GB2380782A

GB2380782A - A fuze

Info

Publication number: GB2380782A
Application number: GB9815101A
Authority: GB
Inventors: Dennis James Hickey
Original assignee: Royal Ordnance PLC
Current assignee: BAE Systems Global Combat Systems Munitions Ltd
Priority date: 1997-07-16
Filing date: 1998-07-10
Publication date: 2003-04-16
Anticipated expiration: 2018-07-10
Also published as: ITRM980471A0; FR2865537A1; DE19831645A1; ITRM980471A1; GB9714947D0; GB2380782B; NL1009621C2; AU7315798A; GB9815101D0

Abstract

A fuze for a round of ammunition, the fuze having a safety and arming device, said safety and arming device comprising <UL ST=" $ "> <LI>a shutter moveable from a safe position in which the round cannot be fired to an armed position in which the shutter position does not prevent firing, <LI>a detent 24 having a locking position in which it interacts with the shutter so as to retain the shutter in the safe position, and a non-locking position in which the shutter can move to the armed position, <LI>detent actuating means 108 operably connected to said detent means so as to move said detent means from the locking position to the non-locking position, said detent actuating means being operable in response to a pressure differential of at least a predetermined magnitude between two positions on said fuze, at least one of which is on its outer surface at a point remote from the nose of the fuze, and said pressure differential being capable of being created by airflow over the said outer surface, and <LI>sensors 102, 104 for detecting the pressure differential between said two positions, and processing means 106 for generating an output when said pressure differential exceeds said pre-determined magnitude, said output being capable of energising detent actuating means. </UL>

Description

A FUZE The present invention relates to fuzes particularly, though not exclusively, for mortar bombs.

For reasons of safety, rounds of ammunition such as mortar bombs for example and missiles require systems which render them safe on the ground during handling and arm them after launch and prior to reaching their target. In the case of present-day mortar bombs a safety pin is removed and an electrolytic power cell initiated by twisting of the fuze nose about its axis prior to dropping the bomb into the launch tube. During launch, a first safety device, often in the form of a so-called"set-back"detent which is an inertia device, operates under the influence of launch acceleration to at least partially arm the fuze.

With modern ammunition coming into service, it is desirable, and indeed required in some safety specifications, that there be a second safety and arming device, independent of the first, which operates after the first inertia arming device has been triggered during launch. With spun rounds fired from rifled bores, safety and arming devices operated by the effects of centrifugal force are easily constructed. However, in the case of unspun rounds such as mortar rounds, ammunition fired from smooth-bore weapons and missiles for example, another actuating environment must be found. Proposals have been made to utilise air pressure effects by means of elevating a pitot tube into the surrounding airflow after launch and during flight of the round, US-A-4 526 104, showing one example. Other proposals have utilised airflow through an orifice in the fuze nose to drive a small air turbine to generate power to fire a small gas motor or generator which frees a detent to finally arm the fuze. However, such known secondary arming safety devices all have the common disadvantage of producing or possessing aerodynamic drag which lessens the range of the ammunition round for any given propellant charge.

It is a requirement of modern rounds that they have maximum range for a given propellant charge and any safety devices which lessen that range due to additional drag are undesirable.

The fuzes of such rounds and which generally form the nose are required to have maximum aerodynamic efficiency. A further disadvantage of the air driven turbine type of fuze is that some rounds where powerful propelling charges are employed have a supersonic muzzle exit speed which prevents efficient operation of the turbine due to the air flow being choked off until the round slows down sufficiently due to drag effects and gravity during its trajectory.

Thus, some rounds which depend upon an arming timer being initiated at launch by turbine generated power are unpredictable as to when they are armed, especially when fired in a flat trajectory as from a mobile armoured mortar system for example. Therefore, a second independent safety system operable after launch, which does not create unwanted drag and is not affected by supersonic muzzle velocities is highly desirable.

It is desirable, though not necessarily essential, that the second safety system is direct acting, i. e. serves to directly arm the round without intervening electrical or electronic systems needing to actuate the arming system.

It is an object of the present invention to enable the provision of a fuze having a second in-flight safety and arming system for an unspun round of ammunition. It is a further object to enable the provision of a fuze having a safety and arming system which produces little or no additional aerodynamic drag in its operation.

