Over the past three decades, we've gotten used to things blowing up in our faces. The advent of airbags has done wonders to protect cars' fragile flesh-and-blood cargo when vehicles go smash.

Lots of people—especially those who've been protected from injury or death by an airbag deployment—must have wondered: What if we put the same technology on the outside of the car? Some of those people must work for German auto supplier ZF Friedrichshafen AG, which has spent 10 years working on the external airbag.

The tech is finally ready for carmakers—that is, if ZF can convince them to buy it.

When the Worst Is Unavoidable

External airbag demo. ZF Friedrichshafen AG

External airbags work just like you'd expect. They are bigger, exterior versions of the airbags that pop out of surprising places in your car's interior—those ugly balloons that fill with weird gases to cushion the human body.

With ZF's system, each side sill (the outside bodywork underneath the car doors) packs one airbag that runs the full length of the doors. Sensors on the car will watch out for any objects that look likely to slam into the side of the car. When the computers decide a crash is imminent and unavoidable, they deploy from the side sill, revealing the airbag. In no more than 100 milliseconds, inflators pump up the airbag to the height of a typical front bumper.

What if we put the same technology on the outside of the car?

One advantage of outside airbags is that they disperse the forces of impact. An oncoming car about the slam into the side of your vehicle would strike with the relatively small surface area of its front bumper—and an even smaller surface if it strikes at an angle. But when a car hits an inflated airbag, the impact force is spread through the airbag and along the length of the vehicle's side structure, which reduces energy loads. ZF says its tech reduces intrusions into the passenger cabin by 30 up to percent, and reduces injury levels by 20 to 30 percent.

ZF would outfit vehicles with a number of kinds of sensors to identify whether and when to fill the airbags, because each type has its own specialty. Radar sensors are excellent for measuring distance and are virtually immune to being tricked by funny lighting or weather conditions, says Uwe Class, director of safe mobility systems at ZF. Cameras, meanwhile, are best at recognizing objects by differentiating between, say, a heavy motorcycle and a lightweight bicycle. They also typically have a wider field of view than the radar sensors, so they may see an incoming car sooner.

External airbag inflated ZF Friedrichshafen AG

Lidar sensors are the final piece of the puzzle. They pulse light waves toward incoming objects and measure how the waves are reflected back. In this way, they create detailed three-dimensional images of everything they see. At the point where a crash becomes unavoidable, the lidar sensors' short response latencies and fast refresh rates allow the system to quickly detect and track whatever fast-moving object is about to hit the car. Even tiny changes in an incoming object's direction during these final milliseconds could drastically change how the airbags should engage, so lidar must be fast and accurate.

Engineered From the Ground Up

If you think this system sounds like a hard thing to jam into a vehicle, you're right. Because the ZF tech integrates into the vehicle's on-board sensor suite and requires space in key structural areas, it can't be retrofitted onto a car that wasn't designed with the external airbags in mind. The vehicle needs to be engineered from the beginning to make room for the airbag module, inflators, sensors, and side sill designs.

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That's a big ask. So rather than convince car companies to add yet another suite of sensors to their vehicles, ZF designed its sensor suite to pull double- or triple-duty. It makes sense; the industry already uses a plethora of cameras and radar sensors in computerized safety subsystems. For example, ZF's external airbag setup could use the same sensors employed in the lane departure warning or active lane correction systems found in new cars.

External airbags could also work in tandem with forward-looking systems, such as collision warning systems that alert the driver when the car is about to hit something, and automatic emergency braking systems, which automatically brake the car if the driver doesn't brake or take evasive action. Adaptive cruise control, which lets the car keep pace automatically with the car in front of it on highways, is another system that uses these types of sensors.

The first ZF external airbags could be on production cars within two years, Class says, though the company must navigate a number of unknowns. Asked about regulatory issues, the company told PM: "ZF is in the early stages of the process to clarify potential hurdles and clear the path for market introduction of this technology."

And while several automakers have shown interest, none have placed contracts yet. Having to engineer a new car around the system is an obstacle in an era where cars typically go seven to ten years between expensive redesigns, and the industry is still feeling out the recent explosion of complex safety systems and sensor suites.

But safety sells these days. If enough automakers decide to design new cars around the external airbag, we could end up finding it quaint that we once ever thought airbags only belonged inside cars.

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