EP0886122B1 - Solid propellant rocket motor with low vulnerability - Google Patents

Description

The present invention relates to the field of risk mitigation munitions, "Insensitive Munitions" for Anglo-Saxons. More particularly the present invention relates to a solid propellant engine having a low vulnerability towards certain external attacks such as a fire in an ammunition depot or the impact of shrapnel or bullets on the ammunition. During an assault, by example the fire of an ammunition depot, the solid propellant loading of a structural engine metallic will ignite after a warm-up time more or less long. The metal of the structure still has significant resistance despite its own heating and the pressure inside the engine will therefore increase provocative, either its bursting or its displacement untimely by the thrust of the gases ejected by the nozzle, which aggravates the fire damage in the depot and in its environment.

We know depressurization devices fast or thrust stop engine solid, by cutting or releasing closed openings arranged on the structure, generally by means pyrotechnics. US Patent 3,052,091 (basis for the preamble of claim 1) describes a device comprising tight plugs closing off openings on the structure and held in place by two half-shells secured by bolts explosive, when the explosive bolts are triggered the half-shells are detached and the caps are driven by the internal pressure of the engine and free the openings; there is rapid depressurization and thrust stop. US Patent 5,166,468 describes applying this technique to reduce the vulnerability of a solid propellant engine. A set of thermocouples suitably arranged in the structure of the motor and associated with a microcircuit electronics detect the heating of the structure by a fire and triggers the operation of the device pyrotechnics causing the release of the openings. This technique has the first drawback that of requiring means of detection and control, more or less bulky and complicated, so subject to default. In addition the operation is accompanied by projections of plugs and often metal holding devices.

We also know structural motors mixed with an interior structure reinforced by an outer winding of fibers impregnated with a resin or more generally of a matrix. The patent FR 2 606 082 describes a mixed structure, the interior structure includes two tube sectors produced by the longitudinal cutting of a tube and a circumferential rewinding ensuring resistance to internal pressure during operation, notice that this engine thus has two long slots longitudinal. The internal structure ensures the holding longitudinal forces. Patent application EP 0 559 436, specified by patent application WO 96/04474, described, to reduce the vulnerability of engines to solid propellant, a mixed structure that includes including a metallic ferrule with a multitude of very narrow slots and a composite outer winding fibers impregnated with a resin. When this structure caught in a fire, overheating fairly quickly destroys the composite winding. And when after heating the solid propellant of the load ignites, the metal structure which does not can withstand that axial forces deform and lets the combustion gases escape through the slots preventing any significant increase in pressure in the engine. However, this device presents the disadvantages of requiring long and delicate machining numerous slits and the risk of tearing the metallic structure.

The object of the present invention is to provide a device without the disadvantages previous.

The present invention relates to a motor with solid propellant, of low vulnerability, comprising a solid propellant charge, thermal protection covering the interior of a metal structure with a front bottom and a back bottom carrying at least one nozzle, said front and rear bottoms being integral a cylindrical shell with large openings circumferentially distributed and temporarily closed by watertight caps; the engine is such that tight plugs are held in the openings between thermal protection and a composite winding of fibers impregnated with a matrix which surrounds the shell cylindrical, the said plugs being released by the accidental degradation of the composite winding.

External aggression degrades, or even destroys, the composite winding, the tight plugs no longer being retained by the winding are expelled by the gases interiors which then escape through the wide openings preventing any significant increase in engine pressure. Possibly the openings are circumferentially evenly distributed to balance the forces due to the gas flow through each of the openings. There are therefore at least two diametrically openings opposite on the cylindrical shell; but, if they are more numerous, its openings can be distributed in several transverse groups along the shell cylindrical, it being understood that in each group transverse the openings are possibly circumferentially evenly distributed.

Advantageously, each tight plug is such that its outer face has the same radius of curvature as that from the outside of the cylindrical shell. This condition avoids flats or growths which could injure the composite winding. Likewise the inner side of each sealed cap has the same radius of curvature than that of the inner part of the cylindrical shell.

