AU2014203268A1

AU2014203268A1 - Active decoy body with an active pyrotechnic composition

Info

Publication number: AU2014203268A1
Application number: AU2014203268A
Authority: AU
Inventors: Arno Hahma
Original assignee: Diehl BGT Defence GmbH and Co KG
Current assignee: Diehl Defence GmbH and Co KG
Priority date: 2013-06-18
Filing date: 2014-06-17
Publication date: 2015-01-22
Anticipated expiration: 2034-06-17
Also published as: AU2014203268B2; IL232582A0; DE102013010266A1; EP2824413A1; EP2824413B8; IL232582B; EP2824413B2; EP2824413B1; ZA201404324B

Abstract

Abstract The invention relates to an active decoy body with an active pyrotechnic composition and with a structure surrounding the active composition, where the structure surrounds the active composition in such a way that gas produced on burn-up of the active composition is hindered by the structure from flowing off from the active composition to an extent such that the gas pressure on at least 65% of the overall surface area of the active composition is higher than outside the structure.

Description

Diehl BGT Defence GmbH & Co. KG, Alte NuBdorfer StraBe 13, 88662 Oberlingen, Germany 5 Active decoy body with an active pyrotechnic composition The invention relates to an active decoy body with an 10 active pyrotechnic composition and with a structure surrounding the active composition. Known from DE 10 2004 047 231 Al is an active body with an active pyrotechnic composition block having specific 15 structures. The effect of the structure is to increase the surface area, thereby allowing control over the burn-up rate of the active composition block and hence over the activity duration of the active body. The problem addressed is that of creating an active body 20 which is active even at high altitudes with low atmospheric oxygen content, and for which performance losses due to flow effects at high ejection velocities are lower. For these purposes, the active composition block may have in its interior one or more channels so 25 as to enable the active composition block to be initiated in its interior with protection from incident flow. Furthermore, the active composition block may have incident-flow protection formed by a protective cap and a protective sheath. In this way it can be 30 ensured that detractions in the IR radiation at high incident-flow velocities, such as occur on ejection of the active body from an aircraft, are reduced. The gases produced in the channels on burn-up of the active composition give rise to a jet effect, which can be 35 utilized at the same time for the propulsion and thus the kinematics of the active body. Known from DE 10 2008 017 722 Al is an active composi tion container with an active composition block. The 40 active composition block here is protected by an - 2 incident-flow protection cap. The incident-flow protection cap may be mounted over a protective sheath of the active composition block. The problem addressed by this embodiment is that of preventing premature 5 tearing of the protective sheath caused by the forces which occur when the active composition container is ejected under flight conditions. Known from DE 10 2009 030 871 Al is an active body 10 which comprises two or more flares, arranged in series, as the active composition. The active body is enclosed in a plastics-like container. This may be a polymeric sheet or a shrink tube. The problem this solves is that of specifying an active body which possess a residue 15 lessly combustible shell, which permits ignition of the active composition from the outside, by thermal, induc tive or laser means, for example. Combustion is accom panied by opening of the active composition container. 20 Known from WO 2011/116873 Al is an encapsulated active body for an IR deceptor or decoy. The problem addressed is that of specifying an active body with optimized ignition characteristics. For this purpose, the active body is accommodated entirely in the interior of a 25 stable, impervious and preferably combustible shell. Ignition may be accomplished via the surface of the active body or through a centrally located ignition along the lengthwise axis. The combustible shell may be ignited by contact with a hot surface, by incoupling of 30 laser radiation, by inductive ignition and by other suitable methods, such as friction, for example. The object of the present invention is to specify an active decoy body which burns up reliably even at high 35 incident-flow velocities, such as on ejection from a fast-flying aircraft, and at high altitudes, preferably with emission of IR radiation resembling (spectrally) that of the target.

