4.1. Case 1: Full Section Layout
This section describes the main results related to the FS layout configurations with one (fore, labeled FS
1) and two sets of tanks (fore and aft, labeled FS
2). The main results in terms of operating performance, i.e., payload and range, are depicted in
Figure 9, where each marker represents a retrofitted hydrogen-based box-wing aircraft, designed by means of the optimization tool detailed in
Section 3; the red marker indicates the value for the baseline configuration (kerosene-based).
Figure 9—left highlights a trade-off correlation between the number of passengers and range; this trade-off depends on two opposite physical properties of hydrogen: high specific energy, i.e., the energy stored for a unit mass, from one side, and low volumetric density from the other. The low volumetric density of hydrogen is the main cause of the necessity of very large tanks when long-range missions are accomplished, as shown in
Figure 9—right, which depicts how the range
X changes with the tank volume
. The necessity to integrate large tanks into the fuselage introduces reductions, even very severe, in the available internal volume for passenger seats; in general, for every tank arrangement, the LH
2-retrofitted BW aircraft exhibit a lower payload than the baseline configuration, even for short or very short ranges. These trends on payload and range highlight the main compromise that the introduction of hydrogen as an energy carrier implies for aircraft transportation: LH
2 allows for the substitution of current fossil fuel, hence boosting aircraft operations’ decarbonization, but the integration of LH
2 storage systems implies a sharp reduction in operating performance. In fact, if we analyze the data in
Figure 9—left in terms of payload–range pairs, it can be seen that long and very long ranges are achievable with cabin configurations that can accommodate only a few dozen passengers; such solutions are to be discarded because they are of almost no practical interest. A less penalizing trade-off occurs for typical medium-haul routes, e.g., 6000 km (or 4000 km) can be achieved with a passenger count of about 150 (or 190); these figures are close to those typical of aircraft currently operating in the medium-range sector. Shifting to short-haul, the hydrogen-retrofitted box-wing is able to cover a distance of 1500 km with a passenger count of about 230.
It is interesting to note that tanks with similar general dimensions can imply different operating performance. In this regard,
Figure 10 shows that, for both FS
1 and FS
2 layouts, there are several tank configurations that have the same total cylindrical length but result in different available volumes for accommodating passenger seats. To discuss this aspect, two couples of configurations, A’ and B’ for the FS
1 group, and C’ and D’ for the FS
2 group, were selected; the tanks belonging to the same FS group exhibit same
and
R values but different
; the latter influences the total length of the tank and hence the available volume for the passenger cabin. The impact of
on the internal cabin volume is more relevant for tanks with larger diameter, and its penalizing effect doubles for the FS
2 layout. It is therefore the total tank length that has a direct impact on the number of passengers, see Equation (4).
A noteworthy aspect of LH
2 tank integration is related to the achievable range, with a fixed number of passengers, and hence total tank length; indeed, aircraft integrating tanks having the same total length can have significant differences in maximum range, see
Figure 11. This is related to the internal tank volume, which is correlated to the solution the optimizer finds for tank integration in the fuselage cross-section, by acting on
and
. Indeed, as presented in
Section 3.2, these two parameters define the position and number of tanks (
) in the fuselage cross-section, and, consequently, the internal tanks’ volume. To underline this aspect, four tank configurations, named A, B, C, and D and shown in
Figure 11, are selected; their main features are reported in
Table 5. Considering FS
1, i.e., cases A and B, the number of tanks in the cross-section
is equal to one for both, and the only difference lies in
, which directly affects the radius of the tanks. The differences in terms of tank position and shape introduce a reduction, for the case B, in internal volume and, hence, in energy stored of 22.1% with respect to case A, causing a range reduction of 22.4%. It is interesting to underline that these variables,
and
, are very useful for the case a catwalk is required by regulation; consequently, it becomes very easy to add a related constraint to the optimization procedure. Considering the selected example for FS
2, i.e., cases C and D, the
are different, one and two, respectively; the larger cross-section area in case
C allows for storing 23% more energy with an increase of 25.2% in terms of range. In this case, both solutions do not allow for a catwalk.
