WO2007003395A1

WO2007003395A1 - Pyrotechnic gas generator intended for motor vehicle safety

Info

Publication number: WO2007003395A1
Application number: PCT/EP2006/006454
Authority: WO
Inventors: Xavier Morvan; Philippe Naulleau
Original assignee: Autoliv Development AB
Current assignee: Autoliv Development AB
Priority date: 2005-07-05
Filing date: 2006-07-03
Publication date: 2007-01-11
Anticipated expiration: 2008-01-05
Also published as: FR2888315A1; FR2888315B1; DE112006001682T5

Abstract

This generator comprises a tubular envelope (1) subdivided internally by a pair of partitions which delimit two combustion chambers situated at the ends of the envelope (1) and communicating, via orifices forming a nozzle, with a central plenum chamber (40), which itself communicates with at least one discharge opening (400) , the partition (5A) of at least one (30A) of the combustion chambers being furnished with a protective member which prevents the inadvertent igniting of the pyrotechnic charge (3A) that it contains by the combustion gases originating from the other chamber; this protective member is a thin and brittle plate (7) pressed against the partition (5A) of the chamber (30A) and normally blocking off the orifice (50A) thereof, which has zones of less resistance, such as grooves, capable of breaking under the pressure of the combustion gases produced in the chamber (30A) so that the parts (70 ') bordering these zones of less resistance deploy like petals towards the plenum chamber (40) , thus allowing the gases to pass (F) into the latter. Safety equipment for motor vehicles, particularly for the inflation of a protective airbag.

Description

Pyrotechnic gas generator intended for motor vehicle safety

The present invention relates to a pyrotechnic gas generator intended for motor vehicle safety.

More particularly it relates to a pyrotechnic gas generator comprising a tubular envelope closed at its ends and subdivided internally by a pair of partitions which delimit two combustion chambers situated at the ends of the envelope and each communicating, via orifices forming nozzles, with a central plenum chamber, which itself communicates with at least one opening for the outlet of the gas, an opening called a "discharge opening", the partition of at least one of the combustion chambers being furnished with a protective member which prevents the inadvertent igniting of the pyrotechnic charge that it contains under the effect of the combustion gases generated in the other chamber.

The gases escaping from the plenum chamber, through the discharge opening, may be used in particular to inflate an airbag intended to protect the occupants of a motor vehicle in the event of an accident.

A device of this kind is described for example in document FR 2 760 710, to which reference may be made if necessary, because it clearly illustrates the prior art on the subject.

In this known generator, one of the partitions is furnished, on the side of the plenum chamber, with a safety valve (reference number 26) which is the abovementioned protective member. It is a valve with a lobe consisting in an annular shim that is fixedly attached, via a flexible tab, to a central stopper pad capable of pressing against the central orifice of the partition, the orifice that serves as a nozzle.

A disadvantage encountered with the known devices is associated with the diversity of the generators, particularly in the dimensions and the shape of the partition, the number of orifices forming nozzles provided in the latter, their position (centred or off- centre) and their profile (cylindrical, conical, bevelled, etc.).

The result of this is a disparity of structure and dimensions of the valve (s) fitted to the generator. This valve must be adapted each time to the specific features of the generator to be fitted. It must also, in certain cases, particularly if there are several nozzle orifices, be oriented in a specific manner.

Finally, the effectiveness of such a valve is not always perfect, because its seal depends on the flexibility of the tab or tabs supporting the stopper pad(s) and the good fit of the pad with the mouth of the nozzle orifice with which it interacts.

The present invention aims to resolve these difficulties by fitting the gas generator with a standard, robust, reliable, low cost and easily mounted protective member.

The pyrotechnic gas generator intended for motor vehicle safety that is the subject of the present invention comprises a tubular envelope subdivided internally by a pair of partitions which delimit two combustion chambers situated at the ends of the envelope and communicating, via orifices forming a nozzle, with a central plenum chamber, which itself communicates with at least one discharge opening, the partition of at least one of the combustion chambers being furnished with a protective member which prevents the inadvertent igniting of the pyrotechnic charge that it contains by the combustion gases originating from the other chamber.

According to the invention, this protective member is a thin and brittle plate pressed against the said partition, while normally blocking off the orifice thereof, this plate having zones of less mechanical resistance capable of breaking under the pressure of the combustion gases produced in the said chamber whose partition is furnished with the protective member, so that the parts bordering these zones of less resistance deploy like petals towards the plenum chamber, thus allowing the gases to pass into the latter.

