I learned this today. An airbag can inflate so quickly because it contains sodium azide, which produces nitrogen gas when it ignites.
When you are in a car crash, a series of steps happen to launch the airbags. An accelerometer measures that the car has rapidly decelerated. Impact sensors work out where the impact has happened. Wheel speed sensors analyze if the car is sliding. Gyroscopes calculate if it is flipping. Seat occupancy sensors work out which seats have people in them, their weights, and how they are sitting. All of this takes about 3 milliseconds.
These sensors send their signals to the central airbag control unit. The ACU analyzes all of the data and calculates which airbags to deploy. Modern cars have more than just a dashboard and steering wheel airbag. They have side ones, central ones, knee ones, and seatbelt ones.
The ACU triggers an ignitor inside the airbag it has decided to inflate. The ignitor lights the sodium azide (about 200g in a steering wheel airbag), which rapidly starts generating nitrogen gas. The conversion happens in an explosion and the airbag is completely inflated in 30 milliseconds.
From crash to complete airbag deployment is less than 40 milliseconds!
A lot of people (myself included) have the mistaken idea that if we are in a crash we’ll be able to react, move out of the way of harm, and protect loved ones. That is laughably not true. It takes the average human 250 milliseconds to react to a stimulus. The car recognizes the crash, analyzes all the data and launches its airbags in 40 milliseconds. That means, the airbags have deployed and saved our lives 210 milliseconds before we have even realized we are in a crash. The fastest reaction time ever recorded by a human was 130 milliseconds.
The airbag concept was invented by two dentists in the UK. They wanted to find a way to help reduce jaw injuries among pilots in World War 1. Often, when a plane would crash, or sometimes even when just landing, the plane would stop suddenly and the pilot would hit their head on the dashboard, breaking their jaw. Arthur Parrott and Harold Round of Birmingham, UK, came up with the first airbag design, which they patented in 1920. This airbag filled with compressed air.
The automobile airbag was invented by American John Hetrick in 1952, but he also used compressed air, which could not inflate the airbag fast enough. In 1964, airbags started to use an explosive to inflate them and in 1967 a better crash detection sensor was invented.
Car manufacturers are working on side airbags, but these have to expand much more quickly. When there is a frontal crash, the various crumple zones mean it takes about 50 milliseconds before the driver hits the steering wheel, giving ample time for the 30-millisecond bag to inflate. With a side-on collision, the driver would hit the door in about 10 milliseconds. Door airbags would have to inflate in this short margin.
So, airbags use sodium azide because it explosively produces nitrogen when ignited and inflates an airbag far more quickly than compressed air would be able to. It takes about 30 milliseconds for all of the sensors on the car to work out what type of crash you are having and to inflate the relevant airbag. It takes you another 210 milliseconds to even realize you have crashed. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Airbag
https://didyouknowcars.com/history-of-airbags/
https://www.racq.com.au/cars-and-driving/safety-on-the-road/car-safety-features/airbags
https://en.wikipedia.org/wiki/Gas_generator
https://www.wired.com/story/the-insane-physics-of-airbags/
