EP0738869A1 - Deconfinement of a military explosive charge by differential expansion - Google Patents

Abstract

The seal, esp. for a casing containing an explosive substance or a propellant fuel, contains a ring (13) of a material which causes the casing to open along a fracture line (21) in the event of a rise in temperature caused e.g. by a fire, allowing the pyrotechnic compound to spill out and burn without exploding. The ring (13), pref. of zinc, is contained within a cavity in the casing with two thrust faces (12,19) lying parallel to the fracture line (21), and the thrust faces of the ring exert an effort against them perpendicular to the fracture line. In a variant of the design the expansion ring can be fitted round a bolt with a shearing head.

Description

The invention relates to the field of envelopes intended to receive a pyrotechnic composition, for example a propellant or an explosive.

These envelopes are now constructed to allow deconfinement of the pyrotechnic material in the event of a temperature rise due to an accidental cause such as for example a fire. The deconfinement allows the non-explosive combustion of the pyrotechnic material.

In this way one does not add to the damage caused by the heat of the fire, damage which would be due to the shock waves generated by explosions.

Deconfinement is obtained by rupture or opening of the envelope in the event of a temperature rise.

The opening of the envelope can be obtained in a known manner, for example by means of holes made in the envelope and plugged with materials melting at relatively low temperature such as bismuth alloys. It can also be obtained by locks made from shape memory materials. These locks unlock a part of the envelope when, as a result of a rise in temperature, they recover an earlier shape. An example of such an embodiment is known from patent application No. FR 2,686,410 in the name of the French State for example. The opening can also be obtained by the shearing, in the event of overpressure, of pins holding a piece of the envelope as described for example in patent US 4,423,683 taken in the name of the Secretary of State for the Navy. For these last two examples, it is noted that they would certainly not be applicable to the case of a propellant casing which, during their normal operation, is brought to withstand high pressures. We know that shape memory materials have poor mechanical properties. The shear pins may shear during operation.

Among the solutions aiming at the controlled rupture of the envelope, one can cite that described in the patent USA 4,084,512 also taken at name of the Secretary of State for the Navy. According to this patent, it is proposed to provide the envelope with an external thermal protection. Through holes of the protection are filled with a conductive pin opening into a part of the envelope, weakened by the presence of blind holes. Heat is conducted by the conductive material to the propellant located in the vicinity of the weakened parts. The start of combustion in the vicinity of these parts causes them to rupture.

Finally, according to US patent 4,458,482 also filed in the name of the Secretary of State for the Navy, an envelope is proposed which is externally covered with a thermal protection layer. This layer is not applied to part of the exterior surface of the envelope. This part has a shape such that in the event of heating internal stresses are generated in the envelope. These constraints may, according to the inventor, be sufficient to cause the envelope to rupture at a calculated point.

It should be noted, however, that the use of external thermal protections involves known drawbacks and described for example in patent FR n 2 656 085 in the name of THOMSON BRANDT ARMEMENTS or in patent US 3 992 997 filed in the name of the Secretary of State to the Navy. It should also be noted that the solution proposed in this latter patent to overcome one of the drawbacks of these protections, the lack of aerodynamics, is not applicable, when the thermal protection comprises a bare area, therefore hollow. This area generates turbulence. Finally, it should be noted that the design of such a bare area of suitable shape to generate rupture stresses in the event of a fire requires a model of heat transmission and the calculation of the internal stresses generated, or else to carry out experiments. . It can therefore be seen that such a simple design in its manufacture is costly in its development and would only find application for specific needs, for materials produced in large series.

According to the invention, in order to cause the envelope to rupture in the event of a temperature rise, there is provided in the envelope a housing intended to confine an expansion body. This expansion body completely fills the housing. It has a coefficient of expansion α ₂ . In case of overheating this confined body exerts on the walls of the housing a thrust. It is this thrust that is used to exert stress on at least one part to be broken. The stress can be exerted directly or indirectly on the part to be broken. Thus in its most general embodiment, the invention relates to a sealed envelope intended to contain a pyrotechnic material and capable of opening by rupture of at least one part, having a coefficient of expansion α ₁ , to break the along a fracture surface when the envelope is subjected to an outside temperature above a predetermined value characterized in that the envelope comprises at least one housing having two bearing faces, a first and a second substantially parallel to the minus one of the fracture surfaces, each housing confining an expansion body with expansion coefficient α ₂ greater than α ₁ , each body having two bearing faces, a first and a second, the expansion of each expansion body transmitting , in a direction substantially perpendicular to each of the fracture surfaces, forces to at least one part to be broken.

