Friday, January 27, 2017

Rapid Transit on Trantor

Second Avenue Subway

In our last exciting episode we visited Trantor, or at least a city world that could pass for Trantor, an ecumenopolis of a trillion people for whom the Galactic Empire is something of an afterthought. But having come this far we ought to see more of it than just the inside of a spaceport gate or elevator foyer.

The Foundation 'verse has no teleportation tech (if you exclude jump-style hyperspace), so to get anywhere on a planet you have to go there. And if our ecumenopolis is a real city - not a mere planetwide suburbia (how boring!) - this means a public transit network.

Cars will not do, not for general use - not even Futuristic[TM] skimmer cars or whatever. Not even robocabs, which did not exist in that 'verse anyway.

It is all about geometry. Cars and highways are fine at low population density, such as tract-home suburbs - say, 5000 people per square mile = 2000/km2. But once you get up even to townhouse urban density, about 10,000/mi2 = 4000/km2, parking and roadway space become a major hassle.

Putting everyone in robocabs instead of private cars would help with parking, and the robo part would allow more roadway crowding before gridlock sets in. (Robots should drive better than we do.) Boxy little cars would help a bit, too. But these measures only get you so far.

Yes, in principle you could have multiple layers of parking garages and underground roadways below the dwellings. And since Asimov's 'verse did have aircars, you could move the garages to the top floors and the traffic jams into the air. In the Future, car accidents never happen.

But in practice, at some point it gets easier and faster to simply take the bus.

Which brings us to transit technology. In rocketpunk days it was taken for granted that even ground vehicle would never use anything so primitive as wheels. As late as c. 1990, the agency building the Los Angeles rail transit lines insisted that artists' conceptions avoid showing that its trains would run on (gasp!) railroad tracks.

Now, of course, tracks are back, including streetcars (trams, to some of you). Depending on the state of the tracks or pavement, streetcars usually have a smoother ride than buses, but don't go any faster. Their chief virtue is that a streetcar, running on rails, can be longer than a bus and thus carry more people.

But this only really matters for very busy lines, which is why most streetcars vanished around the rocketpunk era.* In any case, by the near future - no need to wait for the plausible midfuture, let alone the Galactic Era - technology could blur these distinctions.

The rocketpunk era is associated less with fading streetcars than with two other forms of urban transit. One, monorails, needs little discussion here. They are just elevated (usually) rail lines with a track too narrow for most idiots to try walking along. (The wheels are also neatly hidden from view.)

Much loved in the abstract, monorails never became popular in real life because people hate els running above their street. This is too bad, because you see a lot more of the city from an el than you do from a subway. But most people hate on els anyway, and still hate them even when the tracks are narrower and don't blot out quite as much sun. Which is why monorails remain rare.

The other great rocketpunk transit tech was the slidewalk, a pedestrian conveyor belt resembling a flattened out escalator. Step on and be carried along. These are more interesting, as a real departure from conventional vehicular transit. For one thing, slidewalks run continually, so unlike a bus or train you don't have to wait for it. This is a big deal, because people hate waiting, and long wait times can effectively wipe out the advantage of high speed.

Like monorails, some slidewalks actually exist, but also like monorails they have never really caught on. The problem is that if they are fast enough to save you much time over walking, people will stumble and fall all over when getting on or off.

Heinlein (and probably others) suggested multiple side-by-side strips, so you could start on a slow 'local' strip, then cross over to faster express lanes. Alas, unless Trantor has UBT - universal ballet training - this side-step across a speed differential is also a guaranteed pratfall generator.

With suitable magitech you might improve on the situation. Clarke's far future city of Diaspar has slidewalks made of flowing 'anisotropic matter'** that you can stand/walk on, while allowing a smooth transition from slow edges to the faster center express section.

Slidewalks are still limited in speed unless your magitech also moves the ambient air along so riders aren't facing a gale-force relative wind. Rapid transit they are not, but if you can solve the pratfall problem they might have a place along busy corridors like Seldon Street.

