FR2764689A1 - METHOD FOR CONTROLLING LATERAL DISPERSION OF STABILIZED AMMUNITION BY GYROSCOPIC EFFECT - Google Patents

Abstract

The method consists in modulating the horizontal bypass acceleration due to the gyroscopic effect created by the force of gravity on the munition, by varying the center of aerodynamic thrust of the munition. The effect is obtained by placing retractable fins (4) at the rear (3) of the ammunition, which are released at the instant the correction is to be applied.Application: artillery shells.

Description

The present invention relates to a method for controlling the

  lateral dispersion on the ground of gyro stabilized artillery ammunition and ammunition applying this process. It applies in particular to improving the efficiency of artillery ammunition

  especially in the context of ammunition with increased range.

  The improvement in the accuracy of the point of impact of artillery ammunition is justified by the fact that it allows the treatment of targets by requiring a reduced number of ammunition but also because of the increase in range currently envisaged ammunition future who

  goes along with a degradation of the accuracy of the points of impact.

  To meet these needs, it is known to equip the munitions with devices able to locate them. These implement navigation systems, such as the system known as the Anglo-Saxon GPS "Global Positioning System" or low cost inertial units. These systems allow, as a first step, to correct the pointing angles for future firing from actual trajectory measurements of the first fired shell, without the need to place advanced observers at points of impact and in a second step, to directly correct the real trajectory, using adapted control devices, embedded in the ammunition. The second method is more complex, but offers the advantage of avoiding adjustment shots, which is doubly interesting from an operational point of view by the effect of surprise obtained that does not allow the target to leave the target zone and by reducing the time required to achieve a shot on goal which allows the shooter to quickly leave the firing position, and greatly reduces the

  probability of locating the shooter by the counter-battery radars.

  The ammunition trajectory correction is carried out in two steps: A first step is to correct the errors in scope only. All that is required is an action in the vertical plane, which can be carried out in a relatively simple manner, intentionally aiming beyond the target, and controlling the aerodynamic drag by an airbrake system. The curvature of the trajectory is thus modulated. During this action, the control of the range involves purely axial forces, and requires only

  location information of the ammunition.

  The second step, which is more difficult to achieve, consists in also correcting the lateral errors by implementing, for example, actuators developing forces perpendicular to the speed of movement of the munition, that is to say substantially perpendicular to the axis. of revolution. In this case, the knowledge of the angular position of the actuators with respect to a terrestrial reference, vertical for example,

is required.

  In principle, the course correction is in a relatively little evolutionary direction with respect to a terrestrial reference, at least as long as the amplitude of the correction requested is sufficiently large. When the ammunition rotates in roll, it is necessary that the actuators, in the case where they are related to the ammunition, switch at a double frequency of the roll frequency in order to realize a "recovery" of the force

  generated, to obtain an average value of the resulting non-zero force.

  The direction of the average force is adjusted by adjusting the switching times. The operating mode described above is possible for moderate rolling ammunition up to 30 revolutions / s for example. On the other hand, it seems difficult to apply to ammunition stabilized by gyroscopic effect because their speed of roll is then too high, it is of the order of 300 revolutions / s for a munition of 155 mm and the dynamic performances expected of the actuator In this case, conventional aeromechanical control systems are incompatible

for example.

  At present, two methods exploiting principles of conventional actuators, as described above, appear to be able to be

considered.

  The first method may consist in using piezoelectric devices which make it possible to reach high switching frequencies. However, these may not satisfy the amplitudes of

deformations required.

  The second method may consist in using decoupled correction members in roll from the rest of the munition. But such a method is fraught with problems of making decoupling bearings which must withstand the initial acceleration at the time of firing and have low friction.

  The object of the invention is to overcome the aforementioned drawbacks.

  To this end, the subject of the invention is a method for controlling the lateral dispersion of gyroscopic stabilized munitions characterized in that it consists in modulating the horizontal drift acceleration due to the gyroscopic effect by varying the center of thrust

aerodynamic of the ammunition.

  The invention also relates to an ammunition

application of the aforementioned method.

  The method and the device according to the invention have the advantage that they require a simple arrangement of gyroscopically rotated stabilized ammunition requiring no rapid switching of actuators, no decoupling of actuators and especially no means of

reference measurement of vertical.