According to other features that are advantageous, but nonlimiting, of the invention:

- the other partition is also furnished with a protective member in the form of a thin and brittle plate similar to that associated with the said partition;

- the said zones of less resistance are lines of weakened material;

- the said lines of weakened material are grooves hollowed out in at least one of the faces of the thin plate;

- the said thin and brittle plate is a flat disc;

- the said lines of weakened material are grooves hollowed out in the two faces of the disc, so that it can be reversibly mounted;

- the said thin and brittle plate is a disc dished in the central part, whose convexity is turned towards the plenum chamber; - A -

- the said lines of weakened material are rectilinear grooves disposed diametrically, or substantially diametrically, and crossing in the central region of the disc;

- the zone of the peripheral border of the disc is not grooved;

- the generator comprising a condenser in the form of a cylindrical sleeve mounted in the plenum chamber, the said thin and brittle plate is inserted between the partition against which it is pressed and one end of the said cylindrical sleeve forming the condenser;

- the partition turned towards the plenum chamber has a central zone indented by a recess so that the orifice or orifices forming nozzle (s) emerge at a certain distance from the thin and brittle plate.

This is of value particularly when dealing with several orifices forming nozzles because the pressurized gases that escape therefrom gather in the space situated upstream of the brittle plate (inside the indentation) and help to act upon it; the same phenomenon is obtained with a dished shape as mentioned hereinabove.

According to one possible embodiment of the invention, the generator comprises a tubular envelope closed at its ends and subdivided internally by a pair of partitions, each one separating a combustion chamber situated at one end of the envelope, which contains a pyrotechnic charge associated with an igniter, from a central plenum chamber communicating with at least one discharge opening made in the wall of the envelope, each partition being traversed by at least one orifice forming a nozzle that allows a controlled passage of the hot gases generated by the combustion of the said pyrotechnic charge from the combustion chamber to the plenum chamber, these gases then being able to escape from the plenum chamber via the said discharge opening, particularly to inflate a protective airbag, in which at least one of the said partitions being furnished with a protective member capable of preventing the inadvertent igniting of the pyrotechnic charge of the combustion chamber - called the "second chamber" which is adjacent thereto by the combustion gases discharged at high speed from the other chamber called the "first chamber" - after they have passed through the central plenum chamber, when the charge of this first chamber has been ignited, this generator being characterized in that the said protective member is a thin and brittle plate that is pressed against the partition of the second chamber, on the side of the plenum chamber, so as to normally block off its orifice or orifices -, this plate having zones of less mechanical resistance, such as lines of weakened material, capable of breaking under the effect of a rise in pressure generated by the combustion gases produced inside the second chamber when the charge of this second chamber has been ignited, so that the parts of the plate that border the said zones of less resistance deform, deploying (forming petals) towards the said plenum chamber (without creating debris) , while making an opening opposite the said orifice - or the said orifices - forming a nozzle, thus allowing the hot gases to pass from the second combustion chamber to the plenum chamber.

Other features and advantages of the invention will appear on reading the following description of a preferred embodiment of the invention. This description is made with reference to the appended drawings in which:

- Figure 1 is a schematic view, in axial section, of a pyrotechnic gas generator of the same general type as that of the invention, forming part of the prior art; - Figures 2 and 3 are views in perspective of two protective members consisting of valves, also forming part of the prior art;

- Figures 4 and 5 are views similar to those of Figures 2 and 3 respectively, representing two protective members in the form of disc-shaped thin and brittle plates, according to the invention;

- Figure 6 is a view in section of the protective member of Figure 4, the sectional plane extending diametrically and perpendicular to the plane of the plate, as indicated by the arrows A-A in Figure 4;

- Figure 7 is a view in perspective that represents another embodiment of the protective member, in the form of a dished disc;

- Figure 8 is a partial view similar to that of Figure 1, representing the left part of the generator with its combustion chamber fitted with a protective member similar to that of Figure 4;

- Figure 9 is a view similar to that of Figure 8, illustrating the operation of the device when the charge present in the said combustion chamber is ignited;

- Figures 10 and 11 are detail views representing two possible assemblies of the protective members of Figures 4 and 7 respectively.

The pyrotechnic gas generator illustrated in Figure 1 comprises a tubular envelope 1, cylindrical in shape, whose wall 10 is crimped at each of its ends onto generally discoid-shaped rings HA, respectively HB. Reference numbers 101A and 101B indicate the annular flanges, protruding inwards, that lock the wall 10 onto the rings HA, respectively HB.

