# Spectral project - application of FAST-OAD code to the conceptual design of a hydrogen fuelled commercial aircraft

Mayur Oak, Alice Fabre, Martin Delavenne, Eric Nguyen Van, Emmanuel Benard, Sébastien Defoort

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## ► To cite this version:

Mayur Oak, Alice Fabre, Martin Delavenne, Eric Nguyen Van, Emmanuel Benard, et al.. Spectral project - application of FAST-OAD code to the conceptual design of a hydrogen fuelled commercial aircraft. IOP Conference Series: Materials Science and Engineering, 2022, 1226 (1), pp.012027. 10.1088/1757-899X/1226/1/012027. [hal-05025134](https://hal.science/hal-05025134v1)

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https://hal.science/hal-05025134v1

Submitted on 8 Apr 2025

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# Spectral project - application of FAST-OAD code to the conceptual design of a hydrogen fuelled commercial aircraft

To cite this article: Mayur Oak et al 2022 IOP Conf. Ser.: Mater. Sci. Eng. 1226 012027

View the [article online](#) for updates and enhancements.

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# Spectral project - application of FAST-OAD code to the conceptual design of a hydrogen fuelled commercial aircraft

Mayur Oak<fnref n="1" />, Alice Fabre<fnref n="1" />,<fnref n="2" />, Martin Delavenne<fnref n="1" />, Eric Nguyen Van<fnref n="3" />, Emmanuel Benard<fnref n="1" />, Sébastien Defoort<fnref n="3" />

E-mail :

Mayur.OAK@student.isae-supaero.fr, Alice.FABRE@student.isae-supaero.fr,

Martin.DELAVENNE@isae-supaero.fr, Eric.Nguyen_Van@onera.fr,

Emmanuel.BENARD@isae-supaero.fr, Sebastien.Demoort@onera.fr

**Abstract.** With the increase in the air traffic, reducing the aviation environmental impact is the key challenge of the community. This has called for more research in the area of using alternative fuels which will reduce the greenhouse gas emissions and which can provide similar or better environmental performance. This work presents a preliminary design methodology of a mid-range single aisle commercial aircraft using Hydrogen as fuel which is stored in an integrated tank in the fuselage. The iterative design process was achieved by using overall aircraft design approach through the FAST-OAD open-source software.

This study describes the models used for a first approximation of structural and insulation weight of liquid Hydrogen (LH₂) tank and fuel boil off computations. It then presents the impacts of an LH₂ storage system on the overall aircraft design with an emphasis on the centre of gravity (CG) travel and the additional fuselage length necessary to accommodate the LH₂ tank. An additional parametric study has also been carried out to understand the effect of some design parameters on overall design. Finally, an environmental study has been conducted to evaluate the direct emissions of this aircraft in operation. The only emissions that result from the combustion of hydrogen are nitrogen oxides (NOx) which were evaluated using the Boeing Fuel Flow Method (BFFM2) and water vapor (H₂O) which was evaluated based on the fuel burned. As a result of this study significant reduction in NOx and CO₂ emissions as compared to existing design are envisaged, thus showing the promising potential of Hydrogen fuelled aircraft.

## 1. Introduction and overview

The worldwide consequences of the rapid and significant temperature variations are numerous and have been identified as a threat to many forms of life and activities. Due to climate change, the states that have ratified the Paris Climate Agreement [1] have committed to limit global warming well below +2°C above pre-industrial levels and to pursue efforts to limit the increase to 1.5°C.

