Hydrogen Properties and Their Safety Implications for Experimental Testing of Wing Structure-Integrated Hydrogen Tanks

Abstract

Hydrogen is a promising candidate for addressing environmental challenges in aviation, yet its use in structural validation tests for Wing Structure-Integrated high-pressure Hydrogen Tanks (SWITHs) remains underexplored. To the best of the authors’ knowledge, this study represents the first attempt to assess the feasibility of conducting such tests with hydrogen at aircraft scales. It first introduces hydrogen’s general properties, followed by a detailed exploration of the potential hazards associated with its use, substantiated by experimental and simulation results. Key factors triggering risks, such as ignition and detonation, are identified, and methods to mitigate these risks are presented. While the findings affirm that hydrogen can be used safely in aviation if responsibly managed, they caution against immediate large-scale experimental testing of SWITHs due to current knowledge and technology limitations. To address this, a roadmap with two long-term objectives is outlined as follows: first, enabling structural validation tests at scales equivalent to large aircraft for certification; second, advancing simulation techniques to complement and eventually reduce reliance on costly experiments while ensuring sufficient accuracy for SWITH certification. This roadmap begins with smaller-scale experimental and numerical studies as an initial step.

1. Introduction

The aerospace industry consistently strives for innovations that enhance fuel efficiency [1,2,3], reduce aircraft weight [4,5,6,7,8], and improve overall performance [9,10,11]. A notable advancement in this realm is the concept of Wing Structure-Integrated high-pressure Hydrogen Tanks (SWITHs), pronounced sweets, which are embedded within an aircraft’s wing structure. An illustrative depiction of a SWITH is provided in Figure 1.

The primary distinction between integrated and externally stored fuel tanks lies in their intended purpose. Integrated tanks are engineered to bear external loads and provide support to the aircraft’s load-bearing components, whereas external tanks are employed exclusively for fuel storage and supply. They do not constitute an integral component of the aircraft’s structure and, consequently, are not designed to support the load-bearing structure by absorbing loads. In addition to wing-integrated hydrogen tanks, hydrogen storage can also be incorporated into the fuselage [12,13,14,15,16,17,18,19,20,21,22]. The utilization of hydrogen tanks, whether integrated into the wing or the fuselage, can take the form of liquid or gaseous hydrogen. However, the present contribution is focused on gaseous SWITHs. The main advantage of using hydrogen lies in its potential for more environmentally friendly aviation, as evidenced by numerous studies [23,24,25,26,27]. Moreover, hydrogen’s significance is reinforced by its diverse applications across multiple fields. In the mobility sector alone [28,29,30], it has the potential to power various vehicles, including ships [31,32], motorcycles [31,33,34,35,36,37,38], cars [39,40,41,42,43], heavy-duty trucks [33], submarines [32,44,45,46], trains [31,33,47], and unmanned aerial vehicles (UAVs) [33,48,49,50,51,52].

Figure 1. Illustrative example of a SWITH, similar to APUS i-2 [53].

Figure 1. Illustrative example of a SWITH, similar to APUS i-2 [53].

In the context of aviation, integrating hydrogen storage directly into the aircraft structure offers additional benefits beyond propulsion. The key benefit of integrating tanks as a part of the aircraft’s integral design is the liberation of space that would otherwise be allocated for fuel storage. This freed-up space can then be repurposed for other uses, such as installing additional seating to increase passenger capacity. Alternatively, it could carry more cargo or large, cumbersome items that do not exceed the size of the original tanks. The notion of SWITHs has evolved from a mere scientific concept to a tangible reality. For instance, APUS [53], a company specializing in aircraft manufacturing, is currently engaged in the development of its i-2 model, with the objective of introducing the first commercially available SWITH [54]. Notwithstanding these promising advancements, the relatively nascent SWITH concept remains confronted with challenges pertaining to manufacturing, safety, ambiguous regulatory frameworks, and a paucity of extant literature.

Given the innovative nature of SWITHs, scholarly investigation—particularly regarding hydrogen’s role as a filling agent in structural validation tests for aircraft-scale SWITHs—remains scarce. This gap in academic discourse highlights the importance of this study, which aims to assess the implications of using hydrogen in such tests and its impact on their feasibility. Furthermore, this understanding is especially beneficial when designing new SWITHs, as it is pivotal for addressing key hydrogen safety-related challenges. To achieve this, the study first provides an overview of hydrogen’s fundamental properties, with an emphasis on its beneficial characteristics. This is followed by an analysis of its safety-relevant aspects, supported by numerical simulations and experimental findings. Based on this foundation, the feasibility of conducting static experimental structural validation tests on aircraft-scale SWITHs, with hydrogen as the filling agent, is evaluated. If such testing is found to be impractical in its current form, potential pathways toward its realization are proposed.

Given the complexity and innovative nature of SWITHs, a collaborative effort among researchers is essential to address the challenges associated with their development. As of the writing of this paper, no formal standards or regulations exist for the certification of SWITHs [55]. Consequently, the findings from this study may serve as a valuable reference for the future development of such standards and regulations. The necessity of well-defined standards becomes particularly evident in the context of experimental tests and simulations on SWITHs, as they establish critical benchmarks for validation, safety, and reliability. Without such guidelines, ensuring consistency in testing methodologies and assessing structural integrity under real-world conditions becomes significantly more challenging. Establishing and adhering to structured standards allows engineers and specialists to conduct evaluations in a systematic, transparent, and safety-oriented manner [56,57,58,59,60,61].

2. Fundamental Properties of Hydrogen: An Overview

To understand hydrogen’s implications for experimental structural validation tests of aircraft-sized SWITHs, its basic properties are first introduced. This understanding is crucial in recognizing why hydrogen’s unique characteristics make it a promising alternative fuel. Safety-related properties are examined separately in Section 3. For convenience, all key information from this section is compiled in Table 1.

Table 1. Fundamental properties of hydrogen for aerospace and energy applications.

