Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A BARRIER
This invention relates to a barrier and more particularly to a barrier
suitable for
mitigating the effects of an explosion.
It is known that if an explosive device is detonated, it can detonate another
explosive
device nearby through the effect of the blast wave generated, and/or by the
impact of
fragments of the exploded device. In a situation where there are maiiy
explosives or
explosive devices within a close proximity of one another, it is essential to
protect as
many items as possible from the risk of being detonated by a nearby explosion.
When an explosive device is detonated there are three principle mechanisms by
which
another device may be caused to detonate, termed a sympathetic explosion.
Firstly,
the blast wave caused by the explosion of a first device can impact a nearby
device
with enough force to detonate it or at least damage it. However, the blast
wave
degrades quickly so the risk of devices being detonated falls off quickly with
distance.
Secondly, the fragments of the exploded device and any other items nearby can
become projectiles which radiate out from the device. These fragments, termed
incident material, can impact a device with enough energy to detonate the
device.
The incident material can also cause damage to the device which could render
it
unstable and vulnerable to detonation. Thirdly, the blast wave can lift and/or
carry
objects in its path which then themselves become projectiles which can damage
or
detonate other devices.
An explosive device can be anything from a simple firework to a hi-tech
military
missile or bomb which contains and explosive material. The detonation of such
an
explosive device is caused by a shockwave propagating through the explosive
material contained within a device. The explosive force is released in a
direction
perpenaicuiar to the surface of the explosive which is often shaped to follow
the
internal contours of the device.
Where the explosive device is a bomb, the bomb typically has a casing which
has a
first end and a second end distal from the first end. The first end is a base
plate and
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the second end is often shaped to improve the aerodynamics of the bomb. The
casing
contains an explosive material.
Fragments from a 10001b (454.55kg) bomb can be expected to travel side on to a
radius of 1.3km and end on (from the base plate) to a radius of 1.65km in an
unmitigated event, i.e. an explosion where no barrier is used. As can be
expected, it is
desirable to prevent the fragments travelling such a distance.
Previous trials for mitigation of such 10001b (454.55kg) bombs have shown that
a
second such explosive device, hereinafter termed an acceptor bomb, can be
initiated
by penetrating high velocity fragments from the detonated explosive device,
hereinafter termed the donor bomb, from incident material or from the impact
of the
subsequently driven mitigation barrier. So, it is necessary to slow down the
incident
material to a speed where if they do contact an explosive device it is not
detonated. A
review of current fragment attack literature indicated a fragment strike
velocity in the
region of 600m1s (7.4kJ energy) as the threshold below which no sympathetic
explosion was anticipated for any donor bomb. For experiments using a 1mm
thick
aluminium shield around the explosives, the threshold appeared to rise to
800m/s
(13kJ). Thus for all subsequent research, the inventors have used the region
of 600 -
800m/s as a target to which the barriers must retard the velocity of the
incident
material.
It is known to use a barrier in a situation where explosives are stored or
where there is
the possibility that an explosion could occur. Such barriers may be reactive,
for
example, reactive armour, or passive, which do not have active components.
Concrete has been employed in the past to make a passive barrier to withstand
the
destructive force of an explosion, such as the detonation of a bomb. However,
barriers made from concrete take time to construct and once constructed are
perrnanent. in a conflict situation, for example, it is required that
explosive devices,
such as missiles, are moved around and therefore the concrete baiTiers which
have
been built become disused and further barriers need to be constructed
elsewhere. It is
evident that this practice requires much time and material. One solution to
this
problem has been to use water filled barriers. Water is well known in the art
for
mitigating blasts. A water filled barrier may comprise one or more containers
filled
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with water which is/are placed between the explosive device(s) and the item(s)
to be
protected. The water filled barrier overconies the previous problem as the
barrier can
be removed after use. However, the barrier needs to be erected where there is
an
adequate water supply. In an area where a water supply is poor the water to
fill the
containers needs to be transported. The barriers are often bulky which can
pose
transportation problems and add to the cost of using them. Also, the act of
filling the
containers with water will take time, as will emptying them. Once the water
barrier is
in place it will prevent shock waves and particulates from detonating nearby
explosives by acting as a shield. However, there is the possibility that the
resulting
blast and shock waves can move this baiTier and cause it to impact the item
being
protected which could result in physical damage to the item which in turn
could cause
it to detonate or become vulnerable to detonation.
