For decades, depleted uranium (DU) has been the material of choice for anti-tank projectiles – despite a series of controversies about its potential health hazards. But for the near future, at least, the U.S. military will keep on using DU. Alternatives based on tungsten haven't panned out. Now, the Army is upgrading to a new 120mm Advanced Kinetic Energy round, and about the only thing we know for sure is that it will be made of DU. The generation after that … may be an improved version of DU called Stakalloy.

Kinetic rounds are slim metal darts fired from tanks like the MAA1 Abrams at very high velocity. The preference for DU is not based, as some conspiracy theorists would have it, on a diabolical scheme to dump nuclear waste in developing countries. It's because in addition to its high hardness and density, it has a property called adiabatic shear banding. Essentially, DU is crumbly rather than squishy. During the process of high-speed penetration through metal armor, fragments flake off a DU projectile. This means that a DU projectile is "self-sharpening" (compared to tungsten, which tends to deform in a blunted, mushroom shape.) It also means that DU produces a pyrophoric effect, filling the vehicle hit with a lethal fireball of tiny burning particles. That too makes it more effective than tungsten.

For many years, the Pentagon has been researching alternatives to DU, most notably Darpa's "Liquidmetal" initiative on amorphous tungsten. This is a "glassy metal" without a crystalline structure which is very hard and shows the right kind of behavior under extreme stress. However, there still appear to be difficulties with producing large amorphous-tungsten penetrators.

Darpa wasn't able to comment on the current state of the amorphous-tungsten research effort. However, Peter Rowland, spokesman for the Army's Armament Research, Development and Engineering Center (ARDEC) was able to give a categorical statement: Tungsten still plays second fiddle to depleted uranium.

"At present, there is no tungsten alloy or other material that provides armor penetration performance as good as DU," he told Danger Room. "For some time, there have been efforts to continually improve the performance of tungsten alloys, in an effort to achieve performance comparable with DU. Thus far, DU remains superior."

This is why the requirement for the new Advanced Kinetic Energy round specifies that it must be made of DU.

It should also be mentioned that the idea of tungsten being introduced as a "clean" alternative to "dirty" DU took something of a knock when it was found that military-grade tungsten alloy is highly carcinogenic in rats. A 2007 Department of Defense memo advised considering alternative materials to tungsten in munitions developments. Pure tungsten is not carcinogenic, and amorphous tungsten would be very different to existing applications, but this might be a difficult one to sell to the media.

Meanwhile, research continues into improving the performance of depleted uranium penetrators. From the earliest days of uranium processing, natural uranium was known as Tube Alloy (from "Tube Alloys", a codename for the Manhattan Project), while enriched uranium was Oralloy ("Oak Ridge Alloy") and the depleted remnant was known as Staballoy.

Staballoys containing DU with a small admixture of titanium (from 0.75 percent to 3.5 percent) have been the basis of anti-tank rounds for decades. However, now researchers are experimenting with a new version, known as Stakalloy, which combines uranium with niobium and vanadium. This is said to have improved hardness and ballistic properties compared to traditional uranium-titanium Staballoys.

In 2007, the Army requested the processing of "U-V-X Alloy Ingots," described in the solicitation as Stakalloy. The document noted that "previous development work over the last few years at Aerojet for the Army Research Laboratory (ARL) has produced new alloys with interesting properties and test prototypes for ballistic evaluation at ARL." The idea was to find the best method of turning the ingots into "full-scale kinetic-energy penetrators." (A two-stage quench process is suggested to prevent cracking.)

A detailed description of the new Stakalloy can be found in the patent for it. Stakalloy, incidentally, is named after its inventor, Dr Michael Staker.

Peter Rowland stated that the Stakalloy was not being considered for the new Advanced Kinetic Energy round, but left the future wide open. "There is no way of predicting whether the performance of tungsten alloys or some other alternative material will ever approach that of DU, or if the penetration performance of DU itself can be further improved," he said.

It's possible that a viable alternative to depleted uranium will emerge in the next few years. But – at least from an engineering perspective – the advantage is all with DU. Whether it remains politically acceptable is another matter.

Photo: U.S. Army

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