In a similar manner, the wall 10 is crimped onto a pair of transverse partitions 5A, 5B mounted inside the envelope .

The annular crimping flanges 10OA, IOOB penetrate into a peripheral receiving groove made for this purpose in the partition 5A, respectively 5B.

In its central part, each of these partitions is traversed by one or more orifices forming a nozzle.

In the embodiment illustrated, the partition 5A is pierced by a single central orifice 6A, while the partition 5B comprises two slightly eccentric orifices 5OB and 51B.

The envelope 1, the rings 12 and 11, and the partitions 5A and 5B are conventionally made of metal with high mechanical and heat resistances; the envelope 1 is nevertheless made of a metal that is sufficiently ductile to allow the crimping.

Each partition 5A, 5B divides the inside of the envelope 1 into two adjacent chambers, one end chamber, called the "combustion chamber" 3OA, respectively 3OB, and the other central chamber, called the "plenum chamber" 40.

The generator comprises a pair of igniters 2A, 2B that are each associated with a pyrotechnic charge 3A, respectively 3B; the igniter 2A and the charge 3A are both situated in the combustion chamber 3OA that is situated on the left of the figure, and that will conventionally be called the "second combustion chamber"; the igniter 2B and the charge 3B are both situated in the combustion chamber 3OB that is situated on the right of the figure and that will be conventionally called the "first combustion chamber".

In the embodiment represented, the latter has a greater length than the second chamber, and it contains correlatively a greater pyrotechnic charge.

As can be seen in Figure 1, each charge 3A, 3B assumes the general shape of an annular sleeve, of generally cylindrical shape, that is sleeve-fitted inside the wall of the envelope 10.

The igniter 2A, 2B is mounted in the central part of the ring HA, respectively HB. The latter support, on their inner face, an annular central skirt 12A, respectively 12B, into which the igniter is sleeve- fitted and held by crimping of the free end portion of the skirt.

Each skirt 12 forms, with the associated ring 11, a single piece.

An annular seal 20A, 2OB, seals the igniter 2A, respectively 2B, at the bottom of the annular skirt, against the ring.

Each igniter 2A, 2B penetrates into the inner space of a charge 3A, respectively 3B.

The latter is held in place in the associated combustion chamber 3OA, 3OB by appropriate centring means, not shown, and is immobilized in translation by a grid 13A, respectively 13B, in the shape of a dish that is interposed between the charge 3A or 3B and the partition 5A or 5B.

These grids 13 do not in any way hamper the passage of the gases. Each igniter 2A, 2B is furnished with a pair of pins 21A, respectively 21B, that are intended to allow its connection to a source of electric current.

This igniter, of a known type, is an electropyrotechnic initiator. It comprises an electric initiation system and a breakable cap enclosing a pyrotechnic compound.

In operation, this pyrotechnic compound releases a flame and hot gases which, in their turn, cause the combustion of the charge 3A or 3B.

The latter consists for example of solid propellant, capable of generating a considerable quantity of gas extremely rapidly.

Placed in each combustion chamber is a thin capping plate 6A, 6B that is pressed hermetically against a partition 5A, respectively 5B.

The nozzle orifices 5OA of the second combustion chamber 3OA and 50B-51B of the first combustion chamber 3OB open into the central plenum chamber 40 that is common thereto.

Mounted inside the plenum chamber 40 is a condenser, consisting of a sleeve 4 made of filtering material that is gas-permeable.

Its function is to trap certain particles present in the gas generated by the combustion of the charge 3, preventing these particles from leaving the envelope and entering the airbag.

Opposite the chamber 40 and the condenser 4, the wall 10 of the envelope is pierced by at least one radial hole 400, and preferably by several radial holes, evenly distributed angularly. In the event of an impact of the vehicle, a deceleration sensor produces an electric signal that is transformed by at least one of the igniters 2A, 2B into a pyrotechnic signal, causing the combustion of the propellant charge 3A and/or 3B. In the combustion chamber 3OA and/or 3OB, the propellant generates gases whose discharge is controlled by the nozzle orifices.

This discharge can occur only when the pressure of the gases generated in a combustion chamber has reached a certain predetermined threshold value causing the cap 6A or 6B to burst.

This discharge takes place via the plenum chamber 40, through the filtering wall of the condenser 4, towards the discharge holes 400 inside the airbag.