According to the present invention there is provided a fuze for a round of ammunition, the fuze having a safety and arming device, said safety and arming device comprising a shutter moveable from a safe position in which the round cannot be fired to an armed position in which the shutter position does not prevent firing,

* a detent having a locking position in which it interacts with the shutter so as to retain the shutter in the safe position, and a non-locking position in which the shutter can move to the armed position, and * detent actuating means operably connected to said detent means so as to move said detent means from the locking position to the non-locking position, said detent actuating means being operable in response to a pressure differential of at least a predetermined magnitude between two positions on said fuze, at least one of which is on its outer surface at a point remote from the nose of the fuze, and said pressure differential being capable of being created by airflow over the said outer surface.

In this specification, the term"detent"is to be construed as meaning any lock or interlock which can be released or moved so as to enable the shutter to move to the armed position.

The round of ammunition may be an unspun round, i. e. fired from a smooth bore mortar launch tube or gun barrel, or a missile for example.

Air flow over the (normally) ogival shape of the fuze body generates two aerodynamic effects. At the nose air is compressed to give a static high pressure zone. Further back along the fuze body the flow of the air passing over the curved shape of the fuze causes a pressure drop close in at the surface, similar to the effect which generates lift on an aeroplane wing. The pressure drop along the fuze body or the difference in pressure between the nose and body zones may be used to directly or indirectly cause actuation of the detent by the detent actuating means.

In the case of direct actuation, the higher pressure pertaining at the nose may be bled, by means of a conduit for example, to a position adjacent the detent actuating means which, for example, may be a diaphragm. Thus, a diaphragm may be placed lying substantially flush

with and sealed around its periphery to the fuze body surface and be supplied with relatively higher pressure air at a fuze interior side, by means of a conduit linking the nose region and the interior side of the diaphragm for example, and, in use, may have relatively lower pressure air due to the airflow over the fuze body at an exterior position adjacent the diaphragm; the net positive pressure on the interior surface of the diaphragm causing flexure thereof to operate the detent and causing the round to become armed. In the case of direct actuation, the detent is preferably directly fixed to a diaphragm, for example, the diaphragm being situated adjacent to the shutter for allowing the detent to interact with the shutter.

It is preferred that the interior of the fuze is sealed against the outside environment during storage. Thus, for example, in order to exclude moisture during storage, a conduit may be arranged to operably connect the two positions only upon actuating a power cell prior to firing by twisting of the fuze nose as earlier described. Preferably, even when an operative connection is established by the conduit, only the area adjacent the detent operating means will be exposed to atmosphere, the remaining fuze components remaining sealed against the environment.

Alternatively, the safety and arming system may directly actuate the detent means solely by virtue of the lower pressure generated in the airstream compared with the interior pressure in the fuze body. In this case, the fuze body is preferably hermetically sealed against the outside environment, the detent actuating means again being a diaphragm for example. At launch, the fuze interior is at ambient pressure whereas the airflow passing over the fuze body towards the rear end thereof is at a somewhat lower pressure due to the aerodynamic effects described above. When a pressure differential between the fuze interior and the exterior airflow is established, the net positive pressure inside the fuze body causes the diaphragm to move outwardly and actuate the detent. However, the direct actuation method described

above where the higher pressure at the fuze nose is bled to a position at the fuze interior adjacent the detent actuating means is preferred since a greater net pressure differential is available.

The pressure drop or differential is thus used to directly actuate the detent by the detent operating means causing the round to become armed. Since the diaphragm lies flush with or below the fuze body surface and does not extend out into the air flow, little or no additional drag is caused to reduce the range of the round.

The detent operating means described above refers to a diaphragm. However, other passive detent operating means which respond to pressure drop or pressure differential such as for example a piston in a cylinder which has access to the fuze body surface may be employed.

In the case of indirect actuation, the pressure differential may be sensed by pressure sensors placed at the nose of the fuze and at a position or positions further back along the fuze body for example, the pressure at the fuze nose being higher than that adjacent the fuze body at a more rearward position. The output of the sensors may be used by appropriate electronic means powered by an on-board power cell to cause a detent to retract allowing the shutter to move so as to arm the round. The detent may be caused to operate and arm the round by means of a gas motor or generator for example or any other suitable stored energy device.

A detonator may be housed in the shutter, the arrangement being such that, in the safe position the detonator is held out of alignment with a firing chain through which the round can be initiated, and in the armed position the detonator is brought into alignment therewith.

