, 2025


Some Mitigation Strategies


It's worth mentioning that thermal stratification can also work in your favor in some situations. For example, suppose you have a LH2 storage tank sitting for a period of time and self-pressurizing due to boiloff. If you mix the liquid, the tank pressure will drop as the thermal boundary layer near the interface is disrupted thereby significantly prolonging the vent-free dormancy time. The mixing can be mechanical, or induced by vapor or liquid injection into the tank.

For other situations, a few design and operational mitigation strategies can be considered:
  • Keep the entire LH2 tank near saturation conditions by warming the liquid uniformly with a heat exchanger or heater. This inhibits development of a liquid temperature stratification layer and maintains a constant saturated source pressure in the tank.
  • Define or modify your concept of operations (ConOps) to ensure that any sloshing is below the threshold that would cause an unacceptable drop in tank pressure. Baffles, screens, acquisition devices, or other tank internals are an option if desired slosh control cannot be achieved with the ConOps alone.
  • For cavitation concerns related to draining a tank down to the warmer thermal boundary layer of the fluid, there are two options. The first is to stop flow at that liquid level. The second is to vent the tank (preferably with vapor recovery) to bring the liquid temperature down to a new saturation condition via rapid boiling.
 

Author Bios

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.

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Tuesday, July 15, 2025

Liquid Hydrogen Newsletter (July, 2025)

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Tuesday, July 1, 2025

Hydrogen in Aviation (Myth Busting, Episode 5)

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.

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.

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 these emissions continue to grow over time for most scenarios.^ So, SAFs can help blunt the environmental effects of legacy fuels, but they can't fully decarbonize aviation. In addition, SAFs have some challenges at scale related to sustainable feedstocks, supply chains, and infrastructure build out. Incidentally, some of the most promising formulations use hydrogen as a feedstock.

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. A fundamental drawback is that batteries weigh the same whether fully charged or fully discharged, so there is no reduction in aircraft mass in flight (or 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. This 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 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. 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 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.

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, and 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 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. However, 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.

It's important to note that all combustion engines produce nitrous oxides (NOx) due to 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 aviation from the aerospace-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 build out, 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 of 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 less volume available for payloads or passengers, 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 are 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 and flying wing configurations are examples 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 longer range. Larger fuel storage volumes require holistic integration within the aircraft to minimize the impact on drag and maximize aircraft performance and capabilities.

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 [12]. 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 [13].



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 its 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, much higher upper flammability limit in air, and much higher flame speed

So which fuel is safer? It depends on the situation.# The same can be said for comparing legacy aviation fuels 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. And with none of the toxic fumes, smoke/soot, persistent burning pools of spreading liquid, or extended exposure to high temperatures that jet fuel produces.

On the other hand, an undetected fuel leak and potential ignition event is a generally lower risk for jet fuel than it is for hydrogen. However, hydrogen is nontoxic and can be safely breathed as long as its concentration is below asphyxiation levels. Definitely not the case for any legacy aviation fuels. Bottom line: every potential hazard and operational situation requires careful consideration while keeping in mind the unique properties and behaviors of the specific fuel in question.


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). In the case of 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).
[6]  "Why All Hydrogen Cost Projections Are Wrong", LH2era.com, Aug 5, 2023.
[7] "List of large aircraft", Wikipedia
[8] "Lifting body", Wikipedia
[9] "Why Liquefy Hydrogen?", LH2era.com, Jun 4, 2025.
[10] "Hydrogen Myth Busting (Episode 3)", LH2era.com, Apr 16, 2023.
[11] "Decarbonizing Mobility with Liquid Hydrogen", SAE Research Report, 2024.
[12] "H2-powered aviation – Optimized aircraft and green LH2 supply in air transport networks", Hoelzen, et al., Applied Energy, Vol. 380, 2025.
[13] "Hydrogen Fueling Stations for Airports, in Both Gaseous and Liquid Form", SAE International, issued Nov 11, 2024.

All graphics are from: Liquid Hydrogen Systems Course, Moran Innovation LLC, 2025.

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.




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