Back in 1883, an American engineer named Monroe exploded a slab of explosive against a steel plate. The explosive had letters impressed on it showing that it was the property of the U.S. Navy. After the explosion, Monroe was amazed to find that the impressions on the explosive had been reproduced on the plate. He published a paper describing the phenomenon, then everybody forgot about it. There were much easier ways to engrave inscriptions on steel plates.
Actually, Monroe didn’t discover the effect named after him. The earliest reference to the fact that a depression in an explosive concentrates its force goes back to 1792. Further, there are indications that mining engineers had been using this phenomenon for 150 years without telling anybody about it. But the scientific community put the “Monroe effect” in the space reserved for useless knowledge and went on about its business. Von Neumann rediscovered the effect in 1911, but there still seemed to be no practical use for it.
In 1935, a Swiss chemical engineer, Henry Mohaupt, was on duty as a machine gunner in the Swiss Army. All men in Switzerland serve in the armed forces, take periodic military training and are active reservists until age 45. The tensions that would culminate in World War II were building up, and Mohaupt was disturbed at the ineffectiveness of antitank weapons available to the infantry. Switzerland, for example, was relying on the Solothurn antitank rifle—a semiautomatic 20 mm weapon that would have made hash of the tanks of World War I, but would not even dent such vehicles as Germany’s Pzkw IV medium tank. When his active duty term was up, Mohaupt established a laboratory to develop an antitank weapon for the infantry soldier. He took the Monroe effect as a starting point. At first, he used hollowed-out explosive charges to propel metal disks against a steel target. That led him to line the hole in the explosive with a metal cone. That, he learned, multiplied the penetration of the steel— especially if he moved the charge back from the target for a short distance. At the optimum distance, the shaped explosive charge would drill a small hole in the steel about seven and a half times deeper than the diameter of the cone. Through that hole, a stream of fire and molten metal would cause horrible damage to people and machinery.
Mohaupt, knowing that Hitler was a threat to all of Europe, demonstrated his discovery to French and British military authorities. The French, in turn, passed on the information to the United States. U.S. Government officials invited Mohaupt to come to the United States after the invasion of France and the Low Countries on May 10, 1940. He finally got to the United States in October 1940, after delays caused by other U.S. Government officials because he was not an American citizen. He then took over direction of the “bazooka project”—the attempt to develop a handheld rocket that could penetrate tank armor. Britain was working on a similar weapon, the PIAT (for Projector Infantry Anti-Tank)—a strange-looking weapon that used an immensely heavy main spring to power a massive firing pin that ignited a small powder charge to toss a large shell with a shaped-charge filling at enemy tanks. The firing pin and its main spring absorbed most of the recoil, and the recoil cocked the main spring for another shot. Even so, the PIAT kicked like a blue-nosed mule and was even more unpleasant to fire than the .55 caliber Boys antitank rifle—the second least-popular weapon in the British Army. The bazooka, unlike the two British weapons, would have no recoil at all.
The U.S. Army Ordnance Department was skeptical about the whole idea of penetrating armor with an explosion, but the bazooka proved to be a success. In June 1941, the government labeled the bazooka Secret. Thereafter, its inventor, Mohaupt was excluded from the project because he wasn’t an American citizen. That same month, Mohaupt had filed for a patent on his application of the Monroe effect. (He had previously patented it in Europe.) Because he was a Swiss, not an American, citizen, the Justice Department pursued him for violation of the War Secrets Act. Fortunately, someone with common sense in the government called off the Justice Department. Mohaupt then joined the U.S. Army and was assigned to the bazooka project. He perfected the weapon, which was introduced to the Germans in North Africa in 1943.
For the first time, the individual infantryman had a weapon that would reliably stop a tank. When tank armor got thicker, bazookas got bigger. In the Korean War, the 3.5-inch “super bazooka” made short work of the North Koreans’ Russian-built T 34 tanks, which had defied not only older bazookas but also high explosive shells from the 105 mm howitzer.
The bazooka’s shaped charge became the heart of a huge variety of antitank weapons. Most field artillery guns now use shaped charge shells, known as HEAT (for High Explosive Anti-Tank). All antitank rockets do, too. So do recoilless guns. Some of them use special ammunition that does not require the charge to spin. Spinning, the result of rifling, decreases the penetration of a shaped charge.
