The APPU project aims to lower the threshold of installing hydrogen-driven and boundary-layer-ingesting propulsion systems in the short term. This is done by replacing the Auxiliary Power Unit with an Auxiliary Power & Propulsion Unit (APPU). To study these concepts, the Airbus A321neo is taken as a reference baseline. The implementation of the APPU system on the A321neo aircraft necessitates a redesign of the existing empennage. The research objective of this thesis project is to investigate how an optimal empennage design for an aircraft equipped with an APPU system differs from an optimal empennage design for an aircraft without such a system. Multidisciplinary design optimization is used to minimize fuel weight for different aircraft configurations by optimizing the empennage geometry. Four different disciplines are identified. A weight discipline estimates the empennage weight based on the empirical Raymer equations. Two separate aerodynamic disciplines are implemented with different fidelity levels. The low-fidelity version is based on AVL's vortex lattice method, expanded with a constant friction coefficient drag model to account for viscous effects. The high-fidelity version is based on FlightStream's panel method. FlightStream proved to be infeasible for use in this study due to long run times and limited mesh robustness. Therefore, only the low-fidelity version is used to generate the final results. The static stability and control of the aircraft are ensured through a set of constraints that require specific stability and control derivatives to remain within defined limits. These derivatives are provided by the stability and control discipline that is based on AVL as well. The performance discipline applies the Breguet equation to convert the aircraft weight and aerodynamic performance into an estimate of the fuel load required to complete the design range. ParaPy is used as a multi-model-generator to provide the required input geometries for the disciplines. The study reveals that optimal empennage designs for aircraft equipped with an APPU system differ notably from optimal empennage designs for conventional aircraft. Both configurations benefit from high aspect ratios and reduced tailplane areas. However, APPU-equipped aircraft require a low cruciform tail to accommodate the hydrogen tank and reduced sweep angles to position the aerodynamic center aft without intersecting the propulsor plane. The low-fidelity aerodynamics discipline is unable to optimize the airfoils. AVL’s modeling of only the camber line makes it unsuitable for optimizing symmetric profiles, such as those on vertical tailplanes. Furthermore, the horizontal tailplane airfoils showed limited variation from the initial design. This is likely due to the dual-parameter definition of the camber line, as the class-shape transformation parameterization method is applied independently to both the upper and lower surfaces of the airfoils.

The increasing urgency to mitigate climate change has underscored the need to transition from conventional fossil-based aviation fuels, such as kerosene, to sustainable alternatives. Hydrogen stands out due to its potential to significantly reduce greenhouse gas emissions, making it a promising energy carrier for the aviation sector. However, adopting hydrogen presents substantial challenges, with the development of specialized fuel containment systems being one of the foremost obstacles. Double-walled tanks employing vacuum insulation offer an effective solution for cryogenic hydrogen storage, but they require a robust supporting structure for the inner vessel. Various solutions have been proposed to support the inner vessel of cryogenic tanks; however, despite the diversity of designs, there remains a noticeable lack of comprehensive research focusing specifically on the structural behaviour and feasibility of these inner vessel supporting structures for commercial aircraft applications. This study proposes a novel fibre-based suspension technique for the inner vessel of a double-walled integral tank designed for liquid hydrogen storage in large commercial aircraft. A finite element model was developed to evaluate the structural interaction between the inner and outer vessels and the supporting fibres, enabling structural sizing optimization to assess the impact of added loads. A parametric study was conducted to explore the influence of fibre design parameters on structural performance, mass, displacement, and thermal behaviour. Key design guidelines were established. First, using more than two longitudinal anchoring points results in the unwanted transfer of bending loads from the outer to the inner vessel. Second, while increasing the number of circumferential fibres reduces peak loads and displacements, the associated anchoring mass is the primary limiting factor, as thermal conduction was found to be negligible. Lastly, fibre orientation should prioritize low stiffness in the contraction direction to minimize tensile forces under initial filling. This should be combined with fibres angled in the longitudinal direction to improve longitudinal stiffness and displacement control. The results confirm the structural feasibility of the suspension system, showing only a marginal structural mass increase of approximately 1.88 % compared to a baseline integral tank without internal support. These findings provide practical guidance for the implementation of fibre-based suspension systems in cryogenic tank structures, supporting the development of hydrogen storage solutions for aviation.