It is recommended to have a minimum stand off distance of at least 1 m between
the
donor bomb and any barriers to ensure that weapon fragments strike the barrier
before
the barrier is disrupted by the blast shock. This is the case for the
composite barrier
disclosed in patent application no. PCT/GB2006/004722. The barrier in this
application was invented by the same inventors as the present barrier. It
comprises a
portable barrier which is capable of disintegrating harmlessly in the course
of its
effectiveness. This barrier is suitable for use where the stand-off is about 1
metre or
more and has been proven in trials to mitigate against the effect of
detonations of
various military munitions. With this barrier the fragments of the bomb
overtake the
shockwave where the distance between the bomb and barrier is greater than or
equal
to 1 metre, so the barrier addresses the fragments before the shock wave
disrupts the
barrier. However, when the distance between the barrier and the explosion is
less
than 1 metre, the barrier has to deal with the shock wave first which could,
in some
instances, begin to destroy or disrupt the barrier, thus rendering the item to
be
protected vulnerable to impact by the blast wave and any incident material.
Also,
where the distance between the barrier and explosion is less than 1 metre, the
barrier
itself can present an impact threat to the item to be protected.
The distance between the bombs under a typical military fighter aircraft is
less than 1
metre, and is typically only about 0.78m so a different barrier needs to be
used to
protect these devices.
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It is an object of the present invention to provide a barrier which is simple,
cost
effective, quick and easy to put in place and remove, which is small,
relatively light
and portable and which provides adequate shielding from an explosion, even at
small
stand-off distances where the blast wave arrives either prior to or
simultaneous with
the incident material and which does not itself become a projectile capable of
detonating an explosive device in the event of a blast.
Accordingly, the invention provides a passive barrier for mitigating blast
waves and
decelerating incident material to reduce the risk of sympathetic explosions at
reduced
stand-off distances, which is configured to redirect the force of an incident
blast wave
and reduce the momentum of any incident material wherein the baiTier is
constructed
of a shock absorbing foam material.
The barrier according to the invention will fragment which means that as the
barrier
moves under the force of the blast, it disintegrates and will break up into
smaller
pieces which are not heavy enough to cause detonation of a nearby device as
they do
not have a mass great enough to cause damage if they impact a device. The
barrier
can be designed to fully arrest incident material if the sensitivity of the
acceptor
device requires. Otherwise, it is not always necessary for the barrier to
fully arrest the
projectiles, but to retard the velocity of the projectiles to such an extent
that they no
longer present a sympathetic explosion threat and therefore reduce the
likelihood of a
maximum credible event.
The barrier according to the present invention can, for example, be used under
aircraft
where the distance between the explosive devices is less than 1 metre. Due to
the
preferred material used to construct the barrier, the barrier is able to
disintegrate
quickly and therefore can be used where the distance between the barrier and
the
zxplosive device is less than 1 metre.
The barrier is advantageously constructed of a polyurethane foam. More
advantageously, the foam is a rigid polyurethane foam and is even more
advantageously selected from the Last-a-Foam O FR3700 series produced by
General
Plastics for packaging purposes which are closed cell rigid polyurethane
foams. The
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density of the foam chosen for a barrier is dependent upon the explosive
charge it is to
protect against.
The barrier can be any size depending upon its desired use. For example, it is
advantageous to have a barrier which is the same size as or bigger than the
device to
be protected or to be protected against so as to ensure that as much of the
device(s)/items are protected. Also, if the barrier is bigger than the item to
be
protected then accurate placement of the barrier may not be so crucial. This
will of
course be advantageous where there is little time to ensure accurate
positioning, such
as in a conflict situation or where a threat has only just been identified.
However, the
barrier can be smaller than the explosive device so long as it protects the
assessed
vulnerable area.
The shape of the barrier also depends upon its intended use. For example, in
the
preferred use the barrier is used to protect one bomb from the other under a
typical
military fighter aircraft as it is not known which, if either, of the bombs is
likely to
detonate. In this situation the sides of the barrier which are most proximate
to the
bombs are configured to redirect the force of an incident blast wave. In a
situation
where an explosive device is to be placed so as to protect a non-explosive
device or
object, such as a vehicle or a person, it is only necessary for the barrier to
be
configured to redirect the force of an incident blast wave on the side
proximate to the
explosive device. In either of these embodiments, the barrier can be
configured to
redirect the force of the blast wave by angling the sides proximate to the
items to be
protected. For example, if it is required to redirect the blast wave above and
below
the barrier to the same extent, then an apex is formed which aligns with the
central
line of the device to be protected. If the blast wave needs to be angled up or
down
relative to the explosive device, then the side of the barrier proximate to
the device
slopes out or in respectivelv from the top of the barrier. The side of the
barrier distal
to the explosive device may be of any shape such as planar, curved or angled.
