PH Origins: Airbags

In 1952, John Hetrick was out enjoying a Sunday drive with his daughter when - much to his dismay - he drove over a crest and found a boulder blocking the road ahead.

His instincts took over; he slammed on the brakes, swung the steering wheel of his Chrysler and threw the car off the road. Fortune smiled upon him, for the car simply ended up in a ditch and no one was any worse for wear.

Thoughts of what could have happened, however, continued to run through his mind. After all, cars of that era had nothing but hard, unyielding dashboards for occupants to hit - and if the car had come to a stop more forcefully, both he and his daughter could have been grievously injured.

Hetrick, who had previously served in the US Navy as an industrial engineer, set about designing what he would dub the 'safety cushion'. In the event of a crash, a motion-triggered valve would release compressed air from an accumulator. This air would rapidly inflate a cushion mounted ahead of the passenger or driver, protecting their upper bodies from hitting hard surfaces.

"It is well appreciated that many persons suffer death or serious injury when hurled against an unyielding structural portion of an automotive," wrote Hetrick. "My main object, in devising an inflatable cushion assembly for automotive vehicles, is to provide a means whereby death or injury can be prevented."

His first patent for this innovation was subsequently submitted and then granted in August 1953 - and, although Hetrick's design was unworkable at the time, the likes of General Motors, Eaton, TRW, Daihatsu and Skoda would all later reference Hetrick's design as airbags became a key safety feature.

Hetrick wasn't the only one who had come up with such a concept, though; German engineer Walter Linderer had conceived a very similar configuration and, although he had submitted his patent early in October 1951, it wasn't granted until November 1953. Linderer's airbag patent had provisions for the use of compressed air or an explosive charge - the latter would prove important in the future - as well as manual and automatic means of activation.

It would be some time before an airbag made it to the market, however, despite rapidly increasing public and governmental demand for improved vehicle safety. There were two key issues, the first of which was how to reliably trigger the inflation of the bags. The second problem was with generating enough pressure in the bag, quickly enough, for it to be of use; a system using compressed air was simply too slow to act.

Japanese engineer Yasuzaburou Kobori, who was studying automotive safety systems, suggested using an explosive to rapidly inflate the airbag in 1964. He also proposed the use of side, rear and roof airbags, triggered by sensors in the nose and tail of the car, along with a pedestrian airbag - which was designed specifically to stop a person's head impacting the bonnet of the car.

Alas, Japanese legislation meant that an explosive device couldn't be used in such an application, and the concept was taken no further. Kobori died in 1975, unfortunately, long before airbags made any meaningful mark.

Back in the US, running in parallel with Kobori's developments, engineer Allen Breed had stepped up to the plate to resolve the safe, reliable triggering issue. Breed ran a company that produced specialised safety, fusing and arming equipment for the US Military and had become fascinated with the concept of automotive airbags, seeing both great safety benefits and a huge market. After all, the devices Breed Corporation produced for preventing weapons detonating when dropped could easily be repurposed for automotive use.

Breed's brother, David Breed, held another piece of the puzzle - as he had come up with a simple series of time-delay triggers based on cylinders or spheres suspended in fluid-filled tubes, which he patented in the late 1960s during his time at MIT. These were slowly reworked, over the course of several years, into a series of electromechanical ball-in-tube sensors that could quickly and reliably trigger an airbag in the event of an accident. Breed marketed this concept, along with complete airbag systems, to Chrysler - which was interested enough to begin developing prototype airbags.

Inflating the bag quickly was still problematic, though, so Chrysler approached Talley Defense Systems for help in developing a better, quicker firing system. TDS, which had been around since the 1960s, specialised in designing solid propellant systems for ejection seats - so it had exactly the right kind of expertise required. John Pietz, a chemist working for the company, promptly developed a sodium azide-based propellant. It was an unfriendly concoction but, when safely stored in a container, could be triggered and would rapidly generate nitrogen to inflate an airbag.

Chrysler eventually lost interest in the project, partly because of the prohibitive cost of the overall system. However, as the volume of traffic on American roads continued to expand at an exponential rate - along with the number of accidents - American manufacturers began seeking ways to better restrain people during a crash. After all, seat belts were not required and few wore them - so an automatically deploying airbag would offer some protection and a form of restraint.

TDS, wanting to capitalise on its creation, then went to GM with its new sodium azide-inflated airbag concept. GM - and also Ford - subsequently caved as safety demands increased and began trialling airbags in the early 1970s. Then, in 1973, GM introduced the 'Air Cushion Restraint System' for full-size Cadillac, Oldsmobile and Buicks.

By 1974, airbags were offered for both front-seat occupants. The optional airbag system was dropped in 1976, though, as only some 10,000 customers ticked the box for this expensive new safety system - as prices range from around $180 to $300, which was more than the cost of an AM/FM radio.

As component costs fell and the concept became more widely understood and appreciated, airbags began to be deployed - figuratively and literally - in greater numbers throughout the 1980s. By 1989, it was mandatory to offer either seat belts or airbags in all new cars sold in America. Other innovations followed, such as less harmful non-azide propellants and systems that could vary the pressure in the bag depending on the speed of the collision.

The development of these innovative safety devices wasn't limited to the US and Japan, mind; Mercedes-Benz began experimenting with airbags in 1966 but didn't introduce a production system until 1981, in the W126-gen S-Class.

Breed had persisted with airbags throughout the lull in interest, too, and the payoff was huge; he established a standalone automotive safety company that, by 1995, was supplying 21 car companies with airbags and netting hundreds of millions of dollars in sales.

Fortunately, Hetrick lived to see this and more. He died in 1999, long after the widespread adoption of airbags - and a year after US regulations were revised to make twin front airbags mandatory. Soon, they were an integral part of every manufacturers' safety equipment line-up.

P.H. O'meter

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Comments (18) Join the discussion on the forum

  • Onehp 27 Jun 2018

    Intresting. I had expected some input from the safety minded Swedes, but I suppose Autoliv only came into the picture later on...

  • Lewis Kingston 27 Jun 2018

    Onehp said:
    Intresting. I had expected some input from the safety minded Swedes
    Indeed. Volvo didn't start fitting airbags until '87 and Saab wasn't far behind, fitting them to the 9000 in '88 (I think).

  • Max_Torque 27 Jun 2018

    Of course the most "fun" bit of airbag and their development and testing is the bit where you need to ensure they don't go off when NOT wanted:

    secondhand vw one careful owner


  • ElectricPics 27 Jun 2018

    A mention must go to Audi's alternative Procon-10 that used cables to pull the steering column away from the driver and tension the seatbelts in the event of a frontal collision. Unfortunately any car that might have been repairable was destroyed by this system.

  • richs2891 27 Jun 2018

    Max_Torque said:
    Of course the most "fun" bit of airbag and their development and testing is the bit where you need to ensure they don't go off when NOT wanted:

    secondhand vw one careful owner

    Thats the bit that fascinates me the most - how air bags work successfully with off roaders.

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