In this thesis, a submerged inlet type for the engine core of the boundary layer ingesting APPU (Advanced Propulsion and Power Unit) is designed. The aim is to evaluate key geometric parameters that impact the flow field at the intake region by computing 3D CFD simulations. Various designs are created where entrance thickness, corner radius, duct shape, lip shape and entrance azimuthal range are altered. Assessment criteria include the distortion coefficient, drag coefficient and total pressure recovery. Additionally, wall shear stress, Mach and pressure contours are visualized to conduct comparisons between design iterations and identify (detrimental) flow phenomena. Results show that the total pressure recovery in all cases is below 80%, but losses confined to the interior of the duct can reach values lower than 1%. The findings of this thesis further elaborate on the significance of the various shape parameters and their impact on the flow field.

Growing concerns about the environmental impact of aviation have sparked interest in hydrogen aircraft as a greener alternative. Hydrogen can be used to power existing turbofan engines or electrical motors via a fuel cell, eliminating carbon emissions not only during flight, but also during production, provided renewable energy sources are used. However, adopting hydrogen as fuel introduces technological challenges, particularly with regard to on-board storage. Integral tanks, which are part of the aircraft's main structure, seem promising but existing designs show limitations in their integration with the airframe and insulation capabilities.

To address these issues, this study proposes an integral tank concept featuring a double wall architecture with vacuum insulation. The main advantage of this design is the use of an external stiffened wall that can be directly connected to the remaining airframe. In addition, having stiffeners on the outside ensures the required space for systems routing and addresses concerns with the crash worthiness of the structure. A parametric method, coupled with finite element analysis is developed to size the external load bearing wall, enabling quick analysis and mass estimations of different tank configurations. The method consists of a sizing optimization with the objective of minimizing the structural mass under constraints on the strength, buckling stability and fatigue behaviour.

The feasibility of the concept is then evaluated on an aft tank for a short/medium range aircraft in configurations with and without a forward tank. Preliminary results under this realistic scenario point to fuel containment efficiencies of up to 0.71, which are consistent with existing designs. Moreover, buckling stability is identified as the critical design criterion, highlighting the importance of using a stiffened shell design. These findings show the viability of the proposed concept from a structural standpoint and provide the basis for further research. The optimum solution at an aircraft level can be obtained by integrating the developed framework into a multidisciplinary aircraft design tool.

As society gets more aware of atmospheric pollution and the negative effects of the transport sector on the climate, new concepts are being innovated to mitigate these effects. One of these concepts is the Auxiliary Power and Propulsion Unit (APPU) project, in which the APU of the A321neo is replaced by a hydrogen powered turboshaft system that includes a propulsor and boundary-layer-ingestion (BLI) to alleviate the main engines, causing less kerosene to be burnt whereby less CO2 and soot are emitted.
As in most aircraft, certain parts of the engine system are in need of cooling, most notably the HPT blades. Concurrently, the onboard cryogenic hydrogen needs to be heated up before entering the combustor to avoid high thermal stresses. The cryogenic hydrogen can be used to cool these parts, while at the same time reaching a more appropriate temperature before entering the combustor. Alleviating conventional cooling methods by making use of the cryogenic hydrogen increases the thermal efficiency of the engine system. Moreover, increasing the temperature of the fuel increases the LHV, which increases the thermal efficiency of the engine system even further.
Three different cases are analysed: firstly, the cryogenic hydrogen is used to cool the bleed air, by which less bleed air is required, increasing the core flow, which increases the thermal efficiency of the cycle. Secondly, using the same principle, the TIT is increased, while the amount of (now cooled) bleed air stays constant to the baseline. In this case the thermal efficiency increases due to the higher LHV of the fuel. Thirdly, the cryogenic hydrogen is used to intercool the core flow between the booster and the HPC, after which the pressure ratio is increased accordingly.
It is found that intercooling and increasing the pressure ratio of both compressors to 8, increases the thermal efficiency with 3.6% with respect to the baseline. Performance complications due to increasing the OPR to 64 have not been analysed into great detail and might make this result hard to achieve in reality. The next best option is increasing the TIT 1840K, which increases the efficiency by 0.7%. Cooling and reducing the bleed flow however, results into a thermal efficiency increase of 0.2%.
It is concluded that the largest contributor to the thermal efficiency is the LHV of the hydrogen. This means that any process in which the hydrogen extracts the most amount of heat is most advantageous, efficiency-wise. Furthermore, it is concluded that adding intercooling and subsequently increasing the pressure ratio increases the thermal efficiency more than cooling and subsequently reducing the bleed flow.