With a generator of this type, it is possible to release only a limited volume of gas, and therefore to inflate the protective airbag only partially by initiating the ignition system of only one of the two combustion chambers 3OA, 3OB.

It is also possible to inflate the airbag totally by initiating the two ignition systems simultaneously or with a certain time lag, the lagged ignition making it possible to modulate the speed of inflation of the airbag.

When only the ignition system of the first chamber 3OB is initiated, there is a risk that part of the hot gases that are discharged at high speed and under pressure from this chamber do not leave the plenum chamber 40 via the orifice 5OA (because the latter is axially opposite the outlet orifices 50B-51B) and that then, after having burst the cap 6A, these gases unintentionally ignite the charge 3A of the second combustion chamber 3OA. To alleviate this problem, and according to the teachings of FR 2 760 710 already cited, the partition 5A is furnished with a protective member which consists in a V valve whose function is to resist the passage of the gases from the plenum chamber 40 to the second combustion chamber 3OA, while nevertheless allowing the passage in the reverse direction, when the charge 3A is ignited.

Figure 2 represents a conventional valve with a single lobe Vl, capable of being adapted to a partition provided with a single nozzle orifice.

Figure 3 represents a conventional valve with two lobes V2, capable of being adapted to a partition provided with a pair of nozzle orifices.

Similar valves are provided for partitions having more than two nozzle orifices, particularly three.

The disadvantages of this kind of valve have been explained in the preamble of the present description.

According to the invention, the protective member is a thin and brittle plate.

Advantageously, it is cut from a metal strip of uniform thickness, for example of the order of 0.6 mm, having good cold-forming capabilities while being able to withstand the attack of the hot gases generated in the first combustion chamber.

According to the first and second embodiments illustrated in Figures 4 and 5, the plate is a flat disc. Its diameter corresponds substantially (to within the clearance allowing its installation) to the internal diameter of the wall 10 of the envelope 1.

The discoid plate 7 illustrated in Figures 4 and 6 is grooved on only one of its two faces.

A pair of rectilinear grooves 70 - 71 is provided that can be produced by stamping or by removal of material; they are placed at right angles, crossing in the centre of the disc. They have a length that is less than the diameter of the disc, so that the peripheral zone of the latter is not grooved.

The weakening of the disc arising from this grooving must not be too great so that it can conveniently withstand, without breaking or deforming, the attack of the hot gases generated in the first combustion chamber and present in the plenum chamber.

On the other hand, it must be sufficient to allow the breakage of the lines 70, 71 of lesser thickness, and the deformation of the adjacent parts, when the disc 7 is subjected to the gases generated in the second combustion chamber.

For a strip thickness of 0.6 mm, a groove depth of 0.30 mm and a groove width of 0.35 mm meet these conflicting requirements.

The sectional shape of the groove, for example a "U" or a "V", is of no real importance.

The disc 7 is intended to be mounted in the generator so that its grooved face is turned towards the plenum chamber, towards which the broken parts may deploy "like petals". This poses a manufacturing problem, because the washer must be oriented in a defined manner (front on one side and rear on the other) when it is put in place in the generator.

This requirement slightly increases the manufacturing cost and is a source of error.

On the other hand, the disc may occupy any angular position relative to its central point of symmetry, even if it is associated with several nozzle orifices, which is a worthwhile advantage relative to conventional lobed valves.

The discoid plate 8 of Figure 5 is grooved on each of its two faces, by a grooving similar to that of the first embodiment of Figures 4 and 6.

On one face, two rectilinear "crossed" grooves 80-81 are hollowed out. An identical grooving 80 '-81' is carried out on the opposite face.

The two "crosses" are offset angularly by 45° so that the weakening of the disc is distributed evenly.

This disc is reversible, and the need for a specific front/rear orientation is thus eliminated.

The plate 9 of Figure 7 is a disc that is dished, except in its peripheral border zone that is flat.

Its convex face is grooved.

In the illustrated embodiment, five grooves 90, 91, 92, 93 and 94 are provided placed "like a star", joining at the centre of the disc.

As will be seen hereinafter, with reference in particular to Figure 11, the plate 9 is particularly suitable for being fitted to a partition having several nozzle orifices.

With reference to Figures 8 and 9, the operation of a generator fitted with a plate 7 according to that previously described will now be explained.

The latter is held inside the envelope 1, between the partition 5A and the annular condenser 4. Its grooved face is turned towards the plenum chamber 40.

