This paper presents a calculation methodology of geometrical and mass parameters for shoulder-launched rocket with thermobaric warhead. An optimization of design rocket parameters for specified tactical and technical requirements is carried out by the criteria of the mass efficiency using two calculation methods. According to the first method, the rocket calibre for given a maximum range is determined by the function of payload mass. The second method predicts calibre value and based on that, all rocket sub-assemblies masses can be calculated, which means that total rocket mass is determined either. An optimal calibre value for rocket motor with limited operating time is common and unique solution for both of these iterative procedures. In the concrete example the application of the exposed model is shown -for specified tactical and technical requirements and known material characteristics, carried out calculation and analysis parameters of warhead and rocket motor. For selected rocket design parameters, a simulation of motor operating time and moving rocket throughout its launching tube are done for three typical usage temperatures: -30 ºC, +20 ºC and +50 ºC. The obtained values of the chamber pressure, operating time of the rocket motor and initial rocket velocity are analyzed considering the shooter's safety and realization of required ballistic characteristics.