- 3 The object is achieved by the features of Claim 1. Advantageous embodiments are apparent from the features of Claims 2 to 15. 5 Provided in accordance with the invention is an active decoy body with an active pyrotechnic composition and with a structure surrounding the active composition. The structure surrounds the active composition here in such a way that gas produced on burn-up of the active 10 composition is hindered by the structure from flowing off from the active composition to an extent such that the gas pressure on at least 65%, more particularly at least 75%, more particularly at least 85%, more parti cularly at least 95%, more particularly 100% of the 15 complete surface area of the active composition is higher than outside the structure. This is substan tially different from the construction known from DE 10 2004 047 231 Al, where, as a result of gas ejection through channels, there is an increased gas pressure in 20 the channels, but not through a surrounding structure on at least 65% of the complete surface area of the active composition. As a result of the degree of inhibition of the flow-off of the gas, the burn-up of the active composition can be designed such that the 25 overpressure on the surface of the active composition, relative to the surroundings, is such that burn-up can take place largely unaffected by the external pressure and by wind acting on the active decoy body. To some extent at least, therefore, there can be decoupling 30 from the external conditions, this decoupling being all the more extensive as the difference in gas pressure increases between the gas pressure on the surface of the active composition, and hence within the structure, and the gas pressure outside the structure. For the 35 skilled person there is no problem involved in providing a structure which hinders the flow-off of gas produced on burn-up of the active composition to an extent such that gas pressure present on at least 65% of the complete surface area of the active composition is higher than outside the structure. Since the inhibition of the flow-off of the gas automatically results in a higher pressure during burn-up, any 5 proving of the higher gas pressure on the surface of the active composition is superfluous. It may nevertheless be determined indirectly, for example by recording of the burn-up process with a high-speed camera and measurement of the expansion of the 10 structure during burn-up, and/or by comparison of the speed at which a flame is ejected from the active decoy body of the invention with the speed of a flame determined on the basis of an active composition which burns up without the structure. As a result of the 15 higher pressure, the rate of ejection of the flame is higher for the active decoy body of the invention. A "gas pressure higher than outside the structure" and the reporting of the gas pressure in relation to the atmospheric pressure, as is done below, relates here to 20 the conditions on resting burn-up of the active decoy body on the ground without wind. The atmospheric pressure may be the normal pressure at sea level. The active composition may be an active composition 25 which radiates spectrally on burn-up. Such active compositions are known in the prior art. With active compositions for decoys that radiate spectrally on burn-up, primarily in the medium-wave IR range, it is often a problem that the active compositions, when 30 confronted by an incident flow of strong wind, on ejection from an aircraft, for example, fail to burn, or are extinguished. The active decoy body of the invention, however, allows the burn-up of such active compositions even under these conditions and/or in 35 cases of low air pressure, as present at high alti tudes. At the same time, the active decoy body of the invention allows a more extensive irradiation and hence a greater radiant intensity when the active body known - 5 from DE 10 2004 047 231 Al, where the jet effect caused by the channels means that there is only one-sided irradiation. Furthermore, the higher gas pressure active on at least 65% of the complete surface area of 5 the active composition permits a higher burn-up rate than at atmospheric pressure. Combustion channels or incident-flow protection caps, of the kind known from the prior art, are unnecessary, and the construction of the active decoy body of the invention can therefore be 10 more simple. A goal with spectrally radiating active decoy composi tions is for the ratio of intensity of radiation emitted on burn-up of the active composition in the 15 wavelength range from about 3.5 to 5.0 pm to intensity of radiation emitted on burn-up of the active composi tion in the wavelength range from about 1.5 to 2.5 pm (i.e., spectral ratio) to be as high as possible. This objective can be achieved by means of the active decoy 20 body of the invention, since the structure is able to shield the active composition as it glows on burn-up, meaning that no blackbody radiation from internal parts of a flame produced on burn-up is detectable outside the structure. Moreover, the aforesaid spectral ratio 25 can be increased by the fact that the structure filters out soot from the flame. Soot in a flame increases the blackbody radiation component emitted by the flame. By virtue of the fact that the gas pressure present on 30 the surface of the active composition is determined substantially by the structure, and the consequent inhibition of the flow-off of the gas produced on burn up of the active composition, and that this pressure determines the burn-up characteristics of the active 35 composition substantially, the burn-up characteristics of the active composition can be specified through the selection of the structure. These burn-up characteris tics are then virtually independent of the wind speed - 6 at which the active decoy body is ejected from an aircraft, and of its flying height, or of the prevailing atmospheric pressure. The burn-up charac teristics of the active decoy body of the invention can 5 therefore be predetermined very well. The effect, as a result, is very much more calculable than with existing active decoy bodies, since to predict the effect of the active decoy body of the invention there is no need to take substantial account either of the flying height or 10 of the flying