The U.S. military's toughest trucks are getting a new layer of protection – against rocket-propelled grenades. The Army recently awarded an $8 million contract to equip MRAP (mine resistant ambush protected) armored trucks with Iron Curtain. It's a protection system which blasts incoming rockets before they can hit the vehicle. If the system works, it could go all long way towards neutralizing one of the deadliest threats American troops face overseas.

The contract calls for the Iron Curtain with Darpa's sniper-detection system, CROSSHAIRS. It detects and locates enemy shooters using radar and acoustic sensors, and is intended to work against Rocket Propelled Grenades (RPGs), missiles, bullets and mortar rounds. Iron Curtain itself is currently designed to deal with RPGs; the makers Artis LLC planning future developments to deal with more challenging threats.

Iron Curtain system detects and tracks the incoming rocket with a radar, which then cues an optical sensor – a smart camera, essentially. The optical sensor identifies and classifies the threat – pinpointing the location of the rocket with an accuracy of about half an inch – and selects an aim point.

A row of explosive countermeasures is mounted on a rail running around the top of the vehicle. The system selects the best one of countermeasures, and fires it vertically downwards at the exact moment the rocket is passing. This does not destroy the warhead but 'duds' it so that the warhead deflagrates, rather than exploding properly. By the end of the collision of RPG and countermeasure, Artis claims, the warhead bounces off of the vehicle's side.

In field tests mounted on uparmored Humvees, Iron Curtain has proven its effectiveness against RPGs, so the makers' confidence is understandable. Here's some video, from a corny-but-cool National Geographic documentary:

Iron Curtain is one of many, many competing systems for RPG protection, ranging from metal slats and bars to Kevlar airbags and missile-firing Active protection Systems. Iron Curtain is at the more sophisticated end of the spectrum, and one of its most distinctive features is how the countermeasures work.

Collateral damage is a major issue for "active" protective systems like Iron Curtain. Whatever you fire at the incoming round is going to end up somewhere, and it may do real damage. Systems that spray out shrapnel are likely to be a major hazard for anyone in the immediate vicinity. This can included dismounted troops who have just stepped out of the vehicle, or local civilians if an attack takes place in a crowded area.

One way around this is to use Dense Inert Metal Explosive (DIME), basically micro-shrapnel made of powdered dense metal which only travels a few feet. Another approach is to have shrapnel made of Reactive Material which burns up in the air and only gets a short distance before it complete vaporises. The Iron Curtain approach – of simply firing downwards – could be safer than either of these and should ensure that the countermeasures only hit what they’re aimed at.

It's not clear how the system could defend against attacks from above, but the makers' web site says that Iron Curtain "can be configured to protect almost any surface, from just the sides of a vehicle to all-around protection, including top."

The integration with CROSSHAIRS makes this more than just a system to protect one vehicle. The aim of CROSSHAIRS is to "engage enemy shooters" with both automatic and man-in-the-loop modes. And it might even go one step further than that, according to Darpa: "Additionally, the program is investigating the feasibility of a variety of technologies to detect enemy shooters before the firing of a weapon."

This suggests that in principle, a CROSSHAIRS-equipped vehicle with a convoy could open fire on any potential ambushers before they fire a shot. Or it might just engage them with something like a non-lethal laser dazzler, which could avert an ambush without risking shooting the wrong people.

It could be a useful capability, and one that can't be delivered fast enough: the MRAP vehicles equipped with Iron Curtain and CROSSHAIRS should be ready for testing in Juy 2010. Meanwhile, there are also studies to integrate Iron Curtrain with Hummers and with the LAV's operated by the Marine Corps.

Images: Artis, LLC