WO2015052464A1

WO2015052464A1 - Blast resistant structures

Info

Publication number: WO2015052464A1
Application number: PCT/GB2014/000398
Authority: WO
Inventors: Stuart Richard COWLING
Original assignee: Cowling Stuart Richard
Priority date: 2013-10-10
Filing date: 2014-10-07
Publication date: 2015-04-16
Also published as: GB201317933D0; GB201403664D0

Abstract

A structure (1), such as a portable cabin, a temporary building, a bridge and a vehicle, is provided that has means designed to counter the forces created by blast landmines and improvised explosive devices deployed beneath, adjacent or in the path of the structure (1). The structure (1) comprises a body (2) defining an under-surface (3), which may be wedge-shaped, and side surfaces (4). At least one inclined or curved first deflector (5) is secured to the structure (1) and positioned where the under-surface (3) and one or more side surfaces (4) meet to define a channel (6) between itself and the structure (1) through which blast ejecta from said explosion is channelled in a generally upwards direction. A deflecting means (7, 20, 25) is located within or without the channel (6) and is adapted to deflect or to entrain blast ejecta and align its flow with or in a direction towards a longitudinal axis of the channel (6). The deflecting means (7, 20, 25) may comprise at least one second deflector (7, 20) that extends at an angle to the first deflector (5) to deflect impacting blast ejecta and thereby align its flow with or in a direction towards the longitudinal axis of the channel (6). Alternatively or in addition the deflecting means (7, 20, 25) may comprise at least one ejecta accelerator (25) that is adapted to entrain blast ejecta and to align its flow with or in a direction towards the longitudinal axis of the channel (6).

Description

BLAST RESISTANT STRUCTURES

The present invention relates to structures, such as bridges, vehicles, and buildings, in particular portable cabins or temporary buildings and land vehicles, that may be subjected to blasts originating beneath or closely adjacent to them. More specifically, the invention relates to providing structures with means designed to counter the forces created by blast landmines and explosive devices deployed beneath, adjacent or in the path of such structures. When a buried blast landmine or other similar type of explosive device is detonated the chemical energy contained in the explosive is almost instantaneously released as pressure and heat in the explosive's products of combustion. The energy release causes the earth covering the device to be propelled directly away from the detonating explosive with a trajectory at nominal right angles to the surface of the earth, which can be considered to be directly upwards for a device buried in horizontal terrain. Simultaneously, the earth to the sides and below the device is violently compressed and quickly rebounds so that the bulk of this material is also ejected nominally upwards. Conceptually, the earth can be taken to rebound from the line of the eventual crater left by the detonation of the device. The detonation causes the formation of a rapidly moving vertical column of ejecta with its kinetic energy focussed in the centre above the device.

Typical blast landmines have diameters in the region of 250 mm to 350 mm. Consequently the central portion of the ejecta column with the highest kinetic energy tends to be of a similar diameter and is the first portion to be formed at approximately 0.25 ms after detonation. Over approximately the next 1 ms the ejecta column increases in diameter to around 2 m or more due to the rebound effects described above. The upwards velocity of the ejecta can be in the region of 1000 to 2000 m/s or more. These parameters represent a typical blast landmine detonation; the parameters for other explosive devices can vary somewhat depending on device type, burial depth and ground conditions. When a buried explosive device E is detonated beneath a structure, for example a bridge B as shown in Fig. 1 that spans a ditch or riverbed D, the blast ejecta impacts the under-surface S of the structure B and an exchange of momentum takes place between the ejecta and the lower regions of the structure. If the structure is movable, it is accelerated upwards by the blast ejecta but the time taken for it to actually move more than a few centimetres can be between 5 ms to 10 ms. From the first moment the ejecta impacts the structure and prior to it undergoing significant vertical movement the only escape path for the ejecta is away from the detonating device and along or around the under-surfaces S of the structure B as each element of ejecta is followed by subsequent elements that will prevent a direct downwards rebound. The collective action of these elements causes a flow of ejecta F along the under-surface S of the structure B towards its periphery and beyond in a trajectory largely governed by the geometry of the under-surface. The bulk of this flow F will be directed along paths of least resistance, generally towards the closest parts of the structure's periphery.

Conventional methods o reducing the vertical movement of a structure such as a bridge or vehicle in such circumstances include making the structure heavier so that it is more difficult to accelerate upwards. Another method is to use stiffeners and/or energy absorbing elements beneath the structure as a form of shockwave management. Such methods are described in, for example, US 8146477 and WO 2007/020531. Yet a further method in the particular case of a vehicle involves shaping its under-surface into a wedge shape with the apex of the wedge pointing downwards. Such an arrangement is described in GB 2472718 and GB 2485746. In a vehicle with a wedge-shaped under- surface, the forces required to change the direction of ejecta travelling vertically upwards are lower in magnitude and more horizontal than with a flat-bottomed vehicle. The reaction forces on the vehicle are therefore lower and split between the two facets of the wedge. As the two facets are angled the forces are no longer vertical. The lower reaction forces and angle to the vertical can reduce the vertical acceleration of the vehicle, particularly for vehicles with relatively acute wedge angles but at the expense of interior vehicle volume for given vehicle overall width, length and height limitations.

WO2011/075174 discloses the use of redirecting elements or channels attached along the side edges of a vehicle hull to redirect ejecta from a blast beneath the hull to produce a force on the channels having a component in a downward direction thereby tending to hold the vehicle down. These redirecting channels can be in the form of a thin shell that extends over a large portion of the hull bottom and up along the sides. A blast is intended to rupture part of this shell to allow ejecta to enter and be redirected to produce the desired force. However, the use of such a shell has two main disadvantages. First, for the shell to rupture only locally inwards and then be robust enough to manage ejecta flow, without subsequent outward rupture, it would need to have localised weaknesses and strong points, the strong points including those portions of the shell intended to act as redirecting elements. A generally weak shell susceptible to rupture at any position would tend to collapse in its entirety and block the path between the ejecta and the redirecting elements whereas at localised strong points the shell would not rupture and would thereby be ineffective. Second, rupturing of the shell will cause turbulence and velocity reduction in the ejecta that will also limit the efficiency of the shell.

The aforementioned existing means of controlling vertical blast forces are based on passive techniques as they function without the need to utilise energy stored in sources secured to the structure to power a countermeasure system. Alternative active methods have been designed to reduce vehicle jump and these are based on the detection of the explosion prior to the impact of the ejecta using sensors positioned around the vehicle and a control system to process the input and selectively launch non-gaseous masses upwards to counteract the upward forces caused by the blast. Such an arrangement is described in GB 2480709. However, such measures are complex and involve the use of rapidly acting sensors and controllers for the devices, which makes such active management methods inherently less reliable than passive means of countering vehicle jump.

An object of the present invention is to provide a structure with a primarily passive means designed to counter the forces created by blast landmines and improvised explosive devices deployed beneath, adjacent or in the path of the structure that overcomes or substantially mitigates the aforementioned problems of conventional passive means. According to the present invention there is provided a structure adapted to withstand blast forces from an explosion beneath the structure, the structure defining an under-surface and one or more side surfaces and comprising

at least one inclined or curved first deflector that is secured to the structure and positioned where the under-surface and one or more side surfaces meet to define a channel between itself and the structure through which channel blast ejecta from said explosion is channelled in a generally upwards direction; and

a deflecting means located within or without the channel and adapted to deflect and/or to entrain blast ejecta and align its flow with or in a direction towards a longitudinal axis of the channel.

