Protection from attacking threats is a constant game of measure and countermeasure that has been going on for millennia. Its technological sophistication ratcheted up during World War I, when tanks were first deployed as a means of breaking the stalemate of trench warfare and the futility of massed infantry attacks. Those early tanks had to protect mainly against machine gun bullets and artillery-shell fragments, so less than a half-inch of steel armor protection was sufficient.
But as soon as they were deployed, opposing forces responded by firing bigger guns and using artillery as a direct-fire weapon. Tanks then evolved, in turn, to having as much as three inches of steel armor in the frontal sector. This protected the occupants from all but the very largest guns. But not from shaped charges.
Shaped charges are wonders of science. They are based on the “hollow charge principle,” which in America was discovered accidentally by Dr. Charles Munroe at the Newport Naval Torpedo Station when a TNT block with the manufacturer’s name indented in the charge accidently went off on top of a sheet of steel. The indented letters acted as hollow areas in the TNT and produced deep indentations in the steel resulting in a mirror image of the letters in the steel plate.
That led to the design of a shaped charge as shown here with three explosive charges sitting on steel billets; the first charge having a flat bottom, the second having a conical indentation, and the third having an indentation with a thin metal liner. Upon detonation, the explosive gases and molten metal form a high-velocity jet capable of punching through many inches of steel armor. Optimization of this effect soon led to small weapons such as the bazooka and the rocket propelled grenade (RPG) that allowed even dismounted infantry troops to wield high-penetration tank-killing weapons. And larger weapons, such as anti-tank guided missiles (ATGMs), fired from vehicles or ground mounts, were completely unstoppable.
This situation pertained until the development of reactive armor, which consists of two thin metal plates with an explosive or propulsive layer between them. When struck by a shaped charge jet, the explosive layer detonates, throwing the metal plates obliquely across the jet, disrupting it and greatly reducing its ability to penetrate.
Reactive armor briefly changed the balance between armor and offensive weapons in favor of the former. But that advantage was quickly overcome by tandem shaped charges, in which a small forward shaped charge is used to initiate the reactive armor a few tenths of a millisecond before a large rearward shaped charge arrives. The second charge can then penetrate the underlying structure unimpeded. Another innovation was attack-from-above weapons such as the Javelin ATGM, which engage areas of the tank not generally protected by reactive armor.
The measure-versus-countermeasure contest evolved into the development of “active protection systems” (APS), which use sensors to detect an incoming threat, track it, and then initiate some countermeasure that results in the destruction or dudding (i.e., deactivating) of the inbound threat. Often, this requires a combination of sensors – usually a wide-area radar to detect the incoming threat initially, and either an electro-optic system or a high-precision radar to accurately track the threat and trigger the countermeasure. Countermeasures typically consist of some type of fragmenting weapon that either causes the threat to explode at a distance, duds it, or physically dismembers it.
The Israeli Trophy system is the best-known APS system. It employs a wide-area scanning radar to detect threats, a smaller high-precision radar for accurate tracking, and an array of explosive, formed projectiles – spherical sections made of metal (the liner) with an explosive backing as the countermeasure. When the explosive is initiated, the liner inverts and forms a projectile flying at very high speed. Trophy was a remarkable advance in battle because it enabled the tank crew to eliminate the immediate threat and neutralize the source* and changed the calculus on the battlefield. It first saw combat in 2011 and has saved the lives of many Israeli tank crews and their accompanying infantry.
Active protection systems are very technologically advanced. Because the entire encounter can take place in less than a second from threat launch to impact, the systems must react very quickly and there is no time for human interaction. Moreover, they must be able to discriminate against false alarms, to avoid firing at every bird or bullet that flies by. Fortunately, most anti-tank threats travel at 100-300 meters/sec, which is much faster than any bird, and they are much larger than bullets, providing a basis for threat discrimination.
Also, the countermeasure must minimize the hazard to nearby friendly troops, so it must be very focused in its effect. And under today’s combat conditions, it must be effective against threats coming from higher elevations. Trophy does all these things, while other systems, such as Russia’s Afganit, do not.
The Trophy system had its early beginnings in the US. In 1986, the Defense Advanced Research Projects Agency (DARPA) began a proof-of-concept program to show the viability of active protection. By 1990 this program had displayed significant success. The Israeli Ministry of Defense became interested in this and in 1991 a Data Exchange Agreement with Israel was executed to share technical data on active protection systems. The cooperation took on the name “Trophy.”
After the end of the Gulf War in 1991 the U.S. declared a “peace dividend,” cut the Defense Department budget, and generally lost interest in ground warfare as being outmoded. Israel, however, was under no such illusions regarding the ongoing reality of ground combat and continued the Trophy program on its own. Twenty years later it had a system ready to deploy. Trophy achieved international prominence during the 2011 combat versus Hamas. Currently Hamas is using a new anti-tank guided missile launcher that launches two missiles a short time apart in hopes of overwhelming Trophy. The results remain to be seen.
The postscript to this saga: The U.S. Department of Defense must now buy its active protection systems for the M1 tank from Israel at a hefty markup. DoD deserves no trophy for its lack of perspicacity.
*If you get a “Your Browser is Up To Date” response when you click on the link, click on the blue “Browse” button to view the video.
Tom Hafer developed systems for neutralizing rockets and drones. He currently coaches teenage robotics teams. Henry I. Miller, a physician and molecular biologist, is the Glenn Swogger Distinguished Fellow at the American Council on Science and Health. Hafer and Miller were undergraduates together at MIT.