Advantageously, each watertight stopper is made of a material of the thermal protection material type. It can be the same material as that of the motor thermal protection but we can also use a different material. The material of thermal protection must be compatible with the solid propellant constituting the load. The thermal protection materials are generally elastomers of polymeric or rubbery type, comprising various charges and in particular charges pulverulent and / or fibrous to improve the hold thermal.

Preferably, each waterproof cap is a area in excess of the thermal protection which fills the volume of the opening: the plug makes part of thermal protection. The tightness of the cap is ensured by the contact of its part device with the edge of the opening and it's reinforced by its integration into thermal protection.

Advantageously, the portion of the cylindrical shell where the openings are arranged is less thickness and the composite winding strengthens the the structure. By portion of lesser thickness, it is necessary hear that on this portion of the cylindrical shell which can represent almost the entire length of the ferrule, the thickness of the ferrule is just that necessary to resist axial forces or longitudinal. Resistance to radial forces, especially those due to the operating pressure, is provided by the composite winding which thus reinforces the cylindrical shell.

Advantageously the passage section of the assembly openings is at least ten times the area of nozzle neck. Of course if the engine has several nozzles the condition is calculated with respect to to the sum of the nozzle neck areas.

The openings can be geometric in shape any. Possibly these openings have a shape oblong in the direction of the motor axis, examples of such oblong shapes will be described by the following.

Advantageously, each opening when it is oblong has a major axis parallel to the axis of the ferrule cylindrical, and its greatest width is greater or equal to the fifteenth of the diameter of the cylindrical shell. For example the oblong opening has an oval shape or elliptical. This condition on the width of the oblong opening is intended to avoid spreading crack, risk due to too narrow openings such as slots.

Each oblong opening has a part rectangular of length L, terminated by two parts identical opposites whose shape prevents spread of crack. Advantageously, these opposite parts have the shape, for example, basket handle, half-ellipse or more simply semicircle.

The length L of the rectangular part of each oblong opening is less than or equal to half the length of the thinner portion of the cylindrical shell.

Preferably, the engine has at most three transverse groups of openings distributed along the cylindrical shell. In each transversal group there are has at most eight openings, distributed circumferentially. Preferably the number of openings of each transversal group is between 2 and 4.

Finally the axial distance L 'separating two groups successive transverse on the cylindrical shell is greater than or equal to half the length L, length of the rectangular part of an opening oblong.

The engine according to the invention has the advantages not to require detection devices and command for the release of the openings, not to give rise to projections of metal parts, at most a few pieces of thermal protection and finally the cylindrical shell has a limited number openings that are easy to machine and have no risk of crack propagation.

The present invention is explained in more detail in using the following figures representing a particular achievement.

Figure 1 is a longitudinal sectional view of an engine according to the invention.

Figure 2 is an enlargement of detail II of the previous figure.

Figure 3 is a cross-sectional view III of the motor in line with a group of openings, which are oblong in this example.

Figure 1 shows, in longitudinal section, a motor 1 which has a metallic structure, the inner part is covered with protection 2. This metallic structure is made up in particular a front bottom 3 and a rear bottom 4 integral with a cylindrical shell 5 which comprises several oblong openings 6 closed by tight plugs 8, the shell 5 is externally reinforced by a circumferential winding 7 of fibers impregnated with a matrix. On the front bottom 3 is fixed an igniter device 9. On the rear bottom 4 is fixed a nozzle 10. The loading of solid propellant shown in 11 is of the free loading type. In this example each watertight plug 8 is in fact a zone in extra thickness of the thermal protection which occupies everything the interior volume of each oblong opening 6. The shape of the oblong openings is, in this example very simple: it has a part with edges parallels of length L and distant from 2R, or otherwise says a rectangular part of length L and width 2R, terminated at its two ends by semicircles of radius R.

