GB2624358A - Munition ejected from a ground platform in a substantially vertical direction - Google Patents

Abstract

This invention relates to munitions which are designed for ejection from a ground platform 3 in a nearly vertical direction with a rotation imparted to the munitions. This munition 1 comprises at least one target sensing system 8 whose sighting axis scans the ground along a spiral curve 12; it is characterised in that it comprises devices 11 to slow down the rotation of munition 1 so that ground scanning rate Vb of the sighting axis is minimised in order to reduce the constraints on the sensing system , the ratio of munition axial speed v to munition rotation speed Ω being chosen for optimum target detection probability. Application to bounding mines.

Description

This invention relates to munitions such as those used in bounding mines which are designed to be ejected in a nearly vertical directipn from a platform laid down on the ground with a rotation imparted to the munitions.

Such anti-vehicle munitions are known in particularly by Patent number FR2581175.

Usually, the munition contains a core-forming explosive charge which is initiated by a target sensing system such as an infrared sensing system.

A rotational movement is imparted to the munition by a parachute, for instance. This rotation makes the sensing system sighting axis scan The ground along a spiral curve.

Such a munition has disadvantages.

Thus, the target search by the sensing system only 15 starts when the parachute is deployed, i.e., when the munition has reached a maximum altitude and begins to descend towards the ground.

This induces some delay between the moment when the ground platform senses an approaching target and the moment 20 when the munition can destroy this target and such a delay adversely affects the munition effectiveness.

Patent number FR2682181 describes a bounding mine whose sensing system scans the ground along a spiral path during the ascending trajectory of the munition. However, the rotational speed imparted to such a munition by pyrotechnic impellers is substantially constant along the trajectory under consideration.

Moreover, it will be noted that the ground scanning rate of the sighting axis is proportional to both the 30 munition altitude and the munition rotational speed. Thus, the following equation can be written: Vb (scanning rate) = H.Q.Tangent(0) where H is the munition altitude, 0 is the rotational speed (in radians per second) and 0 is the angle from the 35 sighting axis to the vertical.

When H increases, the scanning rate (Vb) therefore increases as well. Now, an excessively high scanning rate imposes constraints on the sensing system as it shortens the target observation time for the sensing system.

Now, the sensing system passband is all the larger as the target observation time is shorter. Practically, with a constant signal-to-noise ratio, the use of auxiliary devices then becomes necessary, such as a sensor cooling 5 system for an infrared sensing system.

Moreover, as the munition reaches the maximum altitude and its axial speed decreases, the pitch of the spiral described by the sighting axis is greatly reduced and this does not significantly increase the munition 10 effectiveness.

The purpose of the invention is to propose a munition which is not affected by such disadvantages.

Thus, the invention proposes devices which control the ground scanning rate of the sighting axis, thereby 15 allowing the use of sensing systems which are simpler and less costly.

The invention is also intended to control the pitch of the spiral described by the sensing system sighting axis, thereby providing sensing performance which is substantially the same along the entire ascending munition trajectory.

Thus, the scope of the invention is a munition used, for instance, by a bounding mine, which is designed to be ejected in a nearly vertical direction from the ground platform with a rotation imparted to the munition and which comprises at least one target sensing system whose sighting axis scans the ground along a spiral curve. This munition is characterized in that it includes devices which slow down the munition rotation over at least part of the trajectory in such a way that ground scanning rate Vb of the sighting axis is minimized in order to reduce the constraints affecting the sensing system and the ratio of the axial munition speed 70 the munition rotational speed is chosen for optimum target detection probability.

Practically, the devices used to slow down the munition rotation will be such that the ratio of the munition axial speed to the munition rotational speed is substantially constant over at least part of the munition trajectory.

For instance, in the event where the target sensing system is an infrared sensing system, with a munition longitudinal speed smaller than 60 m/s and a munition rotational speed smaller than 15 revolutions/second, the 5 braking devices will be selected so as to obtain a rotation damping constant K between 0.002 and 0.006 in absolute value and preferably between 0.003 and 0.005 in absolute value (that is roughly -0.004 ± 10%).

According to a particular embodiment of the 10 invention, the braking devices include at least two fins which are attached to the munition body and extend radially on the munition; the shape and size of such fins are suitably chosen to provide the desired constant axial speed/rotational speed ratio.