speed. The body of the invention is therefore substantially easier to use than are known active decoy bodies. As a result of the increased pressure, furthermore, it is possible to use active compositions whose oxygen balance is more negative than 15 with existing active decoy compositions, and which would not burn up at atmospheric pressure and/or under incident wind flow. As a result, both the specific intensity of the active composition and the spectral ratio on burn-up can be increased. In addition, the 20 development of the pressure, and the consequent faster burn-up of the active composition, simplify the ignition of the active composition. The active decoy body of the invention can be produced 25 very easily with any desired active composition. The active composition for this purpose may be present in the form of a block, in the form of at least one pressed tablet, in the form of a plurality of pieces, or in the form of granules. The tablet or the block in 30 this case need not have a particularly large surface area in order to achieve sufficiently quick burn-up, since the latter is brought about by the increased pressure in any case. It is also possible to use conventional propellant charge powders in any desired 35 form, without complicated geometry, or incandescent elements which support combustion, as the active composition for the active decoy body of the invention. Even active compositions which otherwise burn very slowly can be used for producing the active decoy body of the invention. Such slower-burning active compositions often exhibit a greater intensity than quick-burning active compositions. 5 The structure may consist, for example, of a combustion chamber with a multiplicity of openings all round, from which the gas formed on burn-up of the active composition is able to flow out. These openings may be 10 of a size and of a selected number such that the pressure within the combustion chamber on burn-up is at least as high as the ram pressure at the maximum wind speed at which the decoy is employed. The openings, however, ought to be small enough that the active 15 composition, at least at the start of burn-up, cannot be slung out of the openings. One to two seconds after the release of the active decoy body from an aircraft, however, it has usually already braked to an extent such that the usual active compositions, at the wind 20 speed then prevailing, continue to burn even without the structure, and so there is also no problem if the active composition is slung out of the openings. It is favourable, therefore, to make the size of the openings such that the active composition, which reduces in size 25 on burn-up, is not slung through the openings too early. A favourable number and sizing of the openings can be determined easily by the skilled person, by means of routine experiments. 30 The structure may consist of a material which withstands a temperature produced on the structure on burn-up for at least a third, more particularly at least half, of a time required for the complete burn-up of the active composition. In one exemplary embodiment, 35 the structure consists of a material which withstands a temperature produced on the structure on burn-up for at least 1.3 s, more particularly at least 1.5 s, more particularly at least 2 s. The active decoy body of the - 8 invention can be produced very easily by packing an active composition into a fine mesh made of heat resistant material. The free surface area in this mesh is selected such that a slight overpressure is produced 5 on burn-up of the active composition. The structure may be present in the form of a metal mesh, more particularly a multi-ply metal mesh, in the form of a woven fabric, nonwoven or wool consisting of 10 an inorganic material, and surrounded more particularly by a metal mesh, or in the form of a combustion chamber which has openings, or may consist of a combustible material or comprise such a material. The inorganic material may be stone, quartz, aluminium oxide or 15 glass. In the case of the combustion chamber, the openings may be distributed over the complete surface area of the combustion chamber. The combustion chamber may consist of a metal or of ceramic, optionally stabilized with a metal mesh. The combustible material 20 is preferably a material which can be combusted with a non-sooty flame, since soot increases the blackbody radiation component emitted on burn-up. The combustible material may be a double-based or 25 multi-based propellant charge powder, a further active pyrotechnic composition, a plastic, more particularly polyacetal (= polyoxymethylene = POM = polyformalde hyde), polyamide, polyethylene, polypropylene, cellu lose nitrate (containing up to 12% nitrogen) or 30 nitrocellulose (containing more than 12% nitrogen). The stated plastics burn with a non-sooty flame, or with at most a weakly soot-forming flame, and are therefore highly suitable for an active decoy body which radiates spectrally on burn-up. The plastic or the active 35 composition may comprise a catalyst which improves the spectral ratio of the flame burning outside the struc ture. The structure may also be coated with a combust ible material, such as a plastic or a surface-coating - 9 material, for example. This combustible material may likewise burn, and additionally generate radiation, on burn-up of the active composition in the air. 5 In one embodiment of the active decoy body of the invention, the structure is designed such that the gas pressure is higher than the atmospheric pressure by at least 0.5 bar, more particularly at least 1 bar, more particularly at least 1.5 bar, more particularly at 10 least 2 bar, on at least 65%, more particularly at least 75%, more particularly at least 85%, more particularly at least 95%, more particularly 100%, of the complete surface area of the active composition, and hence also in the space which forms between the 15 structure and the active composition on burn-up. If the structure is embodied as a combustion chamber with openings, an overpressure relative to the atmospheric pressure of at least 2 bar is advantageous, since by this means the flow velocity at the narrowest points of 20 the openings is able to reach the speed of sound. As a result, the ambient pressure has no effect on the pressure in the combustion chamber even when the airflow incident on the active decoy body reaches the speed of sound. The space on the inside of the 25 structure is then entirely independent of the surroundings when the active composition burns up. As a result, deployment of the active decoy body of the invention is completely independent of the flying height and of the wind speed. 