What it takes to actually outrun an explosion.

<< More bang, less light: Explosions in film and TV (Part 1)

More bang, less light: Explosions in film and TV (Part 3)>>

Hello again! In our last post we looked at how Hollywood transforms dull but dangerous blasts into big, bright, and harmless fireballs¹. This week we are going to keep the focus on film and TV explosions, but focus on how and why the rules of physics get suspended when an explosion happens.

Two fundamentals of physics in particular seem to pose a problem for filmmakers: matter and time. Without further ado, let’s dive in. But first, if you like what you’re reading please hit that “Like” button on the bottom and please subscribe so you never miss a post. Please share this with any like-minded nerds enthusiasts out there. Comments below are always appreciated, and you can email me or comment for any topic requests.

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Size matters

Matter, or “stuff”, seems to pose a problem when it comes to explosions in film and TV. Explosions aren’t magic, they’re just chemistry. They’re a type of combustion whose only differentiator from other types is its speed:

And just like any chemical reaction, they can’t conjure matter out of thin air. The size of the reaction (read: explosion) is determined by the amount of reactants (read: explosives) that you started out with. Despite this fact of physics, we often see explosions that are far too big for the modest amount of charge we started out with:

James Bond is a particular fiend for this, so the above examples barely scratch the surface. A trick which Bond usually does to address this (à la Goldeneye) is to toss the small explosive onto or next to some conveniently located explosive barrels (which never burn or progressively explode, but always detonate along with the initial charge, but that’s a topic for another day).

I’m aware that chemistry is complicated, so I’ve done the hard work. Below is a table containing the results of my analysis into how much explosive is needed to achieve a desired effect:

How big do you want the explosion to be?	How much explosives do you need to achieve this?
Small explosion	Small amount of explosive
Medium explosion	Medium amount of explosive
Large explosion	Large amount of explosive

I’m willing to licence the results of my analysis above to Hollywood studios at a competitive rate, such is my concern for movie accuracy. As it is, Hollywood relies on explosives as simply a portable and scalable Deus Ex Machina, a magic “get-out-of-jail-free card” for characters in trouble.

But hang on, you say, surely some explosives are really powerful? What about that HMX or CL-20 stuff you were complaining about in the Reacher post? Well, yes, different explosives have different properties, this includes their explosive power. Here are some examples of the relative effectiveness of explosives in comparison to TNT (the normal benchmark). The source is Akhavan², unless noted otherwise:

Explosive	TNT equivalence3
TNT	1.00
RDX	1.52 / 1.37 (For C-4, source: TM 5-855-1)
PETN	1.50 / 1.42 (Source: TM 5-855-1)
Nitroglycerine	1.52
HMX	1.50 / 1.70 (source via Wikipedia, no citation given4)
CL-20	1.51 / 1.90 (source via Wikipedia, no citation given)
Ammonium nitrate and fuel oil	0.82 (Source: TM 5-855-1)
Mercury fulminate	0.12

As the table above shows, the variance from the weakest to strongest explosive (aside from mercury fulminate, which we’ll discuss below) is only about twofold. This is significant enough in ammunition design, but not nearly enough to explain the differences seen on screen.

To come back to the Walter White example, mercury fulminate is not a powerful explosive, it’s just a very sensitive one. Mythbusters busted this scene already in terms of the explosive effect (spoiler: it wouldn’t work), so I’ll just add some numerical context: 50 g of mercury fulminate releases about 90 kJ when detonated. This is only equivalent to about 20 food calories⁵ (although, to be fair, we don’t tend to release the energy from food all at once). It’s also equivalent to the energy in 2 ml of petrol, or a kettle running for 75 seconds, or (more impressively), an SUV dropped from the ceiling to the floor. In another metric, gas released, 50 g of mercury fulminate gives us about 11,000 cubic centimetres of gas, which might sound like a lot at first, but is really only 1% of a cubic metre. In a room that size, it’s not going to blow out any windows.

All of this this makes sense, because it’s a primary explosive: its purpose is to impart a shock to a bigger (but still comparatively small) amount of secondary explosive, which in turn will detonate a much bigger charge of secondary explosive. Remember how explosive trains work, from our Expendables post:

What Bryan Cranston (supposedly a masterful chemist) does here is incorporate the worst of both worlds: a very sensitive explosive which has the potential to blow off a few of his own fingers when handling it, but not to do much more than make the baddie and his henchmen startle when it’s detonated. But the real stupidity here is that he makes about a kilogram of the stuff:

Manufacturing primary explosives in that quantity is one thing, but hoping that it won’t detonate when you blow up a smaller amount nearby is utterly foolhardy. Incidentally, the explosion we see from the 50 g chunk would be much better matched to the 1 kg bag. Remember, mercury fulminate is not a powerful explosive, it’s just extremely sensitive.

Now, what Heisenberg could have done is make a big bag of some secondary explosive (if he can access nitric acid to make mercury fulminate, he can make most explosives). Then he could hide his 50 g bit of mercury fulminate in the middle of the bag, as a booster, with a little squib, battery, and key fob receiver as a triggering mechanism. Then all he needs to do is take out his keys and threaten Tuco with this to get the same leverage. If he wanted to impress the baddy with his Crazy Ivan energy⁶ then he could have made two big bags, one with pure explosive (the leverage suicide device) and another with explosive cut with something inert (or even meth) as the demo device. As an added bonus, the filmmakers could still have their cool slow-mo explosion.

