GB2221020A

GB2221020A - Optical proximity fuze

Info

Publication number: GB2221020A
Application number: GB8130655A
Authority: GB
Inventors: Peter Gregory Lloyd
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1981-10-10
Filing date: 1981-10-10
Publication date: 1990-01-24
Anticipated expiration: 2001-10-10
Also published as: GB2221020B

Abstract

A conventional rear-fuzed bomb 10 includes a ground sensing proximity fuze with a plurality of sensors 21 - 23 located in wedge shaped housings attached to a rearwardly tapering portion of the bomb. Each housing is constrained in dimensions such that the thickest part thereof does not substantially increase the overall body dimensions of the bomb. Each ground proximity sensor functions by transmitting pulses of infra-red radiation and timing the return of radiation scattered by the ground and detected in the housing. To obtain maximum sensitivity the housing contains an optical collector formed by a Mangin mirror (Fig 3) or plane mirror and aspheric lens which collector effectively fills the thicker end of the housing, to maximise collection area, and direct the radiation to a detector in the thinner end of the housing. <IMAGE>

Description

OPTICAL PROXIMITY FUZE SENSOR This invention relates to proximity fuzes for missiles and in particular to optical sensors for triggering of proximity fuzes while in flight.

It is common for missile proximity fuzes, operating on optical radiation (that is in the infra red, visible or ultraviolet part of the spectrum) to have one or more sensors each having a field of view directed forwardly and sidewards of the missile and arranged to detect radiation of suitable frequency emitted by an object or reflected from it (possibly as a result of illumination by a source in the missile). Such fuzes are employed in a ground proximity sensing role by which a descending missile can be detonated with an air burst a short distance from impact with the ground.

One example of such a fuze is in US Patent specification No. 4,098,191 in which passive infra-red detectors survey regions ahead and laterally of the missile, in accordance with its expected slant angle of approach to the ground, to determine from a timed relationship between survey of said regions the instantaneous height above ground.

Examples of missiles employing both radiation sources and detectors are described in GB Patent specifications Nos.

2,063,430 and 2,052,021, the latter employing ground detection as a means of missile guidance.

A feature common to all these missile arrangements is that the radiation sensors are normally located within a forwardly tapering nose section such that windows in the nose wall are able to define a field of view both forwardly and laterally of the missile for each sensor.

As such missiles are in the form of projectiles fired from a gun or launcher they are normally designed with fuzing arrangements in a detachable nose cone so that there is little difficulty in producing satisfactory sensor location for a particular design of proximity fuze.

In some missiles, such as air-to-ground bombs, the missile is designed in a well established configuration in which the payload is carried at the front or body portion of the missile ana a tapering tail portion attached thereto for stability, the fuze being located to the rearmost end of the body portion adjacent the tail portion.

In adapting such bomb missile designs to optical ground proximity fuzing it is not readily possible to mount optical sensing means within a forwardly tapering portion, particularly when the remainder of the fuze is constrained to be away from such a position.

Furthermore it is prohibitively expensive to develop and store a radically new bomb construction specifically for ground proximity detonation while being undesirable to make modifications to existing bomb constructions in such a way that overall dimensions of the body are increased substantially to interfere with existing transport arrangements and aerodynamics of the bomb.

It is an object of the present invention to provide an optical proximity fuze sensor suited to use with such a rear-fuzed missile that does not require major redesign of the missile.

According to the present invention an optical proximity fuze sensor arrangement for a missile comprises at least one substantially wedge shaped housing adapted to be mounted on a rearwardly tapering portion of the missile with the thinner end of the wedge directed towards a wider untapered portion of the missile, the housing being dimensionsal such that the thicker end of the wedge does not substantially extend the missile radius at its location beyond that of said untapered portion and including a window for the passage of optical radiation into the housing from a direction forward of the housing location.