However, several routes for reaching those ambitious targets can be envisioned, from new policies to emerging technologies. In the field of aviation, this has called for more research in the area of using alternative fuels which will reduce the greenhouse gas emissions and which can provide similar or better performance. Hydrogen comes as a natural choice of fuel considering it can reduce the nitrogen

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<footnote n="1">¹ISAE-SUPAERO - Université de Toulouse, 31055 Toulouse, France</footnote>

<footnote n="2">²IRT Saint-Exupéry - 3 rue Tarfaya, CS 34436, 31405 Toulouse cedex 4, France</footnote>

<footnote n="3">³ONERA/DTIS - Université de Toulouse, 31055 Toulouse, France</footnote>
oxide (NOₓ) emissions and suppresses entirely all the direct carbon oxide emissions (CO, CO₂, hydrocarbons) in flight. It also has almost three times more mass energy density as compared to kerosene, but it presents a specific challenge in terms of volume management. Existing aircraft design methods must therefore reflect this incoming disruption in aviation. This work presents a preliminary design methodology of a mid-range single aisle commercial aircraft using LH₂ as fuel. The present study is based on a recent open-source software development, FAST-OAD [2] which provides a collection of modules and design logic to size the aircraft upon its top-level requirements and technology features as shown in Figure 1 and it is implemented using the OpenMDAO framework [3]. The latest version is available at the following Github repository https://github.com/fast-aircraft-design/FAST-OAD

<figure><img src="image_5.png" /><figcaption>Figure 1: FAST-OAD disciplines interrelationships.</figcaption></figure>

The main modifications of the base code were related to the geometry, weight, propulsion and performance disciplines. We have also added a thermal module to provide preliminary outputs for the boiled-off quantity of fuel over the duration of flight.

## 1.1. *The choice of mission*

In order to limit the design space following constraints were set for this study:

1. The aircraft range should be 3100 NM.

2. The maximum number of passengers should not exceed 132.

3. There should not be any radical changes in fuselage geometry.

4. The aircraft should maintain the centre of gravity (CG) within current accepted limits.

5. The aircraft should have significantly less emissions.

The reference for this study is a short single aisle medium range aircraft such as Airbus A318 [4] or Boeing 737-600 aircraft [5]. The preliminary studies showed that the initial requirements based on short single aisle aircraft mission allowed to keep the length of the LH₂ variant within an acceptable value as per current industry standards. Hence it was decided to use short single aisle medium range aircraft as a baseline to design the LH₂ variant.

## 2. Design approach and module details

In FAST-OAD code, the conventional aircraft is designed based on iterative process as described in Figure 1, but for hydrogen variant we need to modify few of the existing modules to incorporate the LH₂ tank related computations. The details of which are described in following subsections.

### 2.1. *Geometry module*

The geometry module computes the dimensioning of an entire aircraft by segregating it in five main components: fuselage, wing, horizontal tail, vertical tail and engine nacelle. The statistical models and empirical equations included within the FAST-OAD core code has been used for this step.
As per the literature studies there are two possible ways of incorporating the tank in fuselage: Integral tank design which form an integral part of the fuselage structure and non-integral tank design which will be mounted inside the fuselage supported by the frame structure. The current literature indicates that the use of integral tanks would be more efficient in terms on weight savings, inspection and repair [6]. Hence, we decided to choose the integral tank design located after the cabin pressure bulkhead. The tank volume required gets computed by considering fuel density at 20K and fuel mass based on:

1. Mission fuel as computed by performance model

2. Fuel boil-off as computed by thermal model for entire flight duration of 8 hours.

3. The tank can be filled to a level of only 88.5% of its volume to ensure that 3% of the volume remains gas at the given vent pressure. Hence tank volume needs to be increased by 11.5%. An additional allowance of 3.1% was added further to account for tank contraction and expansion, space for internal structure and equipment, trapped and unusable fuel, gas space for exit pipe as defined by available literature [6].

The insulation strategy used in this study consists of using polyurethane foam and some additional layers as defined in Figure 2.

