A decoy

Abstract

A decoy for protecting a platform against threats comprises a casing 10 housing electronics for generating a RF decoy signal. The decoy has stabilising fins 20 mounted to the casing 10 and configured for rotation relative to the casing 10 about a first axis, between a stowed configuration in which the fins 20 lie alongside the casing 10 and deployed configuration in which they extend laterally away from the casing 10. The decoy comprises a deployment mechanism associated with each fin 20 comprising a first bias means configured to rotate the fin 20 about the first axis and a second bias means configured to urge the fin 20 into a locked position through a translation movement along the first axis. It also includes a retaining mechanism comprising a first surface provided by the fin 20 and second surface provided by the casing 10, the first and second surfaces being oblique to the first axis and configured to engage one another to inhibit movement of the fin 20 about the first axis when the fin 20 is in the deployed configuration.

Description

The invention relates to a decoy. More specifically, but not exclusively, it relates to a decoy for protecting aerial platforms against radar-based threats. GB2516077 describes an active decoy to protect an aerial platform from radar based threats. Tire decoy includes stabilising fins. The following is given to provide context to the invention. It is not admitted as prior art. An expected application of the decoy is to protect aerial platforms travelling at high speeds, e.g. >300kph. As such the decoy is typically ejected into highly turbulent air. The inventors have identified that the stabilising fins need to have a relatively large surface area to stabilise the attitude of the decoy as quickly as possible following ejection from the platform. On the other hand, whilst carried on the platform, the decoy needs to fit within a cartridge whose size is likely to be constrained to provide compatibility with legacy aircraft dispensers. For this reason the fins may be configured to fold outward between a collapsed or stowed configuration to allow the decoy to fit within the cartridge, and an erect configuration once ejected from the cartridge and platform. The inventors have realised that a symmetrical spatial arrangement of the fins around the casing is important to bring the decoy into a stable attitude. To achieve this, the parts of the hinge mechanism that allow the fins to rotate outwards requires to be manufactured within very small tolerances. Equally, sufficient clearances are needed to avoid binding of the fin before reaching the desired orientation. Any variance is likely to lead to large angular deviation from the optimum symmetrical configuration, particularly at the fin tips. According to an aspect of the invention there is provided a decoy for protecting an aerial platform against radar-based threats; the decoy comprising a casing housing electronics for generating an RF jamming and / or RF decoy signal; the decoy further comprising at least one stabilising fin mounted to the casing and configured for rotation relative to the casing about a first axis, between a stowed configuration in which the fin lies alongside the casing and deployed configuration in which it extends laterally away from the casing; wherein the decoy comprises a deployment mechanism comprising a first bias means configured to rotate the fin about the first axis and a second bias means configured to urge the fin into a locked position through a translation movement along the first axis; the deployment mechanism further comprising a retainment mechanism comprising a first surface provided by the fin and second surface provided by the casing, the first and second surfaces being oblique to the first axis to inhibit movement of the fin about the first axis when the fin is in the deployed configuration. The engagement of the oblique surfaces acts to reliably lock, jam or otherwise retain the erect fm once it reaches a desired deployed position, and minimises subsequent rotational movement of the relative to the decoy’s casing to preserve the optimum symmetrical configuration of the fins. Advantageously the invention allows this to be achieved without reducing the manufacturing tolerances of the fin and casing. The fms may be hinged to the casing. In a favourable arrangement, the one or more stabilising fins are situated, more favourably are hinged to the casing, about the second end of the decoy. Where so, this provides the advantage that when ejected rear end first, the fms will leave the cartridge first and so can rotate into the operation configuration earlier, which aids to bring the decoy into a stable attitude more quickly irrespective of the direction it is ejected away from the platform. The first bias means may comprise a torsion spring and may be mounted over a hinge pin through which the fin is mounted to the casing. The second bias means may comprise a compression spring which may be mounted over the hinge pin. Tire first surface may be a concave conical surface. It may be arranged to partially surround an aperture of a gudgeon of the fin about which the fin is mounted to the casing. The second surface may be a convex surface. The second surface may be contiguous with an end face of the case shaped to inhibit movement of the fin along the first axis until the fin has rotated into a predetermined position. Favourably the decoy