Embodiments of the invention will now be described by way of example with references to the accompanying drawings, in which: Figure 1 is an elevational view of a conventional air-to-ground bomb showing the disposition of the fuzing arrangement, Figure 2 is an elevational view of a bomb similar to that of Figure 1 showing the disposition of optical proximity fuze sensors according to the present invention, Figure 3 is a perspective view of a partly sectioned sensor housing showing one form of optical collecting means employed, Figure 4 is an elevational view of a bomb similar to Figure 2 illustrating operation of the ground proximity sensing means, and Figure 5 is a perspective view of an alternative form of optical collecting means.

Referring to Figure 1 an air-to-ground bomb 10 comprises a main body portion 11 and a tail portion 12. The body portion 11 is essentially cylindrical in section having a forwardly tapering nose section 14 ending in a hardened nose cap 13'. The remainder of the body is filled with a payload of high explosives except for a detonating fuze 14, responsive to impact sensing means contained therein, located within a fuze pocket 14' at the rearward end of the body portion. The rearward part of the body is also tapered as shown at 11' which taper is continued by the tail portion 12.

The tail portion 12 is bolted to the body after the fuze 14 is inserted in its pocket and tapers rearwardly of the body towards a rear tip 15 at which are located stabilising fins 16. The tail section may also carry retarding flaps 17, delineated by broken lines 17' and which open out about a tail pivot when the bomb is released (in the manner shown in Figure 4).

Such bombs are normally employed as impact detonation missiles. In accordance with the present invention it is desired to extend the fuzing characteristics of the bomb to include air-burst resulting from proximity detection of the ground targets prior to ground impact. Existing forms of proximity sensors used in airborne missiles and described, for example, in the aforementioned Patent Specifications detect radiation emitted from, or scattered by, the ground and received by a detector, within the nose-cone, by way of a forwardly-looking window.

The direct application of such proximity detection to a bomb of the type described with reference to Figure 1 is precluded by the established bomb design whereby optical sensing means cannot be contained in the tapered nose containing the explosive payload and at the opposite end of the body from the fuze.

The present invention overcomes this as shown in Figure 2 in which the conventional impact operated fuze is replaced by a proximity operated fuze 20 and optical sensing means is carried on the rearwardly tapering section of the body and/or tail portion 12 adjacent the fuze pocket at the rear of the body portion 11.

Preferably, multiple optical sensing means is employed comprising a plurality, say four, sensors 21, 22, 23 (and 24 not shown) disposed around the periphery of the tail section. Each sensor, say, 21 (shown also in Figure 3) comprises a substantially wedge shaped housing 25 with the thinner end 26 of the wedge directed towards the wider untapered body portion.

The thicker end 27 of the wedge is directed towards the tail portion ana the housing dimensioned such that the thicker end 27 stands proud of the tail portion by only by an amount such that it does not extend the radius of the tail portion at its location substantially beyond the radius of the body portion while proviaing between the thicker and thinner ends 27 and 26 a wall 28 having no substantial rearward taper and possibly a forwardly tapering surface. The wall 28 contains a window 29 towards the thicker end of the housing and extending for substantially the width of the housing.

A semiconductor laser emitter 30 is located in the housing and arranged to emit a substantially parallel beam of near-infra-red optical radiation by way of a reflector 31 and the window 29. Most of the thicker end 27 of the housing is occupied by optical radiation collection means 32 by which optical radiation received through the window from forward of the housing is redirected and caused to converge upon a radiation detector 33. The emitter 30 and detector 33 are located with other circuit components (not specifically shown) at the narrow end of the wedge shaped housing such that the thicker end can be devoted to collection of the maximum possible amount of radiation.

The optical collector is in the form of a Mangin mirror 34 which mirror is shaped as a section of a hollow sphere of differing inner and outer radii of curvature. The 'outer' convex face 35 is made reflective of the optical radiation and the curvatures chosen such that to give a focal length such that radiation of the emitted beam scattered from a predetermined distance away and entering the window 29 is reflected and focussed onto detector 33.

Operation of the sensing means will be understood more clearly with reference to Figure 4 in which the emitted beam is shown at 36 emerging from a bomb approaching the ground 37 at a slant angle o with respect to trajectory line 38. The optical arrangement of the sensing means has a field of view bounded as shown by the broken lines 39 and 40 which intersect between distances D1 and D2 from the housing to define a field of view, for the detector, of radiation of beam 36 scattered by engagement with the ground, as at 41. The distances D1 and D2 are selected so that the fuze is unresponsive to radiation scattering from objects or ground too distant or from debris or rain-drops too close to the housing window.

