A rotary jetting apparatus comprising: (a) a housing defining a fluid path for a pressurized fluid; (b) a rotor, at least a portion of which is disposed coaxially within the housing, the rotor including a proximal end and a distal end, the rotor being configured to rotate relative to the housing, the rotor comprising: (i) a fluid inlet disposed at the proximal end of the rotor, the fluid inlet being configured to receive the pressurized fluid from the fluid path, such that a direction of the pressurized fluid as it enters the rotor is coaxial with the rotor; and (ii) at least one nozzle disposed adjacent to the distal end of the rotor, the at least one nozzle being coupled in fluid communication with the fluid inlet and being configured to discharge a jet of the pressurized fluid, thereby causing the rotor to rotate relative to said housing; and, (c) a first pressure balance volume defined by the housing and the rotor, the first pressure balance volume being disposed adjacent to the proximal end of the rotor; the rotary jetting apparatus being characterised in that the rotor is configured to move axially relative to the housing and in that the rotary jetting apparatus further comprises (d) a vent (17,12) configured to selectively place the first pressure balance volume in fluid communication with an ambient volume, as a function of an axial position of the rotor relative to the housing. (a) a housing defining a fluid path for a pressurized fluid; (b) a rotor, at least a portion of which is disposed coaxially within the housing, the rotor including a proximal end and a distal end, the rotor being configured to rotate relative to the housing, the rotor comprising: (i) a fluid inlet disposed at the proximal end of the rotor, the fluid inlet being configured to receive the pressurized fluid from the fluid path, such that a direction of the pressurized fluid as it enters the rotor is coaxial with the rotor; and (ii) at least one nozzle disposed adjacent to the distal end of the rotor, the at least one nozzle being coupled in fluid communication with the fluid inlet and being configured to discharge a jet of the pressurized fluid, thereby causing the rotor to rotate relative to said housing; and, (i) a fluid inlet disposed at the proximal end of the rotor, the fluid inlet being configured to receive the pressurized fluid from the fluid path, such that a direction of the pressurized fluid as it enters the rotor is coaxial with the rotor; and (ii) at least one nozzle disposed adjacent to the distal end of the rotor, the at least one nozzle being coupled in fluid communication with the fluid inlet and being configured to discharge a jet of the pressurized fluid, thereby causing the rotor to rotate relative to said housing; and, (c) a first pressure balance volume defined by the housing and the rotor, the first pressure balance volume being disposed adjacent to the proximal end of the rotor; the rotary jetting apparatus being characterised in that the rotor is configured to move axially relative to the housing and in that the rotary jetting apparatus further comprises (d) a vent (17,12) configured to selectively place the first pressure balance volume in fluid communication with an ambient volume, as a function of an axial position of the rotor relative to the housing.