Small drones are the ultimate smart bombs: They can become portable, personal cruise missiles weighing five pounds or less able put a warhead on target miles away. Some, like the Israeli Hero-30, are already being deployed. But the next generation of such U.S. weapons will have advanced warheads that can hit targets from tanks to buildings. These will be based on a railgun weapons technology descriptively known as MAHEM.

There has been much excitement over the prospect of railguns replacing heavy artillery. The railgun uses electromagnetic force to drive a projectile to phenomenal speeds impossible with gunpowder-style propulsion, but even a small one weighs 250 pounds. With MAHEM, or MAgnetoHydrodynamic Explosive Munition, you can carry the same sort of power in your hand.

We've known since 2008 that the Pentagon that the Pentagon has had MAHEM in development. Yet even this many years later, virtually all information about the program is classified—at least, all information from the U.S. government. We discovered details online from a surprising source—China—suggesting that the country may be reverse-engineering this advanced weapons tech.

The Evolution of Warheads

Explosive warheads have worked in pretty much the same way since Henry Shrapnel's 1784 artillery shell, which was designed to explode and throw out musket balls in all directions. The shaped charge was a 20th century refinement in which the force of the explosion blasted a hollow metal cone into an armor-piercing jet, enabling low-velocity weapons like the bazooka to knock out heavy tanks. Then came the Explosively Formed Projectile. Here, the explosion folds metal into an aerodynamic slug that is less penetrating than a shaped charge but able to do more damage against lightly-armored targets. (It's a larger mass at a lower velocity, and makes a bigger hole.)

MAHEM is different because it combines explosives with electricity. It works in three stages. The first is an electronically modified explosion. The explosion creates an expanding fireball; applying an electrical current to the fireball increases the velocity and pressure of the blast, getting more bang from the same ingredients.

In the second stage, the power of the explosion is converted into electricity. This builds on previous work into "explosively driven flux compression generators" that convert explosive power into an electromagnetic pulse. In MAHEM, a ceramic material produces an intense electric current as the shockwave hits it, in a process known as electromagnetic braking. In contrast to a normal explosion, in which most of the energy is wasted, the makers claim the MAHEM has "superb energy conversion efficiency."

Other explosively driven flux compression generators are designed to produce a high-energy electromagnetic pulse to destroy electronics. In MAHEM, the electrical energy is used to accelerate metal, as with a railgun. Depending on the intended target and how it is triggered, MAHEM can fire shrapnel, an armor-piercing jet, or an explosively formed projectile. And because of its high efficiency, MAHEM can accelerate a projectile to higher speed, or accelerate a greater weight, than existing warheads. It also has more controlled precision; the makers claim it can project multiple jets.

Secrets and Railguns

MAHEM technology has been brewing slowly. It came to the public attention in 2008, when DARPA's plans for the next year included "Develop and conduct experiments to demonstrate feasibility of a self-contained MAHEM in the form of an AT4 shoulder-mounted munition" (The AT4 is a small bazooka used by the US Army with a one-pound warhead). This amazing-sounding weapons tech got a flurry of attention back then, including mentions in Popular Mechanics and a bit part in Call of Duty.

Then, MAHEM disappeared from the public radar.

There is little information available, just a few tantalizing mentions in Pentagon document of how the technology is coming along. One MAHEM research project is investigating a dial-a-yield warhead that could be set to any blast level as needed. Another is a contract for a "Novel Light-weight Warhead for Breaching and Destroying Hardened Structures"— a shoulder-launched bunker-buster that was completed last year. The latest version is the Electromagnetic Explosive Warhead (EMEW), a MAHEM warhead for the US Army's Organic Precision Munitions program, which includes portable lethal drones. EMEW provides "augmented explosion, selectable fragmentation, and controlled blast." The pattern and direction of the effects can be controlled, or it can produce blast only with no fragments, like a giant stun grenade, for "non-lethal effects."

It is difficult to find anyone in the military to talk about MAHEM, or even admit that they are involved. DARPA told me that their MAHEM program "ended in 2012 or 2013 and is now being pursued by the Army Research Laboratory." The ARL would not discuss MAHEM and referred me to the makers, Enig Associates. Enig Associates appears to be unreachable, with calls on several numbers (including cellphone) going to voicemail with no replies. Nor were there replies from their email addresses. Enig's website now contains less information than when enquiries started. True to their name, the company is enigmatic.

All this secrecy might look like a successful attempt to prevent any technical details of MAHEM from getting out. However, there is technical information available if you go to the right place: China.

For $28, I downloaded a scientific paper entitled "Physical Modeling of Magneto Hydrodynamic Explosive Munition and Detonation Control" from the journal Applied Mechanics and Materials. The paper was written by a team at the "ministerial key laboratory" at the Nanjing University of Science and Technology, and is a detailed theoretical breakdown of how MAHEM works. It includes block diagrams of the electronics, the complex "kinematics differential equations of kill element" that indicates how it accelerates metal projectiles, and details of the ferroelectric ceramics in the flux generator. This is more information than you can get from any US source, and appears to be based on the reverse-engineering MAHEM by a team with a very detailed knowledge of magnetohydrodynamics and muntions.

The Chinese paper was published in 2013 and refers to several other theoretical studies, but only limited experimental work. Unless there is other, undisclosed practical side to the work, China is still several years behind the US in developing this type of warhead.

The Buildup

It is easy to see why China might be interested in acquiring this sort of capability. China is by far the world's largest producer of commercial drones, selling over a half a million this year including popular models like the Phantom III. It is a small step from there to building small military drones, and MAHEM could take the striking power of small drones to another level.

If the developers' claims are accurate, MAHEM will be significantly more effective than existing anti-tank weapons while also being deadly against other targets including small buildings, armed pick-up trucks, and insurgents on foot currently targeted by drone strikes.

David Hambling's new book Swarm Troopers: How small drones will conquer the world, is out this month.

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