Could you outrun a fireball, or walk away from an explosion?

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The episode's narration, word for word. Numbers link to the sources below; timestamps play that moment.

The walk-away 0:00

A car erupts into a huge orange fireball, and the hero strolls towards the camera without looking back. Or he sprints, the flames rolling after him a step behind, and dives clear just in time. Audiences have come to expect exactly that big orange fireball, as one Hollywood effects supervisor put it ¹. So could you really outrun a fireball, or walk calmly away from an explosion?

Three things at once 0:28

An explosion is a lot of energy let loose in a small space, very quickly ². It sends out three things ². First, a flash of light and heat ^2,3. Then a shock front, which a classic US government handbook describes as a moving wall of highly compressed air ². And behind it, a fireball of hot gas, which grows and then rises ². In films, the fireball is the star; in physics, worry about the invisible wall ^2,1.

Racing the wall 0:59

How fast is that wall? Even a weak blast wave travels at the speed of sound, about three hundred and forty metres a second in air at twenty degrees ^2,4. Close to the explosion it starts out faster still, several times the speed of sound ². The fastest sprinter ever recorded, Usain Bolt, averaged about ten point four metres a second over his world-record hundred metres in Berlin in 2009 ⁵. The blast, at its very slowest, is about thirty-three times faster ^4,5.

Twenty metres 1:33

Give our hero a twenty-metre head start. The blast wave covers that in about six hundredths of a second ⁴. In sprinting, any reaction faster than a tenth of a second counts as a possible false start ⁶. By the sport's own standard, the blast has gone past before you could react ⁶. Even if you were somehow already running flat out at Bolt's pace, you'd have moved about sixty centimetres ⁵.

Fermi's scraps of paper 2:03

Distance is what buys you time. On the sixteenth of July 1945, at the first atomic test, code-named Trinity, the physicist Enrico Fermi was at base camp, about ten miles, or sixteen kilometres, away ^2,3. First came an intense flash and a feeling of heat on his skin ³. The air blast reached him about forty seconds later ³. He dropped scraps of paper as it passed, saw them drift about two and a half metres, and estimated the explosion's strength on the spot ³. Sound would take about forty-seven seconds to cover that distance, so his count is in the right range ^4,3.

A sudden third of an atmosphere 2:43

So what is it about that wall that hurts? At the shock front, the air pressure jumps from normal to its peak almost instantly ². Your body is squeezed by the difference between the pressure outside and inside it, and a sudden jump does more harm than a slow one ². The handbook's tentative threshold for eardrums rupturing is five pounds per square inch above normal, about a third of an atmosphere extra ². Lungs start to be damaged at around twelve ². And right behind a five-psi front, the air itself is rushing along at over two hundred and sixty kilometres an hour ².

What the blast throws 3:22

Those thresholds are for long pressure pulses, like a nuclear blast's ². A small conventional blast's pulse can be over in about a millisecond, and it takes much higher peak pressures to kill directly ². The blast also throws things at you, like glass and debris, and it can throw you, into a wall or onto the ground ². Standing by a wall is worse still, because the reflected pressure there can be a few times higher ².

Chelyabinsk 3:51

On the fifteenth of February 2013, an asteroid about eighteen metres across tore into the atmosphere at about nineteen kilometres a second and exploded in the sky near Chelyabinsk, in Russia ^7,8. More than twenty kilometres up, it flared brighter than the Sun ^7,8. The flash blinded drivers ⁹. But the shock wave, travelling at roughly the speed of sound, needed more than a minute to come down ^4,7. When it arrived, it blew in windows, and more than a thousand people were hurt, mostly by broken glass ^9,10.

Cool guys don't look 4:27

Now, that walk-away shot. It's so familiar that the comedy group The Lonely Island released a song called Cool Guys Don't Look at Explosions ¹¹. At movie distances, whether you look, walk or run makes no difference to a blast that arrives in hundredths of a second ^2,4. What matters is how far away you were when it went off, and what's around you ². And Chelyabinsk adds one more lesson: after a big flash, the pressure wave can still be on its way, and windows are where it hurts people ^9,7.

The fireball stops 5:01

What about the fireball itself? At first it grows as fast as the shock front, because the front is its edge ². Then the shock breaks away and runs on ahead, and the fireball's growth slows sharply ². For a twenty-kiloton nuclear burst, the handbook says the fireball reaches its full size after about a second; even a megaton fireball takes only about ten ². So a fireball doesn't chase you down the street ². It reaches about as far as it's ever going to within seconds, and then it climbs ².

Seconds, not minutes 5:36

The US Environmental Protection Agency plans for accidents where a pressurised fuel tank bursts into a fireball ¹². In its planning table, the smallest fireballs burn for about three and a half seconds, and the very largest for about twenty ¹². The main danger it plans for is heat radiating from the fireball ¹². If you're outside a fireball when it forms, you're not racing it; you're getting out of its glare.

Shoot the petrol tank 6:04

Which brings us to the movie car that explodes when it's shot. In 2004, the MythBusters shot a bullet through a car's fuel tank ¹³. The tank didn't explode ¹³. In 2005 they came back with tracer rounds, and one did set the petrol alight ¹⁴. There was no explosion, though by then the tank was so full of holes that the team couldn't rule one out from an intact tank ¹⁴.