Property	Description
Atomic structure	Hydrogen (H) is the smallest known atom, with a diameter of approximately 0.07 nm [62,63].
Molecular form	It exists primarily as a H2 molecule [62,63]. The H atom itself is highly reactive [64,65,66]. Generally, references to hydrogen denote the H2 molecule rather than the H atom.
Chemical properties	The H atom forms a stable bond with another H atom to create H2. This stable bond results from fully occupied valence electrons, leading to the low reactivity of H2 [67]. The H atom has a relatively low electronegativity of 2.2 [62,63], which describes an atom’s ability to attract electrons when forming chemical bonds.
Sensory properties	Odorless [68,69,70]. Tasteless [69]. Invisible [68,69,70].
Safety properties	Non-toxic [69,71,72]. Non-carcinogenic [62,63]. Not radioactive [63]. Not caustic [62,63].
Production	Renewable production is possible through water electrolysis powered by wind, solar, or other renewable energy sources [73,74,75,76].
Thermodynamic properties	Hydrogen exhibits a negative Joule–Thomson coefficient at high pressures [77,78,79]. This implies that, under constant enthalpy h, an increase in pressure results in a decrease in temperature (cooling during relaxation in a throttle [62]), and vice versa. This relationship can be mathematically expressed as follows: $${\mu }_{\mathrm{JT}}={\left({\displaystyle \frac{\partial T}{\partial p}}\right)}_h$$ (1) where T represents temperature, p denotes pressure, h indicates the constant enthalpy, and ∂ denotes the derivative or change in temperature with respect to pressure. Hydrogen also deviates from ideal gas behavior.
Physical properties	Hydrogen is the lowest-density element (it is approximately 14 times lighter than air) [62,63]. It primarily exists in liquid and gaseous states. At 0 °C and $1.01325$ bar, hydrogen has a high speed of sound ($a_{{\mathrm{H}}_2}=1261.1m/s$ [62]) compared to air ($a_{\mathrm{air}}=331.5m/s$ [80]). Extensive research is ongoing into alternative storage forms [81,82,83], including physical and chemical adsorption (metallic hydride) [84,85,86], supercritical or cryo-compressed storage [87,88,89], and other methods, as illustrated in Figure 2.
Abundance	Hydrogen is plentiful, including in Earth’s atmosphere [69,70,72,90,91,92]. It is a component of many fossil fuels, including methane (CH4), methanol (CH3OH), ethene (C2H4), propane (C3H8), and benzene (C6H6) [62].
Regulations	Safe handling and storage standards are available and are comparable to those for natural gas [62,63,73,93,94].

Table 1. Fundamental properties of hydrogen for aerospace and energy applications.

Property	Description
Atomic structure	Hydrogen (H) is the smallest known atom, with a diameter of approximately 0.07 nm [62,63].
Molecular form	It exists primarily as a H2 molecule [62,63]. The H atom itself is highly reactive [64,65,66]. Generally, references to hydrogen denote the H2 molecule rather than the H atom.
Chemical properties	The H atom forms a stable bond with another H atom to create H2. This stable bond results from fully occupied valence electrons, leading to the low reactivity of H2 [67]. The H atom has a relatively low electronegativity of 2.2 [62,63], which describes an atom’s ability to attract electrons when forming chemical bonds.
Sensory properties	Odorless [68,69,70]. Tasteless [69]. Invisible [68,69,70].
Safety properties	Non-toxic [69,71,72]. Non-carcinogenic [62,63]. Not radioactive [63]. Not caustic [62,63].
Production	Renewable production is possible through water electrolysis powered by wind, solar, or other renewable energy sources [73,74,75,76].
Thermodynamic properties	Hydrogen exhibits a negative Joule–Thomson coefficient at high pressures [77,78,79]. This implies that, under constant enthalpy h, an increase in pressure results in a decrease in temperature (cooling during relaxation in a throttle [62]), and vice versa. This relationship can be mathematically expressed as follows: $${\mu }_{\mathrm{JT}}={\left({\displaystyle \frac{\partial T}{\partial p}}\right)}_h$$ (1) where T represents temperature, p denotes pressure, h indicates the constant enthalpy, and ∂ denotes the derivative or change in temperature with respect to pressure. Hydrogen also deviates from ideal gas behavior.
Physical properties	Hydrogen is the lowest-density element (it is approximately 14 times lighter than air) [62,63]. It primarily exists in liquid and gaseous states. At 0 °C and $1.01325$ bar, hydrogen has a high speed of sound ($a_{{\mathrm{H}}_2}=1261.1m/s$ [62]) compared to air ($a_{\mathrm{air}}=331.5m/s$ [80]). Extensive research is ongoing into alternative storage forms [81,82,83], including physical and chemical adsorption (metallic hydride) [84,85,86], supercritical or cryo-compressed storage [87,88,89], and other methods, as illustrated in Figure 2.
Abundance	Hydrogen is plentiful, including in Earth’s atmosphere [69,70,72,90,91,92]. It is a component of many fossil fuels, including methane (CH4), methanol (CH3OH), ethene (C2H4), propane (C3H8), and benzene (C6H6) [62].
Regulations	Safe handling and storage standards are available and are comparable to those for natural gas [62,63,73,93,94].

While Table 1 provides essential information about hydrogen, one key aspect warrants closer examination: its gravimetric energy density. Gravimetric energy density defines the usable energy per unit mass, with higher values enabling greater energy storage at lower weight. Due to the ongoing environmental crisis [95,96], most mobility sectors are eager to find an energy medium that offers both a high energy content for economic reasons and a low weight for environmental reasons. The lighter the energy carrier, the less mass needs to be transported, thereby enabling lower overall power consumption.

Figure 2. Illustrative example of different methods for storing hydrogen [81].

Figure 2. Illustrative example of different methods for storing hydrogen [81].

In its pure form, hydrogen possesses a high gravimetric energy density [81,97,98,99]. However, the gravimetric energy densities achieved in practical applications are significantly lower. This reduction is primarily due to storage system specifications. For practical applications, hydrogen must be stored within a tank. In the case of compressed gaseous hydrogen (CGH₂), pressure is a major factor influencing the gravimetric energy density [100,101]. However, there are several considerations regarding pressure and tank design [102,103,104,105,106,107]. Firstly, there is a technical limit on how much pressure can be contained within a tank. Secondly, higher operating pressure exerts more stress on the tank, dictating appropriate tank materials [44,108,109,110]. Furthermore, the mass necessary for pressure containment reduces the overall gravimetric energy density of the hydrogen storage system, as the tank’s mass does not contribute to the energy content. Consequently, if the tank is very heavy but does not allow for the storage of much hydrogen mass, the resulting gravimetric energy density is low. A comparison of gravimetric energy densities between pure forms and practical storage systems for commonly used fuels is presented in Figure 3. In this figure, the suffix G denotes a gaseous state, L denotes a liquid state, and NG denotes natural gas.

When considering only the gravimetric energy density of pure hydrogen, Figure 3 clearly shows that hydrogen provides more energy per unit mass than other commonly used fuel sources, such as natural gas, gasoline, and diesel.