Figure 2 is an enlargement of detail II of Figure 1. We find, in 5, the cylindrical shell which we see the connection between the lower part thickness which comprises the oblong openings 6 and the thicker end part connecting with funds. The outer composite winding 7 which covers the thin part of the shell 5 and in particular the plugs 8 of the oblong openings 6. This winding extends, to end there, on the thick part of the cylindrical shell. Thermal protection 2, which covers the interior of the metal structure, and which has extra thickness areas that fill everything the volume of an oblong opening up to the winding exterior composite; these extra areas of the thermal protection act as a tight plug 8.

Figure 3 shows a cross section schematic of the mixed structure at group level oblong openings. The metal ferrule 5 comprises for this group, four oblong openings 6 evenly distributed, i.e. in this case 90 ° each other. The inner part of the shell cylindrical 5 is coated with a thermal protection 2, which at the oblong openings 6 has zones 8 in excess thickness which penetrate into the oblong openings 6. We see in this figure that the outer face of these thicker areas is shaped so as to have a radius of curvature equal to that of the outer part of the cylindrical shell: this arrangement restores the circularity of the structure and avoids the flat areas corresponding to a part oblong openings.

In this example the shell has two groups oblong opening transverse, each group with four oblong, distant openings angularly 90 °. The oblong openings therefore have a simple shape: a rectangular part of length L = 57.5 mm and width 2R = 10mm, terminated by two semi-circular portions of radius R. The openings oblong are machined by mechanical means simple: in this case for example by milling; the only desirable precaution is cleanliness and sharp edges of openings.

The oblong openings of the two groups are here aligned and separated by the distance L '= 30mm. Knowing that the length Laf of the part of least thickness of the cylindrical shell is 230mm, that its diameter outside D is 140mm and the diameter of the neck of the nozzle is 12mm, we can check that all conditions previously stated are met.

When the engine of the present invention is caught in a fire, the heat of the fire destroyed or seriously degrades the composite winding so that this winding no longer has mechanical resistance or that she is very weak. When, in turn by heating, the solid propellant of the load ignites the combustion gases pressurize very slightly the combustion chamber and the gap between charging and thermal protection. This very slight overpressure is sufficient to punch thermal protection at the openings oblong because this part which forms a stopper is no longer restrained, or very weakly retained by the remains of winding. The openings are released and the gases evacuate avoiding any increase in pressure. These large openings are favorable because the resultant pressure forces on the surface of the cap increases as the area of the opening while the shear strength, to overcome, increases only as the perimeter of the opening. Finally the only projections are those of material plugs of thermal protection so enough materials flexible, in any case non-metallic.

The metal structure is made of all metal usually used in this field, we will cite preferably steels and light alloys with high resistance. The front 3 and rear 4 bottoms are rendered integral with the cylindrical shell 5 by modes of common fixing such as rod, screwing or welding. In this example the cylindrical shell has a thickness reduced over practically its entire length, only the ends where the funds are made have greater thickness. The lesser part thickness of this cylindrical shell can possibly extend over a more limited area of the ferrule, on this portion will be fitted oblong openings. The shell also has missile wing attachment devices or other elements to be fixed on the structure, this which is done by usual techniques since these are metallic elements. The funds before and rear also have bases for fixing the igniter, the nozzle (s), as well as the skirts for connect the engine to the other parts of the missile. The fasteners are not shown or detailed in this figure.

The outside of the cylindrical shell, more particularly its entire portion of reduced thickness, is covered with a circumferential winding of fibers impregnated with a matrix, the composite winding is dimensioned to withstand the radial forces during the nominal operation of the engine, it should be recalled that the metallic cylindrical shell, ensures resistance to axial forces. The materials of the composite winding, or at least the fiber impregnation matrix are chosen so as to degrade or lose their mechanical properties from a temperature higher than the maximum temperature reached during nominal use of the motor, but below the ignition temperature of the solid propellant by heating. Organic materials such as for example aramid fibers, such as those marketed under the name KEVLAR ^®, or carbon fibers and the matrix, for example an epoxy resin, suitable for this application.

