Saturday, 10 February 2007

A Feasibility Study of Tripodal Fighting Machines

Yes I know Ive been on a bit of a WOTW frenzy of late, but this is an interesting article:

A Feasibility Study of Tripodal Fighting Machines
by Professor Ian Stewart

'...and of their appliances, perhaps nothing is more wonderful to a man than thecurious fact that what is the dominant feature of almost all human devices inmechanism is absent - the wheel is absent.' [H.G. Wells, The War of the Worlds, Book Two Chapter 2]

Why do Wells' Martians use legged Fighting Machines, and why three legs? The main reason must surely be that this choice would make them seem very alien indeed, especially to a Victorian audience. But Wells has a reputation as a hard science fiction writer, meaning that he did his best to get the science right - with the exception of whatever exotic gizmo gave the story its zing, such as the little matter of a time machine. So we must assume that he gave the Martian machines legs for a reason that would stand up to a moderate degree of scrutiny.

In this feasibility study I'm going to look at legged Fighting Machines from several different points of view: engineering, stability, control. The area of science into which such questions fall, is called Gait Analysis. The gait of a legged creature or machine is the pattern with which it moves its legs in order to be locomotive. So I will be taking a look at the Fighting Machines' three-legged gait in light of what we currently know about legged locomotion in both animals and robots.

Legs Good, Wheels Bad
Remember, these machines are for invasion. The terrain of the target planet Earth, is diverse. Much of it is unsuited to most of the obvious methods of legless locomotion. Sledges and skis work on snow and ice, but not on rocky ground. The method of mechanical locomotion that we terrestrials currently favour is entirely unsatisfactory for Martian use. It comprises, for the most part, a network of narrow flattened strips, along which improbable vehicles move by means of a cumbersome system of rotating discs. This technology requires the periphery of the disc to remain in contact with the ground, propelling the vehicle forwards by means of inter-surface friction. Such a method of locomotion is clumsy and unresponsive, subject to unexpected slippage, and entirely unsuited to the sudden changes of direction that are so vital in military campaigns. Moreover, such a network can be too easily sabotaged by the terrestrial inhabitants. Martian terrain is mostly rocky deserts, craters, and mountains, so there is little reason why the Martians would have evolved the wheel. Naturally, they would base their designs on the technology that had evolved, over thousands of years, to suit their own planet's conditions - just as we put wheels on the Lunar Rover and on Sojourner, our own Martian invader.

Caterpillar tracks present themselves as a plausible alternative - after all, we use them on our own 'Fighting Machines' for all kinds of terrain. However, there is a further consideration which leads to legs as being by far the most attractive option. That is the fact that Fighting Machines have to maintain adequate surveillance in all directions. This dictates a vertical architecture, whose base, for stability, must be broad - too broad to be constrained within the narrow width of the indigenes' strips (roads). This is yet another argument against wheels. But, very large caterpillar tracks are heavy, which is a major minus, given that initially the machines have to be shipped in from Mars via interplanetary cylinders. They are also clumsy, inflexible, and unreliable because of their mechanical complexity. And aluminium, the Martian's favoured construction material [Author's Note: Wells refers to aluminium three separate times in the text], wears too rapidly and is too brittle to make good caterpillar tracks. The Martians can therefore be expected to reason that Fighting Machines should take the form of a protected Life Enhancement Module (LEM) supported on legs.

What Wells Tells Us

When Wells first describes the motion of the Fighting Machines he presents an image that makes very little engineering sense: 'A monstrous tripod, higher than many houses... Can you imagine a milking-stool tilted and bowled violently along the ground?'

The immediate impression of this description is that the Fighting Machine rotates as it moves ('bowled'). But such a motion is very hard to control - it is rather like trying to balance a car on two wheels like James Bond does in the movies. Moreover, such a gait makes little sense for a military machine which, among other things, is used to carry a Heat Ray projector. I think it is more likely that Wells was having difficulty in finding a metaphor that would convey three-legged locomotion to his readers, which is not surprising given that we hardly ever witness it in real life. (A student of mine observed a three-legged dog, which had lost one hind leg in an accident, which is why I never say 'never'. More of the dog later). Wells' description at this stage of the book is exaggerated, no doubt, for dramatic effect: 'A flash, and it came out vividly, heeling over one way with two feet in the air, to vanish and reappear almost instantly as it seemed, with the next flash, a hundred yards nearer.'

Since we are told elsewhere in the text that the Fighting Machines are about 100 feet high, their stride must be of the order of 30 feet, so they take 10 or so strides (of each leg, 30 altogether) to cover 100 yards. To be fair, Wells is reporting what he sees at successive flashes of lighting in a thunderstorm, but even so 'almost instantly' is an exaggeration. However, 'heeling over one way with two feet in the air' does give what I suspect is an accurate account of the machine's most rapid gait.

Many terrestrial animals use more than one gait: the basic reason is that different patterns of locomotion are efficient at different speeds or under different conditions. It is clear that the Martian Fighting Machines employ the same trick, for they definitely have gaits that differ from the one just described (whatever it actually is!): '...the first party of Martians were crawling slowly towards their second cylinder under cover of a metal shield. Later this shield staggered up on tripod legs and became the first of the Fighting Machines I had seen.'

One cannot 'crawl' or 'stagger' while being 'bowled violently along'. There is other evidence for a versatile repertoire of gaits, too: 'In another moment it was on the bank, and in a stride wading half-way across.'

Water or mud would cause drag on the legs, which again would make 'bowling along' impossible. Less dramatic gaits, however, would be entirely feasible, as we shall see.

Enjoying this article? You'll find the rest online here - goto page 2 of the features:


dafrca said...

Interesting, very smartly written.


Tas said...

Cant take any credit for it myself, but yes I thought so too.

sigh9 said...

tripods! they're even useful for decorating flyers for wierd electronica nights in London.

vide infra:

An exploration of debauchery, vice and other reasons to be a man!

An exploration of debauchery, vice and other reasons to be a man!