The Discovery and Benefits of The 'Munroe Effect'

A hollow cylinder, in which explosives are lined with a metal casing of a particular shape for focusing the results of explosion, is called a shaped charge device. When the explosives are detonated, the resulting explosion forces the casing forward at high velocity in the form of a narrow jet, which can breach hardened and reinforced structures. The shaped charge effect was investigated widely by Charles Edward Munroe in 1888, and is named the “The Munroe Effect” in his honor (also variously known as the Foerster effect or the Neumann effect). The Munroe Effect is used in a variety of situations, both civilian and military.

Perhaps the first mention of shaped-charge explosives comes from the civilian sector. The German mining engineer, Franz Xaver von Baader, proposed the adoption of an empty conical volume in front of a detonating charge in order to increase its efficacy. However, this was in 1792, and the only explosive available at that time was gunpowder. Since this was not a high explosive, it could not produce a shock wave on detonation, and was hence incapable of producing the shaped charge effect (Baader 1792).

A crucial event in the history of explosives occurred in 1867, when Alfred Nobel invented the detonator. But the first implementation of the hollow charge effect had to wait for almost a century after Baader, when the German Max von Foerster created it in 1883. A few years later in 1886, another German, Gustav Bloem discovered a way of detonating caps having a hemispherical lining material within a shell. His invention had a lined cavity, and can thus be considered a true precursor to the shaped charge concept (Walters 2008).

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At this time Professor Charles E. Munroe was working at the U.S. Naval torpedo laboratory, Newport, as a civilian chemist. His main responsibility was to look for better propellants, and he used to collaborate with the laboratories of E. I. du Pont while researching for smokeless powders. The problem that Munroe faced was one of deterioration of the powders when they were stored for a period of time (Carlisle, 2002, p. 21). In 1888, Munroe independently developed the idea of an unlined shaped charge, and he wrote a number of papers on his idea.

While Munroe may not have been the originator of the hollow charge explosion idea, he is remembered for his greatly successful demonstrations of its effects. Beginning 1888, he performed a series of experiments in which the power of both lined and unlined shaped charges were shown. To demonstrate the unlined charge effect, he aligned a steel plate opposite to explosive blocks, with the letters USN (United States Navy) inscribed on the latter. On explosion, the letters were reproduced on the steel plate; Munroe also showed that the indentation amount on the plate increased if the explosives were placed within a cylinder and a cavity was made in them opposite to the point of explosion. The cavity served to focus the products of explosion and could thus form a deeper indentation (Walters 2008). Munroe also used this technique to obtain dramatic engravings of leaves on copper plates, similar to the one shown in Figure 1 below:

Figure 1: Leaf engraving on copper plate using the shaped-charge effect. Source: Davis and Hill, 2002, p. 224.

In another experiment, Munroe demonstrated the puncturing of a steel safe constructed from iron and steel plates. The safe had walls with thickness of 4.75″, and when sticks of dynamite were tied together around a tin can and exploded next to the safe, a 3″ diameter hole was punched right through the top of the safe. Munroe, however, did not properly recognize the role played by the liner, and its significance was realized only much later (Kennedy, 1983, p. 6).

The next references to research on the shaped charge effect come after the turn of the century, from patents filed in Britain and Germany in 1911-12. The patents in Germany were filed by the Westfalische Anhaltische Sprengstoff Actien Gesellschaft (WASAG), and it is possible that the earlier experiments of Foerster in Germany or Munroe in the U.S. were unknown to the patent office employees. The patents filed by WASAG demonstrated both lined and unlined shaped charge effects, while the latter were the subject of patents filed by M. Neumann (in 1911) and by E. Naumann (in 1914). All these patents demonstrated that the depth of penetration could be increased by using a lined shaped charge (instead of a block of explosives), and exploding the charges at an optimal distance away from the target could further increase it.

In Russia, M. Sucharewski, who conducted extensive experiments and reported his findings in 1925 and 1926, investigated this effect. He noted that shells that had shaped charge explosive had lower weights, while at the same time being 3-5 times more powerful in terms of their penetration capabilities. In Italy, D. Lodati carried out similar experiments and he published a paper in 1932. In 1936, Prof. R. W. Wood published a paper, “Optical and Physical Effects of High Explosives”, that reported on advanced effects of high explosives. He discussed the plastic deformation induced in metals by hypervelocity explosions, and also obtained spectrographs of the deflagrating masses (Kennedy, 1983, p. 8).

The Munroe effect is known as the Neumann effect in Germany, and a series of experiments demonstrated its efficacy in 1941. A ships’ armor steel plate was used as the target and a 1:1 mixture of TNT and cyclonite was used as explosives. The experiments used three scenarios – explosives having hollow cavities; those having an iron-lined cavity; and those detonated at a distance from the plate. It was found that the unlined charge had the least penetrative power, about 40% of the cavity diameter. On the other hand, the iron-lined charge had the highest penetrating power, around 120% of the cavity diameter, when it was placed at a distance between 0.5 to 1.5 times the diameters (Walters 2008).

The advent of the two world wars of course accelerated research efforts in shaped charge explosives, together with every other weapon systems. If Munroe was one of the principal investigators of these explosives in the earlier century, then the foremost developers for modern military ordnance purposes were certainly Franz Rudolph Thomanek in Germany and Henry Hans Mohaupt in the U.S. and Britain. These two researchers conducted their experiments independent of each other and gained insights into hollow, lined cavity shaped penetrating charges. Thomanek discovered the lining effect in 1938, while Mohaupt applied for a patent at the end of 1939. The U.S. Army acquired the rights to the discovery from Mohaupt and created high explosive anti-tank (HEAT) grenades as well as projectiles in 1941. The grenade was later modified to fit onto a rocket motor, and it was shoulder-fired to create the celebrated weapon Bazooka. The British Army first deployed it during the desert wars of 1941 in North Africa (Walters 2008).

