Road Trains of Mars

Imaginations have long been captivated by the idea of a manned mission to Mars; colonization, in the form of permanent settlement of Mars, is the logical next step that has also held sway over the minds of men. Yet for all the attention (rightly) given to the technology that will get us to Mars little consideration has been given to the technology Martian colonists will use to get around the planet itself. In this post we will explore the options available to any Martian colonists of the future.

Dune Buggies for Outposts

The earliest stages of colonizing any planet, Mars included, will be characterized by a simple habitation module of the same type used for exploring or visiting outfitted with supplies to last for longer-duration stays. In real life the Apollo missions were actually grading toward this; by Apollo 17 the astronauts were staying on the Moon for 75 hours at a stretch. For comparison Apollo 11’s crew spent only 22 hours on the Moon. The Apollo Applications Program envisaged the expansion of Apollo hardware to enable longer stays, and eventually permanent habitation in the form of colonies. If the Apollo program had continued, much longer stays than 75 hours would have been routine by the end of the 1970s; NASA’s current Artemis program aims to pick up where Apollo left off, envisioning much-longer-duration stays at first, progressing to a permanent lunar base in four years.

Interestingly, the much greater energy and transit time requirements inherent in a Mars mission has inspired many people to propose, especially in recent years, that the first men to explore Mars should go assuming it’s a one way trip and they will stay permanently, an idea which is called “Mars to Stay”. This is a rather different approach than how we have explored and colonized the Moon or indeed most regions of the Earth, but it does have the elegance of eliminating the costly return phase of mission planning. Under this plan exploration and permanent settlement would come simultaneously.

Whether a temporary expedition-style encampment or a permanent habitation module, transportation needs will be fulfilled by vehicles at least somewhat similar to the rovers used by the Apollo astronauts on the Moon. The Lunar Roving Vehicles were four-wheeled battery-powered open-cockpit vehicles designed to enable astronauts to get around and conduct their explorations faster, and were very successful. Designed for a top speed of eight miles per hour (but as Apollo 17 proved capable of at least 11 mph), they were driven an average of 19 miles on each mission. The “Moon buggies”, as they were sometimes known, weren’t designed to last for a long time, whereas any Martian colony’s vehicles would by necessity have to be more robust. Nevertheless Apollo remains the closest real-life comparison.

Pressurized Rovers: The next Step

Most recent designs for Martian rovers are closed-cockpit, enabling a shirt-sleeve environment inside; this would be much more convenient for the drivers and passengers, and enable much better scientific work to be done on the go. This will be especially important over longer distances, as most plans for Martian expeditions or early colonies envision trips days or weeks out from the home base. Extravehicular needs will be provided for by way of detachable spacesuits attached to the outer surface of the vehicle, which conveniently prevents any contamination (like clouds of Martian dust) from coming inside. An especially interesting design along these lines is NASA’s Space Exploration Vehicle (SEV), designed as a vehicular habitation module that can be fitted to a chassis for planetary use or alone for outer-space use.

The first Martian settlement will likely be very compact, much like a village, with every building well within walking distance, probably even connected at some relatively early date by pressurized walkways. Thus rovers will only be used to explore further afield well beyond walking distance, for purely scientific purposes as well as to identify new resources for future expansion of the settlement. These ambitions for long-duration trips raise the possibility that Martian rovers may be routinely equipped with sleeping bunks much like long-haul trucks are on Earth; this will, together with the pressurized indoor environment, represent a major advance over Apollo-era technology. These pressurized vehicles will be the mainstay of long-haul trips, but it is hard to shake the suspicion that much simpler and cheaper Apollo-style dune-buggy-like open-air rovers may still have a role to play in shorter-duration journeys.

Giving Long Haul a whole new Meaning

Where it gets much more interesting is when multiple settlements come into the picture, as inevitably will occur in any colonization effort. While a second settlement may be located very close to the first site, it seems likely that scientific reasons will drive at least some settlements to be located in different parts of the planet to maximize research value. Suddenly the compact planet, in terms of human geography, is no longer so compact. Indeed, at this point there will be a population of thousands at best spread over a planet with a circumference of over 13000 miles. If two settlements are located on opposite sides of the planet, getting from one to the other involves a journey of at least 13000 miles (probably more miles if getting around terrain obstacles is included) with no towns or (likely) service stations in sight. Talk about a long haul.

Admittedly, in the earliest stages colonies at this distance will be more or less isolated, but the desire and need for cross-planet transportation will inevitably emerge, in population-density terms, very early in Mars’s development, especially considering that traveling to another Martian town will, despite the difficulties, be a much shorter trip than traveling to Earth. How early? As I pointed out in a previous post, even a planetary population of five million, comparable to what Elon Musk dreams of achieving in his own lifetime, would still only give Mars the same population density as Greenland. 50 million would yield a planetary population density comparable to Alaska, still very low by the standards of Earth’s more populous regions. Clearly Mars will retain a low population density for a very long time even with a successful colonization effort.

Road Trains: from the Outback to Mars

Earth does, however, have many regions with comparably low population densities that may provide some clues. Most of the vast boreal forest is not connected to the road system because there are too few people to serve to make the cost of building and maintaining the infrastructure worthwhile. Boats, bush planes, snowmobiles, and more rugged four-wheeled vehicles substitute for cars; graded or at best gravel roadways and airstrips substitute for paved roads. The best comparison, though, would almost certainly be the Australian Outback, as both it and Mars are deserts. Aircraft serve the Outback like they do the boreal forest, but the main difference is that in the Outback graded dirt roads traversed by automobiles and trucks predominate and there are no boats or snowmobiles.

