Cage armor is a variant of spaced armor used in vehicle protection. Rather than mounting solid plates at a standoff from the vehicle as in the latter, cage armor is a system of slats or bars placed in frames and mounted on standoff brackets. The system was developed as engineers and designers realized that a solid barrier was not necessary to detonate an incoming shaped charge warhead; a minimal coverage system using slats or bars would also work fine if they were spaced closely enough. There are multiple advantages to the cage armor concept over conventional spaced armor - for one, it can be placed over windows or viewports, as it's feasible to leave enough gaps to see through. For another, it is lighter than full spaced armor, and presents less of a surface to wind, which permits its use on lighter and taller vehicles such as trucks.

The purpose of cage armor and spaced armor in general is not to block the detonation of explosive ordnance, but rather to cause it to detonate further from the main armor or hull of the vehicle. In the case of regular HE rounds, this distance may be enough to prevent the round from penetrating; in the case of shaped charge warheads, the goal is to prevent the plasma jet from forming at the proper distance and angle to the armor or hull. Most shaped charge warheads are maximally effective when detonated directly against the armor in a direction perpendicular to the surface. If they are detonated several centimeters from the hull by the spaced armor, the jet will have begun to disperse/defocus by the time it strikes the main armor.

The use of non-rigid or lightweight materials to protect against smaller ordnance has been traced back to World War II. The Wehrmacht, faced with a plethora of man-portable anti-tank weapons on the Eastern front, developed what was termed schürzen (literally, 'apron', as auraseer was kind enough to tell me). This was lightweight, sometimes flexible metal armor (in the latter case, made of chain netting and called drahtgeflecht schürzen) which was used to protect gaps in the tank armor from the smaller HE warheads of early Soviet MBTs as well as anti-tank rifles. It was typically hung down over the top edge of the treads, protecting the gap between the treads and the armor above, or from the bottom of the turret ring in order to protect the turret join.1

One of the more modern implementations of cage armor is BAe Systems' version, which they call LROD2. It is notable for being constructed not of steel but of aluminum for further weight savings. It is used on several modern combat vehicles such as the U.S. Humvee and MRAP, and was developed in response to a requirement issued by the U.S. agency DARPA for a lightweight protection system to defeat/degrade the effectiveness of RPGs against lighter vehicles.

It is also known as slat armor or bar armor, and sometimes by the more general name spaced armor. For a picture of a U.S. Stryker APC wearing cage armor, see here (it's referred to as 'slat armor' in that article).

Sources:

1 Cornish, Nik. Images of Kursk. Dulles, VA: Brassey, 2002. p. 37. Visible online via Google books.
2 BAE Systems press release, July 17, 2007.

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