The difference between “Real” and “Hollywood” explosions.

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More bang, less light: Explosions in film and TV (Part 2) >>

This week I’m keen to share my thoughts on Hollywood’s idea of explosions and gunfire compared to the real thing. I’m going to try being a bit less sneery this time (difficult for me, I know), since I can actually forgive a bit of visual “sexing up” of guns and explosives. But I’d like to spend a while on it, because I’ve referenced corny Hollywood explosions before.

Since there’s a bit of meat to this topic, I’m going to spread it over three posts. Today we’ll talk about Hollywood vs. Real explosions, and some of the unrealistic effects (or lack thereof) which we see on screen. Part 2 will discuss the scaling of explosions in film and TV, and how feasible it is to run away from an explosion (spoiler: not very), and Part 3 will deal with muzzle flash: how to avoid signalling your presence to the whole galactic cluster.

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It takes energy to make light: a thermodynamics lesson

As any student of physics will tell you, energy doesn’t come from thin air. The universe doesn’t give you a free lunch. It can, however, be converted from one form into another. Explosives and propellants are a useful way for us military folks to store a bunch of energy in a compact space, then release this energy very quickly at a time and place (hopefully) of our choosing:

In the balance above, we can see that some of the converted energy, e.g. gas pressure, is useful to us. It can move a bullet down a barrel or send a fragment into an enemy formation (or break rocks apart: the same principles apply to commercial explosives). Other parts, such as light, are less useful to us¹. The role of the ammunition designer is to maximise the useful outputs and minimise the useless ones.

This is emphatically not the case in film and TV, though, where explosions create bright yellow fireballs and guns light up the sky in broad daylight. This isn’t a bad thing per se: like I said above, I’m not going to sneer so much today. It reflects an artistic choice, a willing trade-off with realism which most people are happy to accept for the sake of film visuals. What is less forgivable (and where I might sneer just a little bit) is how explosions seem to inflict far, far less death and injury on screen than they would in real life. Let’s jump in.

Real explosions are less spectacular but more dangerous

With explosives, or high explosives, to be precise (see my previous post for an explanation of the terms), we see a mismatch between what Hollywood serves up on-screen and what reality looks like:

Look at how bright and fiery the Hollywood explosions are compared to the real ones. It’s as if the designers of the explosive wanted to prioritise light and heat for a big fireball over gas pressure. It’s a new formulation of the energy balance trade-off:

What’s more, we know that this trade-off has happened because we don’t see the same blast and fragmentation (i.e. lethal) effects from the fictional explosions. We covered one example of this before in our critique of The Expendables with their ludicrous artillery vs. heli scene, but it’s a prevalent trend:

As we can see, cool guys, for the most part, clearly don’t look at explosions. Now, it’s time for a quick lesson. Munitions cause death and injury in five distinct ways:

• Primary (blast): The damage to ears, lungs, and other soft bits from a pulse of high-pressure air.
• Secondary (fragmentation): The damage caused by flying projectiles. This is subdivided into:
  • Primary fragmentation: Shards of flying metal from the munition, either natural (the shell casing), or pre-formed (ball bearings or other small bits deliberately packaged in the munition).
  • Secondary fragmentation²:  Objects and fragments which are picked up and propelled by the blast wave, e.g. loose rocks, machine parts, rubble, bits of wood.³
• Tertiary (translation): The damage caused by the blast wave picking you up and throwing you (or “translating” you, in cold physics-speak) against something hard, like a wall.
• Quaternary (burns): The damage from burns and chemical inhalation.
• Quinary (post-event effects): Anything else, e.g. infection, PTSD, radiation exposure.

Of the above, the first two are the most important. Here’s a rough guide to how far their effects range for some common movie munitions:

As you can see above, fragmentation is the real killer with explosions. This is why bombs and shells are designed with casings that break apart into fragments. You’ll rarely see a pile of explosives on its own as a weapon, since its blast effects dissipate quickly enough. You’ll always want it encased in, or surrounded by, stuff that can be sent flying.