Such a structure may comprise a portable or temporary building, a bridge or a vehicle or any other structure that may be subjected to blasts originating beneath or closely adjacent to it. Such structures are vulnerable to attack by buried explosive devices or vertical water spout attack caused by submerged explosive devices

Preferably, the first deflector forms part of a deflector assembly and the channel is defined between opposing surfaces of the deflector assembly. Preferably also, the deflecting means comprises at least one second deflector that extends at an angle to said first deflector to deflect impacting blast ejecta and thereby align its flow with or in a direction towards the longitudinal axis of the channel.

Advantageously, at least one of the second deflectors comprises at least one downwardly projecting strake located beneath the under-surface or adjacent a lower end of the channel to direct blast ejecta into said channel.

Preferably also, at least two projecting downwardly strakes are located beneath the under-surface such that they define a conduit to direct blast ejecta along the conduit and into said channel. Alternatively or in addition, at least one of the second deflectors comprises one or more webs that extend substantially vertically and project outwards from the structure within the channel defined by at least one first deflector. Preferably also, the webs are located between said opposing surfaces defined by the deflector assembly.

Advantageously, at least one web is integrally formed with at least one strake. Alternatively or in addition, the deflecting means comprises at least one ejecta accelerator that is adapted to entrain blast ejecta and to align its flow with or in a direction towards the longitudinal axis o: the channel.

In some embodiments, at least one thruster adapted to counter the force of a blast is preferably secured to a side or to an upper surface of the structure and is activated by a thruster trigger mechanism linked to a thruster sensor that detects the presence of a flow of ejecta in the vicinity of the under-surface of the structure. Such embodiments combine passive with active means of countering blast forces and are particularly suitable for use with movable structures such as vehicles.

Other preferred but non-essential features of the present invention are described in the dependent claims appended hereto.

The present invention will now be described by way of example with reference to the accompanying drawings, in which:-

Fig. 1 is a diagram showing a side view of a bridge and the forces that act on it when an explosive device detonates beneath it; Fig. 2 is a perspective schematic view of a land vehicle in accordance with the present invention; Fig. 3 is a perspective view from below of a modified land vehicle similar to that shown in Fig. 2;

Fig. 4 is a perspective view of a bridge in accordance with the present invention;

Fig. 5 is a perspective view from below of the bridge shown in Fig. 4;

Figs. 6A to 6F comprise a series of diagrams showing the effect of using strakes to manage ejecta flow;

Figs. 7A to 7C are diagrams being end views of three different strakes respectively each showing how the particular strake is profiled and connected to an under-surface of a structure; Fig. 8 is a diagram showing a vertical cross-section of a vehicle with a wedge- shaped under-surface, the forces that act on it and the blast ejecta vector interactions that, occur when an explosive device detonates beneath the vehicle, the strakes shown in Fig. 3 being omitted; Fig. 9 is a schematic view of part of one side of a structure showing a modification of the invention;

Fig. 10 is a diagram showing a deflector sub-assembly for use in an embodiment of the present invention;

Fig. 11 is a schematic vertical cross-section of a vehicle in accordance with the present invention showing modifications to the invention;

Fig. 12 is view similar to Fig. 3 but including the modifications to the invention shown schematically in Fig. 11 ;

Figs. 13a, 13b and 13c are diagrams respectively showing the present invention modified to include three different embodiments of ejecta accelerators; Figs. 14a, 14b and 14c are schematic longitudinal cross-sections of three different embodiments of ejecta accelerators respectively; and Fig. 15 is a diagram similar to Fig. 8 showing the forces that result from use of thrusters similar to the ejecta accelerators shown in Figs 13a and 3b when secured to a vehicle according to the present invention.

The present invention provides structures of various kinds with primarily passive means that are designed to counter the forces created by blast landmines and improvised explosive devices deployed beneath, adjacent or in the path of the structure. The invention is described below by way of example with reference to a structure that comprises a land vehicle although structures in the form of a bridge in accordance with the invention are shown in Figs. 4 and 5. However, it should be appreciated that the invention is suitable for use with many different types of structures, for example bridges, vehicles, and buildings, in particular portable cabins and temporary buildings, that may be subjected to blasts originating beneath or closely adjacent to them. The same principles that apply to the vehicle described below also apply to these and other structures that may be attacked in a simJar fashion. In all cases, references to a side or side surface of a structure refers to any side or side surface including, in a vehicle for example, front and rear sides or front and rear side surfaces.

Structures 1 in accordance with the invention in the form of a land vehicle and a bridge are shown in Figs. 2 and 3 and in Figs. 4 and 5 respectively. Each of these structures 1 comprises a body 2 defining an under-surface 3 and side surfaces 4. Arranged around the periphery of the vehicle and the along the peripheral, parallel sides of the bridge adjacent positions where the under-surface 3 and side surfaces 4 meet is at least one inclined or curved first deflector 5, which is robustly secured to the body 2. The first deflector 5 defines a channel 6 between itself and the body 2 and is shaped so that blast ejecta from an explosion beneath the body 2 is channelled in a generally upwards direction after impacting the under-surface 3. Preferably, therefore, the first deflector 5 is progressively contoured so that the channel 6 initially has an orientation substantially parallel to the likely direction of ejecta flow, which is generally parallel to the under-surface 3 as is explained in more detail below. The orientation of the first deflector 5 then turns to a substantially vertical orientation adjacent the side surface 4 of the structure. Hence, the first deflector 5 is adapted to deflect blast ejecta and align its flow with or in a direction towards a longitudinal axis of the channel 6.

In addition to the first deflectors 5 the structure 1 is provided with at least one deflecting means that may be located within or without the channel and that is adapted to deflect or to entrain blast ejecta and align its flow with or in a direction towards a longitudinal axis of the channel 6, this being the direction that maximizes the subsequent upward turn of the ejecta. The deflecting means may comprise a second deflector that acts to deflect impacting blast ejecta and to align its flow with or in a direction towards a longitudinal axis of the channel 6. Alternatively or in addition, the deflecting means comprises at least one ejecta accelerator that is adapted to entrain blast ejecta and to align its flow with or in a direction towards the longitudinal axis of the channel 6, this again being the direction that maximizes the subsequent upward turn of the ejecta. Both forms of deflecting means are described below.

When the deflecting means comprises a second deflector, these may take the form of strakes 7 and/or webs 20, as described below, that may be used individually or preferably in combination along with the first deflector 5.

In the vehicle and bridge shown in Figs. 2 and 3 and in Figs. 4 and 5 respectively, the under-surface 3 of the structure 1 is provided with at least one strake 7 that is located beneath the under-surface 3 or adjacent a lower end of at least one of the channels 6 in order to direct blast ejecta into the channel 6 or several of the channels 6. Preferably, the strake 7 is connected to and projects downwardly from the under- surface 3 and a plurality of strakes 7 is provided that are orientated such that they define a conduit 8 or a series of conduits 8 between them which directs blast ejecta from an explosion beneath the body 2 along the conduit 8 towards a side surface 4 and into one or more of the channels 6 defined by one or more of the deflectors 5. It will be appreciated that the strakes 7 are preferably parallel to one another or flared outwards or inwards with the intention of directing blast ejecta directly into the one or more of the channels 6. The conduits 8 are therefore open conduits with a view to catching and redirecting the ma.ximum quantity of blast ejecta into the channels 6 of the deflectors 5.