In pursuit of a more nuanced analysis regarding the non-ideal gas behavior of hydrogen, the real gas factor [111], also known as the compressibility factor, is introduced. It indicates the degree to which a gas deviates from ideal gas behavior. It can be represented mathematically by Equation (2), as shown below, where the variables Z and m signify the real gas factor and the mass of the specific gas, respectively.

$$Z={\displaystyle \frac{m_{\mathrm{ideal}}}{m_{\mathrm{real}}}}$$

(2)

When Z equals one ($Z=1$), there is no disparity between real and ideal gas behaviors. If Z exceeds one ($m_{\mathrm{ideal}}>m_{\mathrm{real}}\Rightarrow Z>1$), the ideal gas equation overestimates the mass, whereas, if Z is less than one ($m_{\mathrm{ideal}}<m_{\mathrm{real}}\Rightarrow Z<1$), the equation underestimates the mass. Real gas factors are typically determined empirically and provided as regression-derived analytical equations or terms. An example for hydrogen is outlined in [112] using Equation (3), with the accompanying constants shown in

Table 2. Constants for approximating the real gas behavior of hydrogen [112].

i	${\mathit{a}}_{\mathit{i}}$	${\mathit{b}}_{\mathit{i}}$	${\mathit{c}}_{\mathit{i}}$
1	$0.05888460$	$1.325$	$1.0$
2	$-0.06136111$	$1.87$	$1.0$
3	$-0.002650473$	$2.5$	$2.0$
4	$0.002731125$	$2.8$	$2.0$
5	$0.001802374$	$2.938$	$2.42$
6	$-0.001150707$	$3.14$	$2.63$
7	$0.9588528\times {10}^{-4}$	$3.37$	$3.0$
8	$-0.1109040\times {10}^{-6}$	$3.75$	$4.0$
9	$0.1264403\times {10}^{-9}$	$4.0$	$5.0$

. The variables $p,T,\rho$, and R represent pressure, temperature, density, and the ideal gas constant, respectively, while $a_i$, $b_i$, and $c_i$ are regression model coefficients. Equation (3) extends the ideal gas equation [113] by incorporating regression terms that account for real gas behavior, which varies with both pressure and temperature.

$$Z(p,T)={\displaystyle \frac{p}{\rho RT}}=1+\sum _{i=1}^9{a}_i{\left({\displaystyle \frac{100\mathrm{K}}{T}}\right)}^{b_i}{\left({\displaystyle \frac{p}{1\mathrm{MPa}}}\right)}^{c_i}$$

(3)

Figure 4 illustrates the deviation of hydrogen from ideal gas behavior at various temperatures, with compression from 0 to $700\mathrm{bar}$, represented by real gas factors [62]. The discrepancy between ideal and real gas behavior increases predominantly with rising pressure. Additionally, the effect of temperature between $50\mathrm{K}$ and $100\mathrm{K}$ follows a distinctly non-linear pattern.

A custom tool was developed to analyze the real gas factor and density across a specified range of pressure and temperature. This is particularly useful when incorporating fuel mass into simulation models, such as finite element (FE) models [117,118,119]. The tool features an interactive three-dimensional (3D) visualization, allowing for a detailed examination of temperature–pressure interactions and their impact on hydrogen density. A depiction of the tool in its eye-friendly dark mode, intended to reduce visual strain and fatigue [120,121], is shown in Figure 5. The interactive version is freely accessible via GitHub (https://jav-ed.github.io/H2_Plot/, accessed on 10 March 2025). Additionally, the source code is available on GitHub (https://github.com/jav-ed/H2_Plot, accessed on 10 March 2025), allowing users to customize parameters such as pressure and temperature resolution.

To summarize, the essential properties of hydrogen are cataloged in Table 1, while safety aspects will be examined in subsequent sections. Preliminary assessments indicate that hydrogen possesses primarily favorable properties. However, its invisibility to the human eye due to its small molecular size, along with potential measurement difficulties, remains a challenge [122,123,124]. Additionally, an established analytical function was introduced to facilitate the calculation of hydrogen mass across various pressure and temperature ranges.

3. Safety-Critical Characteristics of Hydrogen

One key argument for why a thorough understanding of hydrogen properties is essential for structural validation tests is the role that standards play in ensuring safety and reliability [56,57,58,59,60,61]. In commercial aircraft, standards are fundamental for maintaining the consistency, reliability, and safety of structural validation procedures. Similarly, for SWITHs, well-defined standards would provide crucial guidance on the conditions under which hydrogen must be stored, handled, and tested to accurately replicate operational scenarios. Moreover, if hydrogen poses specific hazards, standards would ensure that these risks are properly identified, assessed, and mitigated. They would outline important factors contributing to these risks and, where available, methods to reduce them. However, since no specific standards for SWITHs currently exist, such predefined guidance is unavailable [55].

Given that SWITHs encompass both aerospace and high-pressure vessel domains, existing standards from these fields can offer valuable insights [125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142]. They may help identify relevant structural validation tests, highlight the unique properties of hydrogen, and provide general guidance on conducting such tests. One such valuable insight can be gained by examining ISO 11119-3:2020 [143], which specifies the tests required for high-pressure cylinder certification. The mandatory and optional structural tests specified in ISO 11119-3:2020 [143] offer an informative overview of additional tests that SWITHs may be subject to, beyond the standard requirements for conventional aircraft structure. Beyond the types of tests and the corresponding hydrogen parameters, ISO 11119-3 states the following in paragraph 8.5.1.1: When carrying out the pressure test, a suitable fluid shall be used as the test medium. This can include liquids such as water or oil and gases such as air or nitrogen. This stipulation allows for various filling media in pressure approval tests. In the context of SWITHs, the filling medium refers to the substance stored and pressurized within the wing tanks. Additionally, the standard permits variations in testing media for specific tests. For instance, natural gas is allowed in the permeability test (5.5.12.2), whereas the pneumatic cycle test (5.5.16.1) specifically requires hydrogen to be the filling medium. These findings indicate that only certain experimental tests allow for alternative filling media.

Beyond consulting standards, the need for a thorough understanding of the filling medium for experimental tests becomes apparent when considering the factors listed in

Table 3. Examples of factors influencing the choice of filling medium in an experimental and simulative structure test for SWITHs.

Influence on Structural Properties

1.  Maximum tolerable bending moment.     2.  Maximum elongation.     3.  Tensile strength.     4.  Torsional stiffness.     5.  Young’s modulus.     6.  Impact strength.

Impact on Human Health

1.  Carcinogenicity.     2.  Flammability.     3.  Explosiveness.     4.  Olfactory effects (impact on nasal mucosa).     5.  Radioactivity.

Measurement and Detection Characteristics

1.  Visibility.     2.  Olfactory detectability.     2.  Tactile properties.     4.  Responsiveness to electromagnetic fields.     5.  Alternative sensory detection methods.

Test Bench Safety Considerations

1.  Material compatibility (absence of chemical reactions with test bench components).

Practical Considerations

1.  Availability.     2.  Environmental impact.     3.  Economic efficiency.

. These factors represent some key aspects that should be considered when selecting a filling medium, as they may influence structural properties, human health, measurement accuracy, test bench safety, availability, environmental impact, and economic efficiency.

