The paper considers the selection of a main propulsion system of a small space tug, whose development is relevant due to the problem of disproportion between the existing launch vehicles and promising payloads. This problem arose in connection with the active development of multi-satellite constellations. The tendency to increase the number of operated spacecraft turned out to be dialectically related to the tendency to reduce the size of a single spacecraft. The paper introduces the results of the analysis of the functional efficiency of the transport operation and the cost analysis of the options for a small space tug. The analysis was carried out as part of the competition for the development of ultralight launch vehicles, organized by ANO Aeronet. A criterion for selecting a propulsion system for a small space tug from the point of view of efficiency is proposed. The study shows that for a small space tug weighing no more than 80 kg, designed to perform a coplanar transport operation lasting no more than three days with a payload weighing no more than 150 kg and an available reserve of delta-V budget of no more than 500 m/s, the best option is a propulsion system on gaseous oxygen — methane components, which does not require a specialized displacement system and the gas jet engines of the control system work on any of the components.