It will be appreciated that each deflector 5 may comprise a single, one-piece deflector or be a deflector assembly made up of a number of individual deflectors, for example as shown on the side of the bridge in Figs. 4 and 5. In both cases the deflector 5 defines a combination of flat sections and curves and its surface or surfaces 9 opposite the body 2 of the structure 1 is or are preferably progressively curved to ensure low friction and turbulence in the ejecta flow. A plurality of defectors or deflector assemblies 5 may be spaced around the periphery of structures 1 such as cabins and vehicles or along the sides of elongate structures such as bridges but preferably the deflector or deflector assembly 5 is continuous and robustly secured by brackets or other appropriate means to the structure 1. In use, should an explosive device E detonate beneath a structure 1 the strakes

7 act to deflect the ejecta along the conduit or conduits 8 towards one or opposite sides of the structure 1 and into the channel or channels 6 defined by the deflector or deflector assemblies 5. These, in turn, capture the blast ejecta and turn its trajectory from that governed by the geometry of the under-surface 3 to one close to in line with the structure's vertical axis. In practice, on horizontal ground this will generally mean from horizontal to vertical for flat-bottomed structures or from an angle to vertical for structures with a differently angled under-surface, for example a vehicle with a wedge- shaped under-surface. In turning the blast ejecta flow to near vertical, downward reaction forces are generated acting on the deflector or deflector assembly 5 that counteract the upward reaction forces previously generated by the interaction of the ejecta with the under-side 3 and lower regions of the structure 1. For example, for ejecta speeds in the region of 1000 to 2000 m/ s and structures such as vehicles between 2 and 3 m wide the time between generation of the upward and downward reactions will be approximately 1 ms or less, which is typically prior to a vehicle commencing any significant vertical movement owing to the blast.

When the blast ejecta are aligned by the strakes 7, high pressures may be generated in the corner formed by the interface between the strake 7 and the under- surface 3 of the structure 1. This pressure may act on the under-surface 3 and tend to increase the upward forces generated by the blast. It may therefore be advantageous to provide at least one vent 10 (see Fig. 5} through at least one or all of the strakes 7 in order to relieve pressures that may act locally on the under-surface 3 of the structure 1. Although only shown in the drawings in relation to the bridge shown in Fig. 5, it will be appreciated that vents 10 may also be provided in strakes 7 or webs 20, as described below, secured to a vehicle or any other structure in accordance with the invention.

The advantage of the use of strakes 7 in combination with a deflector or a plurality of deflectors 5 is that they take into account the fact that blast ejecta flows in all directions. The strakes 7 therefore channel the blast ejecta to one side or, dependent on the location of the blast relative to the structure 1, to opposing sides of the structure . This enables more of the blast ejec:a to be channelled into the channels 6 defined by the deflectors 5 with a view to maximizing the downward forces produced on the vehicle to counter the upward forces of the blast. This can be appreciated from the series of drawings forming Figs. 6A to 6F as will now be described in more detail.

In Figs. 6A to 6F the direction of ejecta flow is shown in solid arrows in relation to the under-surface 3 of a vehicle with wheels 1 . It will be appreciated that the same will apply for any structure. In Fig. 6A the under-surface 3 is flat, in Fig. 6B the under-surface 3 defines a blunt wedge, in Fig. 6C the under-surface 3 defines an acutely angled wedge and in Fig. 6D the under-surface 3 defines a blunt wedge but is also provided with transversely-extending strakes 7 in accordance with the present invention. Fig. 6E shows a vehicle with a flat under-surface 3 that is provided with deflectors 5 and transversely-extending and lengthwise-extending strakes 7. Fig. 6F is a schematic perspective view of the vehicle shown in Fig. 6E showing the direction of ejecta flow after deflection.

As shown in Fig. 6A, if an explosive device E is detonated beneath the vehicle, the ejecta will flow upwards to hit the under-surface 3 and then radially outwards, tending to skid on the under-surface 3. The ejecta that encounters the wheels 11, or in other structures equivalent obstacles will either become stagnated or will emerge from under the structure with less energy and potentially higher levels of turbulence as the ejecta spreads out in its travel from the detonation position. If the under-surface 3 is wedge-shaped as shown in Fig. 6B, the ejecta tends to be preferentially diverted to opposite sides of the wedge. As the wedge angle is increased, as shown in Fig. 6C, the ejecta will be more strongly directed to opposite sides of the structure, preferentially skidding up the wedge rather than flowing radially. This will cause a greater quantity of ejecta to engage with deflectors 5 mounted on the opposite sides of the vehicle and thereby increase the downward reaction generated as it flows through the channels 6 of the deflectors 5. However, it will be appreciated that strakes 7 secured to the under- surface 3, regardless of its geometry, will act to direct the ejecta away from obstacles such as the wheels 1 and to align and to channel it towards the deflectors 5. As shown in Fig. 6D the use of a series of strakes 7 on each side of a blunt wedge-shaped under- surface 3 between the wheels 11 (or other obstacles). The strakes 7 on each side define a series of open conduits 8 therebetween and act to redirect the ejecta so that it engages preferentially with deflectors 5 mounted on that side of the vehicle and at angles closer to 90° to side surfaces of the deflectors 5. This increases the downwards force generated by the deflectors 5. Similarly, as shown in Fig. 6E, the use of transversely- extending and lengthwise-extending strakes 7 with a flat under-surface 3 increases the quantity of ejecta engaging with the deflectors 5 such that most of the ejecta can be progressively aligned and channelled towards the deflectors 5 and thence upwards, as shown in Fig. 6F.

It will be appreciated that in most structures 1 preferably a plurality of strakes 7 are provided that are connected to and project downwardly from the under-surface 3. The strakes 7 are orientated such that they define a conduit 8 or a series of conduits 8 between them. The strakes 7 may be parallel to one another or define converging or diverging conduits 8 dependent on the geometry of the under-surface 3. In all cases, however, the conduits 8 are such that -Jhey direct ejecta directly into the channels 6 defined by an adjacent deflector 5 or plurality of deflectors 5. Figs. 3 and 5 show such arrangements of strakes 7 beneath a vehicle and a bridge respectively.

The strakes 7 project downwardly from the under-surface 3 of the structure and are preferably in the form of fins, although around vehicle wheels and other obstacles different profiles may be appropriate. Three different strake profiles are shown in Figs. 7A to 7C respectively. In Fig. 7A, the strake 7 comprises a basic plate that is connected by welds 12 to the under-surface 3. Fig. 7B shows a strake 7 that has a substantially triangular cross-sectional profile defining sides 13 that diverge from an apex 14 and that end via curving surfaces in a diickened base 15 which is connected to the under-surface 3 by welds 16. Fig. 7C shows a strake 7 that also has a similar substantially triangular cross-sectional profile but which is integrally formed with the under-surface 3.