Boundary layer ingestion is an airframe-propulsion integration technology capable of enhancing aircraft propulsive efficiency. The Propulsive Fuselage Concept, a tube-and-wing layout with an rear-fuselage-mounted propulsor in the boundary layer ingestion configuration, especially takes advantage this. However, the relation between physical shape and aerodynamic performance, resulting from the complex airframe-propulsor interaction, is not entirely understood. Also, contrary to long-haul aircraft, few studies have investigated the application of the concept on medium-haul aircraft with only 10% cruise thrust contribution coming from the boundary layer ingestion propulsor, which is a top-level requirement of the APPU project. To facilitate parametric studies regarding these research gaps, a parametric model is developed and implemented in an engineering design application that automates aerodynamic analysis to high degree.
This thesis presents a methodology to numerically analyze axisymmetric propulsive fuselage concept designs; an engineering design application using the knowledge based engineering technology is presented that facilitates the implementation of complex engineering design rules in the construction of the parametric model. The application consist of three components.
Firstly, a flexible geometric parameterization in 2D is developed that is proven capable of constructing well-performing designs. A translation mechanism is developed between these geometric input parameters and input parameters for class shape transformation curves, which form the mathematical basis for the geometry.
Secondly, the construction of a C-shaped domain and a multi-block structured mesh are also automated in the application. The mesh density for this application was verified through a mesh convergence study, and can be adjusted to fit other mesh requirements through various mesh control capabilities.
Lastly, the scripted interaction between the application and ANSYS Fluent software is automated. A fan modeling methodology was developed using boundary conditions that requires only fan pressure ratio as input, while mass flow continuity through the fan is ensured. The meshing and simulation routines are validated by comparing the results of the presented routine to that of a status-quo numerical simulation. All relevant aerodynamic output parameters show agreement in a range of 3.3%.
The working of the engineering design application is demonstrated in a design space exploration based on the hypothesis that increased conicity of the rear fuselage and nacelle shape with respect to the longitudinal axis can reduce the required fan power in cruise conditions. To isolate the effect of conicity, a parameter sweep was conducted. Results show that with increasing conicity, the overall viscous dissipation was continuously reduced. Also the total pressure recovery at the fan inlet face increases up to a nacelle conical angle of 11 degrees, after which this decays due to increased wetted area. At 11 degrees conicity, the aerodynamic efficiency (defined as fan power required for a given net propulsive force) was increased by 0.81% relative to a less conical status-quo baseline design with 6 degrees conicity.
The increased fuselage volume and wetted area due to increased conicity introduced the opportunity to shorten the fuselage without decreasing fuselage volume. This increased aerodynamic efficiency by 1.65% relative to the baseline. Also, as the intake diffusion functionality was redundant in this flow field, a third design was constructed with a 29% shorter intake duct, which increased aerodynamic efficiency by 1.81% compared to the baseline.
Demonstrated by these unoptimized designs and the observed physical mechanisms, it is concluded that aerodynamic efficiency could benefit from the direct and indirect effects of an increase in conicity of the propulsive fuselage concept.