Airbag video: the difference 7/100ths of a second make




Nightly cleaning crew must be wondering what this place is all about.


Fruit salad… gone wrong.


Gallagher, your move.


One satisfied customer…


Now I want to see video where the airbag is triggered a few 100ths of a second later, with the watermelon having fallen another half meter or so, such that the expanding airbag hits the watermelon before it (the airbag) is fully deployed.


…and this is your brain on drugs.


I suspect it’s still better than the watermelon actually hitting the deck. The contact area with the airbag will still be bigger because, as the airbag is not fully inflated, it doesn’t have an internal pressure [correction edit - hugely] above atmospheric and will spread around the watermelon.

I expect that the large number of scientists and engineers who developed the airbag might have thought about that because, you know, that’s what people like them are paid to do.

(I worked for years in safety critical product development and it is truly amazing just how much thought goes into something which looks as simple as an airbag, or the thermal trip on an electrical appliance. We’re talking many man-years. Chances are, if you’ve thought of a way of improving a safety critical device, a whole load of other people have had the idea first and found out why it didn’t work as expected.)


“When I said ‘impact-triggered airbag,’ I didn’t mean the impact of the driver’s ribcage on the steering column.”


That’s almost as visually appealing as the watermelon.


Just took the airbag out of one of the cars this weekend. This sure is reassuring. No high speed collisions please.


Me too!

I surely hope so!

I have no notion of how to improve airbags. I’m damned glad they were invented, and I’m grateful for the regulations that have required them to be installed in all passenger cars. I know they have saved thousands of lives and have made it possible for people to literally walk away from collisions that would previously have left them seriously injured, crippled for life, or just plain dead.

I was simply curious about how this video, which is predicated upon the events occurring in a very narrow little sliver of time, would look if that little sliver were to be tweaked a bit.

Did you suspect a different motive for my post :question:

Since you seem to be an authority on this topic, I do have a technical question about the statement you made that I’ve quoted below:

If the airbag, in the fraction of a second between its activation and its becoming fully deployed, has an internal pressure lower than that of the surrounding atmosphere, what makes it expand :question:


There is also the issue of the differential speed. The speed of the airbag surface accelerating towards you will add to the speed of you accelerating towards it. The difference is between falling into a cushion vs the cushion impacting your face like a boxing glove.

Water can be hard as a concrete slab when you collide with its surface in high enough speed. Even if it is pliable and flowy enough that you can walk through it.

Hence the amount of care that goes into the precise timing of the initiation and inflation sequence.

Also, to have some idea about the power in the airbag’s gas generator, check one of the many pranks these things can be used for.

An “off-label” use for the sodium azide based ones could be reacting the azide with lead salt to produce lead azide that’s useful for gun primers. One of the zombie apocalypse survival tricks.


Yup, I made silver azide at school. My chemistry teacher said it was evolution in action; if you survived doing it at school perhaps you would eventually be safe doing actual research. (We had access to silver nitrate for photographic experiments, but lead was considered “too dangerous”).

I was a bit careless; I should have written “very much above atmospheric”, though I didn’t write “below atmospheric”. The pressure has to be enough above atmospheric to push the bag through the air at a high enough speed, but not (obviously) a high pressure that would cause overly rapid inflation.

Thinking about how to explain this best, the problem to be solved is that impact with the airbag has to prevent impact with hard objects with significant relative velocity. The bag must be stiff enough to prevent damage but not so stiff that it becomes a hazard itself. Of course it is a hazard, but a broken nose, black eyes and perhaps dislocated jaw are a lot better than death.
While the bag is inflating, its envelope is unfolding. The internal pressure has to be enough to unfold it with sufficient speed, but at the same time if something hits it while it is partly inflated, air will be pushed into a still folded part of the bag and cause that to unfold.
The ideal, of course, is to hit the bag when it is fully inflated so your neck is protected. Actually the ideal is for that idiot not to use the phone for texting while driving, or to stop turning round to yell at her kids, thus avoiding the accident in the first place. Defensive driving gives some protection against idiots, but evolutionary selection pressure keeps producing better idiots.


Hee. Good times…

If we stick with the theme of salvaging materials from vehicle carcasses, batteries are a rich source of lead; metallic, dioxide, sulfate. Dissolving it into usable form will be somewhat tricky, as nitric acid availability cannot be counted with, but acetic acid dissolves lead (albeit slowly, but even ancient Romans reportedly used this trick) and a bit of peroxide helps to speed it up.

(Then there’s the issue of using the battery lead for casting bullets. It’s a lousy alloy for this purpose, though, and would have to be fluxed to remove the bad additives.)

(If microwave-assisted synthesis of ammonia is added to the fray, because conventional Haber-Bosch is too high-pressure high-temperature for post-apocalypse conditions, we have almost the whole route for production of nitric acid for ammunition works, to make the propellants. The cars will provide us with platinum (from their catalytic converters) for the NH3->NO oxidation catalysis, the battery plates can be used as lead electrodes for production of hydrogen, and some gas stations have membrane-based nitrogen generators for inflation of tires. Then the microwave rig, catalytic oxidizing, absorption and concentration, nitration unit, and we have a propellant production plant ready for the survival compound, and possibly small enough to load on a truck. But I digress here…)

Edit: The microwave generators can be taken from ovens from abandoned RVs/campers, the power inverters as well; the electricity can be made using salvaged alternators and wind or water. The vehicles that annoyingly clog the streets today can be our saviors tomorrow.


Basically you just need to get some air in the acetic acid. Of course, lead acetate is famous for tasting sweet, so resulting in accidental or deliberate poisonings.

No, I was just making a random observation. During my time in the industry, it was usually people from sales and marketing that had the “brilliant ideas”, and seemed not to understand that we had an entire department of engineers and physicists whose job it was to think of these things first - and also think about the problems. Of course, sometimes people from outside an industry do have the ideas, but generally it isn’t the case.


It feels a little like getting punched hard in the face.


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