The theoretical ideal orientation of the surface of a strake 7 is vertical, as shown in Fig. 7A. Such a strake creates reactions owing to ejecta interaction that are horizontal so that they do not contribute to vertical acceleration of the structure. However, the stresses caused in strakes at their interfaces with the under-surface by interaction with the ejecta are high. Consequentiy, the strakes 7 are preferably profiled, as shown in Figs. 7B and 7C to reduce stress concentrations. Some curvature at the interface between the strake 7 and the under-surface 3 (as provided by the base 15 or the integral joint) also tends to smooth ejecta interactions and avoid generating turbulence, which itself can reduce velocities and cause additional jump. Therefore, in a balanced design it may be necessary to depart from strakes that are vertical, as shown in Fig. 7A, so that ejecta velocity is maintained and the strakes 7 are robust enough to function without significant deflection or collapse.

Fig. 8 includes vector diagrams, with vectors labelled T or V dependent on whether the left or right side of the structure is impacted, for the upwardly moving elements of blast ejecta. The blast eject?, starts with an incoming velocity Vi, is turned by the strakes 7 (not shown in Fig. 8) and the under-surface 3 to a skid velocity Vs and then interacts with the curved deflectors 5 to leave the vehicle 1 with an outgoing velocity Vo. The vector diagrams make the simplifying assumption that there is low friction and turbulence in the ejecta fiow such that Vi, Vs and Vo are of similar magnitude. As the facets of the under-surface 3 of the structure 1 in Fig. 8 are substantially flat, the forces acting to turn each element of ejecta resolve naturally into directions perpendicular and parallel to the under-surface 3. Where surface friction is negligible, a structure 1 is only capable of reacting to an impacting flow of material with forces perpendicular to its surface. The interaction between the ejecta elements and the under-surface 3 gives rise to a velocity change 8V1 = Vi - Vs and a corresponding force Fl, which resolves into ejecta and under-surface 2 (belly) force components Fie and Fib respectively. The force Fib causes an equal and opposite reaction Rib which, combining reactions owing to the diverted flow of other elements of ejecta both spatially and temporally, attempts to accelerate the structure upwards. The ejecta element continues in the direction of velocity vector Vs until it engages with the deflector 5. The interaction between the ejecta and the deflector 5 give rise to a velocity change 6V2 = Vs - Vo, the forces acting on the ejecta element is shown in Fig. 8 as F2. The interaction of flow with the deflector 5 gives rise to a progressive application of forces to change the direction of flow around the curved contour of the deflector 5 and, in the absence of significant surface friction, these forces are at all times perpendicular to the surface of the deflector 5. The vector combination of all these progressive forces acting on the ejecta element is F2 and its equal and opposite reaction on the deflector 5 is R2. The reactions owing to all elements of ejecta turned by the deflectors 5 on the left and right of the structure combined temporally and spatially act to resist the forces from the interaction with the under-surface 3 and prevent the vehicle 1 from accelerating upwards.

If the structure 1 has a wedge-shaped under-surface 3, as shown in Fig. 8, a small residual combined vertical reaction on the vehicle Rev occurs owing to reaction with the sides of the wedge being perpendicular to the wedge facets. In reality, an armoured land vehicle utilizing under-surface wedge shaping is likely to have an under- surface 3 with multiple wedge angles, for example a blunt wedge between front and rear wheels and a more acutely-angled wedge between its wheel arches. The height and direction of ejecta flow around the perip ery of any given vehicle will depend to a great degree on the shape of its under-surface 3. Hence, the advantage of using a plurality of strakes 7 over the whole under-surface 3 of the vehicle that guide and channel the ejecta towards the deflectors 5. Deflectors 5 are therefore preferably positioned around the whole periphery of the vehicle 1 to intercept and orientate ejecta emerging from beneath the vehicle and turn it upwards to counteract vehicle jump.

The vector diagram shown within the structure 1 outlined in Fig. 8 show the reaction forces corresponding to left- and right-hand mirrored ejecta elements combined. These diagrams show that the resultant of all forces on the structures 1 integrated spatially and temporally is almost zero. Although some turbulence and internal friction will occur within the ejecta and reduce the velocity of flow, it can be seen that the deflectors 5 in their basic form will generate reactions to counteract the upwards acceleration caused by the ejecta impacting the under-surface 3 of the structure 1. One significant advantage of the invention is that it automatically compensates for explosive detonations offset to the sides or fore/aft of a structure's geometric centre. For most vehicle configurations a larger amount of ejecta will naturally flow towards that part of the vehicle periphery closest to the vertical centreline of the detonating device E, the upwards reaction forces therefore tend to be higher around this part of the periphery. This causes a slighdy lower ejecta velocity, kinetic energy and mass flow rate on the opposite side of the structure, resulting in lower reaction loads to this side. With no deflectors 5 fitted, the reactions driving the structure upwards are dissimilar from left to right, | Rbr | >> | Rbl | . In a vehicle, depending on the position of the vehicle's centre of gravity, this will cause a significant risk of vehicle roll. However, in a structure fitted with deflectors 5 in accordance with the invention, the downwards reactions are similar in magnitude, | R21 | = | Rlbl | and I R2r I ~ I Rlbr I . Again, depending on the position of a vehicle's centre of gravity the similarity of the magnitudes and opposite directions of these reactions will minimise the tendency of the vehicle to roll. Hence, by appropriate positioning and shaping of the strakes 7 and the deflectors 5 the tendency of a vehicle to roll can be minimised or eliminated for a range of detonation positions.

It will be appreciated that in practice the first deflector or deflectors 5 will be fabricated or moulded as deflector assemblies that are fitted together around the periphery of the structure 1 to be protected. In some embodiments, strakes and deflector assemblies may be integrally formed as single units. In other embodiments, strakes and deflector assemblies may be fabricated separately. Hence, existing structures can be upgraded by retro-fitment of strakes 7 and such deflector assemblies as well as incorporation of strakes and deflectors or deflector assemblies into new designs.

So far within this description of the invention the simplifying assumption has been made that the ejecta is turned whilst being subject to conservation of kinetic energy such that the magnitudes of the incoming, skid and outgoing velocity vectors, Vi, Vs and Vo, are roughly equal. However, die direction in which the ejecta is flowing when entering the channels 6 defined by the deflectors 5 is important. In practice an unconstrained ejecta stream will strike the wall of the channel 6 defining the deflector with vertical, transverse and lengthwise velocity components relative to the structure. During the stream's interaction with each of the deflectors 5 mounted at the left and right sides of the structure 1 such as the vehicle shown in Fig. 8, the transverse component of velocity is turned/converted to a vertical component but the lengthwise component remains unchanged, thereby reducing the magnitude of the downward reaction on the structure when compared to an outgoing ejecta stream of equal magnitude but with no lengthwise component.

Also, in a real system there will be some turbulence or buffeting between particles within an ejecta stream and its kinetic energy will reduce as it interacts with the structure. This reduction in kinetic energy causes a reduction in the magnitude of reactions R21 and R2r. This detrimental reduction in ejecta speed can be avoided by various modifications to the invention that are designed to reduce turbulence and avoid stagnation of the ejecta, as will now be described. These modifications also make use of or manage the lengthwise component of the velocity to increase the downward forces on the structure.