Hollywood explosions are big, bright, loud, and pretty harmless at a distance. In comparison, real explosions are (relatively) smaller, less bright, loud, and very dangerous out to great distances. So, is it realistic that our heroes always get away safely? I’m afraid we need to go down a bit of a rabbit hole…

Dangerous does not mean harmful, unhurt does not mean safe

The lines we showed above for fragmentation lethality were for the “likely kill radius”. I was unable to find the exact definitions used, and they might have been slightly different for each munition, but a typical “likely kill” criterion would be 50% lethality. In other words, half of people standing that distance from the munition will die. Many of the remaining half will be injured. Because fragments have ever-more space to expand into as they travel away from their source, the likelihood of lethality falls off very quickly with increased range.

There will be a range, very close to the exploding munition, where the density of fragments is so high that it’s physically impossible to be a person-sized target in that space and not get multiple impacts from high-velocity fragments. You would be peppered with fragments and would disintegrate into red mist pretty quickly. The threshold of this range would be termed a 100% lethal radius.

Then, on the other extreme, there’s a distance at which the chance of getting hit by a fragment is below one in a million⁴ and/or said fragments have slowed down enough that they don’t pose a risk. This is your 100% safe distance. You can stand here, in the open, and watch the bang happen without fear.

The problem is that the gulf between these two distances is vast, as seen above. This is a problem for filmmakers, and it’s why we end up with cool guys walking away from explosions. Putting everyone out of harm’s way (like we do in real life when we use explosives) is BORING. Who wants to see an explosion a kilometre away? It takes a full three seconds to hear the bang!

On the other hand, there’s no point putting the heroes too close to the action, or they will be the aforementioned red mist⁵. So, they put them outside the blast danger area (which, as we’ve seen, is very small) and probably outside the 100% lethal red mist area for fragments, and then have them roll double sixes every time and escape fragment-free.

In fact, people (baddies as well as goodies) don’t really suffer any injuries from explosions. They either get red-misted from being right up close and personal, basically hugging the bomb, or they are fine, with maybe a slight ringing in the ears or a slight singeing (which, remember, is only the fourth-most important injury mechanism from explosions).

Of course, a bit of singeing might be forgiven, seeing as how Hollywood explosions are basically fireballs…

Tell me more about these incendiary explosions

Just in case you think I’m ignoring the incendiary effects of explosives, we’re going to finish up with a quick word on them. But first, a chemistry lesson. You’ve probably seen this fire triangle before:

An explosive reaction follows the same idea, but is much quicker, because the oxidiser⁶ and the fuel are bound up together in the same molecule⁷:

An incendiary explosion (or a fuel-air explosive, FAE) uses the existing fuels and oxidisers above but adds a bunch of extra fuel and then uses the “free” oxygen in the air as an oxidiser, like so:

Here’s a slow-motion research video of a US BLU-96/B 2000 lb fuel-air explosive bomb (link to video):

These explosions are very cool, and they look just like Hollywood explosions. This is for a very good reason: they are the same thing. Film directors want to get this stunning visual effect for every explosion, so they use blast incendiaries to do this. Another upside is that these explosions produce little or no fragmentation, so the danger areas can be greatly reduced. They can get their money shot of the cool guy walking away from the blast:

There’s a great YouTube explainer here where they describe exactly how to make one of these blast incendiaries.

One of the more pleasant jobs we had in the military was running “power of explosives” demonstrations. We would always leave the blast incendiary until the finale. The watching VIPs would nod appreciatively as we went through the various effects of plastic explosive on its own, on a flesh substitute, inside a container etc. They would see the little flashes and the puffs of smoke and heat the chest-thumping bangs and, deep down, die a little inside because standing 100 metres from a hundred grams of plastic explosive is not very exciting.

But once we pressed the button for the last serial and (provided it worked) they saw a big orange ball of flame, and heard the deep “whump” of the fuel-air mixture igniting, and felt the pressure thump in their chest and the wall of heat… Well, then it was a good day’s work.