While the factors outlined in Table 3 underscore the necessity for a comprehensive understanding of the medium used in SWITHs, some of them have already been addressed in Section 2. Based on the findings so far, it can be inferred that only tests requiring an alternative gas would expose potential hydrogen-related hazards. From this perspective, in each potentially hazardous test, hydrogen could be replaced with an alternative filling agent. Following this reasoning, one might initially conclude that all tests requiring hydrogen would be inherently safe and pose no risk to humans or the environment. However, this assumption requires further scrutiny, particularly in the context of aircraft-sized SWITHs. Resolving this uncertainty is one of the main objectives of this work. To clarify the potential risks and challenges associated with hydrogen use, it is essential to first establish some fundamental knowledge and definitions. Sigloch [80] provides the following key concepts:

• Deflagration: a combustion process where the flame front propagates at a speed less than that of sound.
• Explosion: the rapid, nearly instantaneous combustion of combustible materials or explosives, resulting in a large volume of combustion gas that violently displaces the surrounding air.
• Detonation: A combustion process occurring at supersonic speeds, accompanied by a strong pressure increase and anticipated pressure shocks. Due to the intensity of this process, hearing protection is mandatory in proximity to such events.

The combustion speeds and associated pressure increases that are to be expected for the different combustion processes are listed in

Table 4. Combustion types with guide values [80].

Combustion Process	Combustion Rate	Pressure Rise [bar]
Combustion	0.1–50 $\mathrm{m}/\mathrm{s}$	up to ≈0
Deflagration	⪆$0.01\mathrm{km}/\mathrm{s}$	up to ≈10
Explosion	⪆$0.1\mathrm{km}/\mathrm{s}$	up to ≈${10}^3$
Detonation	⪆$1\mathrm{km}/\mathrm{s}$	up to over ≈${10}^5$

. The magnitude of the destructive power for the given pressure increases can be estimated using

Table 5. Estimated pressure thresholds for structural damage [80].

Pressure Rise [bar]	Destruction
⪆0.05	Window panes
⪆0.1	Half-timbered buildings
⪆0.3–0.8	Concrete walls, thickness 12–24 $\mathrm{cm}$

.

A vital consideration in experimental investigations involving hydrogen is its potential for combustion, which can manifest as deflagration, explosion, or detonation. Notably, hydrogen alone is not inherently flammable [62,144,145,146,147]. However, under specific conditions, hydrogen can become flammable or even detonate. The crucial factors, as outlined in

Table 6. Conditions for hydrogen to become ignitable to detonable.

1.  Presence of sufficient ignition energy or attainment of ignition temperature [148,149,150];   2.  Formation of a flammable or detonable gas mixture [144,145,151,152,153];   3.  Special reactive cases: presence of chlorine or fluorine [62,63,154,155].

, create the conditions necessary for hydrogen’s combustibility.

The minimum ignition energy varies with the air-to-hydrogen ratio. However, even a small spark or static discharge can ignite hydrogen in certain gas mixtures. For instance, a static discharge of 20–30 $\mathrm{mJ}$ from rubbing against a carpet and touching a doorknob is enough to ignite a hydrogen–air mixture [62]. Figure 6 illustrates the minimum ignition energy (mJ) of hydrogen (H₂) in humid air (90% relative humidity) and dry air (0% relative humidity). This confirms that a static discharge as low as $20\mathrm{mJ}$ can ignite a hydrogen–air mixture. For comparison with other fuels, such as methane (CH₄) and propane (C₃H₈), the literature provides similar plots of minimum ignition energy across various air compositions [62,63,73]. A comparison of hydrogen, methane, and propane indicates that hydrogen has a lower minimum ignition energy and a broader flammability range.

For further important considerations, let us define the following two technical terms:

• Flash point: For a flammable liquid, this refers to the lowest temperature at which, under specified conditions, sufficient vapors are produced to form an ignitable vapor–air mixture above the liquid surface upon external ignition. Notably, if the ignition source is removed, the flames extinguish [62,63].
• Ignition temperature: this denotes the lowest temperature at which spontaneous ignition of the fuel occurs in an open vessel [62,63].

In both cases, ignition can only occur if a certain amount of air is present.

Table 7. Ignition-relevant properties for hydrogen and various other ignitable fluids [62,63].

Substance	Lower Explosion Limit	Upper Explosion Limit	Flash Point	Ignition Temperature	Minimum Ignition Energy
	[Vol% in Air]	[Vol% in Air]	[°C]	[°C]	[ $\mathrm{mJ}$ ]
Acetylene	$1.5$	82	$-136$	305	$0.019$
Ammonia (L)	15	34	132	651	14
Gasoline (L)	$0.6$	8	>$-20$	240–500	$0.8$
Natural gas	$4.5$	$13.5$	>$-188$	600	$0.3$
Carbon monoxide	$12.5$	75	$-191$	605	>$0.3$
Methane	5	15	$-188$	595	$0.3$
Petroleum (L)	$0.7$	5	55	280	$0.25$
Propane (L)	$2.1$	$9.5$	$-104$	470	$0.25$
Hydrogen	4	$75.6$	$-270.8$	585	$0.017$

offers valuable insights into the ignition or explosion behavior of hydrogen and other fuels. Compared to other gases, hydrogen’s ignition temperature is relatively high, yet the flammability range of a hydrogen–air mixture is exceptionally wide [151,152,158]. While this broad range may be manageable with various safety measures, the fact that hydrogen has the lowest minimum ignition energy among the fluids listed in Table 7 poses a significant challenge for preventing both unintentional and accidental ignitions [151,158,159,160,161,162,163].

Further complicating safety considerations is the fact that hydrogen is invisible to the naked eye. However, experimental investigations conducted by [122,123,124,164] have successfully recorded hydrogen flames. A visual representation of these flames is shown in Figure 7, offering insights into the observable features of hydrogen combustion. This image was captured at a laminar jet velocity of $47\mathrm{m}/\mathrm{s}$ and a Reynolds number of 837 using a Sony DSC D700 (Digital Still Camera, 1344 × 1024 pixels), an f/2.4 aperture, and no filter. Contrary to the common misconception that hydrogen flames are invisible, Figure 7 demonstrates that they are, in fact, visible. Notably, while hydrogen flames are indeed visible, they exhibit lower luminosity than those of burning hydrocarbons [164]. This difference in visibility has significant implications for hydrogen safety and detection methods [122,123,124]. The visibility of hydrogen flames is further influenced by the oxygen content in the mixture, as illustrated in Figure 8. The link between flame visibility and mixture composition provides valuable insights for practical safety measures.

When examining hydrogen flame visibility, the equivalence ratio, denoted as $\varphi$, plays a crucial role. $\varphi$ is defined as the ratio of the actual air–fuel ratio to the stoichiometric air–fuel ratio for combustion [165]. An equivalence ratio of $\varphi =1$ corresponds to stoichiometric combustion, while lower values of $\varphi$ indicate suboptimal combustion conditions. The experimental results show a clear correlation between $\varphi$ and flame visibility: as $\varphi$ increases, the flame becomes more visible. The experimental conditions for these observations are detailed in Figure 8. As [164] highlights, low ambient lighting enhances hydrogen flame visibility, a critical factor in practical flame detection.