Furthermore the angle between the emitted beam and the trajectory line 38, ss, is chosen, having regard to the expected slant angle of approach a for a particular bomb trajectory employed, to be approximately (90 - ) so that the beam 39 is directed to intersect the ground substantially vertically below the bomb.

It will be appreciated that the bomb may be subject to spin during its descent and that each of the sensing means 21 24 produces a similar beam and if the beam strikes the ground, detects scattered radiation from it. In general the bomb attitude will not be as shown in Figure 4, that is, with one housing 21 directed vertically downwards to a point beneath the trajectory but will be to one side of the plane of the trajectory such that beams from two of the housings strike the ground at different angles to the vertical in a vertical plane orthogonal to the plane of the trajectory Each sensing means emits the radiation in pulses, timing the returns of scattered radiation detected in order to determine the range to the ground in the manner well known for laser range finders.It will be appreciated that if ranges are determined by two sensing means the relevant beams are not vertical in the plane orthogonal to the plane of the trajectory but that simple and well known trigonometrical relationships can be used to establish the vertical height of the trajectory line above the ground.

It will be appreciated that height evaluations will be obtained in rapid succession once the ground is within the range D1 - D2, depending upon the optical radiation pulse emission frequency. The height evaluations may be compared as they occur with a predetermined detonation height until such height is attained or may be employed to derive a predicted rate of descent to the predetermined detonation height, which prediction may be updated with further height evaluations or employed to determine detonation should height evaluations cease as a result of interruption of the optical path e.g. by dust.

It will be appreciated that for optical sensing purposes fewer than four sensing means are required but it is prefered to have a symmetrical positioned relationship of the housing with the tail fins of the bomb to avoid any trajectory instability as a result of changes in drag coefficient caused by the additional housings. It may be desirable to extend the housing profile in a manner illustrated by the dotted lined extensions 21' to 23' in Figures 2 and 4, the precise shape depending upon the aerodynamics of the bomb.

The configuration taken by the sensing means, that is, the folded optical radiation path within the housing results from the relatively small portion of the bomb of less than maximum diameter which is close to the fuze pocket but not fouled by retarder flaps 17. Also the requirement to collect a maximum amount of the scattered radiation dictates collection from as large an area as possible so that the collection system is constrained to fill a maximum amount of the housing available, the thicker rearward end while the detection means and circuitry is consigned to the thinner forward end.

An alternative optical arrangement is shown in Figure 4 and comprises a rectangular plane reflector 40 carried by the rear wall of the housing to receive by way of the window 28 and an aspheric lens element 41 with a rectangular aperture located between the plane reflector 40 and the detector 32. Operation is similar to the Mangin mirror arrangement described with reference to Figure 3 in that a similar field of view is provided for the detector, with the larger plane reflector occupying the thickest position of the wedge shaped housing adjacent the window 28 and the lens a narrower intermediate position, whereby the wedge shaped profile of the housing is preserved.

It will be appreciated that the aperture area occupied by the radiation collection means is determined by the thickness dimensions permitted for the housing. If the housing is to be employed on a bomb, or other missile, in which the complete surface of the tapering tail section is available the housing may be located further along the tail section, enabling an increase in overall height, or made longer enabling alternative forms of optical collecting to be employed within it.

Claims

1. An optical proximity fuze arrangement for a missile compirising at least one sensing arrangement contained in a wedge shaped housing adapted to be mounted on a rearwardly tapering portion of the missile with the thinner end of the wedge directed towards a wider untapered portion of the missile, the housing being dimensioned such that the thicker end of the wedge does not substantially extend the missile radius at its location beyond that of said untapered portion and including a window for the passage of optical radiation into the housing from a direction forward of the housing location.

2. An arrangement as claimed in claim 1 in which the housing includes an optical collector arranged to receive radiation by way of the window and direct it to a detector contained in the housing.

