Liquid hydrogen (LH2) is a proposed option to decarbonise long-haul aviation. LH2 aircraft (combustion-based) is expected to be lighter than Jet-A aircraft which necessitates reduction in the engine thrust requirement. Thus, the thermodynamic and energy performance of a LH2 aircraft engine, and its design and optimisation, is of significance. In a first, a conceptual design and optimisation of a future LH2 powered ultra-high bypass-ratio geared turbofan engine is conducted for reduced aircraft thrust requirement, using GasTurb 13 software and implementing future materials and component efficiencies. The thrust specific energy consumption (TSEC) of the optimised LH2 engine is 6–8% lower than Jet-A. The TSEC of LH2 engine is lower than Jet-A due to hydrogen's higher gravimetric energy density during combustion, higher specific heat of combustion products, and reduced thrust requirement. It is observed that optimised LH2 engine has 11% smaller diameter, 5.5–7.5% shorter length, 6–14% lower turbine entry temperature and 7.4–17.6% lower weight, than a Jet-A engine. The results of this work will be useful to future studies on LH2 engine and aircraft design, and LH2 aircraft emissions and contrails modelling.