The safety system and arming device of the present invention may be additional to one or more other safety systems in the fuze.

In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: Figure 1 shows a schematic cross section through a fuze which is in accordance with the invention, showing the major components thereof ; Figure 2 shows a schematic diagram of the arrangement of a shutter of Figure 1 in armed and safe (unarmed) positions; Figure 3 shows a view of the shutter of Figure 2 in the direction of arrow"A"thereof ; Figure 4 shows a schematic side view of a fuze and part of a round attached thereto; Figure 5 shows a graph of pressure vs time during the launch and trajectory phases of a mortar bomb; Figure 6 shows a graph of pressure adjacent the surface of a fuze body relative to air "freestream"pressure vs distance along fuze body from the nose; Figures 7A and 7B show, in schematic form, detent operating means of part of first and second embodiments of the present invention; Figure 8 shows, in schematic form, a third embodiment of detent operating means according to the present invention; Figure 9 shows, in schematic form, a fourth embodiment of detent operating means according to the present invention; and Figure 10 shows graphs of trajectory and velocity of a mortar round vs time.

Referring now to the drawings and where the same features are denoted by common reference numerals.

Figure I shows one example of a fuze denoted generally at 10. The fuze comprises a body 12 having an outer surface 14; an electronics pack 16 for functioning the fuze at a predetermined time during its trajectory and/or functioning the fuze by sensing its proximity to a target; a power pack 18 to supply power to an electronic timer/sensor and to fire a detonator (eg 40, Figure 3); a shutter assembly 20 having a first safety detent 22, and a second safety detent 24 which forms part of a safety and arming device according to the present invention; and, a payload ignition system 26. As they are part of the prior art and known to those skilled in the fuze art, the electronics pack and the power pack will not be described in any detail. The shutter assembly 20 comprises a first safety detent 22, known as a"set back"detent, comprising a pin 32 resiliently biased by a spring 30, which operates under launch acceleration to release one constraint on the shutter body 34 preventing its rotation from a first (safe) position to a second (armed) position (this first detent is part of the prior art and may alternatively enable another mechanism to come into play to have an operative effect on the shutter body other than rotation thereof). The operation of the first detent 22 is part of the prior art and forms no part of the present invention per se. The shutter assembly 20 includes a shutter arm 36 rotatable about a pivot 38 and carrying a detonator 40. Operation of the second detent 24 according to the present invention (after release of the first detent) fully releases the shutter arm 36 from its position shown in Figure 2 and indicated with solid lines, i. e. a safe non-armed position, to rotate about the pivot 38 to a second, armed position indicated by dashed lines in Figure 2 wherein the detonator 40 is brought into line with the remainder of

the initiatory explosive train, comprising"stemming"42 and a booster 44 which can then ignite the main charge 46 on initiation of the detonator 40. The purpose of Figures 1 to 3 is to show the schematic layout of the principal components of one example of a fuze, in accordance with the invention.

Figure 4 shows schematically a side elevation of a fuze 50 and part of a round 52 of mortar ammunition. The power pack 18 as described with reference to Figure 1 is activated by twisting the nose of the fuze 50 relative to the round 52 about the axis 54 immediately prior to dropping the round into a launch tube (not shown). During flight of the projectile there is a static high pressure zone of air at 58 adjacent the nose ; further back along the fuze body at 62 the air flow velocity is increasing and consequently a drop in pressure relative to that at the nose 58 occurs along the fuze body, i. e. a pressure differential is established due to aerodynamic effects which is dependent upon the velocity of the round.

Figure 5 shows a graph of air pressure adjacent the fuze body vs time, the datum line 70 being atmospheric pressure. At launch the mortar bomb causes some compression of the air trapped in the tube above it causing a pressure rise indicated at 72 which is essentially due to inertia of the trapped air. At muzzle exit this air expands but the bomb is surrounded briefly by expanding gas from the burning propellant charge. The bomb travels faster than the gas bubble and breaks through it generating a pressure spike 74 known as muzzle shock. The pressure profile 76 is then largely the inverse of the velocity profile of the bomb, the velocity profile being shown in Figure 10. As shown in Figure 10, the mortar bomb sheds the vertical component of its velocity as it climbs to its apogee indicated generally by vertical line 80 and then accelerates again during its fall. This change in velocity produces a corresponding change in the pressure 76 at the surface of the bomb.