The circular plate 7 closes off virtually hermetically the mouth of the nozzle orifice 5OA.

If the charge contained in the first combustion chamber (not visible in Figures 8 and 9) is ignited, the hot gases that it generates and that arrive at great speed in the plenum chamber 40 are blocked by this plate and can therefore not reach the second combustion chamber 3OA.

If the charge 3A contained in the second combustion chamber 3OA is ignited, the hot gases that it generates induce therein a high pressure that causes the plate 7 to break along the lines of least resistance 70 - 71; the parts 7 ' bordering the broken lines are pushed away axially by the gases coming out of the nozzle orifice 5OA (arrow F) and are deployed "like petals" in the plenum chamber 40 as illustrated in Figure 9.

No debris is formed that might risk entering the plenum chamber and then the airbag.

Because, before breaking, the plate 7 is impermeable to the gases in one direction and the other, its presence makes it possible to dispense with the capping plate 6A, whose function it also performs. Figures 10 and 11 represent devices in which the partition 5A is traversed by two nozzle orifices 5OA, 51B.

In this case the partition is provided with caps (not shown in the drawings) . The use of two nozzle orifices makes it possible to maintain a constant operating pressure in the combustion chamber irrespective of the temperature or type of combustion of the propellant forming the charge.

For example, for a low operating temperature, only one nozzle orifice is uncapped in order to obtain a sufficient operating pressure. Specifically, at low temperature, the speed of combustion of the propellant is relatively low.

On the contrary, at high temperature, the speed of combustion is greater and both nozzle orifices are uncapped in order to prevent an excessive increase of operating pressure.

The caps are therefore calibrated differently to burst at different pressure thresholds.

Whether both nozzle orifices are uncapped or only one is, the plate 7 must break. The off-centring of the orifices 5OA and 51A relative to the central axis of the partition 5A causes the hot gases exiting through one and/or other of these orifices to generate a pressure that is not centred on the plate 7. The latter therefore risks not operating by breaking and deforming in optimum conditions.

Specifically, because the lines of weakened material 70, 71 cross in the centre of the disc, it is important that the pressure is exerted in this weaker region and that the broken parts then deploy symmetrically outwards . This is true in the same way for the plate 8, with both faces grooved.

In order to allow a centred application of the pressure of the gases leaving one and/or other of the orifices 5OA - 5OB, the partition 5A of the device of Figure 10 has a recess 52 formed in its face turned towards the plenum chamber 40.

Therefore, between the flat plate 7 and the recess 52, an indentation is arranged that forms a small "buffer" space receiving the gases originating from the combustion chamber 3OA, which makes it possible to distribute the pressure thereof against the whole central portion of the plate 7.

A similar configuration is advantageously produced with the reversible plate 8.

The solution adopted on the device of Figure 11 is based on the same principle.

Associated with the partition 5A is a dished plate 9 like that previously described with reference to Figure 7.

Here, however, the face of the partition 5A turned towards the plenum chamber 40 is flat and not recessed.

It is the internal space of the concave part of the plate 9 that serves as a buffer to distribute the pressure evenly against the dished wall of the plate, in order to cause it to break evenly and uniformly.

A plate having lines of weakened material similar to those that have just been described could also be associated additionally with the partition 5B of the first combustion chamber 3OB, within a generator in which the ignition may occur first, selectively, in one or other of the two combustion chambers and -not only as a priority in the first chamber.

The zones of weakened material provided in the thin and brittle plates are not necessarily lines; even more so, these rectilinear lines are neither necessarily rectilinear nor made by stamping or removal of material .

They could be curved or bent, joining in the central region of the plate.

They could be formed by moulding, particularly if the plate is made in a moulded material, where necessary a synthetic (non-metallic) material.

The plate could be attached at its peripheral border zone to the partition 5, for example by bonding or welding.

Claims

1. Pyrotechnic gas generator intended for motor vehicle safety, comprising a tubular envelope (1) subdivided internally by a pair of partitions (5A, 5B) which delimit two combustion chambers (3OA, 30B) situated at the ends of the envelope (1) and communicating, via orifices forming a nozzle, with a central plenum chamber (40) , which itself communicates with at least one discharge opening (400), the partition (5A) of at least one (30A) of the combustion chambers being furnished with a protective member which prevents the inadvertent igniting of the pyrotechnic charge (3A) that it contains by the combustion gases originating from the other chamber (30B), characterized in that this protective member is a thin and brittle plate

(7; 8; 9) pressed against the said partition (5A), while normally blocking off the orifice (50A) thereof, this plate having zones of less mechanical resistance capable of breaking under the pressure of the combustion gases produced in the said chamber (30A) whose partition is furnished with the protective member, so that the parts (70') bordering these zones of less resistance deploy like petals towards the plenum chamber (40) , thus allowing the gases to pass (F) into the latter.