Indeed, for window shoppers and flaneurs, Seldon Street might even have slidewalk cafes and such. But this is more tourist attraction urban amenity than practical transit for people who just want to get where they're going.

Otherwise your basic local line, the service that goes everywhere and stops at your corner, is essentially a plain old bus. Even though the smelly diesel bus has surely gone the way of the 19th century horsecar, which also emitted a noxious exhaust.

And so a bus it is, though heavy Trantorian ridership levels - especially along busy Seldon Street - might justify streetcars/trams. We will also suppose that heavy ridership allows TERTA to provide frequent service, so you only have a short wait when connecting between lines.

But the local bus can't be very rapid, not for techological reasons as such, but because it has to fight its way through traffic, automotive or pedestrian. Even if separated from other traffic, it must stop every few blocks to let riders on and off. And it can't get up too much speed between stops because of a basic human limit.

Back in the 1930s, the R&D program for the classic American PCC streetcar determined that the highest comfortable acceleration for transit straphangers is about 0.2 g, or two meters/second^2. The maximum acceleration of the PCC was thus set close to this level - quaintly expressed as 4.75 mphps (miles per hour per second).

For surface vehicles that 'push against' the road or track, power needed for a given acceleration rises with the square of velocity; to avoid wasteful design, average acceleration for all but the most local service will be about half the maximum, a nice even one meter/second^2 or 0.1 g.

Absent magitech pseudo-gravity to allow high acceleration without bowling passengers over, technology cannot dramatically change these constraints, which is why present-day transit lines are not much faster than those of 100 years ago.

Between the acceleration limit and the need for frequent stops (with 'dwell time' for riders getting on or off), the average or service speed of local transit is limited to about 15 mph / 25 kmh or thereabouts. Fighting through traffic makes it a good deal slower, unless the the line runs on its own reserved speedway in a boulevard median - an arrangement both useful and rather elegant.

To get around this practical speed limit, large present-day cities have a two-level transit heirarchy. The local bus runs everywhere. Layered above it - or more often below it, in a subway - is a rapid transit or metro system, unimpeded by other traffic, with lines and stations more widely spaced, typically in the range of a kilometer to a mile apart.

Because the rapid transit trunk lines have heavy ridership they are commonly served by multi-car trains, not individual buses. So we will simply call the rapid transit vehicles trains, without further ado.

Longer runs between stations allow higher top speed for the same acceleration, and rapid transit service speeds are in the range of 25 mph / 40 kmh.

A two layer transit hierarchy is enough for most present day cities. Paris has a third, the RER, and London is developing one, the London Overground, upgrading and connecting suburban commuter lines for frequent all day service.

This heirarchy is not rigid - 'light rail' and 'bus rapid transit' both tend to be intermediate or hybrid cases - but it provides a starting point for discussion. And as commenters on the last post already anticipated, Trantor will need multiple layers in its transit heirarchy. Just how many is hard to say; we don't have even semi-ecumenopolitan examples to guide us.

We can start by considering a performance level that is not remotely magitech. Suppose a train accelerates at an average 0.1 g to a maximum 150 m/s, about 330 mph, then decelerates at the same rate - the public transit equivalent of a brachistochrone orbit. (This only loosely resembles how rail vehicles move, but gives us a first approximation.)

Travel time is about 300 seconds, five minutes, and the vehicle goes 22.5 km. If we let the train cruise at top speed for another 150 seconds, we go 45 km in seven and a half minutes. An express run, passing intermediate stations, can go nearly 160 km - 100 miles - in 20 minutes.

For comparison, the fastest existing transit line, the Shanghai airport maglev, runs 30.5 km in 7:20, hitting a top speed of 120 m/s. So the model performance is only modestly above current rail practice. These high speed lines can form the third layer of the rapid transit hierarchy. Below them, heirarchically, are regional trains that stop every few miles or km, then the primary metro subway (and finally the local bus).