The invention will be better understood by reading the following description of a particular embodiment, taken in connection with the accompanying drawing wherein a munition is schematically represented during the ascending phase according to the invention.

Munition 1 according to the invention is a bounding mine munition which is ejected along a vertical trajectory 2 by a firing platform 3 laid down on the ground.

In a known manner, munition firing is initiated by an approaching target 4 which is detected by means of, for 25 instance, acoustic sensors 5.

Munition 1 has an internal structure which is substantially the same as that described in Patent number FR2695992 and Patent number FR2682181 to which reference can advantageously be made for further details.

Thus, the munition contains a warhead 6 which is a core-forming explosive charge whose direction of action is represented at 7, and an infrared sensor 8 whose sighting axis is represented at 9.

The munition is driven into rotation upon ejection 35 from the firing platform (rotational speed 0) by at least two pyrotechnic impellers 10.

The rear section of the munition is fitted with rotation braking devices 11 which consist of four fins which are evenly distributed angularly.

I II

As an approaching target is detected, munition 1 is ejected in a vertical direction 2 by a propelling charge which will be advantageously housed in firing platform 3. One can refer, for instance, to Patent number FR2685463 and Patent number FR2682181 which describe possible firing platforms.

Both a vertical speed V along vertical direction 2 and a rotational speed Q about the centerline of munition 1 (the munition centerline and vertical direction 2 are 10 superimposed) are imparted to munition 1.

This results in the ground being scanned by the end of sighting axis 9 along a spiral 12 which grows wider as munition 1 rises up above the ground.

Speed V gradually decreases until it nulls out when 15 the maximum altitude is reached, then the munition goes down.

With a munition in the state of the art, there is a relatively small decrease in rotational speed 0 as the munition rises up. Now, the initial rotational speed selection is imposed by the selected ejection speed in order to have a V/Q ratio of about 2 to 4 meters per revolution (with 0 expressed in revolutions per second); this value determines a spiral pitch which is such that target detection probability is optimum. Also, the ejection speed value is also imposed by the desired maximum munition altitude (which determines the munition effectiveness area). 0 will therefore be an important factor because the initial speed is important as well.

The result of these constraints is that the ground 30 scanning rate (Vb) of the sighting axis is significant and increases as the munition altitude.

The equation Vb = H.Q.Tangent(0) given in the preamble can be written, where H is the munition altitude, Q is the rotational speed (in radians/second) and 0 is the angle from the sighting axis to the vertical (see figure).

For example, a munition ejected from its platform at a speed V = 60 m/s, approximately, and rotating at a speed O = 15 revolutions/second reaches a maximum altitude of nearly 180 m and thfl qrAnning rate Vh vafies (with angle 0 = 30°) from 2700 m/s (at an altitude of 50 m) to 6500 m/s (at 120 m).

With the same munition, pitch P of sensing spiral 12 also decreases as the altitude increases. Thus, it varies from 2.3 m upon ejection to 1.3 m at an altitude of 120 m, approximately and equals zero when the munition reaches the maximum altitude.

The munition proposed by the invention is provided with rotation braking devices 11 which have such dimensions that, on the one hand, scanning rate Vb is minimized in order to reduce the sensing system constraints, and, on the other, the ratio of the munition axial speed to the munition rotational speed is chosen to maintain optimum target detection probability.

Practically, both these requirements will be met by selecting braking devices which provide a V/Q ratio within two limit values, a maximum value which ensures adequate target detection probability and a minimum value which is that at which the spiral pitch becomes too small for the detection performance gain to be appreciable.

Efforts will be advantageously made to obtain a V/Q ratio between 2 and 4 meters/revolution for at least part of the munition trajectory which corresponds to the scanning by the sighting axis of the entire munition effectiveness area (the area over which, one the one hand, the sensing system can detect a target and, on the other, the core-forming explosive charge is within a range from the detected target which guarantees perforation).

This useful portion of the trajectory is therefore 30 included between the ground level and the optimum operating altitude (which can differ from the maximum altitude and which is chosen to determine the munition effectiveness area). An optimum altitude of 120 m can be typically selected as in the previous example.

Thus, this V/Q ratio can be considered as substantially constant over at least part of the munition trajectory.