  Other features and advantages of the invention will become apparent

  with the following description made with reference to the accompanying drawings which

  represent: - Figure 1, the trajectory of a munition relative to the horizontal, - Figure 2, the relative provisions on the longitudinal axis of the center of thrust and the center of gravity of a munition, - Figures 3 and 4, the evolution of the lateral acceleration as a function of a stability parameter u; FIG. 5, a graph showing an ammunition bypass as a function of the distance from the point of impact, FIG. example of firing misalignment for centering the corrections, FIGS. 7 and 8, an example of displacement of the thrust centers after correction, FIG. 9, an example of implantation of radial correction fins on the body of the ammunition, FIG. 10, the fins of FIG. 9 in the deployed position, and FIGS. 11 and 12, an example of wedging of fins according to FIG.

  the invention on an ammunition base.

  The gyroscopic derivation of shells at high roll speed is a physical phenomenon resulting in a continuous lateral deviation of the trajectory, which can reach several hundred meters at the end of the trajectory. For example, the lateral deviation of a shell of 155 range 27 km, depending on the horizontal distance traveled is of the order

800 m.

  Qualitatively, the phenomenon can be analyzed in the following way: a) Gravity causes the curvature of the trajectory,

  that is, the rotation of the speed vector downwards.

  b) Since the conditions of gyroscopic stability are assumed to be fulfilled (rotation speed around its sufficient longitudinal axis), the munition follows its velocity vector with a certain drag, but at the same

rotation speed.

  c) The speed of rotation of the munition around its longitudinal axis, also called SPIN, induced by a gyroscopic reaction

  around the vertical axis (gyroscopic torque).

  d) The gyroscopic torque shows a skid angle between the axis of the projectile and its velocity vector. This angle is established at an equilibrium value such that the aerodynamic moment that it induces compensates

the gyroscopic torque.

  The slip angle corresponds to a rotation about the vertical axis and is therefore in a horizontal plane. The lift force

  associated is itself horizontal, and produces the derivation of trajectory.

  The phenomenon of gyroscopic derivation leads to a horizontal deflection which is the natural response of the munition to the vertical stress of gravity. The derivation is to the right of the line of

  shot, for a projectile rotating in the positive direction around its axis of spin.

  Given the order of magnitude of the gyroscopic derivation with respect to the amplitude of the lateral dispersions, it is conceivable according to the invention of

  correct these by modulating the amplitude of derivation.

  The gyroscopic derivation results in a horizontal lateral acceleration FD, which is expressed from the quantities defined below: g: acceleration of gravity A: inertia of the munition around its longitudinal axis (spin axis) m: mass of the ammunition V: ammunition velocity y: slope of the trajectory p: ammunition roll rate XF: abscissa of the aerodynamic thrust center F with respect to

  at the center of gravity G of the ammunition.

  The velocity vector V is as shown in Figure 1 tangent to the trajectory and its direction relative to the horizontal defines the slope of the trajectory. For gyroscopically stabilized ammunition, the aerodynamic thrust center F is in front of the center of gravity G as shown in Figure 2 which shows a munition in the form of a

  shell composed of a cylindrical body 1, a conical head 2 and a base 3.

  By analogy, with aerodynamically stabilized feathered ammunition, the parameter xF is referred to hereafter as "margin

static ".

  The spin speed being assumed to be large enough to ensure stability, the FD acceleration is established after stabilization of the transient motions of the projectile to the following value Ap FrD = gcosy mvxF (1) (Ap 2 + mvxF For a slope trajectory y given, FD only depends on the following parameter: Ap u = - (2) mVxF and is expressed by: U FD = gcosy 2 (3) l + u 2 The condition of stability of the munition imposes more than the coefficient essential stability s, given by: A2p2 s: 2 (4) 2Bpv SCzLxF Verifies the inequality: s> 1 In which: B represents the pitch / yaw inertia p is the air density HD2 S is the reference surface (= with D = caliber) and Czc is the lift gradient Depending on the parameter u, the necessary stability condition becomes: U> 2BpVSCza mAp Usually, the most severe stability condition is at V start of flight the ammunition p, and CzcE being maximum at the exit of the launching tube of the ammunition. For a projectile of 155 mm, the orders of magnitude are as follows: B = 1.715 m2kg A = 0.159 m2kg m = 46.95 kg S = 0.01886 m2 Vo - = 0.5 m / rd in. Czc = 2.78 and p = 1.225 kg / m3 Hence the stability condition: u> 0.0147 u The evolution of FD = gcosy u is represented in FIGS. 1 + u2 3 and 4. FIG. 3 is an overall representation of the function. With the numerical values used above (projectile of 155 mm) the parameter u evolves during the flight in the following range: 0,026 <u <0,041 The range of variation concerns only the beginning of the curve

  FD = f (u) which is represented in FIG.