Figure 12—left shows the trends of the mass of vented hydrogen during the evolution of the design mission for the four example cases listed in
Table 5. It can easily be seen that the vented mass depends mainly on the thickness of insulation
with which the tank is coated. Indeed, the thickness of the insulation material affects the thermodynamics of the tank; in particular, the thicker the insulation material, the greater the thermal resistance. Accordingly, the heat flow fraction (i.e., the ratio between the heat flow
and the maximum heat flow
calculated in the four study cases) towards the liquid hydrogen stored in the tank is reduced in the case of highly thickened insulant foam, as shown in
Figure 12—right. High thickened foam reduces the amount of heat received by the liquid hydrogen, so the boiling effect is mitigated, and the amount of mass vented is reduced.
Figure 12—right also shows that the heat flow depends on the altitude of the aircraft. On the ground, the temperature of the air surrounding the tank depends on the weather conditions at the airport (in this case study, a standard temperature of 15 °C was considered); the heat flow, which is proportional to the temperature difference between the region inside (i.e., the hydrogen) and outside (i.e., the air) the tank, is maximum in this phase. As altitude increases, the temperature of the air decreases and the temperature difference between the air and the hydrogen becomes progressively smaller; consequently, the heat flow is minimum during the cruise phase. An in-depth discussion of the complex thermodynamic evolution of cryogenic hydrogen inside insulated tanks is described in Refs. [
45,
60].
A feature to be discussed regarding LH
2 aircraft is the mass breakdown. Starting from the fuel, LH
2 specific energy is about three times that of the current kerosene; this means that for a unit kg of burned hydrogen, the energy supplied is three times higher than that of kerosene. This is highlighted in
Figure 13—left, which reports the range ratio X/X*, i.e., the ratio between the range accomplished by each LH
2-retrofitted aircraft and the harmonic range of the baseline, see
Table 1, vs. the fuel mass ratio
/
, i.e., the ratio between the hydrogen mass burned and vented by each retrofitted BW and the block fuel of the baseline at the design point. The data show that in the case of the range being close to that of the baseline, namely, X/X* = 1, the hydrogen consumption is almost one-third of the fuel consumption of the baseline. The absolute values of hydrogen consumption for the retrofitted aircraft are provided in
Figure 13—right.
Figure 14 shows the trends of the mass of tanks (
), operating items (
), and OEW as the range varies. In particular, it can be observed that the mass of the tanks (
Figure 14—left) increases as the range raises, as longer tanks are installed to accomplish the mission (cf.
Figure 11), whereas the opposite occurs for the mass of the operating items, which tends to decrease (
Figure 14—center), as these are directly related to the decreasing passenger number. The increase in the mass of the tanks is larger than the reduction observed for the mass of the operating items; as a result, generally, for all the LH
2-retrofitted BW aircraft, the OEW slightly increases for larger ranges, as shown in
Figure 14—right; furthermore, the presence of the tanks introduces OEW increases with respect to the 308-passenger baseline.
As it is possible to intuitively expect, the fuel mass fraction (
Figure 15—left) increases, whereas the payload weight fraction decreases (
Figure 15—center), for configurations with longer range. Nevertheless, the already discussed reduction in payload, together with the significantly lower fuel weight needed due to the three-times higher gravimetric energy density of LH
2 than kerosene, lead to a significant reduction in take-off weight compared to the baseline MTOW, see
Figure 15—right. These data, therefore, allow for the expectation of further reductions in MTOW if the retrofitting approach is discarded for a complete ‘from scratch’ redesign of the hydrogen BW aircraft.
An important aspect to evaluate in tank design is the gravimetric efficiency
, i.e., the ratio between the mass of fuel stored in the tank and the sum of the mass of the tank and the fuel. Given the same design requirements and constraints for the tanks, having high values of
indicates higher storage efficiency. The trend of this parameter as the
ratio changes is shown in
Figure 16—left, which shows that as the
ratio increases (i.e., large tanks and low thickened insulation), tank efficiency increases. The gravimetric efficiency cannot increase significantly because as the internal tank volume increases, the structural mass increases as well, as shown in
Figure 16—center. In addition,
Figure 16—center shows that insulation mass is generally lower than structural mass, and the discrepancy is much more evident as tank volume increases. The thickness of the insulant affects the effective use of the hydrogen stored onboard, as depicted in
Figure 16—right. Indeed, in case of high-thickened foam, the fraction of burned hydrogen
(i.e., the ratio between the burned hydrogen and the sum of burned and vented hydrogen) is almost one, meaning that almost all the stored hydrogen is effectively burned by the engines. In case of low-thickened foam, part of the energy is lost due to the boiling and venting of the hydrogen.
