By David Hambling, Aviation Week
E-bombs, weapons that destroy electronics with an intense pulse of electromagnetic radiation, have been discussed for decades. But despite years of research and development, there is little sign of their deployment. The prospect of knocking out communications and other electronic systems is attractive, but commanders prefer proven weapons with known effects. Now the U.S. Army is developing technology to provide the best of both worlds, by creating munitions that combine conventional and e-bomb effects in one package.
Explosive munitions rely on blast, fragmentation and sometimes armor-piercing shaped charges for their effects. Researchers want to add an electromagnetic pulse (EMP) damage mechanism as well. This is in contrast to previous e-bomb projects that were intended to be nonlethal so they could destroy materiel without causing casualties. The Army program seeks to enhance existing warheads, adding the feature without affecting blast, fragmentation or armor penetration, and with minimal extra weight.
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The power supply in traditional e-bomb design is a magnetic flux compression generator with metal coils carrying current. The coils rapidly compress in an explosion, producing an intense pulse of energy. The generator is bulky and cannot easily be integrated into existing munitions.
An alternative approach explored by the Army is a shockwave ferromagnetic generator. This is a magnet that blows up and spontaneously demagnetizes, releasing energy as a pulse of power. The effect is known as pressure-induced magnetic phase transition, and only occurs with some types of magnets in certain situations. In 2005, researchers from the U.S. Army Aviation and Missile Research Development and Engineering Center (Amrdec), working with contractor Loki and scientists from Texas Tech University, demonstrated an explosive pulsed-power source based on neodymium alloy magnets, a type used in speakers and headphones.
Having proven that the principle works, the researchers moved on to more exotic lead zirconate titanate magnets. This enabled them to reduce the volume of the power generator from 50 cu. cm. (3 cu. in.) to 3 cu. cm., excluding explosives. Army requirements call for assembly of the power generator, power conditioning and aerial in a 1-in. space. Power output will be measured in hundreds of megawatts for microseconds.
The aerial needed to shape and direct the electromagnetic energy is an engineering challenge, due to the intense force of the explosion and the size required. Allen Stults of Amrdec is working on a “conducting aerosol plasma warhead.” A flame conducts electricity due to the presence of charged particles in it. By altering the chemical mixture of a fireball produced by an explosion, Stults aims to turn it into an electrically conductive aerial, a “plasma antenna.”
This builds on previous Army work with explosively generated plasma antennas. Stults is working with military explosives and ensuring that other blast effects like armor piercing are not compromised by the changes. Previous work has also shown that the composition of the fireball needs to be matched to the frequency of the desired output.
An explosion takes the shape of a roughly spherical fireball, but a plasma antenna needs to be more cylindrical. This is why Stults works with shaped charges that produce more linear explosions. An earlier project looked at using the jet of metal produced by a shaped charge as an antenna, but this has been dropped for the plasma antenna.
An enhanced warhead could knock out a tank even if it did not penetrate. The vehicle would be left without ignition, communications or other electronics. A warhead would also knock out other electronic systems, including mobile phones used by insurgents to detonate bombs and circuitry in rocket-propelled grenades.
There is one big question with an EMP weapon: How to tell if it works. Carlo Kopp, an assistant professor at Monash University of Melbourne, Australia, and cofounder of the Air Power Australia think tank, is an authority in this field. He wrote papers that shaped strategic thinking on electromagnetic pulse weapons in the 1990s, and coined the term “e-bomb.”
“Damage assessment for all electromagnetic weapons, be they e-bombs or beam weapons, is problematic,” Kopp says. “Unless the attack fries the power supply and you observe related electrical breakdown symptoms, you will never know whether you fried the target or the victim intentionally shut down. The expectation that such weapons should provide easy-to-observe bomb damage assessment mechanisms is not realistic.”
The multifunction munition provides more signs of its effects than the traditional e-bomb, whose effects are invisible. It is possible to determine whether a target has been hit, and a target within the radius of blast and fragment damage will also have suffered EMP effects. But these are variable, depending on the angle between the target and the pulse, the nature of the electronic component and the amount of shielding. Effects range from temporary disruption and forced rebooting to permanent damage or electrical burnout of components similar to that of a lightning strike.
With their comparatively low power output, the Army’s new small multifunction munitions are for point targets. Two candidate munitions for upgrade are the Tow missile and 2.75-in. rockets fired by helicopter. This is unlike previous e-bomb efforts, which have focused on large air-delivered bombs or unitary artillery munitions that cover a large area, what Kopp terms “weapons of electrical mass destruction.”
A small e-bomb will be qualitatively different than larger versions. Radiated power falls off with the square of distance, so a target 3 meters (10 ft.) away receives 100 times the effect of one 30 meters away. An EMP-enhanced Tow missile would produce a pulse strong enough to destroy what it hits, but should not disrupt electronics over a wide area. The possibilities of electronic “friendly fire” rule out more powerful tactical e-bombs, but Kopp warns that even smaller versions may cause unpredictable collateral damage. If urban electrical power or telephone wiring picks up the pulse, damage could extend over a wide area.
The smallest weapon that the Army is looking to upgrade is the M77 bomblet fired by the Multiple Launch Rocket System (MLRS). A bomblet has a shaped-charge warhead and throws out antipersonnel fragments. Bomblets cover a wide area—one launcher can fire a 12-rocket salvo blanketing an area the size of six football fields—and are used against soft targets. An EMP-enhanced version would cover the same area, providing even destruction over the target zone.
If the M77 can be upgraded, shoulder-launched rockets and similar weapons could be modified to produce an EMP. Small infantry rockets have limited effectiveness against modern armor. An EMP-enhanced round might not penetrate but could provide a “soft kill” capability that immobilizes a vehicle. This damage is hard to repair and would probably require the replacement of electronic systems.
The U.S. Air Force has an interest in this area, but few details are available. Air-to-air missiles might gain considerably with EMP capabilities, if they could be modified without affecting performance. Antiradiation missiles that target air-defense radar would be another market.
The U.S. Naval Surface Warfare Center’s Indian Head Div. wants to build a warhead that knocks out improvised explosive devices (IEDs) with a plasma fireball. The aim would be to produce a controlled explosion, destroying the IED without detonating it, and so minimizing collateral damage.
Tests in 2007 used explosively generated plasma against artillery and mortar rounds, which are often the basis for IEDs. Information about the project has been removed from the Indian Head web site and no details are being released. This suggests the work is at an advanced stage, possibly field-testing.
Multifunction warheads may finally bring e-bombs into the mainstream of armaments, by making a munition effective against all targets as well as electronic ones.