Following up on my September post , which was about the conical shaped charge as it appears in the warhead of a Hellfire missile or RPG round. Conical charges depend on explosives backing a hollow cone, commonly of metal. I wrote an article on the subject for Invention&Technology.
This post finishes up with a description of the other two principal types of shaped charges. The linear shaped charge is a sort of explosive rope that can be draped across or glued onto the object to be cut. When detonated it can cut massive steel beams and open holes in reinforced concrete walls, so it's commonly used in demolition work and other applications where one thing needs to be quickly separated from another, such as spacecraft. The V-shaped metal of a linear shaped charge is hammered into a sort of chisel blade by the longitudinal explosive, and heads out at hypersonic speeds. Here's a link summarizing NASA's experience with them during the Apollo program, including one model called the "guillotine."
The latest shaped charge, the explosively formed penetrator, or EFP, relies on explosives packed behind a shallow metal dish to forge and launch a dense cylinder of solid metal. American troops in Iraq learned to dread the EFP’s ability to disable even the heaviest MRAPs and tanks in the U.S. inventory. The weapon, about the size of a large coffee can, packs so much kinetic energy that the penetrator either smashes through vehicle armor or causes a grenade-like burst inside, from metal splintering off the interior of the vehicle’s own armor.
Unlike the jets produced by conical and linear shaped charges, which are unstable in flight and therefore limited in range to a few inches or a few feet at most, the EFP can destroy targets at a hundred yards or more. Researchers have even tinkered with the explosion to forge little aerodynamic fins in the back.
(While the EFP remains the biggest threat to tank crews at this time, tacticians are also having to think about the kinetic energy penetrator, a fast-moving, dense metal rod that's fired from a tank's main gun and hits an opposing armored vehicle with greater mass and force than an EFP. An example is the Armour-Piercing Fin-Stabilized Discarding Sabot. Radically new defensive techniques, perhaps employing sideways-sliding armor plates powered by their own explosive charges, will be required to stop the long-rod penetrator.)
Shaped-charge researchers of the future are said to be looking at multiple shaped charges, which would simultaneously erupt from the nose of a massive earth-penetrating bomb.
Designers would also like less liner metal wasted in the slow-moving slug and more crammed into the devastating, fast-moving jet. With such efficiency in mind, engineers are looking into a new type of shaped-charge liner using chemically-reactive materials, which might reduce fouling in oilfield applications and be useful for new warheads. As far as I know, there's no breakthrough on that front yet.