Shaping the barrier in this way ensures that the blast wave is deflected
around the
barrier so as to reduce the blast loading on it, thereby reducing the velocity
by which
the mass of the barrier is driven at the item(s) to be protected and
consequently
reduces the likelihood of substantial damage to the acceptor munitions at very
low
standoff distances.
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The baiTier is advantageously substantially diamond shape in the cross section
perpendicular to a first explosive device. The exterior of the barrier forming
the
diamond cross section can be planar, concave, convex or any other
configuration.
Most advantageously, the barrier is shaped to have all four internal angles at
about
90 . The diamond shape is thought to redirect the blast wave from the
explosion
around the barrier. The loading of the blast wave is deflected from the apex
and along
the barrier so that the pressure of the wave is spread over a greater area
than would be
the case with a flat surface. As much of the blast force is deflected there is
less force
left to move the barrier than with a barrier having a flat surface. It is also
believed,
although the inventors do not wish to be bound by theory, that the diamond
shape
causes the fragments of the exploded device to move at different velocities
and hence
the fragments and barrier do not move together at the same speed.
The barrier will be of particular use in situations where there are reduced
standoff
distances e.g. under a typical military fighter aircraft. The barrier may be
suspended
from the central pylon of the aircraft to locate it centrally between the two
bombs.
Alternatively, the barrier may be mounted or supported in a frame, holder or
on
wheels whereby it can be manoeuvred into place. Preferably, the barrier is
mounted
or supported in a frame which is capable of being manoeuvred into place
quickly and
easily. The frame may have height adjusting means so that it may fit under an
empty
or laden aircraft such that the barrier itself is aligned with the explosive
device or item
to be protected. This may be achieved by spring loading the frame or by
jacking or
other such height adjusting means. The frame may further comprise braking or
choking means for maintaining it in place. The frame can be made of any
suitable
material for use in a number of environmental conditions. The barrier may also
be
used to protect weapons on differeiit aircraft. Again, the barrier can be
placed where
it is needed.
[n another embodiment, the barrier may have a casing to support it and to
provide
protection from minor impact damage. It is further advantageous that the
casing is
water tight. The casing may enclose the entirety of the barrier or may
partially
surround the barrier to afford it some protection from being handled, the
environment
in which it is used and the weather. The casing may fit closely around the
barrier or
there may be an airgap formed between the casing and the barrier, as is
evident with
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the square configuration shown in figure 4. It has been shown that if a small
airgap
exists between the barrier and the outer casing there is no effect on the
performance of
the barrier. A preferred material for the casing is polyethylene but any
suitable
material can be used. Further, it is advantageous that the polyethylene is
about 10mm
thick. The barrier casing may include stiffening means which can support the
barrier.
The barrier with casing may be mounted or supported in a frame or holder as
previously described.
Although the invention can be employed in close proximity to explosives and
explosive devices such as bombs and missiles, it is not limited to such use.
The
barrier can be placed between torpedoes in a submarine, between stationary
vehicles
carrying explosive devices or in civil or military explosives stores. The
barrier can
also be used in a vehicle for transporting and/or storing explosives. The
barrier can
further be used to surround a vehicle or vehicles to afford them increased
protection.
The barrier could also be used for packaging explosives or explosive devices
for
transportation and storage. These uses are purely for illustration and do not
restrict
the scope of use of the invention.
The invention will now be described with reference to the accompanying
figures:
Fig. 1 is an end on view of a barrier according to the present invention
suspended
from the central pylon of an aeroplane.
Fig. 2 is a plan view of the barrier positioned between two bombs.
Fig. 3 shows the barrier of the invention with a casing
Fig. 4 shows the barrier of the invention with a square casing
Figs. 5a - 5d show the barrier in different configurations for directing the
blast wave.
Fig. 1 shows the barrier 10 suspended from the underside of an aeroplane 12
which is
positioned between two bombs 14. The barrier is in the advantageous diamond
cross
section configuration which will provide protection to/from detonation of
either bomb
14.
