Aircraft-integrated hydrogen storage systems are tightly coupled, multi-component, multi-physics systems in which thermodynamic behaviour, structural sizing, and control logic interact across disparate time and energy scales. Beyond conventional single-tank concepts, multi-state architectures combining tanks operating at different pressures and temperatures offer a means to exploit complementary advantages of compressed, cryogenic, and cryo-compressed hydrogen storage. At early stages of aircraft design, where architectural decisions remain fluid, computationally efficient and physics-consistent tools are required to explore these interactions and screen candidate system layouts. This work introduces a modular thermomechanical modelling framework for early-stage design-space exploration and preliminary sizing of multi-tank hydrogen storage systems for aviation, developed within the Horizon Europe TRIATHLON project. Mission requirements are prescribed as a time history of hydrogen mass flow demand, governing the thermodynamic response and the resulting structural loads. Each tank is modelled as a thermodynamic control volume governed by a system of differential–algebraic equations resolving mass and energy conservation and transient heat transfer, adapted from generalized hydrogen storage models in the literature. Tank-level thermodynamics are coupled to a filament-wound netting formulation for first-order structural sizing, enabling estimation of tank geometry, mass, and thermal inertia. System-level behaviour emerges through energy-consistent inter-tank coupling laws, allowing arbitrary network topologies, valve-based interactions, and simple control strategies. The framework is demonstrated on a representative two-tank storage architecture interacting with an aircraft powerplant, such as a fuel cell or gas turbine. It enables simulation-driven sizing and simple optimisation of tank volumes, operating pressures, and thermal management strategies under mission constraints. By prioritising architectural insight over component-level detail, the framework supports informed decision-making and system-level optimisation for hydrogen-powered aircraft.