GB2355297A

GB2355297A - Two mode lure payload and munition containing such payloads

Info

Publication number: GB2355297A
Application number: GB0023457A
Authority: GB
Inventors: Jean-Marc Besson; Christian Perut
Original assignee: Etienne LaCroix Tous Artifices SA
Current assignee: Etienne LaCroix Tous Artifices SA
Priority date: 1999-09-24
Filing date: 2000-09-25
Publication date: 2001-04-18
Anticipated expiration: 2020-09-25
Also published as: FR2798927A1; GB2355297B; FR2798927B1; DE10046955A1; GB0023457D0

Abstract

A two-mode payload includes conductive fibres (20) of suitable length assembled into bundles (30, 31, 32) forming at least one pancake (10) impregnated with a pyrophoric liquid and protected from air prior to its functional deployment. This payload produces an infrared signature and a radar signature that are virtually simultaneous in time and in space to lure a guidance system operating in two sensing modes, infrared and radar.

Description

2355297 The field of the present invention is that of countermeasures for

protecting a platform attacked by an enemy threat, for example a guided missile or a munition directed by a predictor. The platform is often protected by various means that prevent the system guiding the threat from directing it toward the platform, and even decoy the threat. The platform to be protected can be a terrestrial, marine or airborne platform.

The present invention relates more particularly to a two-mode payload and a countermeasure munition for protecting a platform under attack from a threat guided by a two-mode guidance system, i.e. a guidance system operating in the infrared band and in the radar band (millimetre to centimetre electromagnetic waves). The above type of guidance system correlates information from its infrared sensor and its radar to determine if it relates to the same platform.

The infrared signature and radar signature (or radar echo) produced by the payload of the countermeasure munition are ignored by the guidance system of the threat unless they plausibly represent a platform of the above kind, in particular because of their size, intensity, spectrum and spatial location.

The problem is to produce a two-mode payload that fulfils the above functions.

Consideration could be given to using for this purpose a payload producing an infrared signature by combustion of a pyrotechnic substance or pyrophoric product and a payload producing a radar signature by deploying metallic or metallized fibres often referred to as "chaff". An approach of the above kind presupposes the production of complex means for deploying both types of payload in space and in time.

US patent 4,815,386 describes sponges, possibly metal sponges, whose cells are filled with pyrophoric 2 powder. However, that type of payload does not solve the problems previously referred to. Although the above type of metallic sponge produces infrared and radar signatures, the radar echo intensity is far from the optimum, because the characteristic dimensions of the sponge are not tuned to the wavelength of the radar, and the size (Radar Equivalent Surface Area) is not optimum, because there is insufficient spatial dispersion of the sponge. Finally, because of its mass, the sponge filled with pyrophoric powder has a relatively high rate of descent and the infrared and radar signatures are too transient in the detection field of the guidance system of the threat.

The present invention relates to a two-mode payload of a countermeasure munition for use as an infrared and radar lure; said two-mode payload essentially consists of fibres of appropriate length whose core is metallized, said fibres are assembled into bundles forming at least one pancake, and said pancake is impregnated with a pyrophoric liquid and protected from contact with air by a sealed wrapper until the payload is functionally deployed.

The countermeasure munition releases at least one such pancake at a given point in space away f rom the platform to be protected. The protective wrapper of the pancake is torn and the metallized fibres are dispersed when the payload is released. The dispersed metallized fibres form a cloud of electric dipoles that reflect radar waves emitted by the threat: the detected radar echo is more intense if the metallized fibres have a length close to half the wavelength of the radar of the threat, and it is in this sense that the expression "appropriate length" is used in connection with the metallized f ibres. The radar equivalent surface area is dependent on the number of fibres dispersed.

3 The pyrophoric liquid dispersed at the same time as the f ibres ignites in contact with air and produces an infrared signature that is detected by the appropriate sensor of the threat. The guidance system of the threat correlates the radar and infrared information from its sensors, interprets the result as corresponding to a plausible potential target, and directs the threat toward that region of space.

The pyrophoric liquid impregnating the pancake advantageously includes at least one pyrophoric "liquid selected from the group comprising trialkylaluminiums (triethylaluminium, trimethylaluminium), trialkylborons (triethylboron, trimethylboron), and organosilanes.

The impregnation pyrophoric liquid can be diluted in a solvent for such liquids and said solvent is preferably a liquid hydrocarbon (hexane, etc). Selecting a single pyrophoric liquid and mixing and/or dilution in a solvent varies the duration, intensity and spectrum of the infrared radiation emitted by the payload and determines its infrared dimension. For example, intense radiation at a relatively high temperature but of relatively short duration can be created to simulate an aircraft type airborne platform or a low level of radiation at a low temperature and of relatively long duration can be created to simulate a terrestrial or marine platform.

