Nib asked
How do airbags inflate so fast?
An airbag is not blown up from a tank of air. A sensor fires an igniter, and in most bags a solid chemical turns into nitrogen gas on the spot, filling the bag in about 30 milliseconds, before you reach it.
7 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the myth
Many people picture an airbag as a balloon connected to a tank of compressed air. In most airbags it is not. A small canister holds a solid chemical charge that, once lit, turns into a large volume of gas in a few hundredths of a second. Some designs, called hybrid inflators, do add stored compressed gas, but the speed still comes from a fast chemical reaction.
Most airbags make their gas on the spot from a solid chemical, not from a tank of air.
a common mix-upAn airbag is not simply a balloon filled from a tank of air; in most, a solid chemical charge produces the gas as it burns.
2the race
From the moment a sensor detects the collision to a fully inflated bag takes about 30 milliseconds, which is 0.03 of a second. An occupant thrown forward reaches the bag roughly 50 milliseconds after the accident. That leaves very little room: the gas has to be ready before your body gets there, which is why a chemical reaction rather than a slow pump does the filling.
The bag is full in about 30 milliseconds; you reach it at about 50.
Why can't a slow air pump fill an airbag?
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You reach the bag about 50 milliseconds after the crash, and the bag must already be full by then.
3the chain
Sensors in the car measure how sharply it is slowing down, not its speed. When the slowdown is sudden enough, a control unit sends an electric signal to the inflator. A small igniter charge fires, and its heat sets off the main chemical charge, called the gas generator, which pours gas into the folded bag.
Sensor, signal, igniter, heat, gas generator, bag: that is the whole chain.
What does a crash sensor actually measure?
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How sharply the car is slowing down, rather than its speed.
4the chemistry
In older airbags the charge is sodium azide. When heated it decomposes: two units of sodium azide become two of sodium metal and three of nitrogen gas. About 130 grams of sodium azide gives roughly 67 litres of nitrogen, enough to fill a standard bag. The leftover sodium is reactive, so makers add other chemicals to tie it up safely. Many newer inflators use different propellants, such as guanidine nitrate, or add stored gas.
Heat turns solid sodium azide into nitrogen gas; 130 grams makes about 67 litres.
Which gas fills an older airbag?
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Nitrogen, made when sodium azide decomposes.
5too hard
Being early is not enough. A bag as hard as a full balloon would stop you abruptly and injure you in its own way. So the bag is designed to give, spreading the stopping over a little time and a lot of body area.
A bag that is full but hard would hurt too, so it must give.
6the vents
Small vent holes in the fabric let the gas escape as the occupant presses into the bag. That turns the bag from a rigid ball into a cushion: your body sinks in, the gas leaks out, and you slow down gently. Deflation starts straight away, which is why the bag is already going flat when you are held by it.
Vent holes let the gas out as you sink in, so the bag cushions instead of bouncing you.
Why does a good airbag have vent holes?
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So gas escapes as you press in and the bag cushions you instead of acting like a hard ball.
7the whole idea
The whole airbag on one page: a sensor feels a sudden slowdown, an igniter fires, a solid chemical turns into nitrogen gas, the bag is full in about 30 milliseconds, you reach it at about 50, and vents let the gas out so it cushions you down.
Sensor, igniter, solid chemical to gas, full in 30 milliseconds, vents cushion you.
Next time you buckle up, remember: a little chemistry is built to get there before you do.