Since its veiled debut at an annual U.S. Army conference last year, there has been little news of the structural blast chimney, a potentially groundbreaking technology that promises to protect Humvees and other military vehicles from roadside bombs.
A BLAST FROM an IED-like explosive envelops a Humvee equipped with a "chimney" structure intended to protect vehicle passengers. The U.S. Test Center has put the chimney through a number of such blast tests. (DARPA)
Now, new information is emerging, including blast test results from the Army's Test Center in Aberdeen, Md., and new data from the Marine Corps on the vehicle's durability in difficult terrain. The test results paint a complicated picture of a promising technology that still has to work out some kinks. And until there is more data, it remains unclear how much the chimney contributes to the safety of the people inside.
Back in October, those invited to an off-site art gallery near the Washington Convention Center could view a blast-tested Humvee and learn of its ability to vent explosive energy up through a blast channel that ran through the center of it.
News of the chimney immediately drew questions from skeptics. There were whispers that Hardwire, the company behind the chimney, and its teammate on the project, Humvee-maker AM General, had oversold the technology's potential.
Others wanted to know how a blast channel running straight up through the interior of a Humvee affected the space and visibility for the soldiers inside.
As the Defense Advanced Research Projects Agency (DARPA), the Army and the Marine Corps proceeded to blast-test the vehicles in Aberdeen, defense officials talked about the technology's potential in congressional testimony and in meetings with reporters.
"Visually, it looks very, very promising," Army Secretary John McHugh told lawmakers March 16.
He said he'd watched classified videos in Army Chief of Staff Gen. George Casey's office of the vehicles being blown up.
Until now, no one outside the program has learned what "very promising" means.
M-ATV PROTECTION, HALF THE WEIGHT
While the tests' results remain classified, DARPA officials say the blasts indicate a Humvee equipped with the structural blast chimney provides the mandatory survivability level required of an M-ATV, the lightest version of the military's mine-resistant ambush-protected (MRAP) vehicles. And it does so at almost half the weight.
The gross weight of the Humvees being tested is 16,300 pounds. An M-ATV weighs roughly 30,000 pounds, while the average MRAP weighs 40,000 pounds.
"The data to date indicates that the occupant survivability of this vehicle is comparable to the M-ATV," said Leo Christodoulou, who directs DARPA's Defense Sciences Office.
There have been 11 survivability tests to date, and eight have been under a joint DARPA-Marine Corps-Army program. The Army Test Center has conducted all of the tests.
Five more survivability tests are scheduled before the end of the summer, Christodoulou said.
A DARPA chart prepared earlier in the year for Congress shows that the goal is to achieve MRAP-like survivability at less than half the weight. The companies behind the effort remain confident that such results will be achieved in upcoming tests.
The structural blast chimney holds a lot of promise for even bigger explosions, according to AM General CEO Charles Hall. He said the structural blast chimney could provide the survivability of vehicles two to three times its weight.
Chris Yunker, who leads the vehicle requirements team at the Marine Corps Combat Development Command, said the service is watching the blast testing closely.
"When they do the blast testing on this rigid cabin we're seeing pretty good results," Yunker said. "That really caught our attention and we said, 'This is a good design; let's keep looking at it.'"
Despite its moniker, there is much more to the new vehicle design than the chimney.
Hardwire CEO George Tunis explained that all of the components working together keep the vehicle intact, on the ground and its occupants safe.
The clutter inside the vehicle has been cleaned up, with new racks designed to hold gear and ammo. Hardwire picked seats designed by Jankel, which Tunis described as the best blast-mitigating seat on the market.
Hardwire has also redesigned the way the Gunner Protection Kit responds during a blast event, greatly reducing the blast overpressure, he said.
The floor has also been redesigned to absorb as much blast energy as possible, Tunis said.
Finally, there is the chimney, which not only creates a hollow space through the vehicle but a very rigid cab structure that can withstand big explosions. The chimney also connects the ceiling to the floor, which reinforces the floor's strength, Tunis said.
But what once seemed like the key feature - the hollow blast channel - might only play a small role in the vehicle's protection.
Christodoulou said DARPA is still running tests and simulations that will help decouple the contributions of the various components. For the chimney, "the jury is still out," he said.
First and foremost, the chimney provides structural rigidity to the cabin, Yunker said. "There's a little bit of debate on whether there is any venting effect of that chimney."
The numbers he's seen show it may contribute 3 to 7 percent in the decrease of the vehicle's acceleration.
"I have the luxury of being a combat developer," he said, "and I tell people, 'I don't care if it's bubble gum on the window that makes the thing work.' If it works, we're interested in it."
He said that of the three ways that Marines and soldiers die in roadside bomb attacks, the chimney may be protecting against two.
The chimney does not mitigate a bomb's energy impulse, which still hits the vehicle's underbody, but it may help decrease acceleration from overpressure, according to Yunker.
"It looks to me like there's some venting, because you see smoke and dirt coming out of that chimney," he said.
Troops are also killed by fragmentation and debris, and some of that appears to be going through the chimney as well, he said.
"No matter what the percentage, if it reduces injury by any percent it's worth it," Tunis said.
SYSTEM'S FLAWS
While DARPA continues to investigate the question of the chimney, the Marines are focusing on the Humvee's structural durability after thousands of miles of driving.
"What we're seeing in the structural blast chimney design is similar to a problem we're seeing in other designs," Yunker said. The rigid cabin structure is putting pressure on the Humvee's frame and this is leading to fatigue problems.
"You can drive it a few thousand miles and you'll have some structural damage and pretty quick you'll have some real component failures, so our next step is to focus now on the automotive issues," he said. "It's kind of like we're looking at a house, but we've got a foundation problem."
The Army and the Marine Corps also have to consider cost. The question becomes how much does it cost to solve the problem versus buying new vehicles.
Meanwhile, both services will have other candidates to consider.
"Honestly, we're seeing other designs in that same weight class that are making M-ATV-level protection," Yunker said.
AM General even has another potential offer for the Army and Marine Corps' upcoming Humvee recapitalization programs.
After canvassing industry a few years ago, AM General decided to throw its luck and investment behind the Hardwire solution and a different one from Israeli armor-maker Plasan.
"This [chimney] program is not supposed to be a point solution for anything; it's supposed to inform us to make the next vehicle decisions," Christodoulou said.
While questions about the chimney remain, officials agree that people should let the data speak for itself.
"Let's follow the data. Let's continue to do experimentation and see where the data and the facts take us," Yunker said.
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