Drive chain for a helicopter incorporating a pyrotechnic assistance drive module and helicopter comprising same Field of the invention 5 The present invention relates to the field of helicopter propulsion. Specifically, the invention relates to the use of rotary pyrotechnic actuators for supplying additional power during difficult flight phases such as autorotation. Prior art I 0 A helicopter is conventionally provided with a main rotor, which forms a rotary wing to lift and propel said helicopter. The helicopter also comprises an anti-torque means that is often formed by a second, rear rotor. Single-engine helicopters have great advantages compared with multi-engine 15 helicopters, in particular in terms of production and maintenance costs. However, if the engine of a single-engine helicopter breaks down or malfunctions, the pilot has to perform the difficult autorotation manoeuvre for an emergency landing. Statistics show that in some conditions this manoeuvre can cause significant damage to 20 the airframe. There is therefore a need to install a means capable of providing potential supplementary power very quickly in order to increase the safety of the autorotation manoeuvre in a single-engine helicopter, while preventing the rotor revolution from 25 dropping during any phase of this manoeuvre. EP2327625 already proposed installing such a system for providing emergency power at the input of the main transmission gearbox that drives the rotary wing of a helicopter. This system uses an electric motor, which has the advantage of being able to 30 quickly start rotating and of having power that can be controlled depending on the driving problem to be fixed. 5 2 However, this kind of electromechanical solution requires batteries, control electronics and an electric motor onboard. All this equipment, especially the batteries, affects the weight estimation for the airframe, despite being used very occasionally. The aim of the invention is to provide a simple alternative to avoid affecting the weight estimation of the helicopter. Disclosure of the invention 10 In this respect, the invention relates to a drive chain for driving the rotor(s) of a helicopter, comprising a main transmission gearbox capable of driving the rotor(s) when said gearbox is moving, a main engine for providing the power for the flight, and at least one assistance drive module. The engine and the assistance drive module are mechanically connected to said main transmission gearbox so as to induce the 15 movement of said gearbox. The drive chain is characterised in that said assistance drive module comprises a pyrotechnic gas generation device for generating a torque on a power transmission shaft mechanically connected to the main transmission gearbox. A first advantage of a pyrotechnic device is its energy density. The assistance drive 20 that uses said device can thus be designed to have a lesser effect on the weight estimation of the airframe while still providing sufficient power for an emergency manoeuvre by supplying a torque for maintaining the movement of the rotors. Another advantage of the pyrotechnic device is that of being able to simplify the 25 onboard electronics for controlling said device. The power curve provided over time depends on the design of the device. When it is produced, the assistance drive module having the pyrotechnic device is thus calibrated such as to provide a suitable power curve for the helicopter without complementary control means. 30 Advantageously, said assistance drive module comprises at least one flyer that can rotate about an axis of symmetry, said flyer comprising a drum rigidly connected to a 5 3 power transmission shaft, at least one gas ejection nozzle positioned on the periphery of the drum and oriented substantially tangentially to the rotation about said axis, said pyrotechnic gas generation device being installed in the flyer and feeding said at least one exhaust nozzle. In other words, the exhaust nozzles produce tangential gas ejection jets for generating a torque on the flyer shaft. The device can thus be used to both provide a torque at the input of the main transmission gearbox if the main engine fails, and maintain the movement of the rotors. With regard to a single usage, the pyrotechnic I 0 device allows gases to be generated in a chamber upstream of the exhaust nozzles at a high pressure and temperature, thus creating thrust and therefore the torques required for driving the rotary wing during the manoeuvre being made. In this case, the main engine is not necessarily restarted, but rather the necessary power is provided to the helicopter in order to complete a manoeuvre or to perform an emergency manoeuvre to 15 allow the helicopter to get to safety. The fact that the pyrotechnic gas generation device is installed in the flyer reduces the transfer problems and the losses during the operation thereof. Moreover, the principle of the flyer means that it can be positioned on the rotary machine and said 20 rotary machine can rotate the flyer during normal operation, i.e. when the assistance drive module is not operating. Indeed, the flyer creates few friction losses and is not at risk of being used prematurely. Preferably, the pyrotechnic gas generation device comprises a block of solid 25 propellant in which there is formed a combustion chamber that feeds said at least one exhaust nozzle. This makes it simpler to maintain the device. It is thus conceivable to replace the pyrotechnic device of the assistance drive module in a simple manner after use. 4 Advantageously, the assistance drive module further comprises a mounting in which the shaft of the flyer rotates, and a volute for recovering the gases, which radially surrounds the flyer and is rigidly connected to said mounting. 