Tests by Navy researchers on a new Army and Marine helmet design, complete with a visor and a jaw protector, showed blast waves could bounce off the added components and produce unexpected pressure, according to a recent research paper.
The Conformal Integrated Protective Headgear System, or CIPHER, prototype came under attack from all sides during the test, conducted by the Naval Research Laboratory, and in all configurations: Helmet only, helmet and visor, helmet and jaw protection, and the full-face coverage of visor and jaw protector.
The findings showed that adding face protection didn't necessarily mean lessening blast-wave impact. For example, according to the report:
• In a front-facing blast, pressures on the forehead were higher with the jaw protector, or mandible, in place and with the mandible-visor combination than they were with the helmet alone.
• Wearing just the jaw protection for a front-facing blast doubled the strength of the secondary shockwave pressure on the forehead from 2 atmospheres (one atmosphere is a little less than 15 pounds per square inch) to 4 atmospheres.
• In a rear-facing blast, pressures on the forehead were more than twice as high for the mandible-visor combination than for the helmet alone.
The tests could help designers mitigate the pressure increases with slight structural changes to the helmet, according to the study's lead researcher. But there is no clear target.
"The military actually has specific criteria that helmets have to meet to be certified for use in ballistic and blunt force," said Daniel Mott, an NRL aerospace engineer. "No such criteria exists for pressure because the medical community is still working on what the injury mechanisms are, and we don't know where to set those desirable levels anyway, at this point."
Follow the bouncing wave
The tests centered on the helmet's "suspension geometry," the scientific term for what's between the wearer's head and the outer shell.
"You need that standoff for that blunt-impact and ballistic-impact protection ... that's the way the helmets work," Mott said. "We had seen that blast waves can infiltrate that gap."
The visor and the mandible may blunt the initial blast, but they can also channel ricocheting blast waves into unexpected spaces around the wearer's head. For example, according to the report, the mandible may "trap" a blast wave ricocheting from the wearer's chest, which could then combine with the initial "incident wave" and lead to a "delayed, stronger forehead peak."
The study recorded a forehead pressure of nearly 10 atmospheres in a front-facing blast with the visor and mandible in place; the top pressure was just above 8 atmospheres in the helmet-only test.
The paper points out that increased pressures in one area generally come with decreased pressures in others. In the front-blast test, the visor-mandible combo dropped the peak of the highest-pressure wave on the back of the head by half when compared with the helmet-only setup, for instance.
It's one of a series of tradeoffs designers must make, Mott said — determining which areas to channel blasts away from, and balancing the need for blast-wave protection against other concerns: A soldier may want to wear the mandible or visor, even with elevated blast-wave pressures in some areas, to keep a bomb fragment from bouncing off his face.
The prototype, which was designed under the Helmet Electronics and Display System-Upgradable Protection, or HEaDS-UP, program by Army researchers in Natick, Mass., is far from finished with testing. Mott offered a series of steps that could improve future findings:
• Blast reaction. "We're moving toward including the material response of the head and the helmet" to the explosion, he said. The latest tests were conducted using "stationary, stiff bodies."
• Torso-tracker. "Although we had a very detailed model for the head and helmet for these calculations, we had a pretty simple torso and shoulders," he said. A more realistic mannequin would yield better data, especially when measuring ricochets off the body.
• More gear. That torso will need a tactical vest, at least — Mott said knowing what soldiers likely will wear in theater will help fully track the blast waves. "We don't have all the relevant geometry in the calculations yet," he said.
Mott and colleagues Ted Young and Doug Schwer published their findings with the American Institute of Aeronautics and Astronautics. While they don't address suggested improvements to the gear, the research "makes us optimistic that we can find combinations of geometry, either for the accessories themselves or for the suspension, that may reduce that threat, reduce those pressure loads that we're seeing," he said.