Since 2008, 28 million cars have been recalled in the United States because they contained airbags that could explode and hurl shrapnel into drivers during even minor fender benders. Ten deaths and more than 100 injuries have been linked to the faulty airbags.
Until a few weeks ago, neither the Japanese manufacturer of the airbags, Takata, nor government safety officials could pinpoint the root cause of the flaw, frustrating safety advocates and lawmakers. Takata airbags are still being installed in cars today, and more accidents have repeatedly forced the National Highway Traffic Safety Administration to expand the scope of the recall.
After Takata was accused of manipulating data and hiding problems during the investigation, a coalition of 10 automakers who installed the airbags called in a team of rocket scientists to perform an independent investigation. Last week they said they found the cause—it’s not just manufacturing problems, as Takata has alleged, but a fundamental flaw in the design that has turned the airbag’s inflator into a bomb.
But why rocket scientists?
“There are some real similarities,” Bob Wardle, a Ph.D chemist by training and 30-year veteran of Orbital ATK, the aerospace company hired to investigate the flaw, told Quartz.
“A rocket motor is big and it flies through the air, but as far as what happens inside of it, there is a signal that comes in, an ignition train that starts a process, a main propellant that burns,” he says. “In the case of a rocket motor you have a single nozzle on the end of it; in the case of an airbag inflator, gasses are used to open an airbag instead.”
Wardle’s team developed a detailed analysis that considered 63 different ways the airbag system could go wrong, using computer models and reams of data to test each possibility.
Ultimately, they found the problem in the solid propellant used to inflate the airbag. Based on ammonium nitrate, that propellant is a cheaper but more volatile fuel than Takata had previously used.
When the fuel is installed in the inflator, it is typically “in the shape of a wafer or bat wing,” Wardle said, but it changes shape when exposed to humidity over time. When the airbag is triggered, the change in the fuel’s shape leads to to an unexpected increase in pressure that detonates the inflator instead of filling the airbag. The force of the explosion hurls pieces of the inflator into the cabin like shrapnel, injuring those in the vehicle.
The report concluded that the inflator’s design is not sufficient to protect it from heat and water. Takata had previously surmised that humidity was behind the flaw, but blamed the issue on aging parts, unusual conditions and a manufacturing plant that hadn’t followed instructions while assembling the inflators. Later, it added a drying agent to the propellant in new airbag inflators that would temporarily keep them from changing shape in humid conditions.
The revelation that the inflator assembly itself is flawed led to criticism of the safety agency for allowing Takata to install airbags with the drying agent as a temporary solution. “Auto manufacturers are installing new live grenades into people’s cars as replacements for the old live grenades,” Florida senator Bill Nelson said at hearing on the issue last week.
But the NHTSA is reluctant to expand its recall to include the 70 to 90 million cars with ammonium nitrate inflators on the market without evidence that the fix is insufficient.
Now, as the government reviews its most recent report, Wardle’s team will begin testing the replacement airbags to see if they really match the lawmaker’s description.