Too rich to burn 6:31

Why is it so hard? Petrol burns as a vapour mixed with air, and only between about one and a half and seven and a half per cent ^15,16. Below that, the mixture is too lean to burn, and above it, too rich ¹⁵. Inside a closed tank, the amount of vapour depends on how volatile the petrol is, and that varies a lot ¹⁵. Work it out, and that air space can be about five per cent vapour, inside the band, or nearly forty per cent, about five times the richest mixture that can burn ¹⁵.

Walking pace 7:04

In 2007, the MythBusters tested another classic: a car leaking a trail of petrol, and a flame racing along it to the tank ¹⁶. They reported the flame moved at about five kilometres an hour, as fast as a brisk walk, and a car at thirty-two kilometres an hour easily outran it ¹⁶. And when Adam Savage and Jamie Hyneman let the flame reach the tank, even in their most favourable test, it still didn't blow up ¹⁶.

Off a cliff 7:33

In 2009 they drove cars off a cliff ¹⁷. No explosion, they reported ¹⁷. A car's fuel tank sits underneath, between the wheels, deliberately out of harm's way ¹⁷. They only got a movie-style fireball by crushing the tank on purpose, and the full film look needed an effects expert to add the bang himself, which is how film-makers usually do it ¹⁷. In other words, the exploding car is something the crew puts in, not something the car does on its own ¹⁷.

Why films do it anyway 8:07

So why does every movie car go up like a bomb? Partly because we expect it ¹. Richard Stutsman, a special-effects supervisor, put it plainly in a 2010 interview: 'the audience has just grown to believe everything turns into these big orange gasoline fireballs' ¹. A fireball reads instantly on screen, and a shock wave you can't see doesn't. A film is telling you, in one picture, that this is dangerous, and that's fair enough.

The film that got it right 8:39

Stutsman was the effects supervisor on The Hurt Locker, Kathryn Bigelow's Oscar-winning film about a US Army bomb-disposal team in Iraq ¹. The film-makers, he said, were adamant about realism ¹. He'd watched real blasts, and noticed that the fire sits in the centre, with the dust and debris out in front of the fireball ¹. So he filmed the shock with a high-speed camera, and you can see the wave make a car jump and the dust lift off it ¹. As he describes it, that's the physics on screen: the wall arrives first, and the flames stay behind ^2,1.

So, could you? 9:17

So, could you outrun a fireball? Not the blast that comes with it: even at its slowest, it's about thirty-three times faster than the fastest sprinter, and it's on you in hundredths of a second ^4,5. The fireball grows for a moment and then stops, so whether it reaches you was settled the instant it went off ². Walk away? Only if you were already far enough away, and even then, the wave may still be coming⁹. And in the MythBusters' tests, a shot petrol tank just leaked, or at worst caught fire ^13,14.

A bigger rock 9:52

Chelyabinsk was a small asteroid, about eighteen metres across, and its shock wave still broke windows up to ninety kilometres either side of its path ^7,8. There are much bigger rocks out there, and Hollywood has a plan for those too: fly up and blow them apart. Next time: could a nuclear bomb split an asteroid?

Sources

Every factual claim in the episode is tied to one of these. Spotted an error? Tell us.

1. Ian Failes, "Hurt Locker Special Effects: Physical Bombs" (Q&A with special effects… — fxguide.com
2. Samuel Glasstone and Philip J. Dolan (eds), The Effects of Nuclear Weapons, 3rd… — osti.gov
3. Enrico Fermi, "My Observations During the Explosion at Trinity on July 16, 1945", US… — dannen.com
4. C. R. Nave, "Speed of Sound in Air", HyperPhysics, Georgia State University — hyperphysics.phy-astr.gsu.edu
5. IAAF, "Bolt again! 9.58 World record in Berlin!", 12th IAAF World Championships in… — web.archive.org
6. World Athletics, Book of Rules C2.1 Technical Rules, Rule 16 (The Start, False Start)… — worldathletics.org
7. NASA JPL Near-Earth Object Program Office, "Additional Details on the Large Fireball… — web.archive.org
8. NASA, "NASA and International Researchers Collect Clues to Meteoroid Science" (summary… — web.archive.org
9. Geoff Brumfiel, "Russian meteor largest in a century", Nature News, 15 February 2013,… — nature.com
10. Nature News Blog, "Russian meteor blast was the largest ever recorded", June 2013,… — web.archive.org
11. The Lonely Island, "Cool Guys Don't Look At Explosions" (video page),… — web.archive.org
12. US Environmental Protection Agency, Risk Management Program Guidance for Offsite… — epa.gov
13. Wikipedia, "MythBusters (2004 season)", episode "Scuba Diver and Car Capers" (first… — en.wikipedia.org
14. Wikipedia, "MythBusters (2005 season)", episode "MythBusters Revisited" (12 October… — en.wikipedia.org
15. NIOSH Pocket Guide to Chemical Hazards, "Gasoline" (flash point, explosive limits,… — cdc.gov
16. Wikipedia, "MythBusters (2007 season)", episode "Trailblazers" (31 October 2007),… — en.wikipedia.org
17. Wikipedia, "MythBusters (2009 season)", episode "Crash and Burn" (11 November 2009) — en.wikipedia.org

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