30 In the case of a further embodiment of the active decoy body of the invention, the structure is designed such that the gas pressure is higher than atmospheric pressure on burn-up of the active composition of at 35 least 65%, more particularly at least 75%, more particularly at least 85%, more particularly at least 95%, more particularly 100%, of the complete surface area of the active composition for at least 1.3 s, more - 10 particularly at least 1.5 s, more particularly at least 2 s. In connection with the maintenance of the gas pressure for a defined time, the size of the openings, in the case of the embodiment of the structure as 5 combustion chamber, is selected such that the flow-off of the gas produced is still sufficiently inhibited during the stated time, even in the case of openings which enlarge as a result of burn-up, and no opening obtains a size sufficient for passage of the active 10 composition before the stated time has elapsed. In a further embodiment of the invention, the structure is coated with a redox catalyst or consists of a redox catalyst. A redox catalyst is, in general, a catalyst 15 which catalyses a redox reaction. The gas produced on burn-up of the active composition is then reacted catalytically as it flows through the structure, and as a result, outside the structure, has a composition which is more favourable for the desired spectral ratio 20 of a decoy. The effect of the redox catalyst alters the structure of the flame and increases the spectral ratio. Moreover, the catalyst is able to catalyse the conversion of soot that is formed into oxides of carbon. As a result there is less blackbody radiation, 25 and the spectral ratio is improved. A further favourable effect of the redox catalyst is that the flame produced on burn-up is stabilized, since the gases which burn off in the flame have a higher hydrogen fraction. Hydrogen burns in air irrespective 30 of pressure and wind. The reaction which takes place over the catalyst, moreover, may cool the structure, meaning that this structure itself emits less blackbody radiation than without a catalyst. The spectral ratio is further increased as a result. 35 The coating or impregnating of the structure may be accomplished, for example, by precipitating the cata lyst from an aqueous solution, as a suspension, and - 11 then filtering this suspension through the structure, leaving particles of the catalyst attached to the structure - quartz wool, for example. The structure must then also be dried in order to be able to act 5 catalytically in the active decoy body of the invent ion. The redox catalyst may comprise a water-gas shift catalyst, at least one organometallic compound, more 10 particularly an organometallic pigment or metal complex, an oxide or a salt of a rare earth metal, a compound comprising a rare earth metal and forming an oxide of a rare earth metal in a flame produced on burn-up of the active composition, zirconium, titanium, 15 aluminium, zinc, magnesium, calcium, strontium, barium, hafnium, vanadium, niobium, tantalum, chromium, nickel, silver, iron, manganese, molybdenum, tungsten, cobalt, copper or thorium or an oxide of one of the stated metals or a compound comprising one of the stated 20 metals and forming an oxide of such a metal in a flame produced on burn-up of the active composition, a platinum metal, rhenium or a compound comprising a platinum metal, rhenium or silver and being reduced to the metal in a flame produced on burn-up of the active 25 composition, or a mixture of at least two of the aforementioned compounds or elements. A water-gas shift catalyst is a catalyst which catalyses a water-gas shift reaction according to the reaction scheme CO + H 2 0 -> CO 2 + H 2 . 30 The redox catalyst may comprise CeO 2 , Ce 2 0 3 , yttrium oxide, ytterbium oxide, neodymium oxide, lanthanum oxide, a mixture of the stated oxides, more particu larly a mixture of CeO 2 and yttrium oxide, a copper 35 doped mixture of aluminium oxide and zinc oxide (LTS catalyst), a chromium-doped magnetite (Fe 3 0 4 ) (HTS cata lyst), a phthalocyanine, more particularly copper phthalocyanine, iron phthalocyanine, chromium phthalo- - 12 cyanine, cobalt phthalocyanine, nickel phthalocyanine or molybdenum phthalocyanine, Vossen blue (= iron ferricyanide = iron(III) ferrocyanide = iron(II) ferri cyanide) or a porphyrin. 5 The active composition may be an active composition which generates at least one secondary flame on burn up. An active composition of this kind is known from DE 10 2010 053 783 Al, for example. Alternatively, for 10 the generation of a secondary flame, the active compo sition may also comprise a fuel containing carbon atoms and hydrogen atoms, and an oxidizer for the fuel that contains oxygen atoms, with the amount of the oxidizer being made such as to be not sufficient for complete 15 oxidation of the carbon. On burn-up of an active composition of this kind in air, a flame is produced that has at least two zones, since the fuel not reacted with the oxidizer then reacts with the air in a second flame zone. A redox catalyst, present in the form of 20 particles, may additionally be distributed in the active composition. The active composition that generates at least one secondary flame on burn-up has the effect of signifi 25 cantly lowering the temperature on the structure during burn-up of the active composition. As a result, different, often more favourable, materials can be used for producing the structure. For example, the structure may be produced from a woven stainless steel fabric or 30 woven quartz fabric. A structure that is itself cataly tically active may be produced, for example, from standard iron or from copper or from a copper alloy. On burn-up, these are strongly oxidized, or already have an oxide layer on the surface, with the iron oxide or 35 copper oxide possibly catalysing the water-gas shift reaction and also possibly serving as oxidizer for soot.

