The renewed interest in lunar exploration and the need to extend the crew's time on the Moon, has led to the conceptual development of several manned landing systems, characterized by numerous architectures that differ mainly in the number of stages and the peculiarity to be re-used or not. Each architecture will depend on the type of assumptions made, the manoeuvres performed by the landing system and the type of orbit used. Generally, the tendency is to choose as an outpost a near-rectilinear halo orbit (NRHO) that serves as a reusable command and service module in lunar space. The purpose of the following discussion is to preliminarily define the requirements for lunar descent and ascent propulsion systems based on data associated with previous Apollo missions.