Fig. 2 is a plan view of the barrier 10 from above, which is positioned
halfway
between two bombs 14. The barrier 10 is centred between the two bombs parallel
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with the side faces of the barrier 11 and aligned so that the noses 16 and
bases 18 of
the bombs are shielded from one another by the barrier 10. The barrier is
placed
halfway between the two bombs so as to protect one from the other. The
distance
between the bombs (x) is only 0.78 metres under a typical military fighter
aircraft.
Fig. 3 shows the barrier 10 surrounded by a casing 20 which is shaped to
follow the
contours of the barrier (shown in a broken line) closely. The casing is close
fitting but
has been shown with a small airgap for clarity of drawing.
Fig. 4 shows the barrier 10 with a square casing 22. There is an airgap around
the
barrier due to the square shape of the casing 22 relative to the diamond shape
of the
barrier 10.
Fig. 5a shows a barrier 10' having an angled side 13 proximate to an explosive
device
14'. The apex 24 of the angled side aligns with the centre line 26 of the
device 14'.
Fig. 5b shows a barrier 10" having a slope 28 which will direct the blast wave
upwards, as shown by the dotted line. The barrier 10" can be inverted so that
the
slope 28 will redirect the blast wave downwards.
Fig. 5c and 5d show the exterior of the barrier 10 having concave and convex
configuration whilst still maintaining a substantially diamond shaped cross
section.
Trials have proven the success of a barrier according to the present invention
when
used to protect against the detonation of a 10001b (454.55kg) bomb. The trial
was set
up to replicate the use of the barrier under a typical military fighter
aircraft with the
separation between the bombs being 0.78m and having a barrier with a diamond
cross
section midway between the two. One of the 10001b bombs was filled with
concrete
to be an acceptor bomb so that the effects of stand-off distance, barrier
thickness,
mass and the physical damage imparted to the concrete filled bomb could be
analysed
and recorded after the controlled detonation of a live bomb to see how much
damage
was caused to the device. Polyurethane barriers with various nominal crush
strengths
were tested as is shown in table 1.
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Foam number Nominal Crush Damage to acceptor bomb
strength (MPa)
3715 5 Slight distortion, fragment
strikes
3718 7 Slight distortion, many
fragment strikes
3725 12.5 Slight distortion, few
fragment strikes
6725 13.5 Slight distortion, few
fragment strikes
3730 17.5 No significant distortion, few
fragment scuff marks. No
detonation when acceptor
bomb was a live bomb.
No barrier n/a Significant fragment strikes
and considerable distortion
either of which is enough to
trigger a sympathetic
explosion
Table 1: Polyurethane foam reference numbers, their corresponding nominal
crush
strength and performance.
All of the barriers tested greatly reduced the number and extent of impact
marks on
the acceptor bombs compared to not having a barrier present. When a 10001b
bomb
was detonated the concrete filled acceptor bomb the other side of the barrier
to the
explosion only received a few scuff marks caused by fragments and was only
slightly
distorted. This amount of damage was deemed not to be enough to cause
detonation
of a live device. The 17.5MPa crush strength foam showed the_best result
in.this_trial
as there was minimal damage to the acceptor bomb.
To prove this, the inventors used a live donor bomb, which was detonated
remotely,
and a live acceptor bomb, each separated from the other by a distance of 0.78m
with a
17.5MPa crush strength foam barrier placed between them. The barrier mitigated
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against a sympathetic detonation by retarding the velocity of the fragments
and
lessening the effects of the blast wave. Only a few fragment scuff marks were
visible
on the acceptor bomb casing and there was no significant distortioil. It was
evident
that the damage was minimal and so the acceptor bomb was unreacted with
minimum
damage. After detonation of the bombs there were no remains of the barriers.
As mentioned previously, the crush strength of the rigid polyurethane foam
depends
upon the size of the possible explosive event, the weapon fragment type and
the
distance between munitions. For mitigating against the sympathetic explosion
of a
10001b (454.55kg) bomb, the preferred crush strength of the polyurethane foam
is
from 5MPa to 17.5MPa and is advantageously in the range of from 12.5MPa to
17.5MPa.
The barrier used in the trials had dimensions of 350mmx350mmx 1350mm and
weighed approximately 36kg. However, the barrier can be shaped or sized to the
required size which the skilled man would be able to determine. The barrier
could be
made to the exact required dimensions, but this would mean that it is down to
the
operator to position it in the exact location for maximum protection. In a
conflict
situation this is not ideal as precious time could be lost. It is therefore
envisaged that
the barriers used will be larger than the devices they are protecting from or
protecting
against detonating.