- 13 The active composition may be present in the form of a block or of a plurality of rods, with at least one end face thereof having possibly been treated with an agent for inhibiting burn-up. Such agents are known in the 5 prior art. The agent in question may be, for example, a flame-retardant coating or paint. The advantage of the block or rods form relative to a powder bed is that the distance of the active composition from the structure can be kept low during burn-up. With a distance that is 10 locally too great, the risk exists of the flame temperature on the structure then becoming so high that the structure is destroyed as a result. It is particu larly favourable if the end face or two opposite end faces is/are treated with the agent for inhibiting 15 burn-up, and the structure is affixed to this end face/these end faces. As a result, an active composi tion block is able to burn up radially, and a rela tively small distance between the structure and the active composition for burn-up can be ensured. 20 In order to accelerate the firing of the active composition it is advantageous if the active composi tion is surrounded by a gastight covering which can be broken by the gas produced during burn-up. This 25 covering may consist of paper, adhesive tape, or a sheet. As a result of the covering, the higher gas pressure within the structure is built up more rapidly than without such a covering, since the gas is prevented from flowing out through the structure at the 30 beginning of the reaction. As a result, burn-up is initially greatly accelerated, and the rise time during burn-up of the decoy is shortened. Such a short rise time would also be possible through the use of a relatively large quantity of a firing charge. That, 35 however, would jeopardize the safety of the active decoy body, since a firing charge of this kind is typically easily ignitable. Strong firing also often generates a non-spectral flash through blackbody - 14 radiation. This may let the seeker head know that the apparent target is a decoy. The invention is elucidated in more detail below by 5 means of working examples. The active compositions specified below were used to press tablets with an approximately 17 mm diameter, 30 mm height, and a weight of 10 g. The ionic liquid 10 used for this purpose, 1-butyl-3-methylimidazolium perchlorate (BMIM-C10 4 ) was prepared as follows: 150 g of BMIM-Cl were dissolved in about 600 ml of dry methanol at 25'C in a 2 litre single-necked flask. A 15 stoichiometric amount of dry sodium perchlorate was likewise dissolved, separately, in 600 ml of dry methanol in a 2 litre single-necked flask. The entire perchlorate solution was then added at once to the BMIM chloride solution. The bottle previously containing the 20 perchlorate solution was additionally washed with 3 x 50 ml of dry methanol, and the methanol was also added to the BMIM chloride solution. After a few minutes, the resulting solution became turbid and yellow, as the sodium chloride formed began to precipitate. 25 The entire solution was then boiled at reflux for an hour. The hot solution was subsequently filtered, using frit, into a 2 litre single-necked flask, and the precipitate was washed with 3 x 50 ml of dry methanol. 30 The filter cake, which consisted almost exclusively of sodium chloride, was disposed of. The single-necked flask was then connected to a rotary evaporator, and the methanol was distilled off under a 35 pressure of about 500 mbar, with the waterbath in the evaporator being heated at 90 0 C. When the methanol had been distilled off, the hot crude BMIM-C10 4 was filtered again from the flask, through the frit, into a 250 ml - 15 separating funnel, since further sodium chloride was precipitated during evaporation of the methanol. The completed BMIM-ClO 4 (a yellowish, viscous oil) was 5 discharged from the separating funnel into a laboratory bottle and weighed. The yield was almost quantitative. All of the tablets produced were burnt in the labora tory, without wind, in two parallel experiments in each 10 case. For these experiments, the tablets were ignited and the spectral intensity and also the burn-up time were determined by means of a radiometer (RM-5650 laser probe) with two RkP-575 measuring heads and high-speed video recording. The results shown are in each case 15 average values from the two parallel experiments. Example 1: Constitution of active composition: 20 Substance Type wt.