Down to Earth: How space science gave us... airbags

27 June 2017

Interview with

Stuart Higgins, Imperial College London

Physicist Stuart Higgins looks at how technologies and breakthroughs made originally for the space race are now helping to make life better for us here on the ground, and this week... airbags.

Stuart - If you want to go to space, you need to get off the planet and you need a lot of energy to overcome gravity’s pull. The technology of choice for years has been the rocket. Take NASA’s space shuttle, for example; it had a big orange fuel tank attached to its belly containing liquid rocket fuel, and two thin rockets strapped to its sides. It was these two rocket motors that provided most of the thrust to get the shuttle off the ground. Each was filled with over 450 tons of solid rocket fuel. Each rocket was heavier than a fully loaded jumbo jet and produced 12 million newtons of thrust.

But how do you set off a lump of rocket fuel that's over 15 metres tall? You need a pyrotechnic initiator; it’s a gadget that starts off a chemical reaction. In NASA rockets, an electrical current is passed through a wire which heats up and ignites a small explosive charge. This in turn sets off a reaction in a chemical called lead azide; it’s a molecule made up of the metal lead and three nitrogen items, that’s the azide part. These nitrogen atoms react to produce heat and nitrogen gas which trigger another more powerful explosive that in turn sets off the rocket and… lift off.

It turns out that this rocket science is used in lifesaving technology here on Earth… the airbag.

Airbags, like rockets, contain a pyrotechnic initiator which causes the rapid breakdown of a chemical producing lots of gas. In fact, one rocket manufacturer in Japan directly used its technology to help a car manufacturer make their airbags.

Nowadays, airbags tend to use chemicals that react more efficiently and cleanly, but many were originally based on sodium azide which, like the lead azide used by NASA, reacts producing nitrogen gas. Two molecule of sodium azide reacts to give three molecules of nitrogen gas, and the gas occupies a volume far greater than the original solid. In fact, only about 130 grams of sodium azide is needed for a 70 litre airbag.

During a crash, an electrical circuit triggers the reaction filling the airbag in less than 30 milliseconds, much faster than the blink of an eye. The airbag increases the time it takes you to slow down during a crash - the longer time taken, the lower the force applied to the body. This is particularly important for delicate organs such as the brain which can strike the inside of the skull.

So that’s how launching space rockets has led to airbags that save thousands of lives each year.

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