A propulsion unit for an aircraft comprising: - a bypass-type turbojet engine, - a nacelle surrounding the turbojet engine, said nacelle comprising a downstream structure comprising an outer structure which defines, with a concentric inner structure surrounding a downstream compartment of the turbojet engine, a flow annular channel, the inner structure being made in the form of two annular half-portions (19a, 19b), - displacement and guide means for displacing (27, 29, 51, 57, 59) and guiding at least one of the two half-portions (19a, 19b), configured to displace the half-portion (19a, 19b) alternately between a working position in which the half-portion (19a, 19b) forms a shell around the downstream compartment of the turbojet engine, and a maintenance position in which the half-portion (19a, 19b) at least partially uncovers said downstream compartment of the turbojet engine, said displacement and guide means comprising: at least one rail/guideway system ensuring a translational guidance of the half-portion, alternately between the upstream and the downstream of the nacelle, and at least one connecting rod connected on the one hand to the half-portion (19a, 19b) and on the other hand to the turbojet engine, the half-portion (19a, 19b) being pivotally mounted on the rail/guideway system, said propulsion unit being characterized in that the rail/guideway system is configured to move radially away relatively to a longitudinal axis of the nacelle at least one portion of the half-portion (19a, 19b) by which the half-portion is pivotally mounted on the rail/guideway system, during a displacement of the half-portion (19a, 19b) from upstream to downstream of the nacelle, and in that said connecting rod is configured to rotatably drive the half-portion about an axis according to which a guide rail of the rail/guideway system extends simultaneously during the displacement of the half-portion along said guide rail. - a bypass-type turbojet engine, - a nacelle surrounding the turbojet engine, said nacelle comprising a downstream structure comprising an outer structure which defines, with a concentric inner structure surrounding a downstream compartment of the turbojet engine, a flow annular channel, the inner structure being made in the form of two annular half-portions (19a, 19b), - displacement and guide means for displacing (27, 29, 51, 57, 59) and guiding at least one of the two half-portions (19a, 19b), configured to displace the half-portion (19a, 19b) alternately between a working position in which the half-portion (19a, 19b) forms a shell around the downstream compartment of the turbojet engine, and a maintenance position in which the half-portion (19a, 19b) at least partially uncovers said downstream compartment of the turbojet engine, said displacement and guide means comprising: at least one rail/guideway system ensuring a translational guidance of the half-portion, alternately between the upstream and the downstream of the nacelle, and at least one connecting rod connected on the one hand to the half-portion (19a, 19b) and on the other hand to the turbojet engine, the half-portion (19a, 19b) being pivotally mounted on the rail/guideway system, said propulsion unit being characterized in that the rail/guideway system is configured to move radially away relatively to a longitudinal axis of the nacelle at least one portion of the half-portion (19a, 19b) by which the half-portion is pivotally mounted on the rail/guideway system, during a displacement of the half-portion (19a, 19b) from upstream to downstream of the nacelle, and in that said connecting rod is configured to rotatably drive the half-portion about an axis according to which a guide rail of the rail/guideway system extends simultaneously during the displacement of the half-portion along said guide rail. at least one rail/guideway system ensuring a translational guidance of the half-portion, alternately between the upstream and the downstream of the nacelle, and at least one connecting rod connected on the one hand to the half-portion (19a, 19b) and on the other hand to the turbojet engine, the half-portion (19a, 19b) being pivotally mounted on the rail/guideway system, said propulsion unit being characterized in that the rail/guideway system is configured to move radially away relatively to a longitudinal axis of the nacelle at least one portion of the half-portion (19a, 19b) by which the half-portion is pivotally mounted on the rail/guideway system, during a displacement of the half-portion (19a, 19b) from upstream to downstream of the nacelle, and in that said connecting rod is configured to rotatably drive the half-portion about an axis according to which a guide rail of the rail/guideway system extends simultaneously during the displacement of the half-portion along said guide rail.