In order to smooth the path of the ejecta and to avoid interaction of the ejector with surface protuberances on the structure, which interactions would otherwise add to the reactions driving the structure upwards, the deflector or deflector assembly may comprise an inner deflector 7 with a smooth outer surface to smooth off projecting or sharp corners on the side walls 4 of the structure, as shown in Fig. 9. The channel 6 for the blast ejecta is therefore defined by an inner surface 18 of the outer deflector 5 and the outer surface 19 of the inner deflector 17. The transverse cross-sectional area of the channel 6 may vary along its length, for example it may increase or decrease at any point in the direction of ejecta flow. It is also possible for the transverse cross- sectional area of the channel 6 to remain the same but its transverse profile may change dependent on the way the deflector or deflector assembly is constructed and constrained by the exterior profile of the structure 1 to which it is fitted. Explosively driven water spouts are substantially incompressible whereas explosively driven dry loose sand has a significant compressible component. Hence, the shape of the channel 6 can be adapted according the nature of the structure 1. Largely incompressible ejecta flows can be accelerated by reducing the cross-sectional area of the channel 6 to produce a venturi. With compressible ejecta flows, the ejecta velocity can be increased by varying the transverse cross-secdonal area of the channel 6 along its length to produce a nozzle, for example a convergent-divergent nozzle, that can accelerate the flow at the expense of its pressure and internal energy in accordance with known nozzle theory. The inner deflector 17 may be fitted directly to the body 2 of the structure 1 or may be integrated into a deflector assembly as shown in Fig. 10. The inner deflector 1 may also be used to gradually reduce the cross-secdonal area of the channel 6 along the longitudinal axis of the channel 6 in the direcdon of ejecta flow to create a venturi effect, as shown in Fig. 9. The reduction in cross-sectional area may also be achieved in addition or in the alternative by the use of second deflectors in the form of webs 20 that form part of the deflector assembly. Such webs 20 extend substantially vertically and project outwards from the structure. They are advantageous for several reasons. First, they may be located between the opposing surfaces 18 and 19 of the channel 6 defined by the first deflector or deflector assembly 5 and thereby form strengthening and/or supporting substantially vertical webs between the inner and outer deflectors 17 and 5. Second, being substantially vertical they will interact with ejecta streams having lengthwise velocity components and ac: to deflect such impacting ejecta, turning the ejecta stream upwards parallel to their surface, in order to align the ejecta flow with or in a direction towards a longitudinal axis of the channel 6. Hence, the lengthwise, horizontal velocity component of such ejecta streams is converted to a vertical component thereby increasing the subsequent downward reaction on the deflectors 5. The provision of such webs 20 is therefore particularly important for this reason.

To achieve the reduction in cross-sectional area of a channel 6, the webs 20 may have a variable thickness along their length, for example being thinner at their base adjacent the channel inlet and thicker at the top adjacent the channel outlet. As elements of relatively incompressible ejecta at the front of the ejecta flow encounter this type of deflector assembly they are kept moving ahead in the same direction by the action of the elements behind them in the flow and are forced to accelerate, maintaining constant mass flow rate over a narrower cross section aperture by increasing their velocity. However, it should be appreciated that overly reducing the cross-sectional area of the channel 6 will create excessive back pressure and begin to slow the ejecta approaching the deflector 5 from below; therefore, there is a limit to how much ejecta acceleration be achieved by this method alone. Hence, in some embodiments it may be appropriate for the transverse cross-sectional area of the channel 6 to vary over its length, sometime reducing and sometime increasing as appropriate to achieve a smooth flow of ejecta through the channel 6.

In some embodiments, the webs 20 may form an integral part of a deflector assembly 5. However, in other embodiments one or two webs 20 they may be integrally formed with a strake 7 in a structure that is fitted around the under-surface 3 and partially up opposing side walls 4 of a structure 1. The inner and outer deflectors 17 and 5 may then be fitted and secured to these structures. It will also be appreciate that a plurality of strakes 7 and webs 20 may be connected to a base plate adapted to form the under-surface 3 as a 'false bottom' for the structure, for example for retro-fitment. Alternatively, the strakes 7 and webs 20 and may be integrated into the body of the structure 1 during its construction.

In other arrangements, an inclined web 20f (see Fig. 2) extending outwards from a structure 1 may be located above or alongside a deflector assembly 5 to intercept ejecta flows that have horizontal velocity components after passing through the channel 6 of a deflector or deflector assembly in order that these horizontal components can be turned into vertical components and generate downward reactions. Such an arrangement can be seen in Fig. 2 where the ejecta stream that emerges from the deflector assemblies 5f at the front of the vehicle may impact against the inclined webs 20f at the ends of the deflector assemblies closest to the front wheels of the vehicle.

In a modification as shown in Figs. 11 and 12, one or more additional first deflectors 5a, 5b, etc., are located in the channel 6. These deflectors 5a, 5b, etc. therefore form intermediate first deflectors as part of a deflector assembly. They have smooth inner and outer surfaces and are orientated substantially parallel to the opposite surfaces 18, 19 of the first deflector 5 defining the channel 6. This reduces turbulence during the turning of the ejecta and maintains ejecta speed, thereby keeping the outgoing velocity Vo high and maximising the vertical component of the downward reaction force R.2. These additional deflectors 5a, 5b may also be used to reduce the overall cross-sectional area of the channel 6 in a manner similar to that described above with reference to Figs. 9 and 10. Fig. 11 shows the intermediate deflectors 5a, 5b, etc., used in connection with a vehicle structure but they are also suitable for use with other structures as described above.

A further modification is the addition of one or more rows of guide vanes 21 that are located beneath and secured direcdy to the structure and/or the strakes 7 as shown in Figs. 11 and 12. The vanes 21 are located above a line of acceptable ground clearance and incorporate a leading edge 22 pointing downwards so that they act to intercept the ejecta in its upwards trajectory and progressively redirect it towards the deflectors 5 around the structure's periphery. The guide vanes 21 reduce turbulence in the ejecta flow, maximise ejecta velocity Vs and turn the ejecta progressively ensuring that the vertical components of reactions Rl are lower than for interactions with flat faceted under-surface geometry thereby reducing vertical jump. Also, the guide vanes 21 intercept the ejecta before it is intercepted by the under-surface 3 of the structure 1, thereby reducing the loads on the under-surface 3. By directing the ejecta so that it engages more reliably with the deflectors 5 the guide vanes 21 make the reaction caused by the deflectors 5 greater than if only a proportion of the ejecta engaged with the deflectors 5.

Yet another modification is the addition of a central ejecta splitter 23. This can be used regardless of the geometry of the under-surface 3. However, if the under- surface 3 is wedge shaped, the splitter 23 may be fitted to the apex of the wedge. Preferably, the splitter 23 comprises a downwardly directed fin 24 and reduces turbulence that would otherwise be caused by ejecta hitting a horizontal flat under- surface or the locally flat surface at the curved apex of a typical wedge in the centre of a vehicle. At this point on such a curved apex, the under-surface 3 is approximately flat and high forces can be generated that may breech the body 2 at this point. The splitter 23 reinforces the apex of the wedge znd is curved on both sides of the fin 24 to progressively turn the central portion of the ejecta to flow towards the deflectors 5 on either side of the structure. This considerably reduces local vertical reactions at the apex of the wedge and avoids ejecta deceleration due to turbulence. The detailed configuration of the splitter 23 will deoend on a particular structure's geometry and construction but the splitter 23 should be shaped to fit to the central portion of the under-surface 3 with curved outer faces defining the fin 24 to encourage smooth ejecta flow with minimum loss of ejecta kinetic energy. Such a splitter 23 can be readily manufactured and may be formed as part of an integrated structure of strakes 7 and webs 20 or configured to key into the facets of a pre-existing wedge to thereby act as a reinforcing spine.

