Update: Welcome, Conservative Grapevine readers. I hope you will look around. If you have a question, just ask. My goal here is to help you understand how the Army works.
You’ve seen on the news how Iranian made EFPs are being used as roadside bombs to attack US vehicles in Iraq. But what is an EFP?
Early IEDs in the Iraq War were mostly artillery shells wired to explode. The first Humvees in Iraq had thin armor that would not protect very well against nearby explosions. As up-armored Humvees became available, these early IEDs lost some of their effectiveness. The insurgents reacted in two ways. First, they used bigger IEDs, wiring together several artillery shells at once. The larger blast was more effective, but took longer to emplace and were easier to spot. The second technique, using EFPs, is more difficult to counter.
EFP stands for Explosively Formed Penetrator. Using the concept behind a shaped charge, a disc of metal on one end of a charge can be blasted in the direction of the target. The charge is usually a steel pipe, 6-8″ in diameter. When detonated, the concave disc is deformed by the explosion, and reformed into a slug. The explosion pushes this slug at phenomenal speeds- up to Mach 6.
One of the biggest advantages of EFPs is standoff. The damage to the target isn’t caused by the explosion, but rather by the slug it fires. This means that the EFP doesn’t need to be right next to the roadside to be effective. This means that US soldiers have to scan a much larger area to detect IEDs.
An EFP can usually penetrate as much armor as the diameter of the charge. That is, a charge 6″ in diameter should be able to penetrate 6″ of armor, more than enough to defeat the armor of any Humvee, and indeed, all but the most heavily armored tanks.
Clearly, the threat posed by EFPs is one of the reasons that the US is putting so much pressure on Iran to stop equipping insurgents. Other countermeasures have included focusing on raiding bombmakers.
Update and Bump:
Because the EFP fires a single slug, timing the explosion is critical. Too soon, the slug goes in front of the vehicle. Too late and it misses behind. To get around this, the insurgents are using a cheap passive infrared sensor, sorta like an electric eye. When a Humvee passes in front of the sensor, off goes the EFP and destroys the vehicle. Ahh, but it didn’t take long for the US to come up with a countermeasure. You can see in the photo below a “Rino” device, designed to trigger the EFP before the Humvee reaches the kill-zone. Normally, it would be lowered so it is in front of the Humvee.
An unsuccessful EFP attack can be seen here.