  The examination of the curve shows that D is almost linear as a function of u and that the lateral acceleration can easily be modified to

  condition to vary the parameter u.

  It is also better to increase u, which offers more possibility of variation and moreover, enhances the stability of the ammunition. From relation (3) and for a trajectory whose slope profile is given, it appears that the derivation acceleration depends only Ap-mms'xrm of the parameter: u = Ap, defined by the relation (2 ) which itself expresses mVxF as a function of the kinetic moment Ap, the momentum mV and the

the static margin xF.

  The momentum mV and the angular momentum Ap result essentially from the initial conditions given by the tube of

launch at the moment of the shot.

  It is not possible to modify the momentum of

  ammunition as this would have a profound effect on its range.

  It may seem possible to modify its kinetic moment. To do this, two ways are possible:

or increase the roll speed.

  A decrease of the roll speed can be obtained for example by aerodynamic brakes. However, these decrease the margin of stability of the munition. In addition, this results in a decrease of

  u parameter, and therefore offers little correction margin.

  An increase in the roll speed can be obtained by an energy method of the impeller type. For example, to double the mid-trajectory drift acceleration, it is practically enough to double the value of u, that is to say to increase the roll speed by a factor of 2. Half trajectory, the variation of p to be communicated is then about Ap = 200 revolutions / s. Given the roll inertia and assuming that the acceleration torque is obtained by nozzles located at the periphery of the projectile, the impulse to be communicated is of the order of: I = 2600 Ns However, this solution has the following characteristics: disadvantage that it requires a mass of powder of about 1.3 kg which, given the filling coefficients of the impellers, risks being unacceptable with respect to

the architecture of the ammunition.

  A third solution implemented by the invention consists of

change the static margin XF-

  The method according to the invention consists in directly varying the position of the center of thrust F which results from the distribution of the pressures due to the flow of air on the surface of the munition. As an indication, for a projectile of 155 mm, the following orders of magnitude can be retained:

  - center of gravity at 2.16 calibres (335 mm) from the base.

  - Variable thrust center as a function of the Mach number M: to M = 3 x F = 1.612calibres = 250mm, S forward of the center of gravity at M = 1 XF 2,826calibres 438mmJ The course correction is in the downward phase of the

  trajectory, that is to say rather around M = 1.

  An exploitable variation of the gyroscopic derivation can be obtained by decreasing the XF static margin of about 1 to 1.5 calibres. Simulations show that it is then possible to increase the diversion to 300 m compared to its normal value of 800 m (see Figure 8: lateral derivation by decreasing the static margin by 30% from t = 30 s) . This gives a correction potential of up to 300 m which is however flexible, by triggering the static margin variation earlier or earlier, implicitly assuming that the variation of the static margin XF is of the "one shoot" type, in English, that is to say, unique, all or nothing and not reversible. The correction is always in the same direction, to the right of the line of fire. In order to refocus the correction, and to be able to catch the errors of any sign, a priori symmetrical around 0, simply shift the azimuth pointing of the launcher tube to the left so as to compensate systematically half of the correction maximum possible. Thus, by denoting by A. the achievable correction amplitude (for example: Ac = 300 m) and Xmax the target range, the target range angle must be shifted by: As = Ac (6) 2Xmax To modify point of application of the resultant pressure forces on the ammunition, it is sufficient according to the invention to deploy, as shown in Figures 9 and 10, at the desired time for the beginning of correction, the wings 4 whose combined lift to that of the ammunition provides the desired static margin. Unlike conventional control systems, which must be oriented to bend the trajectory in the direction of the target, the wings 4 implemented here remain fixed after deployment. Their role is only to modify the center of aerodynamic thrust F in order to

vary the XF static margin.