Table 8. Combustion characteristics and critical values for hydrogen–air mixtures [62,63].

Hydrogen–Air Mixture Properties
Lower explosion limit (ignition limit)	4 Vol% H2
Lower detonation limit	18 Vol% H2
Stoichiometric mixture	$29.6$ Vol% H2
Upper detonation limit	$58.9$ Vol% H2
Upper explosion limit (ignition limit)	$75.6$ Vol% H2
Ignition temperature	585 °C $\left(858\mathrm{K}\right)$
Minimum ignition energy	$0.017\mathrm{mJ}$
Maximum laminar flame velocity	≈$3\mathrm{m}/\mathrm{s}$
Maximum adiabatic combustion temperature	≈2200 °C

contains detailed data on the limits of hydrogen–air mixtures for various types of ignition and other relevant parameters. Under normal conditions (0 °C, 1.01325 bar), hydrogen is flammable at concentrations between $\left[4\mathrm{and}75.6\right]\%$ and detonable between $\left[18\mathrm{and}58.9\right]\%$ [63].

Beyond the challenges related to air content outlined in Table 8, Table 6 highlights two exceptional cases of hydrogen reactivity. First, hydrogen and chlorine can react solely through light irradiation, producing a loud, explosive bang known as the chlorine oxyhydrogen reaction [62,63]. Second, hydrogen undergoes a similarly explosive reaction with fluorine [62,63]. Further studies on the explosion limits of various hydrogen–oxygen mixtures can be found in the literature, such as [166,167,168,169,170,171,172]. These cases underscore the diverse and potentially hazardous reactive properties of hydrogen. The reactive nature of hydrogen not only manifests in gaseous interactions but also in its interactions with solid materials.

Therefore, an important consideration in hydrogen-related SWITH simulations and physical tests is the material compatibility of hydrogen. Hydrogen can accumulate in metal lattices, leading to embrittlement, which degrades structural properties [44,173,174,175,176,177]. Hydrogen incorporation into metal lattices occurs when hydrogen molecules dissociate into atoms at the metal’s surface [63,81]. Certain pure metals, including palladium, magnesium, lanthanum, and aluminum, as well as specific alloys such as TiNi-Ti₂Ni and Mg-Mg₂Ni, can store hydrogen [63]. Hydrogen storage in metal lattices can also be used to produce highly pure hydrogen (99.99%) [81,178,179]. Hence, metal lattices can act as filters, turning a potential challenge into a practical benefit for hydrogen purification.

Although plastics are not susceptible to hydrogen embrittlement [102,103,104,105,106,107], they present significant challenges in retaining hydrogen. In the worst-case scenario, a filled tank could rapidly deplete due to hydrogen permeation through the vessel walls. If a leaking hydrogen tank is stored in an enclosed space, such as a garage, the accumulation of hydrogen could create an undetected explosive atmosphere, where even a small spark or open flame could trigger a fire or explosion, posing a serious risk to occupants. Therefore, two of the most critical properties to consider for hydrogen storage are its flammability and diffusion behavior. For a more detailed discussion on hydrogen diffusion in polymers, see [180].

In summary, hydrogen is not flammable without the addition of oxygen. However, a wide range of hydrogen–air mixtures can ignite. Furthermore, even a very small static discharge is sufficient to ignite a flammable gas mixture, leading to a potential detonation. Hydrogen affects structural properties, particularly in metals, by causing embrittlement; while plastics are not susceptible to embrittlement, they face significant challenges in retaining hydrogen. Given hydrogen’s diffusion behavior and flammability, even a minor undetected leak in a highly pressurized tank could endanger both personnel and instruments during experimental tests for SWITH certification. Consequently, the following section naturally transitions to the experimental and numerical validation of these phenomena.

4. Safety-Focused Hydrogen Experiments and Simulations for SWITHs

This section bridges the theoretical understanding of hydrogen with practical insights from experimental and simulation investigations. It evaluates the feasibility of conducting structural validation tests with hydrogen, which may be essential for future SWITH certification. A critical phenomenon in this context is the formation of shock waves, which arise when local flow velocities exceed the speed of sound, as described in [80,181] and observed experimentally in [182,183]. Shock waves are complex phenomena [184] and their description will be limited to what is necessary for understanding the presented results. Shock waves primarily affect their surroundings in three distinct ways. They generate extremely high pressures, while temperatures can reach extreme levels, sometimes exceeding several thousand Kelvin (e.g., $3000\mathrm{K}$ [185] and higher [186]). Additionally, rapid and significant pressure changes create airborne noise, necessitating the use of hearing protection.

Xu et al. [185] simulated these shock wave effects by compressing hydrogen to $250\mathrm{bar}$ in a pressure vessel and releasing it into the free atmosphere through a nozzle. The simulation output, shown in Figure 9, depicts the system at various time intervals after the hydrogen exits the nozzle. To analyze this output, the Mach number ($Ma$) is defined using Equation (4) below, where a and v represent the speed of sound and local velocity, respectively. The Mach number expresses the ratio of local velocity to the speed of sound. For example, $Ma=5$ indicates that the local velocity is five times the speed of sound.

$$Ma={\displaystyle \frac{v}{a}}$$

(4)

As shown in Figure 9, the Mach scale in this simulation extends up to seven, reflecting the range of the computed results. Notably, shock waves—and their associated consequences—begin to occur at Mach 1. As the Mach number increases, these effects, including high temperatures, high pressures, and increased noise, intensify significantly. For a more in-depth interpretation of these results,

Table 9. Speed of sound in various media, with gases measured at 20 °C and $1\mathrm{bar}$ [80].

Medium	Speed of Sound [m/s]
Steel	5170
Cast iron	3210
Concrete	3730
Polymer: Polyvinyl Chloride (PVC)	1462
Polymer: Polyethylene (PE)	973
Wood	4500
Glass	5300
Water	1437
Diesel	1206
Gasoline	1062
Air (dry)	343
Helium	1005
Hydrogen	1300
Argon	318
Nitrogen	349
Methane	446

can be consulted.

Each medium has a characteristic speed of sound that determines the propagation of pressure changes and sound waves through it. This relationship is expressed by Equation (5), where a, p, and $\rho$ denote the speed of sound, pressure, and density, respectively [80], as follows:

$$a=\sqrt{{\displaystyle \frac{dp}{d\rho }}}$$

(5)

A higher speed of sound in a medium corresponds to faster information transmission. For instance, when one end of a steel rod is struck, the sound propagates to the other end at $5170\mathrm{m}/\mathrm{s}$, making it audible almost instantly. The speed of sound in hydrogen at 20 °C and $1\mathrm{bar}$ is $1300\mathrm{m}/\mathrm{s}$, which is significantly higher than in air ($343\mathrm{m}/\mathrm{s}$). Consequently, information exchange in hydrogen occurs more rapidly than in air. This characteristic is crucial for understanding the behavior of pressurized hydrogen released into the atmosphere. Air undergoes a shock wave at $343\mathrm{m}/\mathrm{s}$, causing a significant temperature rise. In contrast, hydrogen can reach nearly four times this speed before experiencing a shock wave. A simplified explanation for the pronounced temperature increase during shock waves is as follows: when air molecules cannot adapt quickly to pressure changes because the speed at which pressure changes propagate is limited, they are forced to change abruptly. This phenomenon is analogous to a supersonic aircraft encountering air molecules before they can adjust to its approach. The resulting rapid compression and frequent, intense collisions convert substantial kinetic energy into thermal energy, yielding a marked temperature rise. This process predominantly occurs in compressible media like gases, explaining the prominence of shock waves in air and hydrogen. These temperature effects are visualized in Figure 10, which presents the temperature contours associated with Figure 9. Temperatures exceeding $1000\mathrm{K}$ are typically associated with explosions [186,187,188,189].