The Germans adopted a very unusual recoilless gun that used a shaped charge shell. Called a Panzerfaust, it looked like nothing more than a length of pipe. Its shell looked like a trench mortar shell with a long tail. The tail contained its propelling charge. Some of the gas generated by the propelling charge pushed the shell forward, the rest exited through a venturi at the rear of the gun. Gas leaving the venturi blasted out at extremely high velocity—high enough to balance the effect of the low velocity shell leaving the front of the gun. Recoil momentum equals mass times the velocity of what is shot from the muzzle, divided by the inert weight of the gun. The gas had little mass but lots of velocity. Ian Hogg, the British artillery expert, has called recoilless guns like the Panzerfaust and more conventional guns “Newton’s artillery,” because balancing recoil by escaping gas is based on Isaac Newton’s discovery that every action has an equal reaction.
The Panzerfaust was not the German grunt’s favorite weapon. Firing a big shell from a little gun on your shoulder would make anyone nervous, but it was the Panzerfaust’s short range that bothered its users. They had to get close to their targets to use it. Getting close to an enemy tank is not comfortable, and anyone who gets too close could be done in by the explosion of his own shell. The Panzerfaust was effective, though—effective enough to make the Russians adopt an imitation, a gadget they called the RPG 2. They improved the early model enormously by giving the shell a rocket assist. This improved model, the RPG 7, fires the shell from a recoilless gun, like the German and early Russian models. But when it’s a safe distance from the gunner, a rocket motor takes over and carries the shell to a much greater range. With all rocket weapons, a main concern is arranging things so the gunner does not get incinerated by the back blast of his own rocket. The recoilless gun part of the RPG 7 process takes care of this nicely and allows a much more powerful rocket motor than can be used on a bazooka.
RPG 7s have been sold and made all over the world. The weapon is as popular as that other Russian product, the AK 47. It’s particularly popular with Iraqi guerrillas. The United States and its partners in Iraq have far more sophisticated and more powerful antitank weapons than the RPG 7, but the guerrillas don’t have any tanks, so our antitank superiority means nothing.
The Panzerfaust was dangerous because its user had to get close to an enemy tank, but it was by no means the worst in that respect. Every belligerent had a version of the antitank hand grenade, most of them using the shaped charge. The Germans had the Panzerwurfmine, a grenade with four canvas fins to keep it flying point-first so the shaped charge would be effective.
The Soviet Union had the RPG 43, which had a long streamer that popped out when thrown to keep it head-on. And Japan had the Type 3, which had a tail of hemp fibers to insure that the point struck first. Japan also had the ultimate up-close-and-personal antitank weapon: a large shaped charge on the end of a pole. To use it, the soldier ran up to the tank and rammed the charge into it.
In Normandy, the Americans and British used another type of antitank grenade, the Gammon grenade, a sort of bag filled with a plastic explosive. The grenade spreads itself out on the tank, covering any angles, before it explodes. When the Gammon grenade explodes, it is supposed to detach “scabs” of steel from the inner surface of the armor to kill or wound the tank crew, ignite fuel lines and do other damage. It worked quite well on some of the World War I tanks the Germans used in the defense of Normandy. British and Americans have used artillery shells based on this principle. The British call theirs HESH (for High Explosive Squash Head) shells; the Americans use the less colorful HEP (High Explosive Plastic). HESH and HEP shells are seldom used on tanks now. They are more commonly shot at concrete fortifications.
The shaped charge, even more than the land mine, took the Blitz out of Blitzkrieg. Tankers have been able to use a number of defenses against it. Dangling additional armor plates outside the regular tank armor was an early try. That, in effect, moved the charge back from the optimum distance so that full strength of the lethal jet from the exploding charge would not reach the regular armor. The plates, though, added a lot of extra weight and were quickly blown off or askew. Another addition is “reactive armor.” Slabs of explosive are hung on the tank. When a shaped charge explodes, so does the reactive armor. The reactive explosion blocks the effect of the shaped charge. That kind of armor defends against only the first shot. If a second shell hits the tank, there is no more reactive armor to react. And, it has been reported, on some lightly armored vehicles, the reactive explosion crushed the machine that was carrying it. A third, and apparently more effective, defense is laminated armor. This has a layer of ceramic material between layers of armor plate. The ceramic resists the burning effect of the jet of gas and molten metal caused by the shaped charge explosion. It also dampens the shock waves caused by the explosion of an HEP shell and makes less able to break pieces off the interior of the armor.
But with all these defenses, shaped charges and still being used. And, regrettably, even relatively primitively shaped charge weapons are still putting holes in our tanks.