The core of the fibres of the payload is advantageously metallized with a metal that is a good conductor of electricity, chosen from the group comprising aluminium, cadmium, copper, gold, iron, nickel, platinum, rhodium, silver, tungsten and zinc. In the present application nickel is the preferred metal. The thickness of the metallization layer is from approximately 5 pm to approximately 25 pm and preferably from approximately 5 pm to approximately 10 [tm.

4 The fibres used to produce the payload preferably have a circular section core. To obtain a large number of relatively light fibres for a given volume of fibres, the diameter of the core of the fibres is from approximately 10 Jim to approximately 200 pm and preferably from approximately 15 pm to approximately pm.

In a first embodiment of the invention the core of the metallized fibres is made from a material that cannot be pyrolyzed, for example glass, silicon carbide or boron. Glass is the preferred material. The metallized fibres are solid. The pancake is impregnated with the pyrophoric liquid only in the sense that the interstices between the fibres assembled into bundles are filled with the pyrophoric liquid.

This first embodiment of the invention can use bundles of metallized fibres whose core is made of a material that can be pyrolyzed but which has not been pyrolyzed.

In a second embodiment of the invention the core of the metallized fibres is made from an organic material that can be. The core is pyrolyzed: the metallized fibres are hollow and, if the pyrolysis is complete, there remains of each fibre only a tube formed by the metallization coating. The pyrophoric liquid then impregnates not only the interstices between the fibres but also the hollow interior of the fibres. Polymers yielding fibres that can be pyrolyzed at average temperatures from approximately 3000C to approximately 5000C are suited to this type of application. The preferred material is polypropylene.

The present invention also relates to a method of fabricating a two-mode payload according to the invention in which, starting from a pancake made up of bundles of fibres:

- the interstices between the fibres are filled with pyrophoric liquid by capillary action or by immersion, in an inert gas atmosphere, and - the impregnated pancake is protected from air.

In a variant of the above method::

- the core of the metallized f ibres that can be pyrolyzed is pyrolyzed by heating the pancake with hot air to a temperature which pyrolyzes the material of the core and does not oxidize the metal, which temperature is from approximately 400'C to approximately 550'C, - the interstices between the fibres and the interior of the fibres are filled with pyrophoric liquid by capillary action or by immersion, in an inert gas atmosphere, and - the impregnated pancake is protected from air.

The invention also relates to a countermeasure munition for protecting a given platform, said munition including launching means, safety devices and dispersion systems, and includes a two-mode payload fabricated in accordance with the foregoing description. The payloads are assembled into pancakes or even stacks of pancakes whose diameter is close to the calibre of the munition.

The launching means of the munition are of the mortar type, for example, or of the solid fuel rocket type. The safety devices are of the type usually fitted to countermeasure munitions: muzzle safety devices, close impact safety devices.

Preprogrammed or remote controlled dispersion systems firstly tear the protective wrappers of the bundles constituting the pancakes and secondly disperse the metallized fibres in space.

The present invention solves the problem of creating a two-mode effect with a single payload, namely an infrared signature and a radar signature simultaneous in time and located at the same point in space.

6 The very small diameter metal fibres are light and therefore have a very low rate of descent once dispersed. Their radar signature is therefore of relatively long duration. Also, the combustion of the pyrophoric liquid produces upward and turbulent hot air currents that encourage the "flight" of the cloud of fibres and also give the fibres random orientations reducing the horizontal polarization effect of conventional "chaff".

The chosen impregnation pyrophoric liquid controls the duration of combustion and the radiated infrared spectrum.

Finally, the two-mode payload has a very good constructional index because practically all of its apparent volume comprises elements that contribute to the lure effect.

The invention is described in more detail hereinafter with the aid of figures representing one particular embodiment of the invention.

Figure 1 is a diagram showing a metallized fibre in longitudinal section.

Figure 2 shows a pancake in accordance with the invention.

To make Figure 1 easier to read, the transverse dimensions of the fibre are highly exaggerated relative to its length. The fibre 1 has an exterior metallic covering 2; the metallic covering is preferably of nickel and formed by electroplating. Inside the metallic covering is a portion 3 of the polymer core of the f ibre, because in this particular example pyrolysis of the core is incomplete, leaving the tubular part 3. The pyrolysis is explained below. The hollow part 4 of the fibre is filled with a pyrophoric liquid as explained elsewhere.

Figure 2 shows a pancake according to the invention. This embodiment of said pancake 10 is substantially the shape of a circular sector subtending 7 an angle of 600 and the thickness of the pancake corresponds to the length of the metallized f ibres, which length is matched to the wavelength of the radar to be lured, as previously defined. In this figure, the dashed line outline 11 defines the ideal contour of the pancake.

