A process to evaluate fuselage structural loads caused by sloshing in liquid hydrogen tanks

Kurzfassung

The way towards more climate friendly or climate neutral air transportation is one of the mayor challenges in the development of new transport airplane. Different propulsion options may be considered depending on the mission profile. Besides purely electric airplane concepts with batteries for lower design ranges and the usage of SAF (Sustainable Aviation Fuel) for long range airplane, large transport airplane with short to medium range mission profiles may be powered by means of liquid hydrogen (LH2). The hydrogen is either directly burned in modified turbofan engines or used in fuel-cells to generate electric energy for electric engines. In both cases the LH2 has to be stored in tanks at cryogenic temperatures below -250°C. Due to the necessity of a very good isolation and the tank pressure, specific LH2 tanks have to be designed and integrated into the airplanes primary structure. A potential position for two or more LH2 tanks is the aft fuselage section behind the pressurized passenger cabin. Depending on the design requirements (passenger capacity, design range, etc.), that determine the required LH2 volume and fuselage length limitations the OAD (Overall Aircraft Design) may allow a single aisle cabin layout (6 abreast) or require a twin aisle cabin configuration (8 abreast) with a larger fuselage diameter. Loads that may be transferred from the LH2 tanks to the fuselage primary structure include quasi-static loads acting in flight and ground load cases as well as dynamic loads that arise from accelerations acting on the tank system and the subsequent sloshing of the LH2 in the tank. Potential scenarios where the inertia and sloshing loads of the LH2 cannot be neglected are e.g. rejected take-off or emergency landing conditions up to survivable crash conditions. To assess a variety of airplane configurations in early design stages a standardized aircraft description and a (semi-) automated process for a fast model assessment is of great importance. The CPACS (Common Parametric Aircraft Configuration Schema) data format is an already established standard to define the aircraft and its main components and systems [1]. The CPACS schema already includes a detailed description of the fuselage primary structure [2] which can be interpreted by tools such as the DLR modelling framework PANDORA [3] to generate Finite Element models for structural analyses. To integrate the hydrogen tanks and to consider the load transfer from the tanks to the primary airplane structure some extensions of the CPACS schema are necessary. The latest CPACS version 3.5 already includes definitions for the hulls of generic fuselage tanks. However, the connection between the tank hull and the primary structure as well as the definition of baffles used to control the sloshing of LH2 is required. In this paper two aspects of the development of a process to assess tank loads within a flexible airplane fuselage are discussed. In a first part an extension of the CPACS schema to define baffles inside the LH2 tank as well as structural mounts between the tank and the primary structure will be presented together with first steps towards an automatic model generation in the PANDORA environment. In a second part, exemplary sloshing simulations using the Finite Element software package VPS (formerly known as PAM-CRASH) will be presented. Different numerical methods such as SPH (Smoothed particle Hydrodynamics) and FPM (Finite Pointset Method) will be assessed and compared to each other. In addition, the differences between sloshing simulations based on rotated acceleration vectors on a fixed tank and a realistic acceleration of the partly filled tank, which is essential for realistic future crash simulations, will be discussed.