A particularly interesting phenomenon found in the Australian Outback, but also some other places (especially in more remote areas) is the “road train”. Where traffic along a specific route is insufficient to make a railroad line economical, trucks pulling multiple trailers fill trains’ role very effectively. These “road trains” would probably be much more common on Earth than they are today if it weren’t for regulatory barriers, but in any case these won’t exist on Mars. Also on Mars, the only routes where traffic will increase at any point in the near future enough to support a train will be routes into and out of busy mines and routes that go between different parts of cities. Long-range inter-city or inter-colony rail won’t be commonplace on Mars for a very long time. Even when millions of people are on Mars they will be so spread out that any given point along any given highway won’t see more than a few vehicles per day.

So road trains it will be for the foreseeable future. Transportation for a while will be purely off-road but as soon as there is significant traffic between colonies grading a roadway will become worthwhile. Grading and rock removal will likely be the extent of inter-city road-building for a very long time. Vehicles designed for trips on these roads for thousands of miles between services will necessarily be rugged, and will have sleeping cabins for all passengers as well as enough supplies to last in case of an emergency. This, together with the airlock and life support requirements, mean that any long-haul vehicle on Mars will likely be much larger than any comparable vehicle on Earth. It has been suggested, and I believe this is likely to actually occur, that vehicles on Mars may travel in the form of convoys for mutual aid and redundancy in case one vehicle is damaged beyond the crew’s ability to repair.

How efficient can road trains be? Very efficient. Although road trains usually only carry three or four trailers (or “cars” in rail terminology) trains consisting over a hundred trailers have been successfully driven in world record attempts and the like. The low gravity on Mars, one-third of Earth’s, means that all other things being equal three times as much payload can be transported.

The Challenge of Propulsion

Of course, things won’t all be equal. In particular, road trains and trucks on Earth use hydrocarbon fuel extracted from fossil deposits which are unavailable on Mars. The resource closest at hand on Mars is the atmosphere, consisting mostly of carbon dioxide. This carbon dioxide can be extracted and split into carbon monoxide and oxygen; this does burn to produce energy in an internal combustion engine, but is rather unimpressive in terms of power output. “Methalox”, reacting methane with oxygen to produce power, is another alternative that is under serious consideration; the methane would likely be synthesized from refined atmosphere-sourced carbon and hydrogen taken from subsurface water ice. The water ice would also provide the oxygen. This is a much more complex and likely expensive process, but would provide more power than the carbon monoxide-oxygen reaction.

Another alternative under consideration is battery electric vehicles, which have the advantage of not relying on complex chemical reactions. On Mars, a planet without a natural fossil fuel endowment, they may prove economically superior to other methods. Only the advent of gasoline on Earth dethroned electric cars’ initial dominance in the early 20th century, after all. Electric compares unfavorably with gasoline in power output and range, but may be competitive with methalox or carbon monoxide-oxygen. The most obvious disadvantage is that battery electric performs horribly in cold weather, and Mars has a very cold climate compared to the Californian climes electric cars do best in. Nevertheless insulation and heating may be arranged. Electric vehicles may be what Elon Musk is planning for SpaceX’s Mars colonies; there have been interesting suggestions that the technology found in Tesla cars would in many respects do well on Mars. This may be deliberate on the part of Elon Musk.

Giant Nuclear Vehicles: Mars’s Future?

It is worth noting that the combination of large vehicles pulling very heavy payloads far from any services or support is an ideal scenario for nuclear power, which becomes much more efficient relative to other power sources as scale increases. With fossil fuels being impractical, nuclear will look even better relative to the likes of battery electric and methalox. If nuclear is employed bigger vehicles mean greater power density, increased affordability, and interestingly increased speed. When there are large payloads of passengers and cargo to transport large nuclear-powered wheeled locomotives may well be carrying gigantic trailers across the dusty Martian landscape.

How large are we talking about? Probably something comparable to the Juggernaut vehicle from Star Wars; while the Jawa Sandcrawler is better-known, it traverses the landscape using treads, whereas any Martian vehicle would likely use big wheels due to less maintenance being required, especially vehicles designed to traverse graded roads. These wheels would not be the familiar air-filled rubber tires we see on Earth, as the cold of the Martian nights and winters would destroy them. The most likely option would seem to be metallic mesh similar to the Soviet Lunokhod rovers, as that is a cheap, strong, and easy-to-make material that can easily withstand harsh Martian-style conditions.

Nuclear road trains may actually be so efficient that they obviate the development of long-range railroads. After all, electric power (often provided by stationary nuclear reactors in the form of power plants) provides the competitive edge for passenger rail in real life, but on Mars electric power would have to be transmitted thousands of miles. The losses from such long-range transmission means it’s much more efficient to carry your power source with you.

This power source could easily be a nuclear reactor carried inside the locomotive, which was actually studied for Earth use in the 1950s. It was discarded because diesel and (nuclear-provided) electric were superior, but neither of these sources are viable on Mars. Larger-sized trains would be more efficiently powered by nuclear; something like the Breitspurbahn, a gauge of railway twice as wide as standard gauge, would help with this considerably. Indeed, back-of-the-envelope calculations suggest a modern reactor might actually be the best power source for a broad-gauge railroad on Earth; on Mars the case would be much stronger.

Since railroads would only be built once there is heavy traffic anyway, it is very easy to imagine a long-range Martian transportation network consisting of graded roads being traversed by convoys of gigantic nuclear-powered road trains, ultimately in the heaviest-trafficked areas being more or less replaced by broad-gauge nuclear-powered rail trains. Broad gauge was not adopted on Earth due to the expense, but if containers on Mars are manufactured with a colossal nuclear road train in mind, building any railroads to fit the same dimensions is the obvious choice economically. Much like what is said about Texas, everything may be bigger on Mars, at least as far as ground transportation is concerned.

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