The invention relates to general-purpose systems in which fluids, e.g., gases, are transferred by changing their pressure, subsequently transforming it into the kinetic energy of a body (executive device, projectile, etc.). Well known is the idea of gas reducers of the "high pressure-low pressure" type, which arose in 1944: a perforated partition divides a closed space into two cavities, due to which the gas static pressure decreases after throttling the gases through the perforations from the first cavity (chamber) into the second one [I]. This idea was implemented, for instance, in ammunition: in some kinds of 40-mm Austrian and German grenades [1, p.66-73], ammunition for smooth-bore hunting riffles, etc. [2, 3]. The closest to the present invention, in its purpose and the combination of the design features, is a device for body displacement, which comprises cavities separated with a partition perforated with through holes, through which the fluid penetrates from the first cavity into the second one, and a body bounding the second cavity at least on the side opposite to the partition, the body being arranged so that it can move under the action of the fluid in the direction from the partition [4]. This device, as well as the above-described analogues, comprises holes whose lengths (and, hence, the partition thickness) and diameters are in a ratio less than 4:1 (see the drawings). This ratio is sufficient to solve the problem of the pressure reduction. In addition, the perforation factor is significantly lower than its maximum possible value (both geometrically and according to the requirements for the strength of the bridges between the holes); the holes are cylindrical, the partition is rigid, fixed and closed, with a container formed inside the second cavity, whereas the mobile body is a wad-container for a shot charge, the bottom part of the wad-container, having a U-shaped long cross-section, enclosing partly the partition. However, in the prototype device, the significant reduction of the pressure of the fluid (the powder gases produced as products of combustion of the missile powder charge in the first cavity) in throttling through the relatively short perforation holes is not accompanied with a significant conversion of the pressure static component Pi into dynamic component pdyπ = 0,5 p v2 (where ps the fluid density and v is the flow rate). This can be explained by the random nature of the fluid flow and, hence, by the conversion of a considerable portion of energy in the second cavity into heat (pst + pdyn) according to the Bernoulli equation. Accordingly, the fluid pressure is practically uniformly distributed over the surfaces walls forming the second cavity, a part of the partition, and the mobile body. Therefore, the gas energy is spent inadequately to accelerate the mobile body, i.e., mainly via the static pressure in the second cavity; in addition, the pressure becomes lower due to eddy flows and heat losses caused by them. The problem to be solved by the present invention is to improve the technical and economical characteristics of the device by converting the fluid (e.g., gas) eddy flows into straight jets thus intensifying their dynamic impact upon the mobile body and also decreasing the fluid static effect upon other surfaces bounding, along with the body, the cavity. To solve the problem, there is provided a device for body displacement comprising cavities separated by a partition perforated with through holes, through which the fluid penetrates from the first cavity into the second one, and a body bounding the second cavity on at least the side opposite to the partition and arranged so that it can move in the direction from the partition under the action of the fluid, the length to diameter ratio of the holes in the partition being at least 4:1 (four to one). The problem is solved also by using additional design features (the above-specified basic features being the same): - the partition perforation factor can be made as large as possible, the hole length exceeding their diameter five-fold; - the cavities can be formed by the cartridge case and the said partition, the first cavity containing a missile powder charge used as a source of the gaseous fluid, which is coupled with its ignition device; - the above-described features remaining the same, the partition can be made, at least partly, from a material characterized by high erosivity under the action of high-temperature gas flows, providing for an intense increase in the hole diameters in the process of flowing of the missile charge combustion products through them. No known devices or methods exhibit the set of essential features coinciding with that of the proposed device. And it is due to the proposed combination of the features that a novel technical result is obtained in accordance with the