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Explosive Safety

How pyrotechnics and detonators can save your life

Automobile safety is an explosive issue. Literally. New cars have several features that, if employed in a different manner, would earn you a visit by agents of the Bureau of Alcohol, Tobacco, Firearms and Explosives, and, perhaps, a forced, multi-year vacation at taxpayer expense. If you modified these safety features or employed their components for ulterior motives, the ATF would call them "destructive devices," a category that includes things like machine guns, sawed-off shotguns, and rocket-propelled grenades.

Airbags

One of these concealed weapon-like devices is the ubiquitous airbag. Next is the seatbelt pretensioner, just one item on the long list of features automakers and car writers throw around as if they expect everyone to understand. A safety device soon to be fitted to new cars surely will raise a few eyebrows at the ATF: exploding hood hinges.

Airbags are inflated by burning sodium azide, the stuff used in detonators and other applications the ATF would just as soon you not know about, and potassium nitrate, which is commonly called saltpeter and a key component in "blackpowder" gunpowder. But don't be afraid, the result of this reaction is non-toxic nitrogen gas. The "smoke" reported by accident victims is largely the corn starch or talcum powder used to lubricate the folded nylon bag and facilitate its opening.

Many airbags use crash sensors based on high-powered magnets. These magnets are forced apart if the crash is big enough (definition of a "small crash": one you're not in.) and in a direction the airbags can help. For front airbags, the threshold is equal to hitting a solid wall head-on about 12 to 15 mph. When the magnets are flung apart, a signal is sent to a computer. After its digital brain confirms that, yes, indeed this is a crash and not a spurious signal, it sends an electrical charge into the solid propellant. From the instant the magnets move to the moment when the airbag is fully deployed is about 30 milliseconds.

The inflated airbag prevents occupants from slamming into the interior of the car. In a severe crash without an airbag, the driver's head might hit the steering wheel even if he's wearing a seatbelt. Airbags also avert a forward neck-snap that might produce a basal skull fracture of the type that killed Dale Earnhardt.

It's no overstatement to say airbags inflate with explosive force: Some have said it's like taking a Mike Tyson punch. You will better survive the force if you make sure your chest is at least 10 inches away from the steering-wheel-mounted airbag. Though it'll be impossible for most to remember, in the instant before the crash make sure your arms are not crossed over the steering wheel. (Another good reason for "three and nine" hand position.) Also, never allow front-seat passengers to rest their feet on the dash or cross their legs guy-style. Either is asking to have their knee ligaments ripped apart and their size 11s stuffed into their mouth.

An airbag aside: Have you ever wondered what happened to knee room in cars? It's been taken away by knee bolsters designed to keep occupants from sliding under the dash in collisions. Knee bolsters serve the same function as the "crotch straps" on six-point racing harnesses. Along with seatbelts, knee bolsters keep occupants in proper position for the airbags to work.

Pretensioners

Simultaneously with airbag deployment, seatbelt pretensioners cinch seatbelts race-driver tight by employing the force of a pyrotechnic charge. Yes, pyrotechnic, just like Fourth of July fireworks. Some pretensioners move the anchor point of the seatbelts, others wind the reel on which the belts are stowed. One way to wind the reel is with a toothed rod geared to the reel: imagine a bottle rocket with a row of gear teeth on its side. The rocket—I mean pyrotechnic charge—moves the rod, which winds the gear. It's the opposite of rack and pinion steering where the pinion gear moves the steering rack. The resulting super-tight belts better position the occupant for airbag deployment and helps keep them from striking interior components, such as the A-pillar and structure around the side window, both of which cause a surprising number of head injuries. That's why the A-pillars are now seriously padded and many cars features side airbags, which are often installed in the A pillar.

Pyrotechnic pretensioners work fast. From the time the sensor says "Fire in the hole!" to when the belts are tight is about 12 milliseconds.

Some manufacturers are moving away from pyrotechnic pretensioners. For use in the S-Class Mercedes, TRW Automotive recently produced a pretensioner that uses an electric motor to wind in belt slack. In addition to the usual crash sensors, the S-Class' computer monitors the anti-lock braking and stability control systems to predict when an accident is likely, such as when a driver jams the brake pedal to the floor and wrenches the wheel hard over. (We know some who'll spend a lot of time with the belts tight.) If the predicted crash doesn't happen, the belts are slackened.

But tight seatbelts can cause broken bones and internal injuries, so seatbelts incorporate load limiters. These are designed to spread the load over a longer period of time. We're talking milliseconds, but it's enough to soften the blow enough to diminish damage. Some seatbelt load limiters are as simple as a fold of the belt held with stitches. These stitches tear when potential injurious force is reached. Others use a torsion bar, a steel rod that twists when a certain level of force is reached and, thus, reduces the load peak.

Exploding Hinges

The third explosive device is the result of new European Union regulations that will require vehicles to be more "friendly" to pedestrians in low-speed impacts. One way to achieve this is with "crush space." Rather than raising hoods to unattractive dimensions some manufacturers will employ pyrotechnic hood hinges. In an impact with a jaywalker, the exploding hood hinges move the sheetmetal away from hard engine components. This allows the hood room to bend and soften the blow.

Though detonators, pyrotechnic charges and exploding hood hinges may sound like something on Wile E. Coyote's Acme Roadrunner Chaser, they work together to save hundreds of lives every year.