A system comprising: at least two spacecraft, including at least a first spacecraft (300) and a second spacecraft (300), the at least two spacecraft disposed together, in a launch configuration, for launch by a single launch vehicle; wherein: in the launch configuration, the first spacecraft (300) is mechanically coupled with a primary payload adapter of the launch vehicle, and the second spacecraft (300) is mechanically coupled with the first spacecraft by way of an inter-spacecraft coupling arrangement; the system is configured to be deployed, following injection into a first orbit by the launch vehicle, by separating the first spacecraft from the primary payload adapter while the second spacecraft is mechanically coupled with the first spacecraft; the first spacecraft includes (300) a first on-board propulsion subsystem including a first propellant storage arrangement (113, 114); the second spacecraft (300) includes a second on-board propulsion subsystem including a second propellant storage arrangement (113, 114); the first on-board propulsion subsystem includes one or more thrusters (116) configured to execute an orbit transfer manoeuvre from the first orbit to a second orbit; and the system includes a propellant line arrangement (323 (1, 2)) comprising at least one coupling device (325, 327) configured to detachably couple the first propellant storage arrangement with the second propellant storage arrangement; characterised in that the at least one coupling device (325, 327) is disposed between and exterior to the first spacecraft and second spacecraft; and in that the at least one coupling includes a line disconnect including a first portion proximal to the first spacecraft and a second portion proximal to the second spacecraft, each of the first portion and the second portion including a respective valving element and in that the respective valving elements are configured to permit propellant flow when the first portion and the second portion are mutually engaged and prevent propellant flow when the first portion and the second portion are detached. at least two spacecraft, including at least a first spacecraft (300) and a second spacecraft (300), the at least two spacecraft disposed together, in a launch configuration, for launch by a single launch vehicle; wherein: in the launch configuration, the first spacecraft (300) is mechanically coupled with a primary payload adapter of the launch vehicle, and the second spacecraft (300) is mechanically coupled with the first spacecraft by way of an inter-spacecraft coupling arrangement; the system is configured to be deployed, following injection into a first orbit by the launch vehicle, by separating the first spacecraft from the primary payload adapter while the second spacecraft is mechanically coupled with the first spacecraft; the first spacecraft includes (300) a first on-board propulsion subsystem including a first propellant storage arrangement (113, 114); the second spacecraft (300) includes a second on-board propulsion subsystem including a second propellant storage arrangement (113, 114); the first on-board propulsion subsystem includes one or more thrusters (116) configured to execute an orbit transfer manoeuvre from the first orbit to a second orbit; and the system includes a propellant line arrangement (323 (1, 2)) comprising at least one coupling device (325, 327) configured to detachably couple the first propellant storage arrangement with the second propellant storage arrangement; in the launch configuration, the first spacecraft (300) is mechanically coupled with a primary payload adapter of the launch vehicle, and the second spacecraft (300) is mechanically coupled with the first spacecraft by way of an inter-spacecraft coupling arrangement; the system is configured to be deployed, following injection into a first orbit by the launch vehicle, by separating the first spacecraft from the primary payload adapter while the second spacecraft is mechanically coupled with the first spacecraft; the first spacecraft includes (300) a first on-board propulsion subsystem including a first propellant storage arrangement (113, 114); the second spacecraft (300) includes a second on-board propulsion subsystem including a second propellant storage arrangement (113, 114); the first on-board propulsion subsystem includes one or more thrusters (116) configured to execute an orbit transfer manoeuvre from the first orbit to a second orbit; and the system includes a propellant line arrangement (323 (1, 2)) comprising at least one coupling device (325, 327) configured to detachably couple the first propellant storage arrangement with the second propellant storage arrangement; characterised in that the at least one coupling device (325, 327) is disposed between and exterior to the first spacecraft and second spacecraft; and in that the at least one coupling includes a line disconnect including a first portion proximal to the first spacecraft and a second portion proximal to the second spacecraft, each of the first portion and the second portion including a respective valving element and in that the respective valving elements are configured to permit propellant flow when the first portion and the second portion are mutually engaged and prevent propellant flow when the first portion and the second portion are detached.