Mortal Engines London

Mortal Engines: they built this city on rock and roll

Image credit: Universal Pictures

Resources are dwindling and cities are now on the move to win their fair share. Mortal Engines are coming to get you!

Humans were all but wiped out in the 60-minute war – we blasted ourselves into oblivion with “a terrible flurry of orbit-to-earth atomics and tailored-virus bombs”. A couple of millennia (although the timeline is vague) down the line and America is still a wasteland. Elsewhere, life survived and we now live in the Traction Era – the dystopian backdrop to ‘Mortal Engines’.

The technology of ‘the Ancients’ (that’s us, by the way) has never been replicated and is far and away superior to anything existing in the new age – they haven’t got computers for example. Such is the unquenchable thirst for progress inherent within humans, particularly as we Ancients had already laid out the blueprint for the advanced technology, it would be a bit disappointing if humanity ended up living in an ‘industrial-age jungle’, as is depicted in ‘Mortal Engines’. However, let us assume that the 60-minute war has blown away both human creativity and morals and we are now very much into the survival of the fittest.

London is now governed by four guilds (Engineers, Navigators, Historians and Merchants), of which by far the most powerful are the engineers, so at least they have got one thing right. Engineers are now being recognised as the leading profession!

While innovation and technology does not appear to be on the move, villages, towns and even cities are. Huge chunks of civilisation have been loaded on caterpillar tracks and roam the countryside like giant tanks, preying on smaller settlements, gobbling up the resources and spitting out the waste. It’s an arena in which size and speed matter, which is why the engineers’ skills are so highly regarded.

Much of the action takes place in London, although our hero and heroine also get ensnared on the fabulously named pirate town of Tunbridge Wheels for part of their adventure. London is no longer dubbed The Big Smoke, but it might as well be, as the age of fossil fuels is still very much alive, with coal and oil powering these enormous machines. Now even in our modern day as we try and curb our climate-warming activities, coal at the current rate of consumption would only last for about another 150 years and oil a mere 50, so fuel for these mortal monsters would have run out long since. It is this theme of diminishing resources that gave the book its title. Author Philip Reeve borrowed the phrase from Shakespeare, who was describing an unsustainable existence.

However, let’s assume that our climate-warming carbons have made it through to the Traction Age. The more interesting question is could a city such as London practically be made mobile? And what speed could it reach? When would it become too heavy to move?

London’s only surviving building from the 60-minute war was St Paul’s Cathedral, which now sits proudly on the top layer of the new mobile London. It is probably a good time to point out that dome on St Paul’s alone weighs in at 60,000 tonnes. Admittedly the dome (or more accurately ‘domes’, as there are three inside each other) include a bit of lead, but the rest is timber and brick, so not the heaviest construction materials. I am assuming that the dome is about a 15th of the total weight of St Paul’s, as the cathedral is a huge structure and the dome a relatively small part. This conservative estimate brings the total weight to a convenient 1 megatonne.

To give scale to the problem, let’s look at JCB’s largest tracked excavator, the JS370. In standard configuration it weighs in at about 38.6t and the footprint of its twin tracks is 6.43m2. This represents about 6t/m2. At this ratio our 1Mt cathedral would require a footprint of around 170,000m2, just shy of the area of Green Park in London, and about two-thirds of the size of Woodhouse Moor in Leeds. It is also over 17 times the footprint of St Paul’s Cathedral.

That’s just one building – we are talking a fully functioning, seven-level city here, with St Paul’s a mere bauble on the top. Just to use the same 17:1 ratio (which is a bit unfair as St Paul’s is considerably more substantial than a family home in the leafy suburbs), to mobilise London at its size today would require tracks with a footprint the size of all of East Anglia and south-east England.

“The ground pressure is a linear relationship,” says Nick Swift, excavator engineering director at JCB. “The heavier the machine the bigger the footprint, the wider the tracks we use. So the actual pressure will be consistent; all we do is design a machine that spreads the weight out evenly on the tracks. In theory, in terms of weight of machine and footprint, it could be infinite.”

Nor would the materials, bearings, axles suffer. Nick Grills, JCB’s general manager for global excavator sales, joins in: “If you were designing a city on tracks you would need to design the city so that the buildings were evenly spread. Provided you have the surface area calculated correctly, and you have the necessary faces on the steel hardened, it wouldn’t wear out.”

When I dropped in the issue of speed, sadly I cannot capture in words the spontaneous hilarity from both of the JCB Nicks, when I mentioned that in ‘Mortal Engines’ there was mention of a small town fleeing at 80mph (which London still caught). This reaction came from a world where the heavy kit moves at little more than 3mph. Swift comments: “The faster you go, the bigger the earthquakes. If you have driven an excavator across a quarry floor at 5km/h, you will know that you don’t want to go at that speed for very long. There’s a lot of shaking going on. It gets quite uncomfortable.”

Even if we are looking at just speed, rather than size, modern-day tanks, the fastest tracked machines around, can barely scrape beyond 50mph and they typically only weigh 30t or so. And with speed comes wind resistance, which increases in line with the velocity squared. “At 80mph there is not much power required to overcome friction; it is all about wind resistance,” adds Grills. “So my opinion is even if you could make your city travel along at that speed you would need some very clever building design and you would need huge tractive effort delivered by huge power sources. I would say off the top of my head the power generation facilities would dwarf the city to move it at 80mph. And it would need a fantastic suspension system!”

While cities could theoretically one day be on the move, it seems unlikely that they will be rattling along at hunting speeds. Don’t let this put you off the film, mind you – it was the very thought that you would never see a real T-Rex that made ‘Jurassic Park’ so good. I suggest the same applies to a speeding St Paul’s.

Mortal Engines is in UK cinemas now

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