In some embodiments, in particular those for use with vehicles, all of the aforementioned features of the present invention may be incorporated as part of an armour system for the vehicle and configured to protect the vehicle from ballistic attack. In these cases, the various elements of the invention may be constructed from known armour laminates to intercept and degrade the effectiveness of mcoming projectiles prior to their impact on the main structure of the vehicle. As indicated above, in addition to or as an alternative to the second deflectors the deflecting means may comprises at least one ejecta accelerator 25 that is adapted to entrain blast ejecta and to align its flow with or in a direction towards the longitudinal axis of the channel 6. In some cases such an ejecta accelerator 25 may also increase the magnitude of the downwards reactions generated by the deflector 5 by imparting additional kinetic energy to the ejecta flow thereby increasing the velocity of the ejecta. This increase should preferably take place after the ejecta has been turned by the under- surface 3 or guide vanes 21 and during the early stages of the ejecta's interaction with the deflector 5. Acceleration of the ejecta towards the latter stages of its interaction with the deflectors 5 will also cause increased downward reactions but is less efficient than acceleration prior to or in the early moments of the interaction.

Figs 13a and 13b show two possible configurations of ejecta accelerators 25, the purpose of which is to push or to entrain the ejecta in higher velocity flows originating from the accelerator 25 thereby increasing the ejecta's velocity and turning its direction of flow towards the longitudinal axis of the channel 6. In Fig. 13a, the accelerator 25 is aligned with the longitudinal axis of the channel 6. However, in Fig. 13b the accelerator 25 is mounted horizontally and therefore generates no vertical component of thrust. A plurality of such accelerators 25 may be mounted on the body 2 of the structure 1 or to the deflector or deflector assemblies 5 a: appropriate positions around the periphery of the structure 1 and at locations adjacent the inlet to the channels 6, as shown in Fig 13c. Each accelerator 25 is adapted to generate high velocity flows of matter using any suitable means, for example by using pneumatic, pyrotechnic, electromechanical, electromagnetic, explosive or chemical means. The flow generated can be of any form that will efficiently blend with the ejecta flow and not hinder the interaction of the ejecta with the deflector or deflector assembly 5 or damage die deflector or deflector assembly 5. Such flows may be particulate, liquid, gelatinous, gaseous or combinations thereof, for example the products of combustion.

Preferably, the accelerator 25 is activated by a trigger mechanism linked to a sensor 26 that detects the presence of a high energy flow of ejecta in the vicinity of the under-surface of the structure 1. Each accelerator 24 may incorporate its own sensor 26, as shown in Fig. 3a, or a remotely-mounted sensor 26 may be linked to the trigger mechanism of the accelerator 25, as shewn in Fig. 13b. It may be convenient to link a plurality of ejecta accelerators 25 to a single sensor 26. Alternatively, a plurality of sensors 26, typically two sensors 26, linked via an AND logic gate may be connected to the trigger mechanism of a single accelerator 25, as shown in Fig. 3c. However, it is not necessary to provide a control system for the accelerators 25 as the sensor 26 only needs to detect the presence of a potentially damaging ejecta flow to trigger the accelerator 25 to which it is linked. Hence, a preferred location of a sensor 26 is close to the under-surface 3 of the structure 1 so as to intercept the flow of ejecta prior to its interaction with the deflectors 5. With particular reference to the arrangement shown in Fig. 13c, here the accelerators 25 are horizontally mounted below or embedded within cavities defined by the under-surface 3 of the structure 1 in order to deflect the ejecta flow outwards before it engages with the under-surface 3. The ejecta is turned in accordance with conventional fluid dynamics theory by combination of entrainment, inducement and Coanda effects generated by a combination of the ejecta flow and a high energy stream generated by the accelerator 25. There is therefore a benefit in terms of upstream and downstream flow changes, of both velocity vector magnitude and direction. Also, use of one or more accelerator 25 has several beneficial effects, as follows.

1. An increased downstream blast ejecta velocity, increasing downwards thrust at the deflector 5.

With horizontally mounted accelerators 25, an increased turn angle of accelerator efflux and blast ejecta components of flow, causing the deflector to generate increased downward thrust.

An upstream blast ejecta flow redirection away from the under-surface 3 owing to entrainment/inducement effects, reducing upwards thrust on the under- surface 3.

Any form of sensor 26 capable of detecting the flow of blast ejecta is suitable for use with an accelerator 25, for example electromechanical pressure/impact or mechanical and impact actuated pyrotechnic sensors could be used. In this regard, it is important to appreciate that during use of armoured vehicles shock events are common owing to crashes into obstacles, landing after driving over banks or high steps, weapon firing or strikes by a large variety of different high speed, high power projectiles. Equipment in armoured vehicles is designed to survive the forces produced by such shocks as a matter of routine. Hence, the sensor 26 is not intended to react to structural acceleration or air shock which would be likely to give rise to significant false alarms; rather the sensor 26 is intended to react to impact by ejecta flow so that the sensor 26 needs to be located in a position where such flow can be that intercepted. Also, to guard against impact by earth thrown up by the vehicle itself, the sensor 26 should be designed to operate only when impacted by a predetermined threshold level of impulse over a predetermined time period. Impulse is the change in linear momentum of a body and may be defined or calculated as the integral of the resultant force with respect to time, being generally equal to change in the momentum of a body subject to the resultant force. The impulse on a sensor 26 placed in an ejecta flow will depend on the time varying flow momentum, the temporal profile of which will be different for different types of flow. This can be used to differentiate between low level non- damaging flows and high level damaging flows. A slow non-damaging flow could have the same overall impulse as a very fast damaging flow. Also, a very high momentum flow with exceptionally short duration may not have enough time to deform or accelerate the structure and cause damage, whereas a slightly longer duration flow with the same impulse may do, as is well known in the field of dynamic response of structures to varying duration shock pulses. Hence, preferably the sensor 26 is adapted to operate when impacted by a predetermined threshold level of impulse calculated over a predetermined time period. These predetermined values can be determined dependent on the level of impulse that the structure can withstand without significant damage occurring, for example heavily armoured vehicles, such as tanks, can withstand a greater level of impulse than smaller, more lightly armoured vehicles. The response of the sensor can be controlled by adjusting its vibrational characteristic, such as stiffness, Young's modulus and its inertia, to enable it to distinguish between non-damaging and damaging ejecta streams.

A first embodiment of accelerator 25a is shown in Fig. 14a. This is a rocket- motor based ejecta accelerator and comprises a tube of a solid state propellant 27 at the end of which is a nozzle 28. The sensor 26a is integrated with the accelerator 25a and comprises a strike plate 29 with an integral firing pin 30 that is arranged to impact a primer cap 31 or similar pyrotechnic device and ignite it to fire the propellant 27. The strike plate 29 is adapted to operate at a pre-determined threshold level of impulse.

