Achieving efficient and reliable on-board hydrogen storage is a central challenge in the decarbonisation of aviation. The effectiveness of hydrogen as a zero-carbon fuel depends strongly on its physical storage state. While Liquid Hydrogen (LH₂) offers high energy density but requires complex cryogenic systems, Compressed Hydrogen (CH₂) is operationally simpler yet penalised by high structural mass. Cryo-compressed Hydrogen (CH₂) provides an intermediate option. Motivated by the hypothesis that combining tanks operating at different pressures and temperatures (multi-state storage) may offer complementary advantages, this study develops a framework to explore and quantify their potential. We present a modular thermomechanical model for single- and multi-tank hydrogen storage systems that takes mission requirements as input and returns a compatible tank geometry, transient thermal loads, and the pressure-temperature evolution in each tank. The model is verified against reference data and applied to a representative regional aircraft mission to demonstrate effectiveness in modelling single- and multi-tank configurations. These demonstrations confirm that the framework robustly captures inter-tank mass and energy exchange, providing a foundation for future studies aimed at evaluating performance trade-offs and design optimisation.