  The wings 4 must be more angularly staggered, as shown in Figure 9, so as to maintain the rolling speed of the ammunition. For example, for a projectile of 155 mm, whose center of thrust is located very in front of the projectile especially during the phase o the correction is likely to be ordered, Figure 10 shows the location of the aerodynamic force F for Mach = 1 and shows the range in which it must be positioned. The latter is obtained in the example by moving back the point of application of the force F from 1 to 1.45

  calibres to obtain sufficient lateral deflection modulation.

  Examination of Figure 8 shows that to obtain the desired setback for the center of thrust, the only realistic solution is to deploy the

  sails 4 behind the center of gravity.

  In order to maximize the effect of the wings 4 and to minimize their dimensions while maintaining a certain modularity to the corrected ammunition, the wings 4 are placed at the base 3 of the projectile. Designating by: F0, the center of thrust of the FI-finless ammunition, the center of the XFO finned ammunition, the static margin of the XF1 finless ammunition, the static margin of the Pc finned ammunition, the lift of the body without fins (applied in Fo) Pa, the lift of the fins

  and La, the lever arm of the fins lift (La <0).

  The static margin with vanes is calculated by considering the moment of the resulting force. The following is obtained according to the force distribution diagram of FIG. 8: PCXF0 + PaLa XF1Pc + Pa (7) With eight radial fins of unit area Sa implanted as in FIG. 9 the lift Pa is approximately given by the relation: Pa = 4Sa q CNa (8) where q is the dynamic pressure of the flow and CNa is the

  lift coefficient of a fin.

  Given the interactions with the cylindrical body of the ammunition, we can consider that: CNa = 8a

o is the incidence of the body.

  The lift of the ammunition is expressed by: Pc = SC CNC (9) [ID2 where Sc is the surface of the master torque - and CNC the coefficient

body lift.

  Practically: CNc = 2a The area required for a fin 4 is then expressed as follows: Its Sc CNc XFO - XF1 (10) 4CNa XF1 -La Taking as an example: XF0 = 2.8 calibres XF1 = 1.3 calibres (decrease of 1.5 calibres) La = 2.1 calibres The relation (10) gives: Sa = 5.2 cm2 Which corresponds to relatively small fins, easy to integrate at the base 3 by a radial assembly

as shown in Figure 10.

  In this case, the deployment of the fins 4 can be obtained

naturally by centrifugal effect.

  In order not to thwart the rolling speed of the munition, the fins are angularly wedged relative to the axis of the projectile, as

indicates it in figure 11.

  The calibration verifies the relation: pd t9 = PVt: (11) V o d is the distance between the axis of the projectile and the center of gravity

of the fin.

  By way of example, for: p = 200 t / s V = 350 m / s d = 0.09 m

The relation (11) gives: T = 17.9.

Claims (6)

  1. A method for controlling the lateral dispersion of stabilized gyroscopic ammunition (1) characterized in that it consists in modulating the horizontal derivation acceleration due to the gyroscopic effect   by varying the aerodynamic thrust center F of the ammunition (1).   2. Method according to claim 1, characterized in that it consists in placing at the rear (3) of the ammunition retractable wings (4) to change at a given moment of the trajectory the pressure distribution on the munition in moving the center of thrust towards the back of   the ammunition to apply the required dispersion correction.   3. Method according to claim 2, characterized in that it consists in wedging the fins (4) radially to the munition (1) with a determined angle of inclination relative to the longitudinal axis of the munition (1).   so as not to upset his roll speed.   4. Gyroscopically rotatable stabilizing ammunition for the   implementation of the method according to any one of claims 1 to 3,   of the type comprising a cylindrical body (1) disposed between a conical head (2) and a base (3) characterized in that it comprises retractable fins (4) arranged radially on the base (3) to correct when they are deployed, the horizontal derivation of the ammunition due to The gyroscopic effect.   5. Ammunition according to claim 4, characterized in that the fins (4) are inclined relative to the longitudinal axis of the munition at an angle ri such that tgr = pd (11) op is the rolling speed, d is the distance between the axis of the ammunition and the center of gravity of the fin (4) and V is the speed of the ammunition (1).   6. Ammunition according to claim 5, characterized in that each fin (4) has a surface Sa defined by the relation Sa = SCCNC FO -F1 (10) in which Sc is the surface of the master 4CNa XF1 -The pair of the ammunition , CNC the lift coefficient of the body of the munition,   CNa the lift coefficient of the fin, XFo is the static margin of the finless ammunition, XF1 is the static margin of the finned ammunition5 and the lever of the fins lift.