comprises multiple fins, and where so each may have a separate a deployment mechanism a retainment mechanism associated with it. The invention will now be described by way of example with reference to the following Figures in which: Figure 1 is a perspective view of a decoy with its fins folded in a stowed configuration and a cartridge to retain the decoy before its deployment; Figure 2 is a perspective view of the decoy of Fig 1 with fins in a locked erect configuration to stabilise the attitude of the decoy within the air; Figure 3 is a perspective of the decoy of Fig 2 viewed from an opposite side; Figure 4 is a close up view of the erect fin prior to being slid into a locked position; Figure 5 is a close up view of the locked erect fin of Fig 2; Figure 6 is a close up view similar to Fig 5 showing the casing and fin in part crosssection; Figure 7 is a perspective view of a fin in isolation; and Figure 8 is a perspective close up view of the casing looking rearwards to show greater detail of the locking surfaces. Figures 1 to 3 illustrate an active decoy 1 for protecting an aircraft from radar based threats. The aircraft may be a fixed or rotary wing aircraft. Whilst carried by the aircraft, the decoy 1 is retained in a cartridge 2. The cartridge 2 is, in turn, held within a chamber of a dispenser (not shown) carried by the aircraft. Upon detection of a threat, the pilot and / or electronic countermeasure system causes the decoy 1 to be launched from the dispenser, out of the cartridge 2, away from the aircraft and into the air. The decoy 1 may be ejected from the aircraft into the air rear end first. The decoy has a front end 1A, being the end that faces the decoy’s 1 direction of travel through the air following ejection from the aircraft, and a rear end IB. Tire decoy 1 comprises a casing 10 that holds electronic circuitry (represented schematically by box 300) configured to generate a decoy signal, e.g. a radio frequency decoy signal, to be transmitted through an antenna 400, to deceive and / or jam a potential threat. The electronic circuitry 300 may also be configured to process RF signals from potential threats received through an antenna 400 for the purpose of generating suitable decoy signals. In the present example the casing 10 is generally cuboidal, though it may take other forms, such as, for example generally cylindrical. Tire decoy 1 comprises multiple stabilising fins 20, in this example four though this is not to be taken as limiting. Each fin 20 is mounted to the casing 10 through a separate hinge mechanism so that each fm 20 may rotate relative to the casing 10, and independently from the other fins 20, outwardly from a folded stowed configuration represented in Fig 1 to enable the decoy 2 to fit into the cartridge 2, to an erect deployed configuration illustrated in Figs 2 and 3, in which each fin 20 extends radially away from the casing 20 to stabilise the attitude of the decoy 2 as it travels, e.g. gliding and / or falling, through the air towards the ground following the decoy’s 1 ejection from the aircraft. The decoy 1 may omit means for providing powered flight. As best seen in Fig 7. each fin 20 comprises a plate or wing like structure having: a leading edge 20A, that being the edge that faces towards the direction of travel of the decoy 1 when travelling through the air; a trailing edge 20B; a root 20C about which the fin 20 is mounted to the casing 10; and a tip 20D which is distal to the root 20C and casing 10. Each fin 20 comprises a set of gudgeons 21 spaced apart along the root 20C, each having a through hole 22 coaxial with the through holes 22 of the other gudgeons 21 of the fin 20. As seen best in Figs 4,5 and 8, the casing 10 provides a separate set of bracket elements associated with each fin 20. Each set of bracket elements comprises opposing bracket end faces: a forward-facing rear end face 40 and a rear-facmg forward end face 41 located forward of the first end face 40 and face-to-face with the first end face 40. An aperture 42A is provided in the first end face 40, and an aperture 42B is provided in the second end face 41. Each set of bracket elements also includes gudgeons 43, 44 directly between the first and second end faces 40, 41. Each gudgeon 43 44 comprises an aperture 45. The apertures 45 through the gudgeons 43 44 and apertures 42A 42B in first and second end faces 40 41 are coaxial. Each fin 20 is mounted to the casing 10 about a separate pin 30 having a longitudinal axis X-X. The pin 30 is supported at its opposite ends in the apertures 42A 42B of the casing 10 and extending through each of the holes 22 and apertures 45 of each gudgeon 21 43,44 of the fin 20 and casing 10. Through this hinge arrangement, the fin 20 is free to rotate relative to the casing 10 about axis X-X. The separation distance D1 between the first and second end faces 40 41 is greater than the separation distance D2 between the rear end face 24 of the fin 20 and the forwardmost fin gudgeon 21. This allows the fin 20 to move linearly along the axis X-X, relative to the casing 10. Mounted over the pin 30 is a torsion spring 50. The torsion spring 50 is seated and retained between tire inner gudgeons of the fin 20 with a first leg 51 of the torsion spring 50 seated against the casing 50 and a second leg 52 of the torsion spring 50 seated against the fin 20. So arranged, the torsion spring 50 provides a biasing force urging the fin 20 to rotate