Allowing time to get from your home to the high speed rail station, and from the destination station to wherever you're actually going, this type of system - local bus plus a 3-layer hierarchy of rapid transit, will get you pretty much anywhere in the extended neighborhood within an hour, where the extended neighborhood extends a hundred miles or so.

At typically modest Trantorian urban density, up to half a billion people live within this radius (fewer if there are geographical constraints like a coastline, large park, or the Imperial Palace grounds.) So within an hour's ride are a corresponding number and variety of jobs, restaurants, potential lovers, and whatever else the city has to offer.

But to really get around town we need to go faster. Suppose now a one hour 'semi brachistochrone' - 20 minutes accelerating at 0.1 g, 20 minutes cruise, 20 minutes decelerating. This takes you nearly 2900 km, 1800 miles, about the length of Seldon Street. Top speed is 1.2 km/s: rapid transit, indeed!

The currently popular technology for this type of service is a hyperloop. Unfortunately, in current proposals the accent is on hype - not because the tech is modestly speculative, but because promoters tend to shamelessly lo-ball things that are not speculative at all, such as the cost of building elevated viaducts.

(The name hyperloop is unfortunate in another way; it sounds more like an Awesome roller coaster ride than a practical transit service for people who may be package-laden, tired, tipsy, or all three.)

But all that said, some such technology should be viable - essentially a genteel cousin of a mass driver or coilgun - and TERTA knows how to estimate construction costs. With a nod to London, the Mother of Rapid Transit, I will simply call these Tube lines.

For the longest trips, a two hour nonstop Tube takes you up to 11,500 km / 7000 miles, nearly a third of the way around an Earth sized planet. Allowing for all connections and wait times, you can get from most locations on Trantor to most other locations in perhaps five or six hours.

A 'local' Tube running 30 minutes between stops will go a quarter as far as the baseline model, around 700 km / 400 miles. This service thus runs the length of Seldon Street in two hours, with three intermediate stations.

Conventional high speed express trains connect these stations in turn, with another four or so intermediate stops, and so on down the hierarchy to the primary rapid transit that stops every mile or less. Then there are the buses and streetcars, and perhaps slidewalks, along Seldon Street itself.

Thus a two hour trip - about the maximum for casual daily travel, whether commuting to work or meeting a friend for lunch - will get you more or less anywhere within a thousand km / 600 miles. If your destination lies close to a major transit hub you can go two or three times as far, because it will be served by top level lines, and you won't need to work your way back down the hierarchy.

The cityscape will reflect the granulation and heirarchy of the transit system. Most rapid transit stations will be nuclei of urban villages, neighborhood centers for errands, entertainment, and general public social life. Major stations will draw larger and denser condensations of the world city, some perhaps on a scale that would match our grandest Zeerust visions of the urban Future.

And while Trantor falls short of being a single practical commute zone, something in the range of 10-50 billion people probably live within two hours of wherever you are. Long distance travel might be constrained by high fares, but perhaps TERTA runs like the semilegendary subway of Gotham on the Origin World: a nickel takes you all over town.

Although not part of the urban transit system, a word about space elevators. I have argued that they are only suited to truly enormous volumes of space traffic. Well, here we are: If any world has the requirement, Trantor does. We can imagine numerous elevator lines rising from the equator, probably with ring lines connecting them at geosynch level. Commenter Eth noted last post that the elevator cables could also support a ring of solar collectors or radiators if needed for power or heat management.

Enough about the elevators; back to TERTA.

The system is extensive, with bus and rail lines totalling hundreds of millions of miles, served by up to a couple of billion buses and subway cars. The Tube network, serving only long haul trunk routes is a mere million miles or so, interconnecting perhaps a thousand stations - few enough that dedicated enthusiasts will have visited all of them.