The provided explanation, when applied to the described outcomes, indicates that, as hydrogen flows into the free environment, its outflow velocity already exceeds the speed of sound in the surrounding air. Consequently, the air experiences a shock wave, leading to rapid compression and a significant temperature increase. Hydrogen itself, however, would not initially exceed its own speed of sound and, therefore, would not generate a shock wave upon release. The elevated temperatures near the hydrogen, induced by the air shock wave, may not immediately trigger detonation. Subsequently, as hydrogen disperses into the air and diffusion occurs, an ignitable concentration of the oxygen–hydrogen mixture is eventually reached [190]. At this point, the surrounding high temperature becomes sufficient to cause ignition, which, under certain conditions, can escalate to detonation. This explanation is a simplified model illustrating core concepts. The actual physics of shock wave formation and temperature rise involve complex thermodynamics. For a more detailed discussion, refer to advanced texts on nonlinear compressible fluid dynamics [191].

Ignitions that occur without external ignition sources are referred to as self-ignitions or spontaneous ignitions [192]. Various theoretical models for self-ignition have been proposed, including the reverse Joule–Thomson effect, electrostatic ignition, brush and corona discharges, diffusion ignition, sudden adiabatic compression, hot surface ignition, mechanical friction, and impact ignition [151,193,194,195]. Despite the variety of proposed mechanisms, not all are equally probable in real-world scenarios. Compression ignition, Joule–Thomson expansion, diffusion ignition, and ignition by hot surfaces are generally considered unlikely for most unintentional hydrogen releases at ambient temperatures [193]. However, it is important to recognize that several of these mechanisms may collectively contribute to a self-ignition event [193]. Among these mechanisms, diffusion ignition, as proposed by Wolański and Wójcicki [196], has received significant attention. This model describes a spontaneous ignition phenomenon that occurs when pressurized hydrogen is released into a chamber containing either pure oxygen or air [195]. Interestingly, ref. [193] concluded that diffusion ignition, along with compression ignition, Joule–Thomson expansion, and ignition by hot surfaces, is unlikely for most unintentional hydrogen releases at ambient temperature.

Nonetheless, despite this assessment, extensive research has been conducted on the diffusion ignition mechanism. One notable example of intensive experimental investigations into the outflow of pressurized hydrogen into the free environment was conducted by [93]. Additionally, numerous studies using cylindrical shock tubes have been carried out by various researchers [197,198,199,200,201,202,203,204,205,206,207]. Comprehensive reviews and summaries of spontaneous ignition mechanisms in pressurized hydrogen released through tubes are available in [151,195]. The findings from these studies are not only valuable for conducting experimental investigations with hydrogen but also have major implications for the design of SWITHs. In the following paragraphs, these findings are presented in detail, accompanied by an explanation of their relevance to SWITH design and experimental procedures.

When pressurized hydrogen is released from its vessel, certain geometrical aspects have a significant impact on the subsequent spontaneous ignition. One crucial factor is the tube length. Both numerical simulations and experimental studies on releasing pressurized hydrogen indicate that the longer the pipe, the greater the likelihood of spontaneous ignition occurrence is [151,194,195,208]. However, when the tube length exceeds a certain threshold, the likelihood of self-ignition in high-pressure hydrogen decreases [194]. According to [207], this reduction occurs only when the tube length exceeds $1700\mathrm{mm}$; while longer tubes are economically preferable [98,209,210], their potential to increase the risk of self-ignition complicates the determination of an optimal length. Existing experimental studies on self-ignition focus on shorter tubes. However, for SWITH design, it may be necessary to investigate significantly longer configurations. The need for tubes ranging from approximately $5\mathrm{m}$ to $80\mathrm{m}$ [211] is driven by economic and practical considerations. Maximizing flight range is a key economic goal, while the length of wing-integrated tanks in commercial aircraft significantly exceeds that of tubes typically used in experimental research.

The preference for shorter tubes (less than $5\mathrm{m}$) in current experiments is primarily due to cost and time constraints faced by researchers. Previous reviews [151,194,195,207] have indicated that experimental tests are generally conducted with tube lengths of ≤$4200\mathrm{mm}$. Notably, tube lengths of ≤$1000\mathrm{mm}$ are more common than the comparatively long $4200\mathrm{mm}$ tubes used in [202]. A comprehensive list of experimental parameters, including the tube lengths used in various studies, is provided in

Table 10. Experimental parameters and setups for various studies on hydrogen diffusion ignition [151].

Experiment	Burst Pressure [MPa]	Tube Length [mm]	Diameter [mm]	Cross-Section of Tube
Dryer 2007 [93]	1.4–11.3	38.1–3000	4/12.7	cylinder
Mogi 2008 [197]	4–30	3–300	5/10	cylinder
Golub 2008 [198]	2–13	65–185	10	cylinder/rectangle
Mogi 2009 [199]	2–20	3–500	5/10	cylinder
Lee 2011 [200]	10.8–23.5	50/100/200/300	10.9	cylinder
Kim 2013 [190]	6.5–11.3	300	10	rectangle (visual)
Kim 2013 [201]	8–30	10–200	3	cylinder
Kitabayashi 2013 [202]	1–8	100–4200	10	cylinder
Grune 2014 [203]	2.4–24	210–1145	4/5/10	cylinder/rectangle (visual)
Duan 2016 [205]	2–11	80–360	15	cylinder
Gong 2016 [204]	2–9	360	15	cylinder
Jiang 2019 [206]	5–7	2200	10/15	cylinder
Wang 2019 [207]	2–12	300–3000	10	cylinder

. This table highlights the limitations of current experimental setups and underscores the need for tests that more closely reflect the actual dimensions of aircraft-sized SWITHs.