% Ammonium perchlorate ground d 50 = 25 pm 21.9 Nitrocellulose Hagedorn H24 37.9 Diethylene glycol dinitrate own synthesis 10.8 BMIM-C10 4 own synthesis 5.4 Dicyandiamide ABCR crystalline 24.0 Akardit II 0.1 In a first experiment, the active composition tablets were burnt up without the covering structure. In a second experiment, the active composition tablet was 25 enveloped prior to burn-up into a fine stainless steel mesh with a mesh size of 0.15 mm, and in a third experiment into quartz wool. On burn-up, the ammonium perchlorate oxidizer present in the active composition was insufficient for complete oxidation of the nitro 30 cellulose, and so, on burn-up, as well as the primary flame, there was at least one secondary flame formed, - 16 and hence a flame of different temperature zones, with the temperature on the stainless steel mesh and on the quartz wool remaining relatively low. Both were unchanged after burn-up. This shows that the tempera 5 ture directly on the structure has not exceeded about 1000 0 C. The results exhibited on burn-up were as follows: Structure SW MW (MW + SW) MW/SW Burn-up surrounding [(J/(g [(J/(g [(J/(g time active sr))] sr))] sr))] [s] composition None 10 113 123 11.3 24.3 Stainless steel mesh 0.15 mm 19.7 126 145 6.4 13.8 Quartz wool 8.8 115 124 13.1 12.2 10 SW = Intensity in the short-wave channel (about 1.5 to 2.5 pm), MW = intensity in the medium-wave channel (about 3.5 to 5.0 pm) When interpreting the results it should be borne in 15 mind that the stainless steel mesh glowed relatively hot on burn-up and consequently impaired the spectral ratio. On burn-up under deployment conditions, where the active decoy body flies initially at high speed and as a result is subject to strong wind, the wind cools 20 down the flame and the mesh significantly, and so the spectral ratio is then better than that shown here. When the quartz wool is used, a better spectral ratio was ascertained. The reduction of intensity only in the SW band when using the quartz wool shows that the soot 25 was filtered off by the quartz wool and its radiation shielded. Through the stainless steel mesh and the quartz wool, the burn-up rate was roughly doubled. The reason for this is the overpressure at the surface of the active composition, brought about by this structure - 17 on burn-up, and also the temperature back-radiation from the stainless steel mesh or quartz wool onto the tablet. 5 Example 2: The same tablets were used as for Example 1. In a first experiment, the structure covering the active composi tion consisted of a stainless steel mesh with a mesh 10 size of 0.15 mm. Two further experiments were conducted with the same stainless steel meshes, but coated with two different water-gas shift catalysts. For coating, the stainless steel meshes were each immersed repeatedly into an aqueous catalyst suspension, and 15 subsequently dried. One of the catalysts was a so called HTS (High Temperature Shift) catalyst, consist ing of magnetite with 10 mol% of chromium(III) oxide. The other was a so-called LTS (Low Temperature Shift) catalyst, consisting of zinc oxide, aluminium oxide and 20 copper(II) oxide in a 1:1:1 molar ratio. Both catalysts were precipitated from 0.1-molar solutions. The stainless steel meshes were immersed into this suspen sion and dried for half an hour at 120'C. This opera tion was repeated three times in each case. It was not 25 possible to determine the amount of catalyst left on the mesh. In a further series of experiments, quartz wool was used instead of the stainless steel mesh. A weighed 30 amount of the catalysts was suspended in water in each case, and filtered through the quartz wool. As a further catalyst, magnetite was used as well. The quartz wool with the catalyst was subsequently dried for half an hour an 120 0 C. The tablets of active 35 composition were wrapped in this wool and wound round with a 1 mm thick iron wire in order to fix the wool during burn-up. The amount of catalyst was in each case 1% of the tablet weight. Furthermore, quartz wool was - 18 impregnated with 0.01 wt.