The science behind The Hobbit’s fiery dragon
After a few lean years, the tradition of holiday-season blockbuster movies has come roaring back with The Hobbit: the Desolation of Smaug.
This second instalment of director Peter Jackson’s adaption of JRR Tolkien’s much-loved tale is being hailed for its return to themes that made the Lord of the Rings trilogy so thrilling: sinister forces, dark places – and monsters.
The undoubted star is a creature that makes even the flying Nazgul of the Ring movies seem benign. Smaug the Magnificent is a dragon – a gigantic flying, fire-breathing reptile into whose lair Bilbo Baggins, the hobbit hero, must venture.
Like most of Tolkien’s creations, Smaug has his origins in the mythology of many cultures. Tolkien seems to have taken his inspiration from the dragon of Earnaness, which features in the Anglo-Saxon tale Beowulf, written more than a thousand years ago.
Like most of Tolkien’s creations, Smaug is entirely fictitious, our Earth having been spared this dreadful denizen of Middle Earth.
Or has it?
Mention dragons to most people, and at best some may point to the existence of the Komodo dragons of Indonesia. Discovered barely a century ago, they are the biggest extant lizards on the planet. But at no more than three metres long and exhaling nothing more fearsome than halitosis, they are hardly in Smaug’s league.
Tolkien’s own illustrations of Smaug show him to be about 18 metres from head to tail, with a skull about a metre in size. He is far bigger in the movie, like a Tyrannosaurus Rex on steroids.
Could such a flying, fire-breathing creature ever have existed? A little basic science can go a long way towards answering that question.
At first sight, the most implausible of Smaug’s characteristics is his ability to breathe fire. Yet the laws of combustion suggest it is not quite as unlikely as one would think.
The so-called fire triangle rule shows that three key elements are needed to sustain fire: fuel, oxygen and heat. A highly combustible fuel is produced by many large organisms in the form of methane – as the plethora of YouTube videos featuring teenagers and cigarette lighters confirm.
Liquid fuels like natural plant oils would be better still, as they can be squirted out as a fine spray before being lit. This makes them explosively flammable when mixed with the oxygen from the air.
The spark needed to light the fuel could be triggered in several ways. It could come from exposure of the fuel to a natural pyrophoric compound such as phosphorous, which ignites spontaneously.
Or it could be generated by a so-called tribological source, in which flint-like material in the head of the beast releases a spark after being ground together.
As for protecting living cells from the searing heat, that is just a matter of blasting the flames out as far and fast as possible. Heat takes time to travel, and when combined with a protective layer of saliva or sweat, this renders the process harmless, as fire-eaters and fire-walkers demonstrate.
Still not convinced that evolution could cook up such a creature? Then clearly you have never heard of the bombardier beetle. Found on most continents, this astonishing insect sees off predators by mixing two chemicals together in a special “combustion chamber” containing special enzymes.
The resulting reactions create a scalding-hot liquid, which the beetle then blasts out at its attacker, using a nozzle on a swivel mount attached to its body.
Compared to that, the requirements of a fire-breathing creature seems modest. Indeed, the wonder is that such creatures are not common, given the evolutionary benefits they would enjoy.
It seems the reason there are no fire-breathing creatures on earth is not because they are impossible, but because the lottery of evolution has still to hit the right set of mutations.
Surprisingly, the bigger challenge to the reality of Smaug lies in his least surprising trait – an ability to fly.
This becomes clear from an application of a bag of tricks widely used by physicists, known as scaling laws.
These reveal how the properties of an object or phenomenon change as it is scaled up or down in size.
For example, if you found that one pot of paint was enough to decorate a box, then an identical-looking box that is twice as big will require four pots.
The reason it is not simply double the amount is that by doubling each dimension of the box, the total surface area – which is what you are painting – goes up by the square of the increase in scale, and the square of two is four.
In the same way, because volume goes up as the cube of the increase in scale, the double-sized box will carry eight times as much junk, as the cube of two is eight.
For flying dragons, the relevant scaling laws are those of weight and flight. Put simply, the dragon must be able to generate enough lift to keep himself in the air.
For objects of the same density, weight scales as the cube of length. So the lizard-like body of Smaug, being six times longer than a Komodo dragon, will weigh a hefty six-cubed times their 70-kilogram mass, or about 15 tonnes.
That huge weight is going to take some lifting – the scaling laws of aerodynamics confirm this. Getting the necessary lift from wings requires air-speed and a back-of-the-envelope calculation shows that air-speed scales as the square-root of body-size.
A few more scribbles and comparisons with real flying creatures and out pops the air-speed required by Smaug to fly – 200kph.
That is a fantastical demand, and suggests that whatever abilities Smaug may have, powered flight is not one of them. He might be able to glide, but if he threw himself off mountain tops, he would still struggle to get enough air-speed. He is just too big.
The upshot, then, is that flying, fire-breathing creatures are certainly not ruled out by science – but mercifully, they cannot be as colossal as the myths Tolkien and Jackson would have us believe.
So go along and enjoy the movie. But do not have nightmares.
Robert Matthews is a visiting reader in science at Aston University, England
Updated: December 28, 2013 04:00 AM