Each part to be broken is broken by the tensile stress which is exerted at the level of the weakest part of this part. As a general rule, it will be advantageous to prepare the fracture zone by carrying out local weakening by one of the methods known in the art. In the preferred embodiment, this weakening is achieved by means of a rupture initiator constituted by a groove. The bearing faces of the housing and of the expansion body are said to be substantially parallel to the fracture surface, and not simply parallel because it may be necessary for the overall architecture or to ensure better confinement of the expansion body d '' slightly tilt the support faces. The bearing faces are located perpendicular to an axial direction of the expansion body perpendicular to the fracture surface.

• la figure 1 est une demi coupe longitudinale d'une partie d'un propulseur faisant apparaître une enveloppe comportant l'invention ;
• la figure 2 représente un détail de la figure 1 ;
• la figure 3 est une courbe représentant la contrainte engendrée en fonction de la température pour un exemple de réalisation ;
• la figure 4 est une demi-coupe longitudinale d'une partie d'enveloppe de charge militaire incorporant l'invention.

Two exemplary embodiments will now be described with reference to the appended drawings in which:

• Figure 1 is a longitudinal half section of a part of a propellant showing an envelope comprising the invention;
• Figure 2 shows a detail of Figure 1;
• FIG. 3 is a curve representing the stress generated as a function of the temperature for an exemplary embodiment;
• Figure 4 is a longitudinal half-section of a portion of the military payload incorporating the invention.

Figure 1 shows the preferred embodiment. This is the rear part of a propellant 20. This propellant comprises, in a known manner, an envelope 30 made up of several parts and in particular of a lateral ferrule 1 of revolution about an axis XX 'onto which is screwed a rear bottom 2, which bottom comprises a nozzle 3 closed in a known manner by a nozzle cover 4 shown diagrammatically. The interior of the casing 30 comprises a propellant 5.

In a known manner the interior of the bottom 2 is protected by a thermal protection 6. The rear bottom 2 is screwed onto the envelope by means of a screw thread 7. According to the invention, a front part of the bottom 2 comprises a housing annular 8. This housing has the shape of a torus generated by the rotation of a rectangle around the axis XX 'of the propellant. The accommodation is open to the front. It has two lateral faces of revolution around XX ′, an internal lateral face 10 and an external lateral face 11. It finally has a face 12 which constitutes the bottom of the housing. The casing 2 is made of steel with an expansion coefficient α ₁ . A ring 13 of coefficient of expansion α ₂ greater than α ₁ having the same toric shape as the housing 8 fills the latter over at least part of its height. The ring 13 is in this embodiment, zinc. It has four faces, an internal revolution face 14, this face is in contact with the internal revolution face 10 of the housing 8, an external revolution face 15, this face is in contact with the external revolution face 11 of the housing, a rear face 16, this rear face is in contact with the bottom 12 of the housing. The ring 13 finally has a front face 17. When the rear bottom 2 is screwed onto the ferrule 1, a rear part 18 of the ferrule 1 fills the part of the housing 8 which is not filled by the ring 13. When the bottom is screwed onto the ferrule, the ring 13 is thus compressed in a direction parallel to the axis XX 'between the bottom 12 of the housing 8 and a rear face 19 which comes to bear on the front face 17 of the ring 13. On finally signals that the bottom 2 has a groove 21 constituting an initiation of rupture. This throat is of revolution around the axis XX 'and hollowed out from the outer surface of the bottom 2.

The operation of this device in the event of a temperature rise Δθ will now be explained with the aid of FIG. 2. This figure represents the detail A of FIG. 1.

In this figure ℓ represents the dimension of the ring 13 which is parallel to the axis XX '. L represents a length which is taken into account for the calculation of the relative elongations. It is a length where the thickness of material on which the ring 13 will exert a stress is greater than the thickness between the bottom of the groove 21 and the external face 11 of the housing 8 but less than the parts located on either side of a hollow 22 having precisely this length L. S ₁ denotes the surface of a circular crown comprised between two circles, a first circle having the diameter of the diameter of the cylindrical surface constituting the bottom 23 of the groove and a second circle having the diameter of the diameter of the cylindrical surface 24 from from which the bottom of the groove 21 is hollowed out.