% of platinum, based on the tablet weight, by impregnating the quartz wool with a solution of hexachloroplatinic acid, with the entire amount of the solution being absorbed by the quartz 5 wool. The quartz wool was subsequently dried. The results achieved on burn-up were as follows: Structure SW MW (MW + SW) MW/SW Burn-up surrounding [(J/(g [(J/(g [(J/(g time active sr))] sr))] sr))] [s] composition None 10 113 123 11.3 24.3 Stainless steel mesh 0.15 mm 19.7 126 145 6.4 13.8 Stainless steel mesh 0.15 mm with HTS 13 116 129 8.9 12.2 Stainless steel mesh 0.15 mm with LTS 15 110 125 7.4 12.1 Quartz wool 3.8 52.9 56.7 13.8 12.0 Quartz wool with 1% LTS 5.3 64.9 70.3 12.2 10.8 Quartz wool with 1% HTS 3.0 69.6 72.6 22.9 11.3 Quartz wool with 0.01% platinum 4.3 95.5 99.8 22.0 15.9 Quartz wool with 1% magnetite 5.9 108.1 113.9 18.4 17.5 SW = Intensity in the short-wave channel (about 1.5 to 10 2.5 pm), MW = intensity in the medium-wave channel (about 3.5 to 5.0 pm) In the experiments it was found that the catalyst had virtually no adverse effect on the burn-up time, 15 despite the fact that the back-radiation from the stainless steel meshes with catalyst was lower, since the meshes were cooled by the catalytic reaction. This shows that it is almost exclusively the pressure increase caused by the structure that is decisive for 20 the burn-up time. In some cases, a slight acceleration - 19 of burn-up was achieved by the catalyst. The spectral ratio could in some cases be increased considerably by the catalyst. 5 Example 3: Tablets were pressed from the following active composi tion mixtures: 10 Active composition 1: Substance Type wt.% Ammonium perchlorate ground d 50 = 25 pm 21.7 Nitrocellulose Hagedorn H24 37.9 Diethylene glycol dinitrate own synthesis 10.8 BMIM-C10 4 own synthesis 5.4 Dicyandiamide ABCR crystalline 24.0 Akardit II 0.1 Cerium oxide Schuchardt 0.1 Magnetite precipitated in- 0.1 house, particle size < 1 Pm Active composition 2: Substance Type wt.% Ammonium perchlorate ground d 50 = 25 pm 40.8 Nitrocellulose Hagedorn H24 50.15 Cerium oxide fine 0.1 Dioctyl adipate BASF 8.85 Iron phthalocyanine ABCR 0.2 15 The active compositions used here each contain a burn up catalyst and a water-gas shift catalyst. In a first experiment, burn-up took place without a structure covering the tablet. In a second experiment, a 20 perforated tube of polyacetal (POM), of Delrin® type, from DuPont, was used as the structure. Polyacetal - 20 burns with a colourless flame, which exhibits a very high spectral ratio. As a result, the plastic has no effect or a positive effect on the spectral ratio. Moreover, the polyacetal raises the energy content of 5 the active decoy body. For covering, the active compo sition was introduced into the perforated POM tube. The results of these experiments were as follows: Active Structure SW MW (MW + MW/SW Burn compo- surrounding [(J/(g [(J/(g SW) up sition active sr))] sr))] [(J/(g time composition sr))] [s] 1 none 19.7 126 145 6.4 13.8 1 perforated POM tube 13 116 129 8.9 12.2 2 none 3.6 80.2 83.8 22.8 16.0 2 perforated POM tube 2.8 91.5 94.3 32.6 10.1 10 SW = Intensity in the short-wave channel (about 1.5 to 2.5 pm), MW = intensity in the medium-wave channel (about 3.5 to 5.0 pm) From the results of the experiments it is clear that 15 the POM structure has increased not only the specific intensity but also the spectral ratio. The structure has also increased the burn-up rate. Throughout this specification and the claims which 20 follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising!, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or 25 step or group of integers or steps. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that the prior art forms part of 30 the common general knowledge.