Depending on the outcome of self-ignition experiments assessing aircraft-relevant tube lengths and economic feasibility, numerical optimization [212,213,214,215,216,217,218,219,220,221,222] may be required to determine optimal tube dimensions. The need for optimization is further highlighted by the influence of multiple geometrical parameters, beyond just length, on self-ignition behavior. In addition to tube length, diameter is another critical factor. Studies have shown that tubes with smaller diameters are more prone to spontaneous ignition [190,194,195]. Consequently, based on our current understanding, a larger diameter is generally recommended to mitigate the risk of self-ignition. The cross-sectional area is another important geometrical parameter influencing spontaneous ignition [223,224]. Variations in the local cross-section, whether decreases or increases, lower the critical release pressure threshold for ignition compared to tubes with constant cross-sections [195,223,224]. The overall tube shape has little impact on the minimum burst pressure needed for spontaneous ignition [195]. However, in non-circular cross-section tubes, turbulent flow in the corners can enhance the mixing of hydrogen and air. This enhanced mixing increases the amount of hydrogen–air mixture formed [225].

Both diameter and cross-section are key parameters in designing SWITH tubes. Along with tube length, these factors determine the total volume of SWITH tubes. For clarity,

Table 11. Geometrical and environmental factors primarily influencing hydrogen self-ignition, with particular relevance to SWITH design.

Parameter	Operation
Length	Short
Diameter	Big
Compression pressure in the container	Low
Other influencing factors
Geometry of the exit area
Size of the outer surface into which compressed gaseous hydrogen flows out (can gas accumulations arise?)
Gas or medium in the external environment
External pressure
External temperature

provides a concise summary of these findings.

Hydrogen self-ignition has been observed both in theoretical models and real-world applications [226]. This real-world occurrence highlights the need for strict hydrogen safety measures, especially for experimental validation tests of SWITHs. Consequently, further investigations are imperative to establish and refine safe working protocols. To better understand the recommendation in Table 11 for maintaining low pressure inside hydrogen containers, the study reported in [227] is examined in detail. This study investigated the effects of various oxidizing agents on hydrogen’s spontaneous ignition. The experimental setup involved releasing hydrogen from a high-pressure vessel through a nozzle, similar to previously mentioned studies. However, a key distinguishing feature of this experiment was the controlled environment surrounding the hydrogen jet. Unlike earlier investigations conducted in an open atmosphere, the experiment in [227] introduced a constant flow of oxidizing agents perpendicular to the hydrogen jet. The tested oxidizing agents included air, pure oxygen (O₂), nitrous oxide (N₂O), and acetylene (C₂H₂). The experimental apparatus utilized a straight expansion tube with specific dimensions: a diameter of $4\mathrm{mm}$ and a length of $10\mathrm{cm}$. To capture spontaneous ignition, the researchers employed a high-speed camera. Additionally, they measured external overpressures to quantify the effects of the ignition. The experimental findings displayed in Figure 11 illustrate how hydrogen container pressure and ambient gas composition influence the likelihood of self-ignition.

The results depicted in Figure 11 reveal several key insights. First, there is a clear correlation between increasing hydrogen container pressure and a higher probability of self-ignition. At low pressures, self-ignition was avoided in all tested gas environments. This observation is particularly significant, as preventing unintended hydrogen ignition is a key safety requirement for practical applications. When considering only the prevention of self-ignition, the tested ambient gases can be ranked by safety. Oxygen (O₂) presents the most hazardous conditions for hydrogen applications; while pure air and nitrous oxide (N₂O) are less hazardous than oxygen, they are still considered unsafe for hydrogen applications. Among the tested gases, only acetylene (C₂H₂), which is itself combustible, met safety requirements for pressures below $230\mathrm{bar}$.

Revisiting key points from Section 3 and this section confirms that hydrogen possesses both safety-critical and beneficial properties. Despite potential hazards, hydrogen remains a viable option for practical applications when handled responsibly and with a comprehensive understanding of its unique characteristics [62,73,94,227,228]. Notably, Landucci et al. [228] concluded that the risks associated with compressed hydrogen are comparable to those of liquefied petroleum gas and natural gas. The referenced studies are largely based on specialized testing environments or the automotive industry; while their findings offer valuable insights for SWITHs, they differ significantly from aircraft requirements. Understanding these differences requires a closer look at SWITH-specific constraints. In SWITHs, hydrogen tanks are integrated into the aircraft wings, where they must maintain high internal pressure and volume for optimal energy density. Additionally, they must withstand both internal pressure and external loads. The aviation industry’s pursuit of minimal weight and high safety standards favors the use of mature Type IV tanks [102,104,106,108,109,110,229,230,231,232]. However, this design choice introduces additional safety concerns. In the event of rupture, the high-speed dispersion of composite tank fragments could endanger both personnel and measurement devices. Moreover, composite breakdown may release microscopic particles or fibers into the air. Inhalation of these particles can lead to respiratory problems [233,234], a risk that is particularly acute in confined, poorly ventilated spaces.

The goal of maximizing hydrogen load during flight necessitates optimizing tank pressure and volume. This requirement emphasizes a key distinction between SWITHs and most other applications of compressed hydrogen cylinders: the size of the tanks. For instance, the APUS i-2 [54] has a wingspan of $13.2\mathrm{m}$, with approximately ⪆$50\%$ of its length occupied by four cylindrical high-pressure tubes. In comparison, the Airbus A350-1000 has a wingspan of about $64\mathrm{m}$ and a flight range of $16,482\mathrm{km}$ [235]. Assuming 50% of this wingspan could accommodate tubes, the total tube length would be $64\mathrm{m}\times 0.5=32\mathrm{m}$. However, the objectives of maximizing tank size and internal pressure may inadvertently increase the risk of self-ignition, as highlighted earlier in this section.

Another key distinguishing factor for SWITHs, particularly compared to land vehicles, is their operation across a range of altitudes. This variability in operational environment introduces several complex phenomena. At higher altitudes, reduced ambient pressure increases the pressure differential between the hydrogen tanks and the surrounding environment. This greater differential may lead to higher hydrogen release velocities in the event of a leak. Importantly, if the release velocity does not exceed the speed of sound in hydrogen, a hydrogen shock wave is not expected. Additionally, this effect is further complicated by the decrease in the speed of sound in air with altitude. As a result, the probability of an air shock wave forming increases, even at lower hydrogen release velocities. When diffusion causes the released hydrogen to mix with surrounding air at an ignitable concentration, various outcomes—ranging from ignition to detonation—become possible. Conversely, lower air density at higher altitudes [236,237] introduces a counteracting effect. The reduced density may slow or complicate the formation of ignitable concentrations as air and hydrogen diffuse. This adds another layer of complexity to the safety considerations for SWITHs operating at various altitudes.