This, however, brings us on to the next problem…

Time isn’t flexible

Slow-motion shots are cool. Now that most of us have phones in our pockets capable of shooting such shots, we can appreciate them even more. During sports replays they give a satisfying spectacle of goals and points, fouls and tackles. And there’s no denying that slow-motion explosions are among the coolest spectacles out there. The Slow Mo Guys have some of the best. Hollywood makes use of this coolness, but has to show other things happening at the same time (e.g. heroes miraculously escaping flying fragments). But they tend to actually show all these things happening at literally the same time:

Independence Day has the most blatant examples of this (it’s worth watching the whole scene, I can only show limited parts on the gif):

This is where problems arise. At normal speeds, fast things (e.g. explosions) happen fast and slow things (e.g. people) happen slowly. Once we go to slow-mo mode, however, physics takes a backseat and everything moves to a generic “slow-mo” speed. At normal speed, a human could never run or dive away from the effects of an explosion: it’s just too fast. At slow-mo speed, time dilation happens and humans can suddenly do these magical feats:

The clip from Independence Day takes this to an extreme: everything is the same speed! Blast wave, flying cars, people running, Air Force One taking off…

In reality, and rather boringly, slowing down the camera doesn’t magically enable you to outrun blast waves. The speed of a blast wave depends on the size of the explosion⁷, but they travel at at least the speed of sound, which is over 300 m/s. A human jumps at, let’s say 10 m/s maximum. So even if you slow explosion time down by a factor of ten, there’s still a threefold difference in speed (and that’s before getting to the unfortunate fact that slowing down time will also slow down your jumping speed). There is one person who could outrun a blast wave:

A-Train, from Amazon’s The Boys, is said to have a top speed of about 450 m/s. He could probably do most of the feats shown in the clips above. The only way he would be in trouble would be if he was standing close to a very large explosion (hundreds of kilos of high explosive or anything nuclear), in which case the blast wave would be traveling much faster than the speed of sound and would overtake him quickly. But anyone else, nope, they’re toast.

We can see how silly this is when we get rid of the slow motion effect. Here’s the same Sherlock Holmes scene from last week, but in full, and sped up to what I reckon is normal moving speed for Robert Downey Jr. But see how the ragdoll bodies and the explosions themselves are still very slow, because they underwent the special Hollywood relativistic slow-mo effect:

Conclusion:

Last week we cast a critical eye on Hollywood explosions, and concluded that they were too fiery, while also not being nearly lethal enough from a fragmentation point of view. They look the part but don’t have the right effect on people. This week, we see that their size bears little relationship to the underlying chemistry of explosives, and filmmakers use slow motion as a sneaky trick to get around the fact that they’re simply too fast to react to.

Whereas our last post was all about spectacle—the writers want the coolness of explosions, but not necessarily the actual implications—this one is about plot. Explosions have just as much power as they need to get the heroes out of a jam, and they will fit in whatever gadget or container the hero can carry or wear. When they detonate, their effects are slowed down so that the heroes can get out of the way, but only at the very last second, for maximum tension.

As usual with these posts, I’m torn between a shrug and weary understanding and an eye-roll and criticism. On the one hand, of course films need tension and spectacle, and explosions are a great way to do this, even when they’re hammed up a bit. On the other hand, multi-million dollar productions have no excuse for getting their basic research wrong. The pen in Goldeneye could have been much chunkier. Chris Pratt’s redemption move in The Magnificent Seven could have been grittier, with a smaller bang but lots of wooden splinters skewering the baddies.

I’m a big fan of slow motion shots, but I would love to see some powerful, fast explosive blasts and fragmentation whizz past while our heroes are diving away. This would be a great way of highlighting the power of explosives and the reason they’re such a great plot device in the first place.

So that’s nearly it for our Hollywood explosion series, and it’s the end of the “proper explosions” part. Next week we’re going to take a look at muzzle flash—the flames that shoot out the front of guns—and ruin the fun of critically analyse some more unrealistic movie traditions. Thanks for reading, and please remember to Like and subscribe if you haven’t already!

1. As an aside, we spoke about how these explosions are really made, but I also came across this explainer from Film Jams of how they do it with a green screen. Very interesting, and I’ll even politely reserve judgment on the fiery-ness of the explosion. ↩︎
2. Akhavan, J; The Chemistry of Explosives (3rd Ed.); RSC Publishing, 2011 ↩︎
3. The definition of RE is actually a bit woolly, and can differ a lot from source to source, since each one uses a slightly different methodology. The only reason I’ve included the Wikipedia entries for HMX and CL-20 is to illustrate that these are more powerful than more common explosives such as RDX and PETN. I don’t believe that RDX is more powerful than HMX or CL-20, as Akhavan (an otherwise excellent source) implies. ↩︎
4. I know this isn’t good practice, but see note above. ↩︎
5. The “calorie” you see on food packages is actually a measure of kilocalories, or thousand calories. Obviously in an attempt to make the term shorter for simplicity, marketers just lopped off the “kilo” bit. It’s like if we shortened “kilometres” to “metres” on road signs. Sigh. ↩︎
6. This still leaves the problem, which Mythbusters pointed out, that any explosion powerful enough to blow out windows would have done a lot more damage to the people in the room. ↩︎
7. Rabbit hole time. The speed of a blast in its initial stages depends on the amount of explosive. All blast waves start off faster than the speed of sound, since the detonation wave in the explosive itself was faster than the speed of sound. They slow down as they spread, however, and degenerate into a wave travelling at the speed of sound. The image below shows this transition to speed of sound (340 m/s) for two differently-sized explosion data sources (red and blue data points).
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