These major stations should be suitably impressive. The levels of the transit hierarchy must become literal here, the long-haul Tube lines probably running deepest, with local lines being closer to the concourse and street entries. As with major airports today, a transit system might be needed simply to get around the station itself.

And at times it will seem as if all those trillion Trantorians are trying to catch the same train that you are.

But from suitable locations you can look down along some of the lines, with their diverging and converging switching networks and crossovers. The utter coolness of which is justification enough for this visit to Trantor.

To All Trains

* The Great Streetcar Conspiracy was real, but played only a minor role in their demise. Streetcars were unfashionable in the 1950s, and most systems were old and badly run down. So it was simpler to bus convert even the few lines busy enough that streetcar modernization would have been preferable.

** Anisotropic matter is also a term in relativity and cosmology, but I have no idea how it relates to the stuff you would use for slidewalks.


The image of the Second Avenue Subway comes from the New York City transit agency. Because even tiny villages like NYC can benefit from rapid transit!

The eastside Manhattan line, first proposed about a hundred years ago, opened on New Year's Day, and cost about $5 billion for a couple of miles of line - outrageously expensive even for subway lines, which are never cheap. But even at train robbery prices it will be worth the wait for the good citizens of Gotham.

And as a curious example of Google time lag, there are not yet any good post-opening images of the line, which explains the odd absence of New Yorkers on the station platform.

"To All Trains" is from the NY Transit Museum.

Sunday, January 15, 2017

Trantor: the Big Town


As Earth awaits the official debut of America's nightmare comedy, this might be a good time to talk about other planets. So here we go! (Again!)

One minor but durable trope in science fiction is the planetwide city. (For which the geek (and Greek) term is ecumenopolis, 'world city'.) For now - until the Star Wars prequels mercifully fade from popular memory - most people will associate this trope with Coruscant. But accept no substitutes: really it means Trantor, capital world of the Galactic Empire in Asimov's Foundation Trilogy.

Just for the record, this discussion obviously lies waaay beyond the Plausible Midfuture. Also a disclaimer that I am not trying to specifically reconstruct Asimov's Trantor, but a more broadly Trantor-esque world.

I bring up this trope because one of my interests, which has gotten oblique mention here before, is urban rapid transit. And while TERTA - the Trantor Ecumenopolitan Rapid Transit Agency - never got any mention in the books, it reasonably ought to be ... impressive.

As useful background for a transit ride, a few words - well, quite a few - about the overall cityscape, starting with its population. The Good Doctor A slipped up badly on this score. His canonical figure for Trantor - 40 billion - is laughably low, only a few times current world population. We want a global city, not a world of ten-acre exurban ranchettes.

Donald Kingsbury does much better in his unofficial Foundation sequel, Psychohistorical Crisis. His version of Trantor, called Splendid Wisdom, is home to a nice, round trillion people. Spread over the whole surface of an earthlike planet, even this comes out to merely suburban average density. But if we leave the oceans wet and only urbanize the land surface, we get roughly the population density of San Francisco.

Now we're talking. San Francisco, outside the Financial District, lacks the glass and steel canyons look, but cityscapes can vary considerably for a given density. Central Paris apparently has about the same population density as Manhattan, but a very different urban look. Likewise a modest urban density could still have an impressive skyline.

The image of Trantor (nominally Coruscant) above - click to embiggen - hints at one way of finessing this. Most of the city seems to be low-rise, or at least of roughly uniform height, but with monumental structures rising above the rest for added zip.

Another consideration is that an ecumenopolis surely cannot be like an ordinary habitable planet, where humans merely skim the cream off a self-sustaining natural ecosystem. It will need something more like a spacecraft life support system, on a suitably giga scale.

The details are far above my biology pay grade, but may well imply vast sublevels of, essentially, plumbing. The subways could thus run through what amount to basements rather than true tunnels. And the oceans may effectively be sewage treatment / oxygen regeneration ponds - nothing you would want to swim in, which at least would keep shorefront promenades from being impossibly crowded.

