Deadly Spit: Building a Cumulative Ammunition
In 1941, Soviet tankers were faced with an unpleasant surprise - German cumulative shells, which left holes in the armor with melted edges. They were called armor-piercing (the Germans used the term Hohlladungsgeschoss, "shell with a notch in charge"). However, the German monopoly did not last long, already in 1942 the Soviet counterpart BP-350A, built by the "reverse engineering" method (disassembling and studying captured German shells), was adopted, the "armor-burning" projectile for 76-mm cannons. However, in reality, the effect of shells was not associated with burning armor, but with a completely different effect.
Disputes on Priorities
The term "cumulation" (lat. Cumulatio - accumulation, summation) means the strengthening of any action due to addition (accumulation). During cumulation, due to the special configuration of the charge, part of the energy of the explosion products is concentrated in one direction. Priority in the discovery of the cumulative effect is claimed by several people who discovered it independently of each other. In Russia, he was a military engineer, lieutenant general Mikhail Boreskov, who used a charge with a notch for sapper work in 1864, and captain Dmitry Andrievsky, who in 1865 developed a detonator charge made of gunpowder-filled cardboard sleeve with a recess filled with sawdust to detonate dynamite. In the USA, chemist Charles Munro, who in 1888, as legend has it, detonated a charge of pyroxylin with letters extruded on it next to a steel plate, and then drew attention to the same letters that were “reflected” on the plate; in Europe - Max von Forster (1883).
At the beginning of the 20th century, cumulation was studied on both sides of the ocean - in the UK, Arthur Marshall, the author of a book published in 1915 on this effect, did this. In the 1920s, the famous explosive researcher Professor M. Ya. Was engaged in the study of explosive charges with a recess (although without metal cladding) in the USSR. Sukharevsky. However, the Germans were the first to put a cumulative effect on the service of the military machine, who began the targeted development of cumulative armor-piercing shells in the mid-1930s under the leadership of Franz Tomanek.
Around the same time, Henry Mohaupt was doing the same in the United States. It is he who is considered in the West as the author of the idea of a metal lining of a recess in the explosive charge. As a result, by the 1940s, the Germans already had such shells in service.
How does the cumulative effect work? The idea is very simple. At the head of the munition there is a recess in the form of a funnel lined with a millimeter (or so) layer of metal with an acute angle at the apex (bell to the target). Explosive detonation starts from the side closest to the top of the funnel. The detonation wave "collapses" the funnel to the axis of the projectile, and since the pressure of the explosion products (almost half a million atmospheres) exceeds the plastic deformation limit of the lining, the latter begins to behave like a quasi-liquid. Such a process has nothing to do with melting; it is precisely the “cold” flow of material. A very fast cumulative jet is squeezed out of the collapsing funnel, and the rest (pest) flies slower from the point of explosion. The energy distribution between the stream and the pestle depends on the angle at the top of the funnel: at an angle of less than 90 degrees, the energy of the stream is higher, at an angle of more than 90 degrees, the energy of the pestle. Of course, this is a very simplified explanation - the mechanism of jet formation depends on the explosive (BB) used, on the shape and thickness of the lining.
When the funnel collapses, a thin (comparable to the shell thickness) jet accelerates to speeds of the order of the detonation velocity of the explosive (and sometimes higher), i.e., about 10 km / s or more. This stream does not burn through the armor, but penetrates into it, just as a stream of water under pressure erodes sand. However, in the process of jet formation, its different parts acquire different speeds (the rear ones - lower), therefore, the cumulative jet cannot fly far - it begins to stretch and decay, losing its ability to penetrate. The maximum effect of the jet is achieved at a certain distance from the charge (it is called focal). Structurally, the optimal mode of armor penetration is provided by the gap between the recess in the charge and the head of the projectile.
Liquid shell, liquid armor
The speed of the cumulative jet significantly exceeds the speed of sound propagation in the armor material (about 4 km / s). Therefore, the interaction of the jet and the armor occurs according to the laws of hydrodynamics, that is, they behave like liquids. Theoretically, the depth of penetration of the jet into the armor is proportional to the length of the jet and the square root of the ratio of the densities of the cladding material and the armor. In practice, armor penetration is usually even higher than theoretically calculated values, since the jet becomes longer due to the difference in speeds between its head and rear parts. Typically, the thickness of the armor that a cumulative charge can penetrate is 6-8 caliber, and for charges with plates of materials such as depleted uranium, this value can reach 10. Can armor penetration be increased by increasing the length of the stream? Yes, but often this does not make much sense: the jet becomes excessively thin and its armored effect is reduced.