The technology continued to develop after World War II, and significant advances were made through the 1950s. During this time high-speed photography and flash radiographic techniques became available, and numerical simulations aided by powerful computers were used to understand the physics of the expanding jet. A notable development during the Korean War was the 6.5″ anti-tank aircraft rocket (ATAR). These were used by American pilots to attack North Korean tanks and successfully incapacitate them, whereas their earlier munitions were bouncing off the body of the tanks (Kennedy, 1983, p. 28).

The shaped charge effect and its mechanism of punching through resistant material have been extensively studied. A typical configuration for such a device consists of explosives placed behind a lining material, and a detonator placed behind the explosives. The whole assembly is placed within a casing, usually cylindrical in shape. The liner is usually of conical shape, but it can also be hemispherical or other shapes that are symmetric about the central axis. Liner materials that have been used are: aluminum, copper, steel and depleted uranium (Buc, 1991). As the detonator ignites the explosives, a detonation wave propagates through the medium at a velocity of 6-7 km/s; creating a high-pressure front of about 3×1010 Pa. this high-pressure front expands the casing, leading to its rupture and fragmentation. More importantly, the pressure front travels to the head of the explosives within the liner, and collapses it in the direction of the central axis. Because of the focusing of the explosive momentum along this axis, the liner material is transformed into two fast-moving jets, both with negligible radial velocity. The forward moving jet has a thin, lance-like profile and shoots forward at 6-12km/s, while the other jet has a speed of around 1km/s and is thus left behind (it is known as the slug). A longer jet has better penetrating ability, so the distance between the explosive device and the target (known as the stand-off) should be estimated carefully. The tip of the jet should ideally strike the target about 50µs after detonation, with a surface temperature around 500oC. Because of this high temperature, the target undergoes plastic deformation under pressure when the jet strikes it, expanding around the jet and forming a hole. As the jet transfers its momentum to the target it loses energy, and eventually comes to a stop (Poole, 2005, p. 2). The jet forming mechanism is explained in Figure 2 below:

The casing, liner and explosives before explosion

On detonation at the left side, the detonation front begins to propagate through the explosive. The casing begins to expand, and the liner begins to elongate into a jet

As the casing collapses almost completely, the liner forms into an accelerating jet

The jet accelerates into a lance profile that will punch through hard targets

Figure 2: Mechanism of the shaped-charge explosive. Source: Poole, 2005, p. 2.

As already stated, cavity formation in the target occurs not because of the jet temperature, but primarily because of its high pressure. The penetration capacity of the explosive device depends upon several factors, such as: ductility of the liner material (penetration capability varies as the square root of the material density); length of the jet; the stand-off distance); and articulation of the jet, which should be minimized for better performance (this again depends on the micro crystalline structure of the liner material) (Held, 2001, p. 1). Pure metals have been observed to perform better than alloys. A ductile jet, which has a smooth profile and narrows down gradually towards the tip prior to its break up, has been found to have maximum penetration power.

Many variants of the original concept are now used, such as cutting charge, plane charge and explosively formed projectiles (EFPs). A cutting charge acts in the longitudinal direction along the central axis of detonation. In a plane charge, the lining is deployed with a wide angle (100o or more). As a result, the explosion does not form an advancing jet but turns the liner inside out. This results in a weaker blast with less penetrating depth, but a wider diameter of destruction. EFPs are used frequently in the military. In an EFP, the lining is constructed in such a manner that all components of the resulting jet have a uniform velocity. Almost the entire liner is converted into the jet, resulting in higher kinetic energy; this can then be used against targets at a greater distance (Meyer, Kohler and Homburg, 2002, p. 284).

Shaped charges are deployed for piercing hardened substances, both in civilian military applications. They are especially useful for petroleum engineers, and are used for hydrocarbon reservoir characterization as well as for breaching rock strata. In fact bore expansion in oil-fields requires careful optimization of a range of properties, such as rock thickness, deformation characteristics, strength, stress on the rock strata etc. In such cases shaped charges have been found to be effective because they can generate a localized, directed force that penetrates the cement lining of an oil well without disturbing the strata above or below. Figure 3 below shows this effect:

Figure 3: Use of shaped charge explosives for penetrating concrete linings in bore wells. Source: Poole, 2005, p. 4.

Shaped charge explosives are also commonly used in mining, tunneling, excavation works and large civilian projects where precise breaches need to be made under controlled conditions. In the military, all three wings of the armed forces – the army, navy and air force, use shaped charges. The army uses them for manufacturing anti-tank weapons, armor-piercing rounds and bunker buster bombs. The navy uses them in underwater operations and in torpedoes. The air force uses them in making missiles for attacking enemy aircraft and even spacecraft (Walters, 2003).

The shaped charge explosion effect has played an important part in civilian as well as military engineering. It has been used in blasting numerous mines, tunnels, roadways and rail tracks across countless mountains and landscape formations around the earth. Oil companies such as Bloomberg for drilling and expanding oil wells also use it extensively. It has, of course, played an important part in warfare, especially as anti-tank and bunker buster projectile weapons. Shaped charges are used wherever precise breaches in hardened materials are required, and there is ongoing research in increasing their penetrating power as well as precision of the breach. It is expected that they will remain one of the more popular detonation devices at least for the next few decades.