Terror-Stopping Building-Sealer Too Much for Darpa

The recent anti-terrorist operation in Mumbai highlighted the difficulty of clearing hostiles from a large building. This is exactly the sort of situation that Darpa's Reversible Barrier program was intended to help. But the challenge appears to have been too daunting, even for the Pentagon's premiere research agency.

The problem is the sheer scale of the building compared to the number of troops available. "For example," Darpa notes, "U.S. Army doctrine states that for a large two-story building, 60 soldiers are required to safely clear the building. Many of these troops are 'leave behinds' who guard doorways and hallway intersections."

What is needed is a means of sealing off access points – in particular doorways and windows – to ensure that once an area is cleared, it stays cleared. Hostiles can't simply re-occupy an area that has been swept.

The requirements for the portable barrier are that it should weigh no more than 11 pounds and it should be possible to fit two in a standard issue rucksack. The installation time has to be under thirty seconds; and it has to be reversible, so that it can be removed again in under a minute.

But the real challenge is with how tough it has to be. Not only should the thing be able to "stand up against forces equivalent to attack by a fully equipped infantryman for one hour." It has to "resist commonly available chemicals and fire," as well. Which is not exactly a simple task.The document specifies that it has to be resistant to everything from crowbars and pickaxes to Molotov cocktails and paint thinner. (Not to mention gunfire.) The "fully equipped infantryman" is considered to have a bayonet, entrenching tool and AK-47.

The pictures and some of the documentation suggests that the favored approach was some sort of chemical foam that could be sprayed into the opening and which would rapidly harden. Another approach was something like the "sticky foam" already in use, would simply prevent anyone from getting through. A third suggestion was microwire:

Thin steel wires compacted under tension. When deployed the wires expand to assume predetermined geometric shapes formed from a tangle of the wire itself. Expansion ratios of 1:6000 are achievable.
The devices could be used to prevent access by vehicles or by personnel to designated areas. Micro wires represent a means of erecting an impenetrable barrier to deny access to an area or building. Sharp edges could lead to cuts.

But none of the ideas appear to have panned out. The program was quietly put on ice last year, and is no longer progressing. Apparently too challenging even for Darpa, at least until some new technology comes along. It's hard to see what the answer is here, but surely
Darpa's famed out-of-the-box thinking can come up with one. A mass of
Kevlar microbubbles? Polymer razor wire you can spray like crazy string and dissolve afterwards? Self-healing liquid mimetic alloy?
Fast-growing GM poison ivy? Bulletproof interlocking flatfish?

Or maybe Danger Room readers have some better suggestions?