Conclusion: Frag is an unintended victim of the Coconut Effect

Real explosions aren’t nearly as impressive as Hollywood explosions, which tend to use a blast incendiary effect (which is a subset of explosion in its own right) for cool points, as well as for safety.

We, the viewers, actually don’t have a huge problem with this. We are used to stuff on-screen being a little bit different to reality. Guns make a “click” sound when drawn and pointed at someone at close range. Landmines also give a click when someone steps on them. Lasers make a “pew pew” sound. There’s even a name for this phenomenon: The Coconut Effect, named after the sound of two coconuts banging together which accompanied every on-screen depiction of a horse. The Simpsons even referenced this:

However, we need these film and TV coconuts to stand in for something real. Real horses do make a noise, even if it’s less distinctive than the coconuts. You would notice a gun being pressed against you, even though it doesn’t click. Landmines… Well, there’s no excuse for that one actually.

But for the explosions, we have the cool (and, remember, unrealistic) blast incendiary effect, but we don’t actually see the real effects of a shell or grenade or bomb or whatever it is. Everyone gets away scot-free from the lethal fragmentation effects of the munition. Even when we see the fragments flying through the air all around our heroes, they still escape without a scratch (and, in the example below, looking as gorgeous as ever):

Granted, this might be tough to get right. Like we discussed above, the effects are statistical, it’s not quite the same as one character shooting at another character, with a binary outcome: a hit or a miss. With a grenade or a shell or a car bomb, there’s an X% chance that the hero or villain will get the good news, depending on the munition and how close they are (although in the Sherlock Holmes example above, I would argue that that chance is pretty close to 100% for all three characters⁸). In that respect, heroes never being hit by shrapnel is a bit like never being hit by a bullet fired by a nameless goon (as opposed to bullets fired by main baddies, to which they are vulnerable).

As always with film and TV, I wouldn’t mind if this only happened from time to time. But I think screenwriters and directors are missing a trick by not incorporating frag effects into their explosions, getting out from under the comfort blanket of cliché and creating a scene which makes us grimace in pain and come away with a greater appreciation for the power of explosives.

Let me know what you think in the comments below: am I being too harsh? Next week I want to stay on this topic and look at some other aspects of explosions in Hollywood, namely the relationship between the amount of explosive and the size of the explosion. Plus, we’ll be exploring that perennial question: can you run or dive away from an explosion as it comes toward you? Watch this space.

Thanks, as always, for reading, and I’ll see you again next week.

1. The exception, of course, is certain pyrotechnics which are specifically designed to give out light, e.g. illumination compositions. ↩︎
2. I’m sorry, I know it’s confusing to talk about “primary frag” and “secondary frag” within the overall level of “secondary” effects. We (ammunition design students) always distinguished between primary frag and secondary frag, since they are important design differences, but never looked beyond this, even at tertiary effects. On the other hand, doctors only talk about “secondary” effects to encompass all fragmentation, since (from the point of view of treatment) they don’t care whether the frag came from the bomb or from nearby debris. My list is a mish-mash of both categorisations, just for completeness. ↩︎
3. This category of fragmentation is the reason that exploding cars, buildings, etc. cause the same problems as exploding shells or bombs: there’s lots of fragments to throw out. ↩︎
4. This is one definition of “safe”, you could increase or decrease this based on your tolerance for risk. ↩︎
5. Although some, like The Expendables, flout even this norm. ↩︎
6. Nitrogen is a great oxidiser, despite not being oxygen. There’s a chemical explanation as to why this is the case, but I’ve always just taken it as an article of faith. ↩︎
7. And the “Heat” part, in case you’re wondering, is supplied by the detonator. ↩︎
8. And no, the fact that Robert Downey Jr.’s ears were slightly ringing after this ordeal does not give the filmmakers a free pass to ignore every other mechanism of injury. ↩︎