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Ammonium nitrate in airbags? Are you out of your minds?

Most car recalls are pretty tame. But the recent Takata airbag recall is not one of them. It’s not your ordinary situation where something might malfunction in a mild way — say, a wire shorting out which might lead to a fire, which in turn might injure someone who handled the situation wrong. With these airbags, the risk is that when set off, they might explode and send shrapnel into you.

A recent article about this stated that Takata hasn’t yet found the root cause of the failures; it went on to say that, according to Takata, “the ammonium nitrate used in the airbags was safe and stable”.

Wait, what? Ammonium nitrate, in airbags? Did I read that wrong? Did the news site get it wrong? No; checking, plenty of other sources confirm.

Well, they may not know what the root cause of their airbag malfunctions is, but I do: they used ammonium nitrate. Ammonium nitrate is hygroscopic. It’s “stable” in the sense that it won’t decompose under normal circumstances; but it absorbs water from the atmosphere and, above a certain humidity, turns itself into a little puddle. Then as it dries out, it recrystallizes into a different form. Presumably they tried to seal water out of the airbags; but seals often fail over time, or are flawed from the start; and lots of materials (like plastic films) which seem impervious to water actually let it slowly diffuse through. Once moisture has gotten in, this process (absorbing water and turning into a liquid, then drying out and recrystallizing) would repeat itself every day, due to the humidity dropping during the day then rising at night. (With a constant amount of moisture present, humidity falls when the temperature rises.)

In the airbags, they must have mixed the ammonium nitrate with something else, as pure ammonium nitrate is not a suitable propellant. And the exact nature of the mixture must have been important. The more finely divided the material, the faster the boom. The changing shape of the ammonium nitrate must have played havoc with whatever they’d done to control the speed of the boom.

I don’t know how they could ever have thought this would work. If they didn’t get problems with exploding too fast, they’d have gotten problems with exploding too slow. (And I wouldn’t be surprised if the latter also sometimes turned out to be a problem with these airbags.) But ammonium nitrate is among the cheapest of materials, which is presumably why they tried to make it work.

Still, you just can’t field millions of devices, and expect to keep moisture out of all of them. On a laboratory scale, that might work; out in the world, subjected to all sorts of abuse and to the forces of aging, it never will.

(Most of this was originally posted as a comment to the article linked above.)

Addendum (Jan 17, 2015): On further reflection, there is one technology for sealing out water that has been widely and successfully fielded, namely enclosure in a glass capsule. Vacuum tubes generally work fine even after sitting in storage for decades; and they were produced by the billions and sent all over the globe. Their technology includes, of course, electrical connections through the glass, as would be necessary here. Having airbag propellant sealed inside glass might be a bit tricky, as the flying glass shards would have to be filtered out before they reached the bag itself; but that could be done. My guess is that the expense of such an assembly would eliminate ammonium nitrate’s cost advantage, though.


Where did all these silly “similar to”s come from?

Something I’ve noticed a fair bit, recently, has been the use of the phrase “similar to” at the start of a sentence, where one would ordinarily use “like”. Say, instead of writing

Like dogs, cats have four legs.

someone would write

Similar to dogs, cats have four legs.

When I first encountered this usage, I parsed it wrong — meaning that I parsed it by the normal rules of the language, where the leading clause is a parenthetical remark that cats are similar to dogs. But as I ran into more such instances, I realized that such sentences are never meant to be parsed that way — that “similar to” is just being used as a synonym for “like”. Under the rules for “like”, the sentence is not saying that cats and dogs are similar, just that they share the property of having four legs.

Well, okay; if people want to extend the rules for “like” to “similar to”, who am I to stop them? Language changes, sometimes for the better and sometimes for the worse. But it leads to the question: what is it about “like” that is causing people to avoid it?

I have several theories:

  1. “Like” is something that, like, teenage girls say, and so is inappropriate for pompous, pedantic writing — which is commonly where I’ve seen these strange “similar to”s. In particular, I’ve seen a lot of them in formal medical and biology articles. In those fields, there are a lot of women who don’t want to seem like teenage girls in their writing. (Not that they are alone in this strange usage; males have picked it up too.)

  2. “Like” is too short and simple, and inappropriate for this era of obfuscation, so the same sorts of people who write “utilize” instead of “use” write “similar to” instead of “like”.

  3. People don’t really know where to use “like” any more, as opposed to alternatives such as “as with” or “as in”, so they just use “similar to” whenever any of them is called for, in the hope that it will do. To illustrate this distinction, the sentence

    Like everything else, the more practice you have the better you can become.

    should really be

    As with everything else, the more practice you have the better you can become.

    since “everything else” isn’t like “more practice”, “you”, or any other subpart of the sentence — not even in some particular way, as in the first example, where cats and dogs both have four legs. But it seems like some people, vaguely sensing that “like” isn’t quite the right word, would make it even worse, by writing

    Similar to everything else, the more practice you have the better you can become.

    (Not that I’ve seen that particular sentence in the wild, but I’ve seen analogous ones.) Unlike the simple substitution of “similar to” for “like”, this sort of muddling actually subtracts information as compared to a proper phrasing, so is substantially more objectionable.

These three theories are not mutually exclusive.


Unknown unknowns

When Donald Rumsfeld came out with his line about there being “unknown unknowns”, a lot of people laughed, and in response his defenders sneered at the laughers. But I didn’t see on either side a real appreciation of the phrase — indeed, I still haven’t, from anyone.

These are unknowns.

They are not the “known unknowns”, which “we know we don’t know”.

Instead they are things which “we don’t know we don’t know”.

So these were things he (and others) thought he knew, but he didn’t know — in simpler words, things he was wrong about.

This makes Rumsfeld’s line one of the most unusual things said by a politician in recent memory: an admission of error. Not just that he had been wrong in the past — as in the line, which politicians hate, but are sometimes forced into, “yes, that was a mistake, but now I know better”. This, though presented confusingly, was an even rarer admission: that he was wrong in the present and going to be wrong in the future. If he’d wanted not to obfuscate but to put it dramatically, he could have turned one of Shakespeare’s lines against himself, saying “There are more things in heaven and earth than are dreamt of in my philosophy”.

Which indeed turned out to be the case.


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