How Do Airbags Deploy So Quickly in a Crash? The Physics, Explained
Arthur T Knackerbracket writes:
https://www.slashgear.com/2197980/how-airbags-deploy-so-quickly-in-a-crash-physics-explained/
Front airbags have been required in new passenger vehicles since the 1999 model year. While side airbags aren't specifically mandated, auto manufacturers install them to meet other federal safety requirements regarding side protection. The National Highway Traffic Safety Administration (NHTSA) claims that airbags saved over 70,000 lives in the U.S. since their implementation, so they work. But how do they work?
Airbags, which have recently become targets of theft, are part of the vehicle's passive safety system designed to help keep passengers safe during an accident. On average, an accident happens in roughly 200 milliseconds - less than 1/5 of a second. So, the system needs to detect, react, and deploy faster than that to be effective - usually just 10 to 30 milliseconds, which is quicker than you can blink. The deployment of an airbag has been described as "engineered violence" because it essentially contains and directs a literal explosion.
First of all, the term "airbag" isn't accurate since they don't actually use "air" per se. Today's systems use guanidinium nitrate with a copper nitrate oxidizer to produce nitrogen gas. When guanidinium nitrate is ignited, it breaks down into nitrogen gas, water, and carbon. The copper nitrate oxidizer is included to help reduce the temperature of the expelled gas. Older airbag systems once used ammonium nitrate, a chemical that didn't play nicely with humidity and moisture, and ended up causing several injuries and even some deaths. Guanidinium nitrate isn't affected by moisture.
Airbags are designed to deploy at various speeds depending on the scenario. If a car hits something narrow (think tree or pole), bags can deploy at just 8 mph. Impacts involving larger objects (such as other cars) can cause bags to unfurl at 18 mph. But the technology and methods used in the front passenger compartment are different than those used in other parts of the car.
Front airbags use small electronic accelerometers that can detect when a car suddenly decelerates, which is technically what occurs when it's involved in a crash. Using a technology called MEMS (micro-electro-mechanical systems), the onboard impact sensors determine - in the proverbial blink of an eye - changes in the vehicle's speed, how fast the car was going, what hit it, and whether the occupants were wearing seat belts. Passengers wearing seatbelts are considered safer, so airbags won't deploy unless the speed exceeds 16 mph. However, those not wearing them are at greater risk, so the system typically triggers bag deployment at speeds between 10 and 12 mph.
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