This dissertation exposes the theoretical design of a rocket upper stage using a coolant method based on a liquid tank surrounding the combustion chamber. The project involves the calculation, design and simulation of the performances of the rocket engine and the coolant method as well. The aim of this work is to show that a simple cooling method for a certain stages can be easily carry it out without using the classic expansive methods: regenerative, ablation, radiation and film cooling. For this reason, the first chapters of this thesis will introduce these cooling methods: what they do, how they do it, their advantages and disadvantages. The research of this project starts explaining the concepts of the new cooling methods. How does it work and which can be their advantages for an upper stage. Afterwards, in order to prove the system an upper stage has been designed concerning the new tendency for small payloads around 300 and 500 kg for low orbits and using a proper propellant to fulfil the requirements of the cooling system without leading to suppose a risk or either a high increase of the cost without sacrificing very much the performance. During the design of the stage, two variants are going to be presented for the same rocket engine. A first variant of a rocket chamber made of copper in order to test how behave a very thermal conductive material and a second variant made of Inconel-718, and alloy mainly made out of nickel for aerospace applications, less thermal conductive but with a higher melting point and mechanical properties. Simulations of rocket performance have been carried out with SolidWorks CFD add-on to verify the design was correctly made. The results of these simulations show a very accurate theoretical performance if compared with the calculations previously made. The results obtained by the coolant system iterative simulation show that the copper is not a suitable material for this system because it transfers the heat so fast to the coolant that before the burn time is finished, all the liquid is already vaporized and the final temperature of the walls is too high. On the other hand, Inconel-718 behaves much better and has a good relation between the energy absorbed and the one transferred to the coolant which prevents it to reach a critical wall temperature that could decrease its mechanical properties and ruin the whole mission. Finally, as conclusions, the results are exposed, discussed and possible changes explained to improve the system.