Pros and cons
Cumulative ammunition has its advantages and disadvantages. The advantages include the fact that, unlike sub-caliber projectiles, their armor penetration does not depend on the speed of the projectile itself: you can shoot cumulatively even from light guns that are not able to disperse the projectile at high speed, and also use such charges in rocket-propelled grenades.
By the way, it is precisely the “artillery” application of cumulation that is associated with difficulties. The fact is that most shells stabilize in flight by rotation, and it extremely negatively affects the formation of a cumulative jet - it bends and destroys it. Designers seek to reduce the effect of rotation in various ways - for example, using a special texture of the cladding (but at the same time the armor penetration is reduced to 2-3 caliber).
Another solution is used in French shells - only the body rotates, and the cumulative charge mounted on the bearings practically does not rotate. However, such shells are difficult to manufacture, and besides, the capabilities of the caliber are not fully used in them (and armor penetration is directly related to the caliber).
It would seem that the shells fired at high speed from smoothbore guns do not rotate - their flight stabilizes the plumage, but there are problems in this case too: at high speeds when the projectile meets armor, the jet does not have time to focus. Therefore, the most effective cumulative charges in low-speed or generally stationary munitions: shells for light guns, rocket-propelled grenades, ATGMs, mines.
Another disadvantage is that the cumulative jet is destroyed by explosive dynamic protection, as well as when passing through several relatively thin layers of armor. To overcome dynamic protection, a tandem ammunition was developed: the first charge undermines its explosive, and the second pierces the main armor.
Water instead of explosives
In order to simulate the cumulative effect, it is not necessary to use explosives. We used ordinary distilled water for this purpose. Instead of an explosion, we will create a shock wave using a high-voltage discharge in water. We made the rated sportsman from cutting of the television cable RK-50 or RK-75 with an external diameter of 10 mm. A brass washer with a 3 mm hole (coaxially with the central core) was soldered to the braid. The other end of the cable was stripped 6–7 cm long and the central (high-voltage) core was connected to a capacitor.
The role of the funnel in our experiment is performed by the meniscus - it is precisely this concave shape that the surface of the water takes in the capillary (thin tube). A large depth of the “funnel” is desirable, which means that the walls of the tube should be well wetted. Glass does not fit - a water hammer during discharge destroys it. The polymer tubes do not wet well, but we solved this problem by using a paper liner.
Water from the tap is not suitable - it conducts a good current, which will pass through the entire volume. We use distilled water (for example, from ampoules for injection), in which there are no dissolved salts. In this case, the entire discharge energy will be released in the breakdown region. Voltage - about 7 kV, discharge energy - about 10 J.
Connect the spark gap and capillary with a piece of elastic tube. Pour water inside with a syringe: there should be no bubbles in the capillary - they will distort the picture of "collapse". After making sure that the meniscus is formed at a distance of about 1 cm from the spark gap, we charge the capacitor and close the circuit with a conductor tied to the insulating rod. In the area of breakdown, a lot of pressure will develop, a shock wave (shock wave) will form, which will “run” to the meniscus and “collapse” it.
You can detect a cumulative stream by its poke in the palm of your hand, extended at a height of half a meter above the installation, or by the spreading drops of water on the ceiling. It is very difficult to see the thin and fast cumulative jet with the naked eye, so we armed ourselves with special equipment, namely the CASIO Exilim Pro EX-F1 camera. This camera is very convenient for shooting fast-moving processes - it allows you to shoot video at up to 1200 frames per second. The first test shots showed that it is almost impossible to photograph the formation of the jet itself - a spark of the discharge “blinds” the camera.
But you can shoot the "armor penetration". It is not possible to break through the foil - the speed of the water stream is small for liquefying aluminum. Therefore, we decided to use gelatin as armor. With a capillary diameter of 8 mm, we managed to achieve “armor penetration” of more than 30 mm, that is, 4 calibers. Most likely, experimenting a bit with focusing the jet, we could achieve more and even, perhaps, break through the two-layer gelatin armor. So next time, when the army of gelatin tanks attacks the editor, we will be ready to give a worthy rebuff.
We thank the CASIO representative for providing the CASIO Exilim Pro EX-F1 camera for the experimentThe article was published in the journal Popular Mechanics (No. 9, September 2008). Do you like the article?
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