Auxiliary system for driving a shaft of a helicopter propulsion system

Auxiliary system for mechanically driving a shaft, called relay shaft, of a helicopter propulsion system comprising at least one turboshaft engine connected to a power transmission box adapted to drive in rotation a rotor of the helicopter, said system comprising:

• a hydraulic motor mechanically connected to said relay shaft and adapted to be able to drive it in rotation,
• a hydraulic circuit for supplying a pressurised liquid to said hydraulic motor,
• a controlled, fast-opening hydraulic valve arranged on the hydraulic circuit and adapted to be placed on command at least in one open position wherein the liquid can supply said hydraulic motor to enable a driving in rotation of said relay shaft, or in a closed position wherein said hydraulic motor is no longer supplied with pressurised liquid,
• wherein said system further comprises a pyro-hydraulic storage unit driven by a control unit and connected to said hydraulic valve of said hydraulic circuit by a liquid supply line, said pyro-hydraulic storage unit comprising:
• an enclosure comprising a piston that defines two sealed compartments, a main compartment comprising at least one solid propellant cartridge, and a compartment for a liquid reserve opening into said liquid supply line,
• an ignition charge connected to said main compartment and adapted to initiate the combustion of at least one solid propellant cartridge upon receiving a command from said control unit.