He Swerved to Avoid Something in the Road With His Family in the Car, and Patented the Airbag Three Months Later

US2649311 was filed by John W. Hetrick on August 5, 1952, under the title Safety Cushion Assembly for Automotive Vehicles. It is the airbag, described clearly, roughly a quarter century before anyone would sell one.

The drive

Hetrick was an industrial engineer who had served in the Navy, where he worked with compressed air systems, including torpedoes. That background is the reason this patent exists rather than remaining a vague wish that cars were safer.

In the spring of 1952 he was driving outside Newport, Pennsylvania, with his wife and his seven year old daughter. Something appeared in the road, accounts differ on whether it was a rock or an animal, and he swerved hard into a ditch to avoid it.

Both he and his wife instinctively threw their arms out in front of their daughter to stop her going into the dashboard. Nobody was seriously hurt.

What he thought about afterward was that the reflex was correct and the mechanism was hopeless. A human arm cannot decelerate a child in a collision. But the shape of the response, getting something soft between a person and a hard surface very quickly, was right.

He connected it to a memory from the Navy, of a canvas cover being blown off a torpedo by compressed air and shooting into the air. Air stored under pressure can move a piece of fabric faster than anything a person can do.

He filed within a few months.

The core insight

The patent describes a folded cushion stored in a compartment, in the steering wheel hub and in the dashboard, connected to a supply of compressed air released on impact.

The idea depends on one physical fact that is easy to state and was not obvious in 1952. A collision takes a long time by mechanical standards. The occupant does not hit the dashboard at the instant of impact. The car stops first, and the body continues forward and arrives some tens of milliseconds later.

That gap is the entire opportunity. It is not much time, but it is enough time to inflate a bag, provided the inflation is driven by stored energy rather than a pump.

Hetrick had the concept and the timing right. What his version did not have was a workable trigger, and that turned out to be the hard part.

Why it took twenty-five years

Hetrick sent the patent to the major automakers. He got no interest. He never made money from it, and the patent expired in the early 1970s, before airbags reached the market.

The reasons for the delay are worth separating, because "the industry ignored safety" is only part of it.

Some of it was genuinely technical. A compressed air cylinder is a poor fit for a car, since it can leak over years of service and its inflation speed varies with temperature. The problem was eventually solved chemically rather than pneumatically, using a solid propellant that deflagrates and generates gas in milliseconds. And the trigger has to distinguish a real collision from a pothole in a few thousandths of a second, using mechanical or early electronic sensing, without ever firing by mistake, since an airbag deploying on the highway is itself a crash.

Some of it was not technical. Manufacturers were reluctant to market safety, on a widely held theory that emphasizing crashes reminded buyers that cars were dangerous. Seat belts had a similar slow adoption for similar reasons.

Airbags appeared in limited numbers in the 1970s, and became widespread only after federal rulemaking in the 1980s and 1990s made them effectively mandatory.

The credit

Hetrick is not the only claimant. Walter Linderer filed a similar compressed air concept in Germany at almost the same time, and the technology that actually shipped owes a great deal to later work, notably Allen Breed's sensor and inflator systems in the 1960s, which solved the detection problem Hetrick had not.

That is the ordinary shape of this. A patent can be correct, early, and specific, and still not be the thing that reaches production, because being right about the concept is a smaller part of the job than it looks.

Hetrick died in the 1990s, having received no royalties. He said in interviews that he was satisfied the idea had gotten where it was going.

See the original

The full text and figures of US2649311 are on patents.us.