principal object of the invention.. The proposed device for body displacement is illustrated in the drawings, in which: Fig. 1 is a longitudinal section of the device version with a flat partition between the cavities, cylindrical perforation, and fluid supply (under pressure) into the first cavity from an external source, where d and 1 are the diameter and length of the partition perforation holes, b is the distance between the partition and mobile body, D is the diameter of the transverse (midlength) cross-section of the mobile body, and V is the mobile body motion speed; Fig. 2 is a longitudinal section of a similar device with a powder missile charge as a source of fluid (powder gases) under pressure; Fig. 3 is fragment of longitudinal sections of the devices presented in Fig. 1 or Fig. 2 including the illustration of the conversion of fluid pressure and dynamic effect upon the mobile body, where pi, piCT, p2, p2CT and p2ΛHH are the total fluid pressure in the first chamber, its static component in the first cavity, total pressure in the second chamber, and its static component in the second cavity, respectively; Fig. 4 is a fragment of the partition between the cavities viewed in the axial direction; Fig 5 is a fragment of the longitudinal section of the device version with conical holes of the partition perforation, where φ is the angle at the hole cone vertex, and dav is the average hole diameter; Fig. 6 is a longitudinal section of the device version with a closed partition, the container being formed inside the second cavity; Fig. 7 is a fragment of the device presented in Fig. 6, the version with the II-like cross section of the mobile body; Fig. 8 is a fragment of the longitudinal section of the device version with a convex- concave system "partition - mobile body", where α is the angle between the long axes of the partition perforation holes and the long ax of the mobile body; Fig. 9 is a fragment of the container version with perforations on a separate part made of a highly erosive material; Fig. 10 is a fragment of Fig. 9, which is an enlarged picture of the perforation hole with the scheme of the gas jet formation and the process of the hole wall erosion; Fig. 11 is a fragment of the container version with the perforations in the form of bushings made of highly erosive material and built in the container wall; the figure presents a magnified picture of the perforation hole with the scheme of the gas jet formation and the process of the hole wall erosion; here di is the bushing diameter; Fig 12 is empirical dependencies of the mobile body initial speed V0 versus mass ω of the powder missile charge in the devices with perforated container walls made of titanium alloy BT3-1 and steel. The device for body displacement comprises (see Figs. 1, 2) first cavity 1, second cavity 2, and partition 3 that separates the cavities (chambers). Cavities 1 and 2 are partly bounded by housing part 4. Also provided is body 5 (the so-called "mobile body") which either bounds cavity 2 on the side opposite to partition 3 (see Figs. 1- 6) or, in addition, partly encloses partition 3 (see Fig. 7). Body 5 is arranged so that it can move (with speed V) in the direction from partition 3 under the action of fluid (e.g., gas or gas mixture, mainly combustion products of the powder missile placed directly in cavity 1) as it enters cavity 2 under pressure. In particular cases, mobile body 5 can be a machine actuator, projectile, bullet, shot container (as in the prototype device), and so on. Partition 3 is typically perforated with a great number of parallel through holes 6 with diameters d and long axes perpendicular to the opposite surface of body 5 (the preferable shape of the surface is flat). The ratio between the hole 6 length (and, typically, the partition thickness 1) and hole diameter d is at least 4 : 1 (four to one). In other words, diameter d of holes 6 is not larger than 1/4 (one fourth) of their length 1: the length is larger than the diameter by a factor of 4, 5, 6 and more (or a factor equal to any number within this range). The most preferable ratio is 1 : d = 5 : 1. The excess more than 6 is not reasonable. Absolute value of diameter d is about 0.6 to 2.0 mm. If fluid is not supplied under pressure to cavity 1 from any external source 7 (a gas cylinder, receiver, etc.) via duct 8 (pipe, hole, etc.), as is the case in the device illustrated in Fig. 1, and instead is produced by burning powder 9 or similar substance (mixture) directly in cavity 1 when the powder charge is initiated by blasting cap 10 or a similar detonator (see Figs. 2, 6, 7), then diameter d is decided upon depending on the average minimal size of the powder (or its analogue) particles 9, since perforations 6 are to hold back still unburned particles of powder 9 in cavity 1. Holes 6 may be either cylindrical (Figs. 1-4, 6, 7) or conical with the cone base directed towards cavity 1 (Fig. 5). In the latter case, the diameter d values are specified (see above) as a mean of the base and vertex diameters of the cone (dm). The preferable values of angle φ at the vertex of the hole 6 cone (see Fig. 5) range from 16 to 24 degrees. With cylindrical and conical shapes of holes 6 alike, advisable is to ensure the maximum possible perforation factor of partition 3 (see Fig. 4), i.e., the bridges between holes 6 should be as small as it is allowed by the requirements for their strength (the number of holes being large, not just two or three holes). Under some specific conditions, it is not ruled out for the device to comprise a partition with only one hole 6; in this case the hole diameter should be, as a rule, larger than the values from the above-specified range 0.6 to 2.0 mm. In this case the hole 6 shape can be more complex than cylindrical or conical. Flat partition 3 (the simplest design) can be made immovable by fixing it in the axial direction with pins or similar fasteners 11 (see Fig. 2). If partition 3 is closed so that a container inside cavity 2 is formed (see Figs. 6, 7) as in the prototype device, then it is fixed in the rear (bottom) part of the device directly at wall 4 or via an adapter (adapters), for instance, a part with a threaded joint 12 (see Figs. 6, 7). Perforations 6 should be typically made only in the flat end wall of the partition, which is opposite to mobile body 5 (as in the prototype). Notice that the initiating hole behind the blasting cap, which is fabricated as an extension of the blasting cap socket, for instance, in rear plug 13, is not a part of the said perforations. In any case, it is recommended to make partition 3 rigid and immovable in order to prevent its bending or other deformation resulting in non-parallelism of the fluid flows and, hence, in formation of eddies, non-perpendicular interaction with the body 5 surface, and high heat losses. Just to ensure the interaction between the fluid flow coming from holes 6 and the body 5 surface under the straight angle, a permissible version of the device, which comprises convex partition 3 and fanlike perforation 6, should also comprise a suitably concaved surface of body 5 (see Fig. 8). However, in this design, the dynamics of fluid in cavity 2 is somewhat worse than that in the case of flat partition 3. The efficiency of the interaction between the fluid flows and body 5 is also lower. The convex-concave system "partition 3 - body 5" has better characteristics if body 5 longitudinal cross-section is U-shaped (see Fig. 7). Distance b (see Figs. 1-3, 6, 7) between the opposite surfaces of partition 3 and body 5 should not, as a rule, exceed 8 equivalent diameters D of the body 5 transverse (midlength) cross-section (if body 5 is cylindrical, D is the cylinder diameter). Mobile body 5 may be secured against premature motion in the axial direction by, for instance, light curling 14 until the known forcing units are arranged. In one of the recommended (expedient) device versions, partition 3 is fully or partly (see Figs. 9-11) made of a material that is highly erosive under the action of high-temperature gas flows, providing for diameter d of holes 6 to increase intensively while the combustion products of the missile charge (powder) 9 are flowing through them. Such a material is, for instance, titanium (Ti) or its alloys (e.g., BT3-1); the Applicant has obtained empirical data showing that these alloys are characterized by higher erosivity along with sufficiently high strength as compared with steel (see Fig. 12). If necessary, only a part of the partition, the perforation zone, can be made of highly erosive material. In this case the front end wall (see Figs. 9, 10) or its part (not shown), which includes the entire perforation zone, is fabricated from a highly erosive material in the form of a separate perforated part 15; part 15 is secured (the figure shows the fastening with an interference fit and, in addition, with radial screw pins 16). If the mobile body 5 diameter D is large (more than 100 mm), it is permissible (and, in any cases, reasonable) to use the design version (see Fig. 11) with perforations in the form of bushings 17, whose outer and inner diameters are di and d (the perforations themselves), respectively; the bushings are built in (for instance, pressed in) the front end wall of container 3 so that they cannot move in the axial direction (as a whole) under pressure pi. In this design, only bushings 16 are made of highly erosive material. While particular embodiments of the present invention have been described, it is understood that other versions of the proposed device are possible within the scope of the appended Claims. The device operates as follows. Fluid under high pressure, which