The stresses σ ₁ and σ ₂ generated in the parts of section S ₁ and S ₂ respectively obey the equation below in which E represents the Young's modulus of steel. $$\begin{array}{c}\begin{array}{c}\mathit{\text{L}}{\text{Δθα}}_{\text{2}}\text{- [(}\mathit{\text{L}}{\text{-1) <figure-callout id="2" label="Δθα" filenames="imgf0001.png" state="{{state}}">Δθα</figure-callout>}}_{\text{2}}{\text{+ ℓΔθα}}_{\text{1}}\text{] =}\left[\frac{{\text{σ}}_{\text{1}}\mathit{\text{e}}}{\text{E}}\text{+}\frac{{\text{σ}}_{\text{2 (}\mathit{\text{The}}\text{)}}}{\text{E}}\right]\\ {\text{we also have <figure-callout id="2" label="σ" filenames="imgf0001.png" state="{{state}}">σ</figure-callout>}}_{\text{2}}{\text{= σ}}_{\text{1}}\frac{{\text{S}}_{\text{1}}}{{\text{<figure-callout id="2" label="S" filenames="imgf0001.png" state="{{state}}">S</figure-callout>}}_{\text{2}}}\end{array}\end{array}$$

The stress σ ₁ in the weakest section is therefore $${\text{σ}}_{\text{1}}\frac{{\text{ℓΔθ (α}}_{\text{2}}{\text{-α}}_{\text{1}}\text{) E}}{\text{e +}\frac{{\mathit{\text{S}}}_{\text{1}}}{{\mathit{\text{S}}}_{\text{2}}}\text{((}\mathit{\text{The}}\text{)}}$$

It is therefore possible to choose ℓ, L and e to obtain, for a temperature rise Δθ, a stress σ ₁ greater than the tensile strength of the surface section S ₁ .

The preferred embodiment relates to a 150 mm caliber propellant. The ferrule 1 and the bottom 2 are made of steel, the ring 13 is made of zinc.

The numerical values are as follows: Rear bottom 2: Steel resistance: 108 <R <128 hbars Young's modulus: E = 21,000 hbars Length L: L = 20 mm Break groove width: e = 2 mm Bottom section: S ₂ = 2714.3 mm ² Breaking section: S ₁ = 879.6 mm ² Coefficient of expansion : α ₂ = 1.22. 10- ⁵ Expansion ring 13: Ring length: ℓ = 15 mm Coefficient of expansion : α ₁ = 3.10- ⁵

The pressure in hectobarres exerted on the surface S ₁ as a function of the temperature is shown for this example in FIG. 3.

For this example, the tensile strength of the steel should be reached between 140 and 180 ° C.

To raise this temperature, it would suffice for example to reduce the length ℓ.

For example, for ℓ = 12 mm, the rupture should take place between 180 and 230 ° C.

FIG. 4 illustrates a second embodiment of the invention. This figure represents a longitudinal section of part of an envelope of a military charge, in which the invention is incorporated. This envelope 40 comprises a cylindrical ferrule 31 of revolution about an axis XX '. This ferrule is closed on its rear part, the only one shown in FIG. 4, by a flange 29 by means of studs 28, for example eight studs distributed regularly on the flange 29. This stud has a head 27 and a rod 26 terminated by a threaded part 25. The rod has a weakened part 32 by digging a groove 33 shown excessively deep in FIG. 4. This groove is hollowed out from the external surface 35 of the rod 26. Each stud 28 contributes to maintaining the flange 29. Each stud 28 is screwed into a filter 34 of the shell. This thread is located at the bottom of a blind hole 35 of axis YY 'of the ferrule. This blind hole is hollowed out from the contact surface 36 between the ferrule 31 and the flange 29. The end of the blind hole 35 located on the side of the surface 36 therefore opposite the bottom has a part 37 whose diameter is greater than diameter of the bottom of the blind hole comprising the threaded part 34. This part 37 has a cylindrical lateral surface 38 and a bottom 12 constituted by a circular annular crown having as axis the axis YY 'of the blind hole. The small circle of this crown has the diameter of the clearance near the diameter of the rod 26. The volume delimited by the surfaces 38, 12 and a cylinder centered on the axis YY 'of the blind hole and having the diameter of the rod diameter 26 constitutes a housing 8. This housing 8 is filled by an expansion body in the shape of an O-ring 13 made of a material with a coefficient of expansion α ₂ greater than the coefficient α ₁ of the material constituting the ferrule 31 or the rod 26.