The curvature of the exterior part 12 corresponds to the inside radius of the container of the payload and the curvature of the interior part 13 corresponds to the outside radius of the dispersion device, generally an axial dispersion rod containing a pyrotechnic charge. In this example the payload is made up of assemblies of six pancakes 10 disposed around the dispersion device and stacked to the full height of the container. The pancake 10 is made up of bundles 30, 31, 32 with a diameter of a few millimetres; the bundles 30, 31, 32 consist of a multitude of parallel deformable fibres 20 which can be grouped together for optimum filling of the volume delimited by the dashed line outline 11. The pancake, which is impregnated with a pyrophoric liquid, is protected from air by a thin airtight wrapper which is not shown but effectively corresponds to the outline 11. Several pancakes can be stacked in a common wrapper to form a kind of part-payload, several of which are assembled around the dispersion device to form the payload, and the shape of the protective wrapper is adapted to suit to this configuration.

Fabricating a payload pancake according to the invention starts with a long fibre or filament (for example made of glass or polypropylene) which is metallized by depositing a metal, preferably nickel. The relatively thin deposit, from approximately 5 pm thick to approximately 10 pn thick, is formed in the usual way by electroplating or any other process. The production of a payload pancake then comprises the following steps, of which the initial steps are also used to fabricate 8 conventional "chaff" payloads:

- the metallized fibre is wound onto a cylinder in one or more layers, the fibres are cut along a generatrix of the cylinder, - they are dipped in a lubricant bath and all of the fibres are wrapped in tinned paper to form a bundle, - the bundle is cut to the required length (the length of fibres is matched to wavelength of the radar to be lured and the resulting bundles therefore have a diameter of approximately 1 cm and a height from a few millimetres to a few centimetres), - the bundles are assembled into pancakes whose diameter is close to the calibre of the countermeasure is munition, often from approximately 40 mm to approximately mm, - if the metallized fibres have a polymer core, the polymer is pyrolyzed by heating the pancake with hot air to a temperature chosen so that, firstly, the polymer is destroyed and, secondly, the metal covering the core of the fibre is not oxidized; the temperature is preferably from approximately 4000C to approximately 5000C, - the pancake is impregnated with the pyrophoric liquid prepared elsewhere, by capillary action or by immersion in the pyrophoric liquid, in an inert atmosphere; if the metallized f ibres have a core that cannot be pyrolyzed, and are therefore solid, impregnation with the pyrophoric liquid relates only to the interstices between the f ibres and the periphery of the pancake, but if the f ibres have a core that can be pyrolyzed and has been destroyed by the method previously described, and are therefore hollow, impregnation with the pyrophoric liquid relates also to the interior of the fibres, - the impregnated pancake is enclosed in an 9 airtight wrapper made of a material compatible with the pyrophoric liquid, and - the pancakes, protected by their wrapper, are assembled into the countermeasure munition.

Claims

Clai.ms:

1. A two-mode payload for use as a radar and infrared lure, the payload including metallized core fibres of suitable length assembled into bundles forming at least one pancake impregnated with a pyrophoric liquid and protected from air by a sealed wrapper.

2. A two-mode payload according to Claim 1, wherein the impregnation pyrophoric liquid includes at least one pyrophoric liquid chosen from the group comprising trial kylaluminiums, trialkylborons and organosilanes.

3. A two-mode payload according to Claim 2, wherein the impregnation pyrophoric liquid is diluted in a solvent.

4. A two-mode payload according to Claim 1, wherein the fibres are metallized with a metal that is a good conductor of electricity.

5. A two-mode payload according to Claim 4, wherein the thickness of the metallization layer is from substantially 5 jim to substantially 25 pn.

6. A two-mode payload according to any one of Claims 1 to 5, wherein the metallized fibres have a core with a diameter from substantially 10 Vn to substantially 200 jim.

7. A two-mode payload according to Claim 6, wherein the metallized fibres have a core of a material that cannot be pyrolyzed and the impregnation of the pancake with the pyrophoric liquid relates in particular to the interstices between the fibres.

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8. A two-mode payload according to Claim 6, wherein the metallized fibres have a core made of a material that can be pyrolyzed and which is destroyed by pyrolysis before the pancake is impregnated with the pyrophoric liquid, impregnation with the pyrophoric liquid relating to the interstices between the fibres and a hollow part of the fibres.

9. A two-mode payload according to Claim 8, wherein the metallized fibres have a polypropylene core.

10. A two-mode payload for use as a radar and infrared lure substantially as herein described with reference to the accompanying drawings.

11. A method of fabricating a two-mode payload according to any one of Claims 1 to 7, wherein:

the interstices between the fibres are filled with pyrophoric liquid in an inert gas atmosphere, and the impregnated pancake is protected from air.

12. A method of fabricating a two-mode payload according to any one of Claims 1 to 5, 8 and 9, wherein:

the core of the metallized fibres that can be pyrolyzed is pyrolized by heating the pancake with hot air to a temperature which pyrolyzes the material of the core and does not oxidize the metal, the interstices between the fibres and the interior of the fibres are filled with pyrophoric liquid by capillary action in an inert gas atmosphere, and the impregnated pancake is protected from air.

12

13. A countermeasure munition for protecting a platform, including launching means, safety devices and dispersion systems, characterized in that said munition includes at least one two-mode payload according to any of Claims 1 to 10.