<figure><img src="image_6.png" /><figcaption>Figure 2: The insulation structure for the foam-based insulation [6].</figcaption></figure>

As the heat transfer takes place between the outer environment and tank, LH₂ will convert into gaseous phase, thus increasing the inner pressure of tank. Hence some portion of the gaseous hydrogen needs to be vented out of the tank. In order to minimise the boil-off of LH₂, the vent pressure of 3.5 bar and the fill pressure of 1.2 bar was used for this study. The choice of vent pressure and fill pressure leads to average fuel density of 63 kg/m³[6]. The tank material chosen is Aluminium (4.4% Cu) 2014–T60 which has the density of 2800 kg/m³. The wall thickness ($t_w$) required for the tank structure to hold this vent pressure (P) was computed for a cylindrical tank with hemispherical end caps of radius (r) based on factor of safety (FOS) of 4 and material strength ($\sigma$) using

Equation (1):

$$t_w = \frac{P*r*FOS}{2*\sigma} \qquad \text{Equation (1)}$$

This results in a wall thickness of 3.34 mm. The length of the tanks is then computed based on the volume computations for a cylindrical tank with hemispherical end caps and this length gets added to the fuselage length. Geometry module also provides a 'vspscript' file which can be opened in OpenVSP software to view the 3D geometry of the converged design of aircraft generated by connecting the FAST-OAD outputs to OpenVSP.

## 2.2. Thermal module

The geometry module provides the dimensions for the tank and insulation geometry to thermal module. The thermal module then computes the total amount of heat transferred from atmosphere to LH₂ by considering following heat transfer modes:

1. External convective & radiative heat transfer

2. 1-D conductive heat transfer through insulation

3. Internal convective heat transfer

The thermal module is based on the calculation of thermal heat resistance for all the modes that are mentioned above based on the equations suggested in the literature [6].
Finally, based on the total heat transferred per hour ($Q_{total}$) and the latent heat of vaporisation for LH₂ (446,592 J/kg) the total quantity of boiled-off fuel (kg) gets computed for the duration of mission (8 hours).

$$Total\ boil-off = \frac{8*Q_{total}}{latent\ heat\ of\ vaporisation} \qquad Equation\ (2)$$

This quantity of fuel will be wasted during the flight and hence this quantity of extra fuel needs to be added in the mission fuel computation in performance module.

## 2.3. Weight module

The conventional FAST-OAD weight module computes the overall empty weight (OWE) based on the weight calculations of airframe, power plant, systems and fixed installations, operational items and crew. Maximum take-off weight (MTOW) is also computed based on the OWE, fuel and payload weight for the mission. For this work, following additional weights are considered to account for the weights related to LH₂ storage and fuel systems.

### 2.3.1. LH₂ tank weight: The LH₂ tank weight consists of empty LH₂ tank weight and insulation weight. An additional 10% mass penalty is added to consider the support structure weight. A truss type structure can be used to attach the LH₂ tank to the cabin pressure bulkhead [7].

### 2.3.2. Refuel/Defuel system + Vent pressurisation system weight: These weights were taken as constant values from a previous NASA study [8] which detailed the design for LH₂ propulsion systems.

### 2.3.3. Cryogenic systems weight: The mass of cryocooler for the required cooling capacity ($Q_c$) and the cryogenic temperature (T) of 20K was computed based on the relations obtained from the cryocooler survey [9]. The relations are mentioned below. The cooling capacity was computed in thermal module based on total heat required to be removed from the tank at ground conditions.

$$mass = \alpha(T) * Q_c^{\beta(T)} \qquad Equation\ (3)$$

with

$$\alpha(T) = 157 * e^{(-0.0533*T)} \qquad Equation\ (4)$$

$$\beta(T) = 0.009 * T + 0.1275 \qquad Equation\ (5)$$

As the fuel is now located at the rear part of fuselage it is important to acknowledge the shift in CG throughout the flight. This is done by computing the most aft centre of gravity for several load cases by varying the passenger number, weights in front and rear freights and the fuel mass in LH₂ tank. The empennage is resized in order to maintain the longitudinal stability. The results for the most extreme centre of gravity travel are mentioned in Section 3.2.1.