is supplied into cavity 1 from source 7 via duct 8 (Fig. 1) or produced directly inside cavity 1 by burning the powder missile charge 9 (or its analogue), the burning being initiated by blasting cap 10 (see Fig. 2), flows through holes 6 into cavity 2. According to the Bernoulli equation and due to the said optimal hole 6 diameter to length (d:l) ratio, pressure pi (pist) in cavity 1, which is mainly static (dynamically multidirected), is converted into the mainly dynamic pressure p2 in cavity 2 (p2dyn is considerably larger than p2St)- After passing through the system of holes 6, fluid is shaped as parallel jets; after leaving the faces of holes 6 (i.e., partition 3), fluid flows in cavity 2 in the form of parallel jets (see Fig. 3) and produces dynamic pressure upon the body 5 surface, thus accelerating it in the axial direction, body 5 overcomes the friction and resistance of curling 14 and moves (at a speed of V). The ratio 1/d = 5 (established experimentally for cylindrical holes 6) ensures the best conditions for the jet formation in the perforation. The minimum distance between the outlet faces of holes 6 ensures minimum disturbances from the fluid remaining between the jets, thereby providing for longer duration of the fluid flow character formed. In the section b < 8D (this value has also been found experimentally), no considerable eddies occur nor reduction of the dynamic component p2dyn of the total pressure p2. At the same time, as a positive side effect, reduction of pressure upon the external wall 4 of the device (i.e., wall, forming cavity 2) takes place due to the reduction of static component p2St of the total pressure p2- This is consistent with one of the basic trends of the prototype device, namely, with the attempts to make casing 4 lighter and to make it possible to use housing parts 4 made of plastic or thin metal. Another positive side effect arising due to re-distribution of the pressure p2 components in favor of the dynamic component p2dyn is the improvement, under certain geometric conditions, of the fluid obturation in the system "body 5 - wall 4". Conical shape of holes 6 provides almost the same efficiency of the jet formation with a lower 1/d ratio. However, in this case the dynamics in cavity 2 is somewhat worse because of a large distance between the hole 6 outlet faces. If partition 3 is closed (and forms a container), forces of pressure pi close mainly on partition 3, thereby reducing the undesirable "breakdown" to the device outer elements and surrounding bodies. High-temperature (about 2000-3000 K) and high-speed jets of gaseous products of combustion of missile charge 9 give rise to intense erosion of the material the wall with holes 6 is made from: the material particles are washed-out by the jets into the space behind the projectile (see the scheme of the material washing-out in Figs. 10, 11). Metal inclusions increase the mean density of the jets in the space behind the projectile, owing to which the dynamic effect of the jets upon the projectile becomes larger, and the negative effect of the eddy stratum in the vicinity of the projectile surface becomes less significant. Since at the end of the process of the missile charge 9 burning and flowing of gas through the perforation diameter d becomes larger due to the erosion against the background of the decrease in pressure pi (pist)> a compensating (in respect to the effects upon body 5) increase in the gas flowrate through the perforations takes place. In Applicant-performed comparison experiments with a steel front end wall and the wall made of titanium alloy BT3-1, with a variable (smaller) powder weight (see Fig. 12), an evident effect, i.e., a significant increase in the body 5 initial speed Vo was gained, on the one side, due to an increase in the mean jet density (gas plus metal produced by the erosion), and, on the other side, due to the erosion-induced increase in diameter d from 0.87 to 1.2 mm, i.e., by about 38% against the "background" of the first cavity pressure decreasing with time. The erosion-induced increase in the diameter d is a positive factor for both the proposed devise with the optimal perforation geometry and the analogues with the "high pressure - low pressure" system. Therefore, the use of the proposed device can improve the technical and economical characteristics of the device due to the conversion of the fluid eddy flows into straight jets and, hence, the intensification of their dynamic effect upon the mobile body, and also due to the reduction of the fluid static effect upon other surfaces. References:

1. Ya. Hogg. Ammunition: cartridges, grenades, shells, mortar bombs. Moscow, EKSMO-Press, 2001, 144 p., Figures - pp. 66-73. 2. US 3575113, F 42 B 7/02, 26.02.1968. 3. SU 1821618 Al, F 42 B 7/00, 21.11.1990. 4. US 3687078, F 42 B 7/02, 31.03.1970.