2. Generator according to Claim 1, characterized in that the other partition (5B) is also furnished with a protective member in the form of a thin and brittle plate (7; 8; 9) similar to that associated with the said partition (5A).

3. Generator according to Claim 1 or 2, characterized in that the said zones of less resistance are lines of weakened material.

4. Generator according to Claim 3, characterized in that the said lines of weakened material are grooves (70-71; 80-81, 8O'-81'; 90-91-92-93-94) hollowed out in at least one of the faces of the thin plate (7; 8; 9) .

5. Generator according to one of Claims 1 to 4, characterized in that the said thin and brittle plate (7; 8) is a flat disc.

6. Generator according to Claims 4 and 5, taken in combination, characterized in that the said lines of weakened material are grooves (80-81; 80 '-8I¹) hollowed out in the two faces of the disc (8), so that it can be reversibly mounted.

7. Generator according to one of Claims 1 to 4, characterized in that the said thin and brittle plate (9) is a disc dished in the central part, whose convexity is turned towards the plenum chamber (40) .

8. Generator according to one of Claims 4 to 7, characterized in that the said lines of weakened material are rectilinear grooves disposed diametrically, or substantially diametrically, and crossing in the central region of the disc.

9. Generator according to one of Claims 5 to 8, characterized in that the zone of the peripheral borders of the disc is not grooved.

10. Generator according to one of Claims 1 to 9, that comprises a condenser (4) in the form of a cylindrical sleeve mounted in the plenum chamber

(40) , characterized in that the said thin and brittle plate (7; 8; 9) is inserted between the partition (5A, 5B) against which it is pressed and one end of the said cylindrical sleeve forming the condenser (4 ) .

11. Generator according to one of Claims 1 to 8, characterized in that the face of the partition

(5A) turned towards the plenum chamber (40) has a central zone indented by a recess (52) so that the orifice or orifices (5OA, 51A) forming nozzle (s) emerge at a certain distance from the thin and brittle plate (7, 8) .

12. Pyrotechnic gas generator intended for motor vehicle safety, comprising a tubular envelope (1) closed at its ends and subdivided internally by a pair of partitions (5A, 5B) each one separating a combustion chamber (3OA, 30B) situated at one end of the envelope (1), which contains a pyrotechnic charge (3A, 3B) associated with an igniter (2A, 2B), from a central plenum chamber (40) communicating with at least one discharge opening (400) made in the wall (10) of the envelope (1), each partition (5A, 5B) being traversed by at least one orifice (5OA, 50B) forming a nozzle that allows a controlled passage of the hot gases generated by the combustion of the said pyrotechnic charge (3A, 3B) from the combustion chamber (3OA, 30B) to the plenum chamber (40), these gases then being able to escape from the plenum chamber (40) via the said discharge opening (400), particularly to inflate a protective airbag, in which at least one (5A) of the said partitions is furnished with a protective member (V) capable of preventing the inadvertent igniting of the pyrotechnic charge (3A) of the combustion chamber (30A) - called the "second chamber" which is adjacent thereto by the combustion gases discharged at high speed from the other chamber (30B) - called the "first chamber" - after they have passed through the central plenum chamber (40), when the charge (3B) of this first chamber has been ignited, characterized in that the said protective member is a thin and brittle plate (7; 8; 9) that is pressed against the partition (5A) of the second chamber (30A), on the side of the plenum chamber (40), so as to normally block off its orifice - or orifices - (50A), this plate (7; 8; 9) having zones of less mechanical resistance, such as lines of weakened material (70-71; 80-81, 80 '-8I¹; 90-91-92-93-94), capable of breaking under the effect of a rise in pressure generated by the combustion gases produced inside the second chamber (30A) when the charge (3A) of this second chamber has been ignited, so that the parts of the plate that border the said zones of less resistance deform, deploying towards the said plenum chamber (40) while making an opening opposite the said orifice - or the said orifices -

(50A) forming a nozzle, thus allowing the hot gases to pass from the second combustion chamber (30A) to the plenum chamber (40).