## 2.4. Propulsion module

The propulsion module generates engine database and fuel burn, later used in the performance module, based on parametric engine modelling. For this study all other characteristics of the hydrogen turbofan engine are assumed to be similar to the conventional engine except the computed Specific Fuel Consumption (SFC). The SFC is modified with an additional factor of 2.8, which is the ratio of mass energy density of conventional fuel and LH₂. This modifies the computed SFC for hydrogen engine as per following relation:

$$SFC_{H_2} = SFC_{Kerosene}/2.8 \qquad Equation\ (6)$$

This factor of 2.8 is subject to change as per the respective mass energy densities of conventional fuel and hence a parametric study is done by varying this factor to understand the effects on overall aircraft design. The results are detailed in Section 4.1.
## 3. Sizing results

Initially a reference aircraft running on kerosene fuel was sized with the mission profile and top-level requirements of single aisle medium range aircraft. This aircraft served as baseline for comparison with results obtained from sizing of new hydrogen propelled aircraft. The results in terms of geometry and weight comparison are described in subsequent sections.

### 3.1. Geometric sizing comparison

Table 1 shows the geometric parameters that have been computed for both conventional and hydrogen aircraft along with the percentage delta change in hydrogen aircraft with respect to reference aircraft.

<table><caption>Table 1: Geometric sizing comparison between conventional and hydrogen-propelled aircraft</caption><thead><tr><th>Geometric parameter</th><th>Conventional kerosene aircraft (m)</th><th>Hydrogen aircraft (m)</th><th>Delta change (%)</th></tr></thead><tbody><tr><td>Fuselage length</td><td>32.67</td><td>45.76</td><td>+40 %</td></tr><tr><td>Wing span</td><td>34.06</td><td>33.92</td><td>-0.4 %</td></tr><tr><td>Wing Mean Aerodynamic Centre length</td><td>4.17</td><td>4.16</td><td>0 %</td></tr><tr><td>Wing aerodynamic centre position with respect to nose</td><td>14.44</td><td>26.38</td><td>+83 %</td></tr><tr><td>Horizontal tail span</td><td>12.56</td><td>14.52</td><td>+15.6 %</td></tr><tr><td>Vertical tail span</td><td>6.95</td><td>7.74</td><td>+11.5 %</td></tr><tr><td>LH<sub>2</sub> tank length</td><td>NA</td><td>13.09</td><td>NA</td></tr></tbody></table>

### 3.2. Weight comparison

Figure 3 shows the weight comparison between conventional and hydrogen aircraft along with the percentage delta change in hydrogen aircraft with respect to conventional aircraft. Although the tank weight (2,039 kg) constitutes to 10 % increase in airframe weight as compared to similar conventional aircraft with no rear tank, the airframe weight is also dependent on MTOW and since MTOW for hydrogen aircraft is lower than conventional aircraft, it requires lighter wings and fuselage structures. Hence this results in only 3% increase in OWE.

<figure><img src="image_7.png" /><figcaption>Figure 3: Weight comparison between conventional and hydrogen-propelled aircraft</figcaption></figure>

3.2.1. Centre of gravity (CG) shift: The maximum CG shift is computed with 132 passengers (PAX) and maximum freight weight between start of flight with maximum fuel and end of flight with minimum fuel. The maximum travel is 1.31 m or 31 % in terms of wing MAC length. It is important to note that the empennage has been resized for this variant and the wing position is shifted aft as compared to the conventional aircraft. As a reference, we can compare these results with an A320 which is 37.57m in length where the maximum distance along which the CG position may move is 1.34m [10]. The CG travel of LH2 variant seems to be in the acceptable bounds.
# 4. Parametric studies

As described in next sub-sections, two parametric studies were carried out to study the effect of SFC ratio variation and passenger number variation on overall aircraft design.

## 4.1. SFC ratio variation

The SFC ratio was varied between 2.4 to 2.8 which produced the results in terms of weight and geometry changes as shown in Figure 4. This study shows that the overall aircraft design is very sensitive to the SFC variation and hence the detailed designs of propulsion systems will be required to get the true sense of impact of using LH₂ as fuel.