Claims

1. Active decoy body with an active pyrotechnic composition and with a structure surrounding the 5 active composition, where the structure surrounds the active composi tion in such a way that gas produced on burn-up of the active composition is hindered by the struc ture from flowing off from the active composition 10 to an extent such that the gas pressure on at least 65% of the overall surface area of the active composition is higher than outside the structure. 15

2. Active decoy body according to Claim 1, where the gas pressure is higher on at least 75%, more particularly at least 85%, more particularly at least 95%, more particularly 100% of the overall surface area of the active composition 20 than outside the structure.

3. Active decoy body according to either of the preceding claims, where the structure consists of a material which 25 withstands a temperature produced on the structure on burn-up for at least a third, more particularly at least half, of a time required for the total burn-up of the active composition. 30

4. Active decoy body according to any of the preced ing claims, where the structure consists of a material which withstands a temperature produced on the structure on burn-up for at least 1.3 s, more particularly 35 at least 1.5 s, more particularly at least 2 s.

5. Active decoy body according to any of the preced ing claims, - 22 where the structure is present in the form of a metal mesh, more particularly a multi-ply metal mesh, in the form of a woven fabric, nonwoven fabric or wool consisting of an inorganic mate 5 rial, more particularly stone, quartz, aluminium oxide, ceramic or glass, and surrounded in particular by a metal mesh, or in the form of a combustion chamber having openings, more particu larly of a combustion chamber having the openings 10 distributed over the complete surface area of the combustion chamber, or consists of a combustible material, more particularly a material combustible with a non-soot-forming flame, or comprises such a material. 15

6. Active decoy body according to Claim 5, where the combustible material comprises a double based or multi-based propellant charge powder, a further active pyrotechnic composition, a plastic, 20 more particularly polyacetal (POM), polyamide, polyethylene, polypropylene, cellulose nitrate or nitrocellulose.

7. Active decoy body according to any of the preced 25 ing claims, where the structure is designed so that the gas pressure is higher by at least 0.5 bar, more particularly at least 1 bar, more particularly at least 1.5 bar, more particularly at least 2 bar, 30 than the atmospheric pressure on at least 65%, more particularly at least 75%, more particularly at least 85%, more particularly at least 95%, more particularly 100%, of the complete surface area of the active composition. 35

8. Active decoy body according to any of the preced ing claims, - 23 where the structure is designed so that the gas pressure on burn-up of the active composition is higher for at least 1.3 s, more particularly at least 1.5 s, more particularly at least 2 s, than 5 the atmospheric pressure on at least 65%, more particularly at least 75%, more particularly at least 85%, more particularly at least 95%, more particularly 100%, of the complete surface area of the active composition. 10

9. Active decoy body according to any of the preced ing claims, where the structure is coated with a redox catalyst or consists of a redox catalyst. 15

10. Active decoy body according to Claim 9, where the redox catalyst comprises a water-gas shift catalyst, at least one organometallic compound, more particularly an organometallic 20 pigment or metal complex, an oxide or a salt of a rare earth metal, a compound comprising a rare earth metal and forming an oxide of a rare earth metal in a flame produced on burn-up of the active composition, zirconium, titanium, aluminium, zinc, 25 magnesium, calcium, strontium, barium, hafnium, vanadium, niobium, tantalum, chromium, nickel, silver, iron, manganese, molybdenum, tungsten, cobalt, copper or thorium or an oxide of one of the stated metals or a compound comprising one of 30 the stated metals and forming an oxide of such a metal in a flame produced on burn-up of the active composition, a platinum metal, rhenium or a compound comprising a platinum metal, rhenium or silver and being reduced to the metal in a flame 35 produced on burn-up of the active composition, or a mixture of at least two of the aforementioned compounds or elements. - 24

11. Active decoy body according to Claim 9, where the redox catalyst comprises CeO 2 , Ce 2 0 3 , yttrium oxide, ytterbium oxide, neodymium oxide, lanthanum oxide, a mixture of the stated oxides, 5 more particularly a mixture of CeO 2 and yttrium oxide, a copper-doped mixture of aluminium oxide and zinc oxide (LTS catalyst), a chromium-doped magnetite (Fe 3 0 4 ) (HTS catalyst), a phthalocyanine, more particularly copper phthalocyanine, iron 10 phthalocyanine, chromium phthalocyanine, cobalt phthalocyanine, nickel phthalocyanine or molyb denum phthalocyanine, iron ferricyanide or a porphyrin. 15

12. Active decoy body according to any of the preced ing claims, where the active composition is an active composi tion which radiates spectrally on burn-up. 20

13. Active decoy body according to any of the preced ing claims, where the active composition is an active composi tion which generates at least one secondary flame on burn-up. 25

14. Active decoy body according to any of the preced ing claims, where the active composition is present in the form of a block or plurality of rods, where at 30 least one end face thereof is treated with an agent for inhibiting burn-up, and the structure is affixed on the end face or two end faces.

15. Active decoy body according to any of the preced 35 ing claims, where the active composition is surrounded by a gastight covering that can be broken by the gas produced on burn-up.