As is well known, the energy specifications of a liquid rocket engine (LRE) determine the structural scheme, optimal geometric, mass, thrust, flight specifications and the efficiency of the rocket. Therefore, in the calculation of the LRE energy characteristics special attention is paid to thermochemical modeling of processes and the thermodynamic design of the engine chamber. Analysis of researches show that the incorrect account of the nature of chemical reactions along the length of the LRE chamber lead to errors in the assessment of the specific impulse and the formation of the engine geometry. Consequently, the problem of correctly simulating of the flow parameters distribution over the chamber volume and taking them into account for nozzle profiling arises. With known length (L=xmax) and diameters of the LRE chamber main sections is possible to form the distribution of the number of moles of components, heat capacity and isentropic index (N=f(x), c=g(x) and γ=h(x)) of the gas mixture. Thus, has been developed a new system for the LRE design, which is based on the thermodynamic calculation equations, the method of characteristics and the sigmoidal function of the flow velocity distribution. This system provides optimum geometry forming of the chamber and improving of the LRE energy parameters at the preliminary design stage.