Furthermore, considering the increased pressure difference and reduced air density in tandem, the behavior of released hydrogen becomes more nuanced. The speed at which hydrogen is released is not only enhanced, due to the decreased ambient pressure, but also due to an increased mean free path for gas particles. In other words, hydrogen molecules can traverse longer distances before colliding with air molecules, leading to a delayed reduction in speed. This delay occurs because fewer collisions reduce the conversion of kinetic energy into thermal energy. Consequently, lower air density at higher altitudes allows hydrogen molecules to maintain high velocity over greater distances, potentially enabling a more rapid dispersion and coverage of larger areas compared to releases at lower altitudes. The decreased temperature at higher altitudes further complicates hydrogen behavior. The interplay of temperature, pressure, and density affects diffusion rates, ignitable mixture formation, and the likelihood of ignition or detonation. These altitude-dependent factors significantly impact hydrogen release dynamics and safety, emphasizing the need for further research. Comprehensive studies are required to fully understand these effects and address the following two key aspects: first, ensuring the safe design and operation of SWITHs across their entire operational altitude range; and, second, replicating these conditions for ground-based experimental testing.

To facilitate the use of hydrogen in SWITHs, developing experimental structural validation test methods is essential to ensure safe working conditions for personnel, instrumentation, and the SWITH itself. However, based on the findings presented in this paper, immediately fulfilling this requirement for large-scale SWITHs, such as those comparable to the Airbus A350-1000 [235], may not be feasible. Instead, a more prudent strategy involves taking smaller, incremental steps towards the larger goal. This gradual approach offers several advantages: it enhances risk management, facilitates knowledge accumulation, and allows researchers to identify and resolve issues at smaller scales before progressing to larger, more complex systems. Additionally, it enables a more cost-effective development process. By following this incremental strategy, researchers can systematically build upon their findings, ensuring a robust and safe development pathway for SWITHs.

The experimental observations presented by Jallais et al. [227], as illustrated in Figure 11, suggest a potential way to mitigate a critical safety concern. To prevent self-ignition or detonation, the working environment could be maintained in a protective gas atmosphere; while acetylene demonstrated enhanced safety properties, helium could be a more suitable option due to its high speed of sound ($1005\mathrm{m}/\mathrm{s}$, according to Table 9) and its noble gas characteristics. The inert nature of noble gases, attributed to their fully occupied outer electron shells, makes them highly unreactive. Other noble gases, including neon, argon, krypton, xenon, radon, and oganesson, should also be considered [238,239,240]. The proposed experimental approach involves a structured, incremental process. Initial small-scale experiments should be conducted, systematically increasing either the internal pressure or cylinder volume based on the results. If the proposed inert gases prove ineffective as a surrounding medium, alternative options should be explored. Upon identifying combinations of maximum internal pressure, maximum volume, and surrounding gas that prevent ignition, the research should progress to testing with the addition of external loads. This subsequent phase should be initiated with reduced internal pressure, volume, or a simultaneous decrease in both parameters.

Throughout this critical stage, it is of paramount importance to rigorously investigate various combinations of internal pressure, volume, load type (static or dynamic [241,242,243,244,245]), and external load distribution. Maintaining meticulous records of configurations that can be executed without ignition is vital. This stringent testing strategy ensures a comprehensive understanding of the key variables and helps establish the safest experimental conditions for hydrogen-filled SWITHs. Concurrently with these experimental efforts, simulations should be developed to encapsulate the observed physical behavior. The ultimate objective is to refine experimental testing methods for SWITHs at the scale of large aircraft like the Airbus A350-1000 [235]. This effort must be accompanied by the development of reliable simulation tools to minimize reliance on highly cost-intensive experiments [246]. If substantial discrepancies arise between simulations and experimental results, one possible factor to examine is the influence of the surrounding medium on measurement equipment.

In conclusion, hydrogen self-ignition has been shown to be a tangible concern in real-world applications. This section outlined the primary geometrical factors that can precipitate ignition and explained the modifications necessary to mitigate this risk. Despite the acknowledged risk, the literature supports the feasibility of safe hydrogen applications, provided its properties are thoroughly understood and properly managed. However, caution is crucial when directly applying hydrogen applications from the automotive industry to SWITHs at the scale of large commercial aircraft, such as the Airbus A350-1000. A more judicious strategy involves conducting a series of smaller-scale experimental and numerical studies, incrementally approaching the scale of a large aircraft. This approach enables the systematic accumulation of knowledge and careful risk management when scaling up hydrogen technologies in aviation. A potential pathway to achieving this goal, emphasizing the importance of a gradual, methodical research and development approach, has been outlined.

5. Conclusions

This study investigates the feasibility and safety considerations of using hydrogen as an energy carrier in Wing Structure-Integrated high-pressure Hydrogen Tanks (SWITH), with a particular focus on structural validation tests for aircraft-scale implementations. To establish a foundational understanding of hydrogen’s properties and its viability as an alternative fuel, its fundamental characteristics were examined in detail. This was followed by a theoretical analysis of safety-critical characteristics, highlighting potential hazards such as flammability, detonation risks, material compatibility, and leakage concerns. Additionally, key mitigation strategies were outlined, aiming to support the safe integration of hydrogen into aircraft structures. To validate these insights, findings from numerical simulations and experimental studies available in the literature were reviewed, providing empirical support for the identified safety concerns.

Building upon these insights, a structured framework was proposed for conducting experimental tests on SWITHs, with a strong emphasis on safety. The framework aims to establish testing conditions that minimize risks to the SWITH itself, surrounding personnel, measurement technology, and other critical components required for structural testing. However, in light of these findings, the immediate initiation of large-scale SWITH experiments—without prior risk assessment and gradual scaling—is strongly discouraged. Instead, a structured roadmap should be followed, beginning with small-scale experiments and advancing systematically toward aircraft-sized implementations to ensure safety and reliability. This roadmap underscores the critical need for a thorough experimental and numerical investigation of key variables, including internal pressure, internal volume, load type, and external load distribution. A comprehensive understanding of these factors is essential for the design, operation, and experimental validation of this new aircraft concept. The results of this study align with findings from previous research, which suggest that hydrogen can be utilized safely when its properties are well understood and managed responsibly. A key example of such a responsible approach is the gradual and cautious progression in experimental testing—specifically, refraining from conducting structural tests with high-pressure gaseous hydrogen on large-scale SWITHs at the outset.

The findings of this study have notable implications for the future of aviation, particularly in the context of sustainable fuel alternatives. SWITHs address a key challenge in aviation by acting as a more environmentally friendly and efficient energy solution. However, several technical and safety challenges must be resolved to ensure their successful implementation. Future research should further explore the impact of increasing altitude on the likelihood of ignition transitioning to detonation. Additionally, more advanced investigations into the structural behavior of pressure vessels under external loads are necessary. Developing reliable simulation tools to complement these studies could help reduce reliance on costly experimental testing. Advancements in these areas, alongside an effective certification framework, would pave the way for the safe and scalable adoption of SWITHs in aviation.

Finally, certain key decisions regarding the execution of a preliminary validation test of a small-scale SWITH were guided by the findings of this study. These tests were conducted at IMA Materialforschung und Anwendungstechnik GmbH in Dresden, representing a pioneering endeavor with the potential to shape future structural validation efforts.