It is the lowering of pressure above the fuze body back from the nose during the flight trajectory or the pressure differential between the nose region 58 and freestream 62 which is utilised in the fuze of the present invention to operate the second detent 24.

Figure 6 shows, for two different values of muzzle velocity (M. V. ), i. e. 170m/s and 70m/s, a graph of pressure adjacent the fuze body surface 14 relative to the air freestream pressure, i. e. pressure differential or pressure drop caused by aerodynamic airflow over the fuze nose and body vs distance from the nose. As may be seen a pressure drop of about 3kPa is produced at a bomb velocity of about 170m/s, at about 130mm from the nose.

Figure 7A shows schematic view of a first embodiment 90 of the second detent 24 in its rest or unarmed position. Detent actuating means are provided, in the form of a flexible diaphragm 92 which is dished and resides slightly below the fuze body surface 14. It is protected by a gauze 94. The diaphragm 92 is sealed around its entire periphery, to the body of the fuze. After launch, the pressure drop in the boundary layer adjacent to the diaphragm 90 causes it to be pushed outwardly by the greater pressure inside the fuze body, pulling the detent 24 with it (Figure 7B), and thus releasing the shutter 36 as described with reference to Figures 1 to 3. The positive net pressure inside the fuze body is due to the fuze body being sealed and having an internal air pressure substantially at the ambient air pressure at the moment of launch.

A second embodiment described with reference again to Figures 7A and 7B comprises the diaphragm 92 and detent 24. However, the region 96 behind the diaphragm 92 is connected by a conduit 98 (indicated by dashed lines) to the nose region adjacent the higher pressure air 58 described with reference to Figure 4 (in which the conduit 98 is also shown by dashed lines). The higher pressure air 58 is bled to the region 96 behind the diaphragm 92 to provide a pressure differential between the nose region and the site of the safety and arming device rearwardly of the fuze nose. Thre pressure differential produced according to this second embodiment is normally greater than the pressure drop produced in the first embodiment.

Figure 8 shows a second embodiment 100 of a fuze according to the present invention wherein a first pressure sensor 102 is positioned at the high pressure region 58 adjacent the nose and a second pressure sensor 104 is positioned at the surface of the body 14 further back away from the nose. Outputs from the two sensors are fed to suitable electronic processing circuitry 106, which may or may not be housed within the electronics pack 16. When a suitable predetermined pressure differential exists between the sensors 102,104, the processing circuitry 106 generates an output signal through which suitable detent actuating means are energised by the power pack 18 to remove the detent 24 and thus release its constraint on the movement of the shutter to the armed position. Suitable detent actuating means might for example comprise an electrically initiated gas motor or generator 108, actuable by an electrical pulse from the processor 106.

Figure 9 shows a fourth embodiment 120 of part of a fuze according to the present invention. Since the pressure differential or drop is caused by the aerofoil effect of the fuze shape in a similar manner to that of the aerofoil section of an aeroplane wing, a localised enhancement of the aerofoil effect may be produced by the addition of a surface feature 122 such as a bulge or other local surface irregularity which acts to speed up the relative airflow 124 and cause a greater pressure drop in the region 126. Although shown as a dashed line, the feature 122 must allow any enhanced pressure drop to be communicated to the diaphragm and consequently, a portion of the feature 122 may comprise an air-permeable portion such as a gauze for example in a suitable position. The interior of the fuze body may be sealed as described with reference to the first embodiment or the region 130 behind the diaphragm may be connected to the higher pressure air by means of a conduit (not shown) for example as in the second embodiment to enhance the pressure differential.

From the embodiments described above it will be seen that the invention makes possible the provision of safety and arming systems which do not depend upon sensing of spin, and which produce little or no additional drag on the fuze body or round allowing maximum range to be achieved for a given propelling charge.

The embodiments described with reference to Figures 7 and 9 are direct acting mechanical detent releasing systems and therefore there is no need for power packs or electronic devices specifically for the arming system per se. In these circumstances, the presence of power packs or electronics will be for other purposes which the fuze has to fulfil.

