For Liquid Rocket Engines which employs cryogenic fluids, chill-down process represents a fundamental phase before turbopumps operation. The propellant, before being sent to the combustion chamber, is employed to perform cooling function of both fuel and oxidizer fluidic transfer lines. In this way, pumps cavitation and correct ignition during engine operation are ensured. This thesis, carried out at the Avio Company based in Colleferro, has the main objective of analysing and optimizing chill-down processes in cryogenic LREs, using EcosimPro software. The thermal conditioning process of cryogenic propellant transfer lines is essential due to the high temperature difference existing between the cryogenic liquid in the storage tanks and the wall of ducts at the ambient temperature.