A second embodiment of accelerator 25b is shown in Fig. 14b. Here the sensor 26b is separate from the main body of the accelerator 25b, which is a water-shot gun based ejecta accelerator that comprises a propellant 32 and a thin shell water-shot canister 33. The sensor 26b is linked to an electrical power source, for example a vehicle's battery, via circuitry 34 and comprises a strike plate 35 that is arranged to impact the electrical contacts 36 of a trigger unit 37 that is powered by the power source. The strike plate 35 therefore acts as a switch. Once the strike plate 35 makes contact with the contacts 36 the circuit is energized and the trigger unit 37 activates an electrical primer 38 of the remote accelerator 25b to ignite the propellant 32 and thereby fire the water-shot canister, which breaks open to shoot water droplets at high speed into the blast ejecta. The strike plate 35 is again designed to operate at a predetermined threshold level of impulse.

A third embodiment of accelerator 25c is shown in Fig. 14c. This is similar in construction to that shown and described above with reference to Fig. 14b with a sensor 26c separate from the main body of the accelerator 25c but it differs by replacing the trigger unit 37 with a piezoelectric crystal 39 that will produce a current at a predetermined level of deformation. Hence, no independent power source is required.

It will be appreciated that other embodiments of accelerator are possible. For example accelerators generating high aspect ratio streams of efflux to enhance a Coanda effect in the ejecta may be used.

The firing of the accelerator 25 quickly with minimal delay is important. Techniques for achieving a fast firing are known and well developed in the fields of gun system internal ballistics, design of timed burst explosive ammunition and other time critical complementary fields. For example, firing mechanisms comprising exploding wire detonators for explosives and fast acting electric primers for gun systems may be used.

Arrays of sensors 26 and accelerators 25 may be arranged around the periphery of a structure 1 and set up to operate at different actuation thresholds. The advantage of such a configuration is that if, for example, an explosive device is detonated under one side of a structure the ejecta would predominantly flow and have more energy to that side so that the accelerators 25 on that side of the structure 1 would tend to actuate in preference to the accelerators 25 on the opposite side of the structure. In this way the reactions generated in the deflectors 5 would be higher on the side of the structure 1 subject to the highest upwards reactions caused by interaction of the blast ejecta with under-surface 3 of the structure 1. Also, if a low level explosive device is detonated under the structure 1 only those accelerators 25 set up with a low level threshold would fire, whereas all accelerators 25 may be made to fire for high level explosive devices. The use of arrays of accelerators 25 spaced around the structure's periphery and set at different actuation thresholds therefore provides a proportionate acceleration of the ejecta into and through the deflectors 5, both from side to side and front to back, for different sizes of explosive device.

The arrangement uses the intensity of ejecta flow and multiple simple trigger mechanisms to determine its response, rather than sensor signals fed through a complex control system that would tend to be less reliable. This arrangement also has a high level of redundancy and damage tolerance compared to an arrangement fed through a single central control system.

The combination of deflectors 5, strakes 7, webs 20 and accelerators 25 is a hybrid system, containing passive and active mechanisms to respond to an attack by an explosive device. However, different structures will interact with ejecta differendy and in certain positions around a structure's periphery the ejecta flow will tend to be weak necessitating the addition of accelerators to maintain high ejecta flow rates through the deflectors 5. In the extreme, around the periphery of some structures the intensity of ejecta flow may be so low that no useful downwards reaction can be generated using deflectors. This may be the case for certain designs of flat-bottomed tracked vehicles fitted with panniers, for example. In other cases, the attachment of strakes 7 and a deflector or deflector assembly 5 in certain places around a structure, for example adjacent the wheel-arches of a vehicle, may not be practical. In such circumstances in addition to the use of deflectors or deflector assemblies 5 at other locations around the periphery of the structure 1 , at least one thruster 40 adapted to counter the force of a blast may be secured to a side surface or to an upper surface of the structure 1 and adapted to be activated by a thruster trigger mechanism linked to a thruster sensor 41 that detects the presence of a flow of electa in the vicinity of the under-surface of the structure 1. Such a thruster 40 and thruster sensor 41 may be constructed identically to the accelerators 25 and sensors 26 described above but they may have a larger energy output. The same type of integrated or separately mounted sensors 26 that react to local ejecta flow impacting on the sensors 26 can be used. Hence, a single thruster sensor 41 may be linked to the trigger mechanisms of a plurality of thrusters 40 or a plurality of thruster sensors 41 may be linked to the trigger mechanism of a single thruster 40. Such arrangements are intended to be self-stabilising and proportionate depending on the size and position of the device E detonated under the vehicle. Fig. 15 shows two examples of thrusters 40 fitted to the side surfaces 4 of a vehicle 1 to counteract reactions acting through the under-surface 3 of the vehicle 1 in positions where there are no deflectors 5 fitted. On the left side of the body 2 shown in Fig. 15 a sensor 41 is shown placed in the path of an ejecta stream, whereas on the right side of the body 2 shown in Fig. 5 the sensor 41 is shrouded and fed with samples from the ejecta flow via an ejecta scoop 42 that is fitted to the sensor 41. The use of the scoop 42 has the advantage that the sensor 41 is only subject to impacts coming directly from the underside of the vehicle and is therefore less prone to false alarms from random impacts.

Such a scoop 42 may also be used in combination with the sensor 26 of an accelerator 25 and has the same advantage as with a thruster 40.

It should be appreciated that it is not necessary to provide a control system for the thrusters 40 as the thruster sensors 41 are adapted to detect the presence of a potentially damaging ejecta flow to trigger the thruster or thrusters 40 to which it is linked direcdy.

As above, Fig. 15 includes vector diagrams, with vectors labelled T or Y dependent on whether the left or right side of the vehicle 1 is impacted, for the upwardly moving elements of blast ejecta. In the locations around the structure where the thrusters 40 are fitted, the blast ejecta start with an incoming velocity Vi and are then turned by the under-surface 3 of the vehicle 1 to an outgoing velocity Vo. The vector diagrams again make the simplifying assumption that there is low friction and turbulence in the ejecta flow such that Vi and Vo are of similar magnitude. The interaction between the blast ejecta elements and the under-surface 3 gives rise to a velocity change 8V1 = Vi - Vo and a corresponding force F, which resolves into ejecta and under-surface 3 (belly) force components Fe and Fb respectively. The force Fb causes an equal and opposite reaction Rb which, combining reactions owing to the diverted flow of other elements of ejecta both spatially and temporally, attempts to accelerate the structure upwards. The vector combination of the left-hand and right- hand blast ejecta is shown by the vector diagram in the body 2 of the vehicle 1.

Again, in Fig. 15 the vector diagram shown within the vehicle outline shows the reaction forces corresponding to left and right hand mirrored ejecta elements combined with the thruster forces. The result of all forces on the vehicle 1 integrated spatially and temporally is nominally zero. However, some turbulence and internal friction will again occur within the ejecta and reduce the velocity of flow but it can be seen that the thrusters 29 will generate reactions to counteract the upwards acceleration caused by the ejecta impacting the under-surface 3 of the vehicle 1.

