Last month, Pentagon placed a $5.4 billion rush order for 3,900 Mine Resistant Ambush Protected vehicles, or MRAPs, pushing to get at least 3,500 into Iraq by the end of 2007.

The MRAPs have a V-shaped reinforced hull designed to withstand the blast of IEDs and EFPs, the leading killer of US troops in Iraq. The Pentagon's MRAP program is "the fastest moving major program in the Defense Department," according to John Young, chairman of the Pentagon's MRAP task force.

The military plans to order as many as 23,000 MRAPs over the next few years, with about 6,500 currently on order, and the Pentagon seeking other manufacturers prepared to ramp up production on their own designs.

With the ascent of the MRAP, Humvee sees its popularity on decline, and its manufacturer, AM General, is working with the Army to develop a new "Frag Kit No. 6" armor package to fit some of its Iraq-bound vehicles with additional protection to protect against the EFPs that can punch through their standard armor.

According to Military.com, the Frag Kit No. 6 armor uses various metal combinations and spacing to absorb the ballistic impact of an EFP without penetrating the crew cabin. It will be placed over the armor plating of an M1151 Humvee, the most protective Humvee design fielded in combat.

The Frag Kit No. 6 is stronger than the recently-fielded Frag Kit No. 5, which was primarily designed to protect Humvee crews from roadside bombs that detonate under the vehicle or ones with force enough to split an armored troop carrier in two.

The Frag Kit can add a layer of protection against EFPs, but the Army Research Lab-designed package does have drawbacks, adding about 1,000 pounds and 12 inches to each side of the vehicle.

The weight makes the doors so heavy that they may have to add a mechanical device to help open and close the doors, and drivers will need help judging if they can fit the modified Humvee through narrow spaces.