Chimney Sweep

A story published in Friday's New York Times highlights a "blast chimney" technology designed to vent an underbody explosive through a channel in the middle of a humvee and out the roof:

The Humvee fell out of favor in Iraq and Afghanistan as homemade bombs, the biggest killer of American troops, ripped through its light armor and turned it into a death trap.

But recent blast tests show that Humvees built with the new chimney could provide as much protection as some of the heavier, and more costly, mine-resistant vehicles that have replaced them in many uses.

And if the final tests go well, the invention could save billions in new vehicle costs and restore much of the maneuverability that the Army and the Marines have lacked in the rugged terrain in Afghanistan, military officials say. Engineers say the chimney, which rises through the passenger cabin, releases some of the explosive gases — traveling at twice the speed of a fighter jet — that have mangled and flipped many of the vehicles.

Inside the Army had the story on this earlier this month, however:

The Pentagon's chief weapons tester said earlier this year that he wanted to put the brakes on an effort to rush so-called "blast chimney" vehicle technology to the field in U.S. humvees this summer, stating in his initial assessment that the design failed to provide a significant boost in protection against improvised explosive devices and could expose a vehicle gunner to blast damage, according to an internal memo obtained by Inside the Army.

The chimney technology -- designed to vent an underbody explosive through a channel in the middle of the vehicle and out the roof -- is still in testing and is being offered as a potential solution to a planned humvee recapitalization program by AM General, the Defense Advanced Research Projects Agency and Hardwire LLC.

The Pentagon's director of operational test and evaluation, Michael Gilmore, questioned the push to field the technology in a February memo. At the time, the chimney had undergone three tests.

George Tunis, the chairman of Hardwire, said in a July 8 interview that content of the memo does not accurately reflect where the program is today. Tunis said he was unaware of the memo before being questioned about it by ITA, but was aware of the criticisms raised in it. "It is inaccurate and not current," he said.

But a Defense Department spokeswoman said July 8 that Gilmore has not updated his assessment of the chimney since February and is following test results as they become available.

"To date, DARPA has conducted three test events with the [blast mitigation system] installed on a High Mobility Multipurpose Wheeled Vehicle," Gilmore wrote in his Feb. 14 memo to acquisition executive Ashton Carter. "The data from these tests do not demonstrate Hardwire provides Mine Resistant Ambush Protected (MRAP) levels of protection. . . . Unresolved issues remain regarding the protection to crew provided by Hardwire against underbody attacks, as well as other ballistic threats, and the effects on operational effectiveness and suitability of Hardwire's increased weight and reduced space for mission equipment and passengers."

Gilmore also noted that it could not be determined if the humvee gunner would be protected by such a design. "The protection (if any) provided to the gunner is unknown because of a lack of instrumentation (the gunner stands in a position subject to the blast energy transmitted up the chimney)," he wrote.

Tunis, however, said Hardwire had since found a way to instrument the gunner area and asserted that the gunner was protected from the blast. He declined to lay out specific steps to that effect, citing sensitivities surrounding the issue.

"The gunner is not subjected to blast energy transmitted up the chimney," he said. "We use the standard instrumentation setup. The first three [did] not. It would be kind of silly to subject the gunner to blast energy."

In addition, Gilmore's memo noted that "these tests demonstrated the potential for a Hardwire-equipped HMMWV to meet the MRAP threshold for under wheel mine blasts, but did not provide sufficient data for conclusions to be reached regarding Hardwire's performance against the MRAP threshold for underbody blast."

Gilmore also noted that the design of the chimney-equipped vehicle was modified as the first three tests proceeded. "Data were insufficient to identify the particular aspect's of Hardwire's design that contribute most to blast mitigation (i.e. the chimney itself, the structural beam connecting floor and roof, or the underbody shovel plates)," he wrote.

In the memo, Gilmore warned Carter that he might be asked to authorize the purchases of long-lead items so the chimney could fielded this summer. Gilmore recommended that the chimney undergo at least nine more live-fire tests.