about the axis X-X outwards into the erect configuration. Also mounted over the pin 30 is a compression spring 60. The compression spring 60 is retained between a fin gudgeon 21 and a casing gudgeon 44, the casing gudgeon 44 being forward of the fin gudgeon 21 so that the compression spring 60 tends to urge the fin 20 to slide relative to tire casing 10, along the pin 30, and thus along the axis X-X against the first end face 40. The decoy 1 will typically encounter highly turbulent airflow as it leaves the aircraft. To stabilise the decoy’s 1 attitude as much as possible it is desirable for each fin 20, once erect, to be held in position with as little play as possible against the forces from the environment. To achieve this the decoy 1 comprises a locking mechanism to inhibit rotation of the fin 20 once erect. The locking mechanism comprises complementary profiled contact surfaces 23 46 of the respective fin 20 and casing 10 which engage one another, and each lie at an angle oblique to the axis X-X to inhibit movement of the erect fin 20, including against the bias force of the torsion spring, back towards the stowed configuration. As best seen in Fig 7, the contact surface 23 of the fin 20 is provided at the root 20C, adjacent and rearward of the rear most gudgeon 21A. The contact surface 23 is a concave conical surface that extends approximately half way around the rear side opening of the aperture 22 of gudgeon 21 A, and thus also axis X-X. The concave conical contact surface 23 slants away from the X-X axis with increasing distance from the gudgeon 21A to meet end face 24 of the fin 20. As best viewed in Fig 8, the first end face 40 has a first edge 40A and a second edge 40B that meet at a curved interconnecting edge 40C. The contact surface 46 of the casing 10 extends around and is conjoined to the first end face 40 about each of the first edge 40A, second edge 40B and interconnecting edge 40C. The contact surface 46 comprises a first generally planar portion 46A conjoined to the first edge 40A, and a second generally planar portion 46B conjoined to the second edge 40B. The first and second planar portions 46A 46B are conjoined by a convexly curved interconnecting surface portion 46C that meets the end face 40 at the curved interconnecting edge portion 40C. Each of the first, second and interconnecting surface portions 46A 46B 46C lie at an oblique angle from the end face 40 and thus also at an oblique angle to the axis X-X, which is approximately the same as the slant angle of the contact surface 23 to the axis X-X. In operation, following ejection of the decoy 1 from the cartridge 2, the fins 20, no longer retained in the stowed position by the cartridge 2 rotate outwardly about axis X-X under force from the torsion spring 50. Translation movement of the fin 20 rearward along X-X is initially restricted by contact of the end face of the fin gudgeon 21A against the bracket end face 40. Rotation about axis X-X continues until the end face 24 of the fin 21 no longer contacts the bracket end face 40, as illustrated in Fig 4, whereupon under the bias force of the compression spring 60, the concave contact surface 23 of the fin 20 slides directly across the interconnecting surface portion 46C until the fin 20 is wedged fast inhibiting further rearward translation movement of the fin 20 as well as rotational movement of the fin 20 about axis X-X. In the above described example, the gudgeons and pivot bracket are features defined by the shape of the casing 10. Alternatively, the decoy 1 may comprise additional hinge elements fastened or otherwise mounted to the casing 10 to provide these features. Instead of a through pin 30 that extends through multiple gudgeonsthe hinge may comprise multiple pintle-gudgeon pairs.

Claims

1. A decoy for protecting an aerial platform against radar-based threats; the decoy comprising a casing housing electronics for generating a RF decoy signal and / or jamming signal;the decoy further comprising at least one stabilising fin mounted to the casing and configured for rotation relative to the casing about a first axis, between a stowed configuration in which the fin lies alongside the casing and deployed configuration in which it extends laterally away from the casing;wherein the decoy comprises a deployment mechanism comprising a first bias means configured to rotate the fin about the first axis and a second bias means configured to urge the fin into a locked position through a translation movement along the first axis;the deployment mechanism further comprising a retainment mechanism comprising a first surface provided by the fin and second surface provided by the casing, the first and second surfaces being oblique to the first axis and configured to engage one another inhibit movement of the fin about the first axis when the fin is in the deployed configuration.

2. A decoy according to claim 1 wherein the first bias means comprises a torsion spring.

3. A decoy according to claim 2 wherein the torsion spring is mounted over a hinge pin.

4. A decoy according to claim 1, 2 or 3 wherein the second bias means comprises a compression spring.

105. A decoy according to claim 4 wherein the compression spring is mounted over a hinge pin.

6. A decoy according to any previous claim wherein the first surface is a concave conical surface and wherein the second surface is a convex surface.10

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