CIRA, the Italian Aerospace Research Center, manages the HYPROB (HYdrocarbon PROpulsion test Bench) Program, supported by the Italian Ministry of University and Research, with the aim of improving the national capabilities to develop rocket engines for future applications. A system line, named HYPROB-BREAD, focusing on liquid oxygen/liquid Methane (LOX/LCH4) technology and relative breadboards, is included. A step-by-step approach has been adopted to validate critical design aspects by simplified technological breadboards. The reliable operation of an engine is ensured by thermally efficient cooling jackets, which require the in-depth comprehension of the coolant behavior. For this purpose, a specific breadboard has been designed and tested. Another important issue is the evaluation of the thermal loads, transferred by the combustion hot gases to the thrust chamber walls. In this view, a Subscale Calorimetric Breadboard has been designed; 13 disks surround the chamber: they are fed up by water and provide the cooling and the measurement of the exchanged thermal power. The final article is a 3-ton-class LOX/LCH4 regenerative demonstrator, whose coolant is liquid methane, flowing in a cooling system, made by several axial channels. This paper aims at describing the thermal investigations, conducted in the design and the verification phase for the aforementioned breadboards and demonstrator cooling jacket.