The inner part of the metal structure is, in known manner, covered with a thermal protection fabricated with a thermal protection material compatible with the solid propellant of the load. This thermal protection is sized according to the mission of the missile. This thermal protection is put in place by casting, draping or by any other technique appropriate to the nature of the material retained. For example, the thermal protection material is an elastomer obtained by crosslinking of a polybutadiene with hydroxytelechelic termination and loaded with refractory materials (SiO ₂ , etc.) in the form of powder and fibers.

The solid propellant charge can be of the type free or molded glued, it can include one or more solid propellant blocks, either frontal combustion either with radial combustion, the appropriate arrangements: inhibitor, sticking and / or timing are known to man business.

Any type of propellant may be suitable for carrying out loading and preferably the propellants do not not giving rise to transition phenomena combustion-detonation during the attacks envisaged here.

In this example the loading consists of a solid propellant block with central channel, so at radial combustion. This block is mounted free in the structure, this type of mounting leaves a gap between thermal protection of the structure and the face outside of the block coated with an inhibitor. This gap is maintained by wedging devices details of which are not given here.

Claims (13)

1. Low-vulnerability solid propergol propulsion unit (1), comprising a solid propergol charge (11) and a metal structure with a front base (3) and a rear base (4) which supports at least one nozzle (10), the said front and rear bases being integral with a cylindrical collar (5) which contains wide apertures (6), which are distributed circumferentially, and are temporarily shut by sealed stoppers, which propulsion unit is characterised in that it comprises a thermal protection (2) which covers the interior of the said metal structure, in that the sealed stoppers (8) are held in the apertures (6), between the thermal protection and a composite winding (7) consisting of impregnated fibres of a matrix which surround the cylindrical collar (5), the said stoppers (8) being released by accidental deterioration of the composite winding (7).
2. Propulsion unit (1) according to claim 1, characterised in that each sealed stopper (8) is such that its outer surface has the same radius of curvature as that of the outer surface of the cylindrical collar (5).
3. Propulsion unit (1) according to claim 1 or claim 2, characterised in that each sealed stopper (8) is made of material of the thermal protection type.
4. Propulsion unit (1) according to claim 3, characterised in that each sealed stopper (8) is an area of additional thickness of the thermal protection (2).
5. Propulsion unit (1) according to any one of the preceding claims, characterised in that the portion of the cylindrical collar (5) where the apertures (6) are provided is less thick, and the composite winding (7) reinforces the cylindrical collar (5).
6. Propulsion unit (1) according to claim 5, characterised in that the cross-section of passage of the apertures (6) in total is at least equal to ten times the area of the neck of the nozzle (10).
7. Propulsion unit (1) according to claim 6, characterised in that each oblong aperture has a major axis which is parallel to the axis of the cylindrical collar (5), and its greatest width is greater than, or equal to, a fifteenth of the diameter of the cylindrical collar (5).
8. Propulsion unit (1) according to claim 6 or claim 7, characterised in that each oblong aperture (6) comprises a rectangular part with a length L, which ends in two identical opposite portions, the shape of which prevents propagation of cracks.
9. Propulsion unit (1) according to claim 8, characterised in that the length L of the rectangular part of each oblong aperture is smaller than, or equal to, half the length of the portion of the cylindrical collar (5) which has the smallest thickness.
10. Propulsion unit (1) according to claim 6, characterised in that it comprises at the most three transverse groups of apertures (6) distributed along the cylindrical collar (5).
11. Propulsion unit (1) according to claim 10, characterised in that each transverse group of apertures comprises at the most eight apertures.
12. Propulsion unit (1) according to claim 11, characterised in that the preferred number of apertures in each transverse group is between 2 and 4.
13. Propulsion unit (1) according to claim 8, characterised in that the axial distance L' which separates two successive transverse groups in the cylindrical collar (5) is greater than, or equal to, half the length L, which is the length of the rectangular part of an oblong aperture.

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