5 The volute helps to expand the gases exiting the exhaust nozzles, and thus, by means of the thrust from said nozzles, contributes to the torque provided by the flyer. It is therefore possible to improve the performance of the flyer by optimising the shape of this volute. Another advantage of this volute is that of the hot gases exiting the exhaust nozzles being discharged radially with respect to the axis of the flyer, thus limiting the I 0 extent to which the equipment surrounding the flyer heats up. These gases can then be directed to the outlet of the volute towards a suitable discharge region. If necessary, said assistance drive module can comprise at least two flyers arranged in a line for driving the same power transmission shaft. A first advantage of this 15 arrangement is the ability to provide a particular power by combining a plurality of standard flyers. Another advantage is that of being able to adjust over time the power provided by the assistance drive module by controlling the successive start-up of the flyers such that it is adapted to the requirements of a manoeuvre. 20 Said assistance drive module can comprise a mechanical output arranged to directly drive a mechanical input of the main transmission gearbox or to drive the same power transmission shaft connected to the main transmission gearbox as the main engine. When the main engine is a turbine engine, said assistance drive module can 25 comprise a mechanical output coupled to the spindle of a turbine of the turbine engine. Advantageously, said turbine is the power turbine of the turbine engine. Depending on the installation selected, this option can make it possible to integrate the assistance drive module in the turbine and to further improve the weight estimation. 30 Advantageously, the assistance drive module further comprises a system for igniting the or said pyrotechnic gas generation device(s), said ignition system comprising a 5 control system that can be placed in an armed mode or a deactivated mode. In particular, this prevents the system from being ignited at the incorrect time. The invention also relates to a helicopter comprising a drive chain as described 5 above. The invention also relates to a method for driving the rotary wing of such a helicopter, in which the assistance drive module ignition system can be placed in an armed, deactivated or triggered mode, said method comprising a step of arming said I 0 ignition system when a helicopter pilot orders a predetermined manoeuvre, for example autorotation. This step corresponds in particular to the case in which the safety conditions for triggering are satisfied. This enables the system to react quickly when necessary, and to avoid the risk of the system being triggered during normal flight conditions. 15 Brief description of the drawings The present invention will be better understood, and other details, features and advantages of the present invention will become clearer upon reading the following description, given with reference to the accompanying drawings, in which: 20 Fig. 1 is a perspective view of a flyer for an assistance module according to the invention. Fig. 2 is a section through half a flyer according to the invention, in a plane that is perpendicular to the axis of rotation and passes through the exhaust nozzles. Fig. 3 is a longitudinal section through an assistance drive module according to 25 the invention prior to use. Fig. 4 is a schematic perspective view of one arrangement of the means for discharging the gases on an assistance drive module according to the invention. Fig. 5 is a schematic section, in a plane perpendicular to the axis of rotation, through the volute for discharging the gases and through the flyer of an assistance drive 30 module according to the invention. 6 Fig. 6 is a longitudinal section through an assistance drive module according to the invention towards the end of its ignition. Fig. 7 is a schematic view of a first embodiment of a drive chain according to the invention for a helicopter. 5 Fig. 8 is a schematic view of a second embodiment of a drive chain according to the invention for a helicopter. Fig. 9 is a schematic view of a third embodiment of a drive chain according to the invention for a helicopter. Fig. 10 to 12 show alternative embodiments of an assistance drive module 10 according to the invention, which can be used in the various embodiments of the drive chain. Detailed description of the invention The invention relates to the use of a pyrotechnic drive module such as an assistance 15 drive module in a drive chain of a helicopter. In the example described, as shown by Fig. 1 to 3, this drive module comprises a flyer 1 consisting of a cylindrical drum 2 and a power transmission shaft 3, which are rigidly interconnected and have the same axis LL about which the assembly is intended 20 to rotate. With the drum 2 having a given width 0 along the axis of rotation LL, a plurality of exhaust nozzles 4 are arranged on a narrower strip, of width d, of the peripheral cylindrical wall 5 of said drum. This strip is located at one side of the cylindrical wall 5 of 25 the drum 2. With reference to Fig. 1 and 2, if, for example, the left transverse surface is denoted the upper surface 6 of the drum 2 and the right transverse surface is denoted the lower surface 7 of the drum, the strip in which the exhaust nozzles 4 are located can, for example, be off-centre as shown, and close to the upper surface 6. The exhaust nozzles 4 are oriented tangentially to the cylindrical wall 5, all facing the same direction. 