<figure><img src="image_8.png" /><figcaption>Figure 4: Variation in MTOW and fuselage length with variation in SFC ratio.</figcaption></figure>

## 4.2. Passenger number (PAX) variation

The computed fuselage length for reference conventional aircraft is 32.6 m. In order to retrofit current geometry of reference conventional aircraft with LH₂ tank we will need to reduce the passenger number from 132 to 55. This may not be an economically viable option considering manufacturing and operational costs added due to transition to hydrogen aircraft.

<figure><img src="image_9.png" /><figcaption>Figure 5: Required reduction in number of passengers to maintain the same length of reference conventional aircraft</figcaption></figure>

# 5. Environmental computations

As stated previously, hydrogen combustion only generates water vapour and nitrous oxides. In this section, the method deployed to compute those emissions is explained, as well as a short comparison with the emissions of a kerosene aircraft.

## 5.1. Water vapour emissions

To get to the amount of water vapour generated by the hydrogen combustion, a simple chemical balance of the hydrogen combustion equation has been used in order to obtain the emission index. The chemical reaction of hydrogen with oxygen is as follows:
$$2H_2 + O_2 \rightarrow 2H_2O \qquad \text{Equation (7)}$$

From the stoichiometry of this reaction, we obtain an emission index of 8.937kg/kg, which leads to a total amount of 51.7 t of water vapour emissions for a mission of 3100NM (5,783kg of hydrogen fuel burned during mission) as compared to water vapour emissions of 21.3 t from reference conventional aircraft.

## 5.2. Nitrous oxides emissions

Quantifying the amount of NOx generated during this mission is more complicated than for water as emission index of NOx highly varies during the flight. Therefore, the empirical Boeing Fuel Flow Method (BFFM2) [11][12] coupled with the ICAO engine database [13] and a detailed mission profile, have been used to get to the total amount of NOx. By operating several transformations to account for altitude and for the change of fuel, the amount of NOx emitted at each time of the flight can be deduced, as well as the total amount of NOx. In the end, an average emission index of NOx of 0.020 kg/kg has been found, leading to 117.7 kg of NOx emissions as compared to 149.3 kg of NOx emissions from reference conventional aircraft.

## 5.3. Emissions profile and comparison with kerosene aircraft

The amount of NOx, water vapour and CO2 emitted for both hydrogen and kerosene aircraft during the main segments in flight profile is presented in Figure 6.

<figure><img src="image_10.png" /><figcaption>Figure 6: Comparison of emissions for main flight segments</figcaption></figure>

The computations show that the kerosene aircraft emits less water but more NOx than the hydrogen aircraft, while the hydrogen aircraft does not produce any CO2 nor CO, unburned hydrocarbons, (UHC), volatile organic compounds (VOC) and sulphur oxides (SOx). In the troposphere, water vapour remains between 10 to 15 days [14] whereas CO2 remains about 4 years [15] and NOx for 120 years [16]. However, this study does not take into account the effect of altitude regarding the NOx emissions (that can have a warming or a cooling effect), but also regarding the contrail formation due to water vapour. Finally, from a life cycle perspective, a significantly reduced environmental footprint could be achieved if the hydrogen was produced from low-carbon or zero-carbon processes [17].

# 6. Conclusion and future work

To conclude, this study shows that the current set-up of FAST-OAD application can be successfully used to develop preliminary design of hydrogen-propelled aircraft using multidisciplinary analysis. The preliminary design shows 15% reduction in MTOW for a typical short single aisle aircraft mission despite the 40% increase in fuselage length owing to the fact that the fuel mass required for the mission is decreasing significantly. But this reduction might get negated once the detailed design studies for additional H2 technologies are conducted. The use of LH2 as an aircraft fuel seems promising in terms of its significantly less environmental impact (21 % less NOx emissions) and it is worth investigating the design in further details.
The future steps for this work would be to develop the code for unconventional geometries such as
blended wing body (BWB) in order to maximise the weight advantages given by usage of LH₂ fuel.
The challenges still remain in terms of developing a detailed design of propulsion, storage and
cryogenic systems for LH₂ fuel such that all the safety regulations are followed in order to make the
future flights safer, reliable and sustainable.

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