3. An arrangement as claimed in claim 2 in which the window extends across the housing and the collector extends substantially the full width and thickness of the thicker end of the housing to collect radiation from a maximum area for the detector.

4. An arrangement as claimed in claim 2 or claim 3 in which the collector comprises a converging mirror operable to reflect optical radiation received by way of the window into a photodetector located at the thinner end of the wedge shaped housing.

5. An arrangement as claimed in claim 4 in which the mirror is a Mangin mirror.

6. A missile substantially as herein described with reference to, and as shown in, Figures 2 to 4 of the accompanying drawings.

6. An arrangement as claimed in claim 2 or claim 3 in which the collector comprises a plane mirror element operable to direct radiation entering the housing by way of the window towards a photodetector located at the thinner end of the wedge shaped housing and a lens 'arrangement located adjacent the mirror element to cause radiation reflected by the mirror element to converge on the detector.

7. An arrangement as claimed in claim 6 in which the lens arrangement comprises an aspheric lens.

8. An arrangement as claimed in any one of claims 2 to 7 in which the housing includes a source of optical radiation adapted to emit radiation in a substantially non-divergent beam in a direction forward of the housing to define, by its intersecting with the field of view of the detector a sensing region extending for a predetermined distance laterally of the missile and forward of the housing.

9. An arrangement as claimed in claim 8 in which the source is a laser operable to emit pulses of radiation.

10. An arrangement as claimed in claim 9 in which the laser is part of a laser rangefinder.

11. An arrangement as claimed in any one of the preceding claims in which the fuze arrangement comprises a plurality of sensing arrangements arrayed around the periphery of the missile and operable in parallel to define a plurality of sensing regions around the missile.

12. An optical proximity fuze sensing arrangement substantially as herein described with reference to and as shown in Figures 2 to 4 or Figure 5 of the accompanying drawings.

13. A missile comprising a body attached to a rearwardly tapering tail portion said missile including a fuze pocket in the body, a fuze arrangement carried in the pocket, and mounted on a rearwardly tapering portion of the surface adjacent the fuze pocket a proximity fuze sensing arrangement as claimed in any one of claims 1 to 12.

Amendments to the claims have been filed as follows 1. A missile comprising a body, containing a payload and a fuze, a rearwardly tapering tail portion and, mounted on the surface of said rearwardly tapering tail portion, at least one fuze proximity sensor, said sensor comprising a wedge-shaped housing mounted with the thinner end of the wedge directed towards the forward untapered portion, dimensioned with respect to the dimensions of the missile such that the thicker end of the wedge shaped housing does not substantially extend the missile radius, at its location, beyond that of the untapered portion, a window extending across the housing for the passage of optical radiation from a direction forward of the housing location and contained within said housing a photodetector, located at the thinner end of the housing, an optical collector, located at the thicker end of the housing to reflect and converge the radiation received through the window from forward and laterally of the housing onto the photodetector, and a source of optical radiation adapted to emit radiation in a non-divergent beam in a direction forward of the housing to define, by intersection with the field of view of the photodetector, a sensing region extending for a predetermined distance laterally of the missile and forward of the housing, said source of optical radiation comprising a laser forming with the photodetector a laser rangefinder operable to determine the distance of an object detected in the sensing region by reflection of said emitted optical radiation.

2. A missile as claimed in claim 1 in which the collector extends substantially the full width and thickness of the thicker end of the housing to collect radiation from a maximum area for the detector.

3. A missile as claimed in claim 1 or claim 2 in which the collector comprises a Mangin mirror.

4. A missile as claimed in claim 1 or claim 2 in which the collector comprises a plane mirror element operable to reflect radiation entering the housing by way of the window towards the thinner end ofthe housing and an aspheric lens located to cause said reflected radiation to converge on the photodetector.

5. A missile as claimed in any one of claims 1 to 4 including a plurality of proximity sensors arrayed around the periphery of said tail portion of the missile, said sensors being operable in parallel to define a plurality of sensing regions around the missile and said fuze being responsive to contemporaneous detection of an object within more than one sensing region, indicative of approach to ground, to resolve each of said distances with respect to the angle between them to define a local vertical and the effective distance in said vertical plane.