What’s an EFP?

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.


13 thoughts on “What’s an EFP?”

  1. Xbrad,
    I know nothing about this, but is the copper liquefying or just bending? If it is liquefying, What happens in the next 2 or 3 frames of figure 1?
    Am I confusing a efp with some of our armor piercing stuff that uses molten copper?

    Thanks for a intelligent update.

    And the foodie thing everyone can relate to, I need to pick up a box of mre’s for hurricane season.

  2. V,
    EFPs generally use a steel disc. The copper lining is usually found in a “HEAT” shaped charge warhead.
    In an EFP, the force of the blast deforms the disc, and molds it into a slug. The third picture shows a “before” and “after” picture of a disc. It stays a solid, but given the pressures involved, acts a little like a liquid- the technical term is super-plastically formed. It is this solid slug that gives the EFP its standoff range.
    EFPs are weird precisely because they are a blend of shaped charges and kinetic energy penetrators.
    HEAT rounds and kinetic energy penetrators (often called sabot rounds) will be topic in the near future. And go read the Apache post up top.

  3. Xbrad,
    How does ship armor compare to tank (or Bradly) armor. Is ship armor hardened steel? I have not worked in a machine shop in 20 years but hardened steel came in different grades. (A4 O6 etc) Does the Abrams use hardened steel in addition to the ceramic? I am not asking for details just generalities.

  4. V, Argent, two answers for the price of one.

    1. EFPs were actually the predecessors of heat rounds, but weren’t very successful due to the difficulty of aiming and timing, and they were a lot more expensive that regular mines. EFPs are heavy for a given armor penetration and what was wanted was something that could be thrown as a hand grenade. Really. Development continued and produced the HEAT round ( or shaped charge) which could penetrate roughly 6 times its diameter ( a 4 inch round penetrates about 24 inches of armor.) Someone then got the bright idea of sticking a rocket up the ass of a heat round, and the bazooka was born. (really)

    2. When we say a weapon penetrates 6 inches of armor, we really mean 6″ of Rolled Homogenous Armor (RHA) as a benchmark.
    Most ships today have NO armor. There has been a trend to shift from using aluminum for the superstructure back to steel, but that’s mostly because of the fire fighting properties of aluminum.

    Back in the old days, when battleships and cruisers had armor, they usually used Face Hardened steal, with a hard face to break up the round, but the backside of the armor was NOT hardened, to keep it from being brittle and shattered by rounds. My reference on the Japanese Navy shows about 40 different grades of steel, including HT, VH (Vickers Hardened) and NVNC ( New Vickers Non Cemented). I don’t know the exact grades back then, buy US Los Angeles and Ohio subs were made of HY-80, and I think newer subs are from HY-100.

    Bradleys are made of aluminum. For a given weight, aluminum armor provides better protection and stiffness. They were designed to stop bullets up to 14.5mm and fragments from artillery. The front half of the later models is designed to stop up to 30mm (because its opponent, the BMP-2 had a 30mm cannon). An RPG will penetrate. I had a friend in Desert Storm was killed by one, and the guy next to him didn’t get a scratch. Because of the threat of mines, Brads also have a 3/8″ steel plate bolted on to the front half of the bottom of the vehicle. For high threat areas, additional armor can be bolted on, but it costs speed and maneuverability. We usually try not to be in a position to get shot.

    Abrams, as you know have ceramic armor. The outside layer is Face Hardened steel, for the same reason as ships. Inside, there are layers of soft steel, hard steel, ceramic, a mesh of depleted uranium, and styrofoam. Yes, stryofoam. Pound for pound, it is the best armor ever found against heat rounds. The exact composition is secret. I had a glance at a tank that had hit a mine, but even looking at it, I couldn’t really tell you which was which.

    Keep in mind, the thickness of the armor varies over the vehicle. The front is the thickest, followed by the sides, then the bottom, rear, and top. The top of an M-1s armor isn’t very thick at all.

  5. Ahh thanks for that. Yes it’d be unevenly armoured based on expected hits. There are so many alloys and ways to structure them. Ok i read up on HEAT and understand it a bit better now.

    I had no idea ships were not armoured I suppose that emphasises not being targeted and hit in the first place.

  6. I’ll put something up on HEAT rounds soon, especially since I also want to cover kinetic energy rounds, usually called sabot. When you hear about the horrors of the US spreading death through the evil of depleted uranium, that’s a sabot round. More on that later.

    Ships aren’t armored in the sense that they won’t stop a large shell or missile. They do have some armor, but it is mostly to stop the fragments once the shell or missile has hit. Kevlar is becoming popular for this.

  7. Xbrad,
    For some reason I like the Navy. I consider it a crime that our ships are not armored.
    I know that we have not lost a ship in the past 40 + years. Except submarines to accidents. Would the Cole have had so much damage if it was armored? What about the Starke?
    I am required by my bank to have flood insurance. (I live in a flood zone) on the off chance that the creek behind my house floods during a hurricane. I live in Florida, it is not unusual for hurricanes to come close.
    By the same reasoning, shouldn’t the Navy that is supposed to fight in a war have armor, you know like just in case?
    It takes Cahones to go into battle in a aluminum vehicle with little to no armor.

    Ha, DU is the biggest bunch of crap I have seen in forever. People think OH! Uranium, it is bad! Uranium will kill you! Depleted = not radioactive.

    I was looking at a bike (Honda 919 that was Uranium green)

  8. V, I’m partial to the Navy somewhat also, being a Navy brat.

    It’s not so much a decision to be cheap and not have armor. There’s a tradeoff in everything. If you armor a ship, the enemy just makes a missile with a bigger warhead. That’s a lot easier to do than up-armoring a ship.
    Instead, the money (and more importantly, the weight) goes into active defenses.
    Think about a cruise missile attack on a carrier group. It has to get through the screen of fighters, long range SM-2 missiles from the escorts, Sea Sparrow short range missiles, SeaRAM very short range missiles and the CIWS close-in gun system. And it has to do this while we are jamming radars, and sending off a cloud of chaff.
    The Stark was caught with it’s pants down. But you don’t hear about the times the Navy actually did defeat cruise missile attacks. At least two Iranian missiles were defeated in 1988.
    Adding armor to ships costs a lot. Just the armor is expensive. Then the ship needs more horsepower, which means a bigger engine, which means a bigger ship, which needs more armor… it’s a losing proposition.

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