Parts of the life support system might rise to or above surface level, and some of the megastructures could well be the equivalent of rooftop air conditioning equipment.

Also, the population density need not be uniform all over town. If you really don't like urban living, Trantor is not the planet for you. But many neighborhoods (totalling millions of km2 and a couple of hundred billion people) might well be at suburban density, balanced by high-rise urban districts, thus accommodating people who want a yard as well as those who prefer living near good shopping and restaurants.

Kingsbury mentions one such commercial district on Splendid Wisdom that is suitably Trantorian in scale. I forget its name, but since Splendid Wisdom is a rebuilt Trantor of the Second Empire, I will call it Seldon Street. Though technically a pedestrian mall, functionally it is a suitably giga-scaled version of Market Street or Wilshire Boulevard, extending for some 3000 km.

That being a long, long stroll, expect some serious transit lines to run along the Seldon Street corridor.

But before we ride, a few more thoughts on the cityscape. First, a couple of annoying practicalities. Realism[TM] is not really a key issue in this exercise, but we should give it a superficial nod.

On a world without farmland, how do you feed a trillion people? Asimov's Trantor imports its food from 20 agricultural worlds, but they could only supply this Trantor with delicacies. For basic food supply you need a planetwide array of oscillating hands: something something hydroponics, something something algae.

And, of course, all this stuff should really be on the surface, with the city life below, but we want a planet that looks urban. We will delicately assume that technology originally developed for spacehabs and such will solve food supply along with the rest of the life support challenge.

Energy supply turns out to need less handwaving than food supply. An earthlike planet absorbs on order of 10^17 watts of insolation (instellation?) from its parent star. Current average US energy usage is about 10 kw/capita, scaling to 10^16 watts for a trillion people. So cover the rooftops with solar panels and you more or less get there. Waste heat disposal is not a problem, because you are merely using energy the planet would absorb anyway.

And energy consumption on Trantor can be relatively modest. It is not an industrial world; as the Imperial capital, its chief manufactured product is government. Large cities also tend to be energy efficient - not least because people ride the subway instead of driving.

From these material concerns we can turn to social considerations. An ecumenopolis must have substantial overall social stability to function at all, but Trantor presumably has its good neighborhoods and not so good, perhaps including slums the cops only enter in army-corps strength.

Or - another familiar urban SF trope - class stratification might be literal, with the down and out living among the plumbing sublevels, while the upper classes live on upper floors, the richest in penthouses. This lends itself to a transit subtrope that goes back to 1890: dismal subways for the poor, elegant els for the rich.

Note that the district shown in the image above must be served only by subways; there are no hints of elevated lines.

Given a world of a trillion people none of this needs be an either/or: Within vary broad limits, Trantor's cityscape and social life can be as varied - including the charmingly urbane and the dystopian - as you want them to be.

On yet another note, Isaac Asimov was famously agoraphobic, and his Trantorians rarely went up to the open surface. Depending on how the life support system operates, 'rarely' might be never, at least without a quasi space suit.

But this too is not a given. An ecumenopolis, or neighborhoods thereof, may have rooftop gardens and dining patios under the solar awnings, and even (shock!) open air streets instead of roofed over corridors. When I speak of the Seldon Street corridor I say nothing about its architecture, only that it is an elongated urban district.

Trantor might even have parks, though the only open space on Asimov's version was the Imperial palace grounds. But we have not come all this way to an ecumenopolis to visit a park. You can find those on any garden colony world. So in our next exciting episode we will head for Seldon Street.

And since it is already mostly written, you won't need to wait until the Galactic Era to read it.

Yes, knowing this blog's commenter community - if you have not all given up and deserted me - the discussion will work itself around to space battles.

The cityscape image comes from a blog review of Second Foundation. Alas, I know nothing of the artist who created it.