30 This direction is the same as that of the gas jet that should exit said nozzles, and therefore, in response, it causes the flyer 1 to rotate during operation in the opposite 7 direction to that of the gas jet. In the example, the exhaust nozzles 4 are distributed evenly in azimuth, and there are three of them, with two being visible in Fig. 1. Still referring to the example, the exhaust nozzles 4 are two-dimensional. This means 5 that they are defined by their shape in a sectional plane transverse to the axis of rotation LL. With reference to Fig. 2, the exhaust nozzle 4 forms a duct of length dz that diverges starting from a neck 8. This neck 8 is located on a radius R of the axis LL of the flyer 1, and the exhaust nozzle 4 is oriented along an axis ZZ that is substantially perpendicular to the radius passing through the neck 8. 10 15 20 Alternatively, it is possible, for example, to design the exhaust nozzles 4 to have an asymmetric shape, depending on the required ease of design and production. In this case, said exhaust nozzles are still defined as a diverging duct oriented along an axis ZZ. Via the neck 8, the exhaust nozzle 4 is in communication with a combustion chamber 9, which should generate pressurised gas when the flyer 1 is in operation. In the example shown, this combustion chamber 9 is shared by the three exhaust nozzles 4 positioned on the cylindrical wall 5 of the drum 2. Therefore, the combustion chamber 9 has to be supplied with pressurised gas. With reference to Fig. 3, which shows the flyer 1 prior to use, it can be seen that the drum 2 forms a cavity between its cylindrical wall 5 and its upper surface 6 and lower surface 7. The internal cavity in the drum 2 is filled by a solid block 10 of a material designed to 25 produce high-energy gases when set alight by an ignition device, which is positioned in the region of the combustion chamber 9 but not shown in the drawings. This material is generally made of solid propellant. The space left free in the drum 2 between the strip occupied by the exhaust nozzles 4 and the lower surface 7 is of such a size as to form a sufficient store of propellant, the combustion of which will generate gases for the 30 necessary period of time for the emergency manoeuvre. 8 In the flyer 1, before use, the combustion chamber 9 which feeds the exhaust nozzles 4 and is intended for receiving the gases produced by the combustion of the propellant is dug out of the propellant block 10 and occupies less space in the region of the exhaust nozzles. Preferably, the exhaust nozzles 4 are sealed by a membrane 11, 5 which is ejected by the pressure of the combustion gases during ignition, thus preventing dust and moisture from entering the combustion chamber 9 when not in the triggered state. To form a drive module, the flyer 1 is incorporated on a mounting 12 comprising I 0 bearings 13, 14, in which the shaft 3 rotates. As shown, the shaft 3 is intended to be coupled to a shaft 15 that drives another mechanical system. The shaft 15 can be an intermediate shaft, referred to as a "shear shaft", that is designed to break if the transmitted torque accidentally exceeds a maximum permissible value. Furthermore, said shaft is coupled, for example by means of splines, on the shaft 3 of the flyer 1. 15 As shown in Fig. 3 to 5, the mounting 12 preferably includes a volute 16. This volute 16 radially surrounds the flyer 1. The volute is designed to allow the gases exiting the exhaust nozzles 4 to expand before discharging them. Together with the portion of the mounting 12 that surrounds the drum 2, the volute forms a duct 16 which winds around 20 the flyer 1. The internal wall of this duct 16 is open opposite the passage for the exhaust nozzles 4 in order to collect the gases exiting said nozzles. In the example shown, the radial cross section of the duct formed by the volute 16 is substantially rectangular. With reference to Fig. 5, the cross section of the external wall of the volute 16 has a 25 spiral shape around the axis LL of the flyer 1. If q> denotes the azimuth around the axis LL, the distance from the external wall of the volute 16 to the axis follows a law S(q>), which increases steadily in this example, as a function of q> between a point A and a point B in the direction of rotation corresponding to that of the flyer 1 during operation. In Fig. 5, the direction of rotation is anticlockwise and corresponds to exhaust nozzles 4 30 oriented as in Fig. 2. 9 In addition, the width of the volute 16 along the axis LL increases in this example from A to B. This is shown by the sections shown in Fig. 3 and 6, which show the cross section of the volute 16 in the longitudinal sectional half-planes passing through point A (at the top) and point C (at the bottom), which is an intermediate point between A and B 5 and shown in Fig. 5. The cross section of the duct formed by the volute 16 thus steadily changes (increases in the example given here), according to a law S(