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A crash test dummy in a test car

Airbags

by Chris Woodford. Last updated: July 24, 2014.

Bang! We think of explosions as terrible, dangerous things—but that's not always the case. Every day, explosions are helping to save people's lives. If you're unlucky enough to be involved in a car accident, a carefully controlled explosion will (hopefully) fire an airbag out from the dashboard, cushioning the impact and helping to reduce the damage to your body. Airbags are very simple but also amazingly clever, because they have to open up at over 300 km/h (200mph)—faster than a car can crash! Let's take a closer look at how they work.

Photo: Airbags save lives thanks to the selfless dedication of crash-test dummies, which have been a feature of car design since the very first dummy, Sierra Sam, made his original test drive in 1949. If we couldn't test new safety innovations with dummies, we'd never be able to deploy them in our cars for real. A typical crash-test dummy has over 130 different sensors packed inside. This dummy, whose name is "Adam," is even designed to sweat like a real person so researchers can test the climate conditions inside a car! Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/ NREL).

The trouble with momentum

Sir Isaac Newton, English physicist

Like everything else in the world, car crashes are controlled by the laws of physics—and, more specifically, the laws of motion. Anything that moves has mass (very loosely speaking, this means how much "stuff" an object contains and it's closely related to how heavy it feels) and velocity (loosely, this is the same thing as speed, but strictly it means speed in a certain direction). Anything that has mass and velocity has kinetic energy, and the heavier your car and the faster you're going, the more kinetic energy it has. That's fine until you suddenly want to stop—or until you crash into something. Then all the energy has to go somewhere. Even though cars are designed to crumple up and absorb impacts, their energy still poses a major risk to the driver and passengers.

The trouble is, people inside a moving car have mass and velocity too and, even if the car stops, they'll tend to keep on going. It's a basic law of physics (known as Newton's first law of motion, after brilliant English physicist Sir Isaac Newton who first stated it) that things that are moving tend to keep on moving until something (a force of some kind) stops them. Cars have had seatbelts for decades, but they're a fairly crude form of protection. The biggest problem is that they restrain only your body. Your head weighs a surprising 3–6kg (6–12lb)—as much as several bags of sugar— and isn't restrained at all. So even if your body is fastened tight, the same basic law of physics says your head will keep on going and smash into the steering wheel or the glass windshield (windscreen). That's where airbags come in.

Photo: Sir Isaac Newton (1642–1727) formulated three basic laws describing how forces work. Picture courtesy of US Library of Congress.

How airbags help

An airbag is more correctly known as a supplementary restraint system (SRS) or supplementary inflatable restraint (SIR). The word "supplementary" here means that the airbag is designed to help the seatbelts protect you rather than replace them (relying on an airbag to protect you without fastening your seatbelt is extremely dangerous).

The basic idea is that the airbag inflates as soon as the car starts to slow down in an accident and deflates as your head presses against it. That's important: if the bag didn't deflate, your head would just bounce back off it and you'd be no better off.

How airbags work

How an airbag works: simple line artwork showing how an airbag is triggered by an impact

  1. When a car hits something, it starts to decelerate (lose speed) very rapidly.
  2. An accelerometer (electronic chip that measures acceleration or force) detects the change of speed.
  3. If the deceleration is great enough, the accelerometer triggers the airbag circuit. Normal braking doesn't generate enough force to do this.
  4. The airbag circuit passes an electric current through a heating element (a bit like one of the wires in a toaster).
  5. The heating element ignites a chemical explosive. Older airbags used sodium azide as their explosive; newer ones use different chemicals.
  6. As the explosive burns, it generates a massive amount of harmless gas (typically either nitrogen or argon) that floods into a nylon bag packed behind the steering wheel.
  7. As the bag expands, it blows the plastic cover off the steering wheel and inflates in front of the driver. The bag is coated with a chalky substance such as talcum powder to help it unwrap smoothly.
  8. The driver (moving forward because of the impact) pushes against the bag. This makes the bag deflate as the gas it contains escapes through small holes around its edges. By the time the car stops, the bag should have completely deflated.

Who invented airbags?

Original airbag patent drawing by John Hetrick, US patent #2649311, August 18, 1953

If you search around online, you'll find quite a few different people are credited with inventing airbags. Who thought of them first? It appears to have been John W. Hetrick of Newport, Pennsylvania, who came up with the idea after an accident in which he swerved his car off the road into a ditch to avoid hitting a rock, almost throwing his daughter through the windshield. Hetrick filed his patent for a Safety cushion assembly for automotive vehicles on August 5, 1952 (it was granted as US Patent #2,649,311 on August 18, 1953). Although a German inventor named Walter Linderer filed an airbag patent several months before Hetrick, it was granted after Hetrick's, and it seems likely that the two men came up with the same idea independently. Many other inventors have built on the idea since then, notably Allen K. Breed (1927–2000), who developed a variety of different ways of triggering the explosion of gas inside an airbag just before the impact of a crash. According to Breed's New York Times obituary, he made his first airbag design in 1968, and filed numerous patents for improvements, helping to turn Breed Corporation into one of the world's largest suppliers of car safety systems.

Artwork: John Hetrick's original airbag design from 1953, which I've colored to make it easier to follow. There are three separate drawings here, showing the main mechanism (occupying most of the picture), a driver's perspective view of the steering wheel (bottom right), and a view of the inflated bag from the side (top right). The bag is triggered by a heavy weight (blue) restrained by a spring (yellow) inside the red cylinder on the right. After an impact, the weight pushes the spring to the right, opening a valve inside a pipe (turquoise) that allows compressed air to flow out from a cylinder (green) and inflate the airbag cushion. Artwork courtesy of US Patent and Trademark Office. Read a full description in Safety cushion assembly for automotive vehicles (via Google Patents).

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Text copyright © Chris Woodford 2008, 2011. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008) Airbags. Retrieved from http://www.explainthatstuff.com/airbags.html. [Accessed (Insert date here)]

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