Claims

CLAIMS 1. A fuze for a round of ammunition, the fuze having a safety and arming device, said safety and arming device comprising * a shutter moveable from a safe position in which the round cannot be fired to an armed position in which the shutter position does not prevent firing,

* a detent having a locking position in which it interacts with the shutter so as to retain the 0 shutter in the safe position, and a non-locking position in which the shutter can move to the armed position, and 'detent actuating means operably connected to said detent means so as to move said detent means from the locking position to the non-locking position, said detent actuating means being operable in response to a pressure differential of at least a predetermined magnitude between two positions on said fuze, at least one of which is on its outer surface at a point remote from the nose of the fuze, and said pressure differential being capable of being created by airflow over the said outer surface.
2. A fuze according to claim 1 wherein the first of said two positions is on the outer surface of the fuze body and the second of said two positions is in the interior of the fuze body.
3. A fuze according to claim I or claim 2 wherein the fuze body is hermetically sealed.
4. A fuze according to claim 1 wherein both of said two positions are at the outer surface of the fuze body, the first being in the region of the fuze nose, and the second being rearward thereof along the fuze body.

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5. A fuze according to claim 4 wherein there is provided duct means for transmitting air

I pressure from the said first position in the region of the fuze nose, to a position adjacent said detent actuating means within the fuze body.
6. A fuze according to any one preceding claim wherein the detent operating means includes a flexible diaphragm or a piston and cylinder and is directly operable by the said pressure differential.
7. A fuze according to claim 6 wherein said piston or cylinder, or said diaphragm, is located adjacent to or below the profile of the fuze body.
8. A fuze substantially as hereinbefore described with reference to the accompanying description and drawings.

8. A fuze according to Claim 7 wherein said detent is directly fixed to said piston or cylinder or said diaphragm, said piston or cylinder or said diaphragm being adjacent said shutter for allowing said detent to interact with said shutter.

9. A fuze according to any one preceding claim comprising aerodynamic means rearwardly of the fuze nose for increasing the said pressure differential in use.

10. A fuze according to claim 9 wherein said aerodynamic means, in use, locally increase the air speed relative to the fuze body.

11. A fuze according to any one preceding claim comprising sensors for detecting the pressure differential between said two positions, and processing means for generating an output when said pressure differential exceeds said predetermined magnitude, said output being capable of energising detent actuating means.

12. A fuze according to claim 11 wherein one of said sensors is positioned at the fuze nose region, and another is positioned rearwardly of the fuze nose.

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13. A fuze according to claim 11 or claim 12 wherein the detent actuating means comprises a gas motor or a gas generator.

14. A fuze substantially as hereinbefore described with reference to the accompanying description and Figures I to 3; or Figures 4 and 7; or Figure 8; or Figure 9 of the drawings.

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Amendments to the claims have been filed as follows 1. A fuze for a round of ammunition, the fuze having a safety and arming device, said safety and arming device comprising 'a shutter moveable from a safe position in which the round cannot be fired to an armed position in which the shutter position does not prevent firing, 'a detent having a locking position in which it interacts with the shutter so as to retain the shutter in the safe position, and a non-locking position in which the shutter can move to the armed position, 'detent actuating means operably connected to said detent means so as to move said detent means from the locking position to the non-locking position, said detent actuating means being operable in response to a pressure differential of at least a predetermined magnitude between two positions on said fuze, at least one of which is on its outer surface at a point remote from the nose of the fuze, and said pressure differential being capable of being created by airflow over the said outer surface, and 'sensors for detecting the pressure differential between said two positions, and processing means for generating an output when said pressure differential exceeds said pre-determined magnitude, said output being capable of energising detent actuating means.

2. A fuze according to claim 1 wherein the first of said two positions is on the outer surface of the fuze body and the second of said two positions is in the interior of the fuze body.

3. A fuze according to claim 1 or claim 2 wherein the fuze body is hermetically sealed.

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4 A fuze according to claim 1 wherein both of said two positions are at the outer surface

of the fuze body, the first being in the region of the fuze nose, and the second being 0 rearward thereof along the fuze body.

5. A fuze according to any one preceding claim comprising aerodynamic means rearwardly of the fuze nose for increasing the said pressure differential in use.

6. A fuze according to claim 5 wherein said aerodynamic means, in use, locally increase the air speed relative to the fuze body.

7. A fuze according to any one preceding claim wherein the detent actuating means comprises a gas motor or a gas generator.