Overall, the present invention provides a structure, for example a portable cabin, a temporary building, a vehicle, and a bridge, that may be subjected to a blast originating beneath or closely adjacent to it with a passive means of generating reaction forces to counteract upward forces owing to blast landmines or other similar explosive devices. In some embodiments a passive means of generating reaction forces may be combined with active ejecta accelerators 25 and/or thrusters 40, as described above, to provide a hybrid system. Unlike the prior art, in particular WO2011/075174, the present invention takes into account that blast ejecta flows in all directions by providing a deflecting means such as second deflectors 7, 20 or accelerators 25. The direction in which blast ejecta is flowing when entering the channels 6 of the first deflectors 5 is important. For example, a blast ejecta stream's interaction with the side-mounted first deflectors 5 converts transverse components of velocity to vertical components but its lengthwise component remains unchanged, thereby reducing the magnitude of the downward reaction on the structure when compared to an outgoing ejecta stream of equal magnitude but with no lengthwise component. The use of a deflecting means such as strakes 7 and webs 20 and/ or accelerators 25 to deflect and/ or entraining blast ejecta and to align its flow with or in a direction towards the longitudinal axis of the channel defined by the first deflector 5 converts lengthwise velocity components into ones that can be acted on by the first deflector and converted to vertical components thereby increasing downward reactions.

Claims

A structure adapted to withstand blast forces from an explosion beneath the structure, the structure defining an under-surface and one or more side surfaces and comprising

A structure as claimed in Claim 1, wherein the channel defined by the first deflector is defined by a surface or surfaces that is or are progressively contoured from an orientation substantially parallel to said under-surface of the structure to a substantially vertical orientation opposite a side surface of the structure.

A structure as claimed in Claim 1 or Claim 2, wherein said surface or surfaces of the first deflector are progressively curved.

A structure as claimed in any of Claims 1 to 3, wherein the first deflector forms part of a deflector assembly and the channel is defined between opposing surfaces of the deflector assembly.

A structure as claimed in any of Claims 1 to 4, wherein a plurality of first deflectors is located around the periphery or along peripheral sides of the structure.

6. A structure as claimed in any of Claims 1 to 5, wherein the deflecting means comprises at least one second deflector that extends at an angle to said first deflector to deflect impacting blast ejecta and thereby align its flow with or in a direction towards the longitudinal axis of the channel.

A structure as claimed in Claim 6, wherein at least one of the second deflectors comprises at least one downwardly projecting strake located beneath the under- surface or adjacent a lower end of the channel to direct blast ejecta into said channel.

8. A structure as claimed in Claim 7, wherein at least two projecting downwardly strakes are located beneath the under-surface such that they define a conduit to direct blast ejecta along the conduit and into said channel.

A structure as claimed in Claim 7 or Claim 8, wherein the strake or strakes directs or direct said blast ejecta towards opposite side surfaces of the structure.

A structure as claimed in any of Claims 7 to 9, wherein a plurality of strakes are located beneath the structure and define a plurality of conduits that are each adapted to direct blast ejecta towards opposite side surfaces of the structure and into the channel or channels defined by one or more deflectors.

A structure as claimed in any of Claims 7 to 10, wherein one or more of the strakes comprises a fin with a substantially triangular cross-sectional profile.

A structure as claimed in any of Claims 7 to 11 , wherein at least one strake is integrally formed with the under-surface of the structure.

A structure as claimed in any of Claims 7 to 12, wherein at least one vent is provided through at least one strake.

14. A structure as claimed in any of Claims 7 to 13 when dependent on Claim 4, wherein at least one strake is integrally formed with at least part of one of the deflector assemblies.

A structure as claimed in any of Claims 1 to 14, wherein at least one of the second deflectors comprises one or more webs that extend substantially vertically and project outwards from the structure within the channel defined by at least one first deflector.

A structure as claimed in Claim 15 when dependent on Claim 4, wherein the webs are located between said opposing surfaces defined by the deflector assembly.

A structure as claimed in Claim 15 or Claim 16 when dependent on any of Claims 7 to 14, wherein at least one web is integrally formed with at least one strake.

A structure as claimed in any of Claims 15 to 17 when dependent on any of Claims 7 to 14, wherein a plurality of strakes and webs are connected to a base plate adapted to form the under-surface of the structure.

A structure as claimed in any of Claims 1 to 18, wherein the cross-sectional area of the channel defined by at least one first deflector increases or decreases in the direction of ejecta flow.

A structure as claimed in any of Claims 1 to 19, wherein at least one intermediate deflector is located in the channel defined by at least one first deflector and defines surfaces orientated substantially parallel to a surface of the first deflector defining said channel.

A structure as claimed in any of Claims 1 to 20, wherein at least one guide vane is located beneath the structure and is adapted to direct blast ejecta towards the channel defined by at least one of the first deflectors.

A structure as claimed in any of Claims 1 to 21, wherein a splitter is provided beneath the structure and is adapted to direct blast ejecta towards opposite sides of the structure.

23. A structure as claimed in Claim 22, wherein the under-surface of the structure defines a wedge-shape with the apex of the wedge pointing downwards and the splitter is secured to the apex of the wedge.

24. A structure as claimed in Claim 22 or Claim 23, wherein the splitter comprises a downwardly directed fin.

25. A structure as claimed in any of Claims 1 to 24, wherein the deflecting means comprises at least one ejecta accelerator that is adapted to entrain blast ejecta and to align its flow with or in a direction towards the longitudinal axis of the channel.

26. A structure as claimed in Claim 25, wherein the or each ejecta accelerator is activated by a trigger mechanism linked to a sensor that detects the presence of a flow of ejecta in the vicinity of the under-surface of the structure.

27. A structure as claimed in Claim 25 or Claim 26, wherein one sensor is linked to the trigger mechanisms of a plurality of ejecta accelerators or a plurality of sensors are linked to the trigger mechanism of a single ejecta accelerator.

28. A structure as claimed in Claim 27, wherein two or more of the sensors are linked to the trigger mechanism of a single ejecta accelerator via an AND logic gate.

29. A structure as claimed in any of Claims 25 to 28, wherein at least one ejecta accelerator is horizontally mounted below or embedded within a cavity defined by the under-surface of the structure.

30. A structure as claimed in any of Claims 25 to 29, wherein the or each sensor comprises a strike plate adapted to operate at a predetermined threshold level of impulse.

31. A structure as claimed in any of Claims 25 to 30, wherein the or each accelerator is either a rocket motor based accelerator or is a water-shot gun based accelerator.

32. A structure as claimed in any of Claims 25 to 31 , wherein the or each ejecta accelerator is located adjacent an inlet of the channel.

33. A structure as claimed in any of Claims 26 to 32, wherein the sensor is shrouded and is fed with samples from the ejecta flow via an ejecta scoop.

34. A structure as claimed in any of Claims 1 to 33, wherein at least one thruster adapted to counter the force of a blast is secured to a side or to an upper surface of the structure and is activated by a thruster trigger mechanism linked to a thruster sensor that detects the presence of a flow of ejecta in the vicinity of the under-surface of the structure.

35. A structure as claimed in Claim 34, wherein the thruster sensor is shrouded and is fed with samples from the ejecta flow via an ejecta scoop.

36. A structure as claimed in Claim 34 or Claim 35, wherein a plurality of thrusters is arranged around the periphery of the structure and set up such that each thruster operates at its own individual actuation threshold.

37. A structure as claimed in any of Claims 34 to 36, wherein the or each thruster is activated by a thruster sensor located remotely from said thruster.

38. A structure as claimed in Claims 37, wherein one thruster sensor is linked to the trigger mechanisms of more than one thruster.

39. A structure as claimed in Claim 38, wherein a plurality of sensors is linked to the trigger mechanisms of one thruster.

40. A structure as claimed in any of Claims 1 to 39, comprising a portable or temporary building, a bridge or a vehicle.