The use of hydrogen as an aviation fuel can be beneficial for the reduction of CO2 emissions, if renewable energy sources are used for hydrogen production. Pure hydrogen fuel produces no CO2 in flight. NOx emissions can be significantly lower for hydrogen fuelled combustors than for current kerosene fuelled combustors. Other advantages derive from the high energy content, which reduces the necessary fuel mass, and from the availability of a valuable heat sink, useful to improve cycle performance. The present paper (based on the EU Cryoplane Project) focuses on the use of hydrogen in aero gas turbine engines. It studies the differences in performance produced by of its cryogenic properties in unconventional cycles. Three novel concepts are applied to a turbofan aero engine; for each cycle the improvement in performance at take-off and cruise is presented. An estimation of the weight and size of the engine is then made.