A first spacecraft comprises a drive mechanism. A second spacecraft comprises a valve for filling/draining the second spacecraft. The coupling assembly comprises a first connector comprising an actuating member configured to engage and open the valve and a coupling member configured to couple to the drive mechanism. A second connector comprises a stop portion and a bore for transferring fluid. Operation of the drive mechanism rotates the coupling member and the second connector together to screw the second connector onto the valve until the stop portion engages the valve, the actuating member being prevented from moving relative to the second connector to open the valve whilst the lock is in the locked state. Once the stop portion engages the valve, operation of the drive mechanism to exert a torque on the coupling member greater than a predetermined torque level moves the lock to an unlocked state wherein the actuating member is moveable relative to the second connector to engage and open the valve.