This material has a face 16 in contact with the face 12 of the housing 8. The opposite face 17 of this ring is in tight contact with the flange 29. When the stud 28 is in place the housing 8 is completely closed.

It is physically delimited on the one hand, by the cylindrical surfaces 38 belonging to the ferrule and by a part of an external surface 35 of the stud 28, these two faces being centered on the axis YY 'of the blind hole. It is also limited by a face 39 of the upper flange 29 and finally by the bearing face 12 of the ferrule 31.

In the event of a rise in temperature, the expansion bodies 13 exert by their face 17 sufficient pressure to break the rods 26 of the studs 28.

Naturally, it is not compulsory for the housings 8 to be annular and situated around the stud rods. It could for example be blind holes not threaded filled by the bodies 13 and closed by the establishment of the flange. It suffices that the flange is rigid enough at the temperatures envisaged to transmit the force to the studs or to weakened parts of the flange.

It could also be the same annular form of ring, the central part of the ring 13 constituting the expansion body being filled by a protruding pin from the flange, the flange being moreover fixed to the ferrule. Finally, note that the solution described in Figure 1 for a propellant could apply to a military charge, the bottom then being replaced by a flange. Similarly the solution described in Figure 4 could be applied to a propellant.

Claims (10)

Watertight envelope (30, 40) intended to contain a pyrotechnic material (5) and capable of opening by rupture of at least one part (2, 28), having a coefficient of expansion α ₁ , to be broken along a fracture surface when the envelope is subjected to an external temperature above a predetermined value characterized in that the envelope (30, 40) comprises at least one housing (8) having two bearing faces (12, 19, 39) a first (12) and a second (39, 19) substantially parallel to at least one of the fracture surfaces, each housing (8) confining an expansion body (13) with a coefficient of expansion α ₂ greater than α ₁ , each body (13) having two bearing faces (12, 17) a first (12) and a second (17), the expansion of each expansion body transmitting forces, in a direction substantially perpendicular to each of the surfaces fracture, at least one part (2, 28) to be broken. Envelope according to claim 1 characterized in that at least one housing is a blind hole (8) having a bottom (12) constituting a bearing face, and an opening, the hole being hollowed out in a first part (2) of the 'envelope (30, 40) an expansion body (13) occupying this hole, a bearing face (16) of the body being supported on the bottom (12), a second part (1, 29) having a face ( 19, 39) coming to bear on the second bearing face (17) of the expansion body (13). Envelope according to claim 2 characterized in that the blind hole (8) and the expansion body (13) have an annular shape centered around an axis. Envelope according to claim 3 characterized in that the first part is a rear bottom (2), the blind hole (8) being hollowed out from a front surface of the bottom (2) and in that the second part is a part comprising a rear part (18) in the form of a ferrule, a rear face (19) of this part in the form of a ferrule coming to bear on a rear bearing face (17) of the expansion body (13), the blind hole, the expansion body and the ferrule being of revolution about the same axis. Envelope according to claim 4 characterized in that the first part (2) comprises a weakening (21) of annular shape having the same axis of revolution as the blind hole (8) of annular shape. Envelope (40) according to claim 1 characterized in that a housing (8) has an annular shape having two faces of revolution of axis YY ', a first (38) of cross section diameter larger than a second (35 ), this second face constituting at least a part of an external surface (35) of a part (26) of a part to be broken (28). Envelope according to claim 6 characterized in that the part to be broken (26) of the part to be broken (28) has a weakening (33). Envelope according to one of Claims 1 to 7, characterized in that the expansion body 13 is made of zinc. Propellant comprising an envelope according to one of claims 1 to 8. Military charge comprising an envelope according to one of claims 1 to 8.

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