An additional element to be discussed is the payload–range energy efficiency (
PREE), a metric commonly used to assess aircraft productivity [
55,
73];
PREE is defined as the product of flight distance
X and payload weight per unit of energy spent
E, see Equation (9).
In the case of LH
2 aircraft, to compute the energy spent to fly, we consider both burned and vented hydrogen. The results, depicted in
Figure 17—left, show that the retrofitted hydrogen aircraft generally exhibit a significantly lower
PREE than the reference aircraft; the maximum hydrogen aircraft productivity is located in the range of 2000–5000 km; then, it decreases. The degraded performance of LH
2 aircraft with respect to kerosene-based aircraft is mainly related to a lower operating performance in terms of payload–range;
Figure 17—center shows that the payload–range product for hydrogen-retrofitted aircraft
divided by the value of the baseline at its design point
is always significantly lower than 1: this highlights that the payload reduction to allow for tank integration is more impactful than the possible range extensions. Potential reductions in energy spent to fly, achieved for retrofitted aircraft for ranges shorter than 7000 km (see
Figure 17—right) do not compensate the degraded operating performance.
In the end, what emerges is that a loss in operating performance is needed if conventional fuel is to be converted to LH
2, and thus if it is to cut CO
2 emissions. The large disadvantage in terms of volumes required for LH
2 storage results in reductions in the payload–range combinations that significantly lower the aircraft’s operating envelope. For this specific reason, in this study, it was decided to retrofit a high-capacity aircraft such as the box-wing developed in [
49]; in this case, although limitations in operating performance of the retrofitted configurations are apparent, payload–range combinations typical of current short–medium-range (SMR) aircraft can still be achieved. On the other hand, retrofitting current SMR aircraft would result in excessive payload–range reductions, undermining the practical potential of such a retrofit. This aspect will be discussed in
Section 5.
4.2. Case 2: Partial Section Layout
This section provides a comparative analysis for the FS (both with one or two groups of tanks) and PS layouts focusing on operating performance and weight. As shown in
Figure 18, in the PS layout tanks are located below the cabin floor, in the hold; the constraint imposed by the dimension of the hold implies a strong reduction in tank radius, and, consequently, in the available volume.
Figure 19—left highlights that the configuration retrofitted with the PS layout can fly significantly shorter distances than the FS layout. On the other hand, the PS configuration’s tank integration does not reduce passenger cabin volume. The amount of stored hydrogen is much lower in the case of the PS layout (see
Figure 19—right), and, consequently, the range is strongly reduced.
The advantage of installing LH
2 tanks in the PS layout, hence, lies in the hypothetical non-reduction in the number of passengers, as the tank integration does not affect passenger cabin volume. It has been specified that the non-reduction of the number of passengers is
hypothetical because the installation of the PS tanks affects the cargo hold’s volume, which is necessary to allow for each passenger to travel with at least one piece of baggage.
Figure 20—left shows the estimates of the available hold volume
and the required hold volume
calculated by considering one unit of luggage per passenger with a volume of 0.113 m
3 [
74], for both FS and PS configurations. All the FS configurations comply with the constraint on
due to the fact that the installation of tanks also leads to reductions in the number of passengers. The number of passengers for PS configurations, on the other hand, is constant, and for longer tanks, the available hold volume decreases; for some configurations, therefore, the constraint is not respected, and the layout could be considered unfeasible. The maximum range for feasible PS configurations with 308 passengers is about 930 km, see
Figure 20—right. For some configurations for which the constraint is slightly exceeded, some practical solutions could be found, such as a slight reduction in the number of passenger seats to reduce
or to allocate some of the cabin volume for baggage loading. In general, however, the volume of the hold is an aspect that cannot be overlooked when dealing with large tank integrations.
In terms of OEW, there are no significant differences between the FS and PS cases (
Figure 21—left). This is because the aircraft retrofit only affects the operating items’ weight and that of the installed tanks, as depicted in
Figure 21—center, whereas the main structural weight of the aircraft remains unchanged. The differences in W
TO, see
Figure 21—right, are basically related to the differences in mass of hydrogen stored and the number of passengers, and hence in payload weight.