Braking devices 11 give the munition a rotational damping factor (Clp) which inserts a rotation damping constant K such that: = Qo Exp (K(Vot-gt2/2)) (where Qo is the initial rotational speed, herein expressed in radians/second).

The V/51 ratio is going to vary with time (and altitude) and is therefore equal to: V/Q = Exp (-K(Vot-gt2/2)) x (Vo-gt)/Qo Practically, considering a munition with an infrared sensing system whose speed Vo is about 60 m/s and rotational speed Q is about 15 revolutions/second (94.2 rad/sec), braking devices will be selected to provide a damping constant K between 0.002 and 0.006 in absolute value (K is negative), and preferably between 0.003 and 0.005 in absolute value (that is substantially K = -0.004 ± 10%).

Thus, a nominal value of -0.004 ± 10% for K yields a 38% reduction in scanning rate at 120 m of altitude; this 20 permits a substantial reduction in the required infrared detection system passband.

The resulting scanning rate at 120 m of altitude is 4000 m/s ± 6.5% (as compared with 6500 m/s without the device according to the invention). The V/52 ratio is kept constant to within ±8% and the spiral pitch is nearly constant since it varies from 2.3 in upon firing to 2.2 in ± 4.5% at 120 m of altitude.

From the aerodynamic standpoint, for a given munition shape, a craftsman can readily determine the shape of the braking devices which yield such a damping constant value. The following equation can thus be written: K = 1/2 p Clp S12/J where: Clp is the damping factor, J is the munition moment of inertia along the axis of rotation, S is the reference surface area (in terms of aerodynamics), 1 is the ire-f-eettee---1-efitt-iii-(414-t-ens of aerodynamics), p is the specific gravity of the air.

With a constant K --0.004, a 200mm caliber, 200mm long munition then yields a Clp of -0.2, approximately, which can be obtained using, for instance, four fins which 5 are evenly distributed angularly on the rear section of the munition; such fins are nearly 100mm long (L) and 40mm high (H) (see figure).

The fins will be advantageously deployed upon firing by fin deploying devices of a known type (typically by 10 springs).

It is possible for the craftsman to tailor the devices of the invention to any type of munition whatever the target sensor characteristics and the munition geometry may be.

In particular, the craftsman can design rotation braking devices which yield a substantially constant axial speed/rotational speed ratio over a single well-defined portion of the ascending trajectory. The braking fins may be deployed, for instance, only from a given point of the trajectory.

The braking devices may also be given several shapes which yield the desired braking characteristics (fins, flaps, studs, powder rocket motors, gas generators, etc.). Finally, the invention can be tailored to types of 25 munitions other that bounding mines such as anti-tank or anti-vehicle munitions launched from a stationary structure such as a building.

Claims (1)

1. CLAIMS1-Munition (1) used by a bounding mine and designed for ejection from a ground platform (3) in a nearly vertical direction (2) with a rotation imparted to the munition which comprises at least one target sensing system (8) whose sighting axis (9) scans the ground following a spiral curve (12), characterized in that it includes devices (11) which slow down the rotation of munition (1) over at least part of the trajectory so that ground 10 scanning rate (Vb) of the sighting axis is minimized in order to reduce the constraints imposed on the sensing system, the ratio of the munition axial speed (V) to the munition rotational speed (Q) being chosen for optimum target detection probability.

   2-Munition according to claim 1, characterized in that the munition rotation braking devices are such that the axial speed/rotational speed ratio is substantially constant over at least part of the munition trajectory.

   3-Munition according to claim 2, characterized in 20 that the target sensing system is an infrared sensing system, the munition longitudinal speed is smaller than 60 m/s and the munition rotational speed is smaller than 15 revolutions/second and characterized in that the braking*devices yield a rotation damping constant K between 0.002 and 0.006 in absolute value.

   4-Munition according to claim 3, characterized in that the braking devices yield a rotation damping constant K between 0.003 and 0.005 in absolute value.

   5-Munition according to claims 2 to 4, characterized 30 in that the braking devices include at least two fins (11) which are fitted to the body of munition (1) and extend along a radial direction on the munition, such fins have shapes and dimensions which are suitably chosen to provide the desired constancy to the axial speed/rotational speed ratio.