The Linear Fallacy

The hazards of prediction are many - particularly, as Yogi Berra observed, about the future. One such hazard is tropes such as monorails that even the full Rasputin treatment won't kill. But another, and my closing theme for this year, is the impulse to project current trends into the future.

When the above image of a midfuture New York City was imagined - presumably in 1899, though possibly 1900


Update ...

Blogger glitched badly and ate most of this post! I'll try to recover it, but it may be gone along with 2010 ....

Update II ...

A (considerably revised) version of the lost post is now up as the first post of 2011.

Transport Nexus III: I Brought My Heart to San Francisco

Truth to be told, in all but the narrowest technical sense (driving the car) she brought me; it was my wife Paula's inspiration and effort that got us here. In any case the move and settling-in process account for the lack of posts here in the last couple of weeks, but now RM is up and running again.

In itself all this has nothing at all to do with space travel, but it does inspire some further thoughts about space stations. Recent discussion threads have included noteworthy heresies on this point.

In the traditional understanding that we all grew up on, an orbital station had two primary functions. One was to serve as a center for orbital operations such as communications, weather observation, and so forth; the other was to serve as a transport nexus, the meeting point between shuttles coming up from the surface and deep space craft arriving from other worlds.

Time and technology spoiled the first of these. All the observation and communications relay functions that Clarke and Heinlein expected space stations to perform are instead done by a host of satellites, and no crews are needed to change burned-out vacuum tubes.

The comment thread heretics challenged the second function as well. For a long time to come, spacecraft (or the modules that make them up) will be built and serviced on the ground, where the industrial infrastructure is. Work on orbit will be limited to final assembly, requiring no large staff of orbital workers. Deep space ships may well arrive and depart from individual parking orbits, with no need and no advantage to matching orbits with a big fixed orbital facility.

Space stations, in short, may have become obsolete before any had been built. The ISS, so far as I can tell, serves exactly none of the traditional functions of a space station. For practical purposes it is not a space station at all but a sort of training ship for future deep space missions.


Being obsolete is, in a surprising number of cases, no bar to success. San Francisco was technologically obsolescent from the very beginning of its history as a major city.

From a pre-industrial perspective it is the logical location for a seaport, a transhipment point between oceangoing ships and craft serving the vast inland waterway formed by San Francisco Bay and its outliers, which in turn provides access to rich agricultural regions: the wine country, Santa Clara (now Silicon) Valley, above all the Central Valley.

The railroad era - already well established by 1849 - changed all that, at any rate in principle. San Francisco, at the end of a rugged peninsula some 60 km long, is not a convenient rail terminus (except from the south). The original transcontinental railroad had its western terminus far inland at Sacramento, accessible to water transport; the line was later extended to Oakland, accessible to seagoing ships. And indeed Oakland eventually did supplant San Francisco as a seaport, though it took more than a hundred years, and the physical transformation of port facilities by the container revolution, to accomplish it.

What Oakland has not yet managed to supplant is Gertrude Stein, whose quip, "there's no there there," is practically her sole claim to fame. (Along with Alice B. Toklas brownies.)

San Francisco did not become a suburb of Oakland because of a combination of local circumstances and sheer inertia. Steamboats long remained more economical for regional transport around the Bay Area, and railroads were expensive to build so far from existing industrial centers. By the time these factors changed, San Francisco was already a major port, and network effects took over. It had infrastructure and port services, and the availability of these more than made up for the potential freight charge differential for east-west rail traffic.

Even when the port finally declined a broader network effect continued. In the current era San Francisco is, functionally, the downtown core of the Bay Area metropolitan region, accounting for about a tenth of the regional population but a much larger proportion of metropolitan services. These services to and beyond the region have only an incidental connection its original function as a seaport.


Unfortunately for space stations, the particular circumstances that allowed San Francisco to grow as a port even in the railroad era do not seem to apply in space. On the other hand, cities have always been defined less by their initial primary functions than by the secondary and tertiary services that they are uniquely suited to provide. If - for whatever reason - there are a large number of people in Earth's orbital space, they will probably aggregate in ways that allow them to have lunch together without having to undertake space missions just to get to a restaurant.

Where people go, cities will probably follow.



The image of San Francisco and environs was taken from the ISS.

The Unspecified Drive

Deep space propulsion is, unsurprisingly, a major concern of this blog. I regularly specify the performance of interplanetary craft fitted with some form of high specific impulse drive. Sometimes I describe it as a nuclear electric or solar electric drive, sometimes simply as electric, often not even that much. Sometimes, especially when discussion takes us to the wide open spaces beyond Jupiter, I allude to fusion.

Since I got myself in a bit of hot water, or some more exotic (and much hotter) coolant, by some snide remarks about fission power plants, a few comments on deep space propulsion are in order.

First of all it is not the main barrier to widespread interplanetary travel. That would be the sheer amount of costly design engineering needed to build a fleet of prototype spacecraft, followed by the cost of getting them all into space.

But once we are up there, how we get around is an important concern. The current limit to human space missions is about six months, beyond which the health consequences of prolonged microgravity become severe. Longer missions require a spin hab, adding cost and complexity. Even with spin habs, radiation and ordinary human factors limit practical mission duration to a couple of years or so.

Within these constraints we could reach Mars with chemfuel (and a spin hab), but the Hohmann round trip to the main asteroid belt is two and a half years, without any stay time at the destination, while to Jupiter and back is five and a half years. This is too long for regular human travel.

So for a human interplanetary presence we need fast orbits. These are above my math pay grade to calculate, but a klugewerks of flat space modeling, sketching orbits, interpolation, and sheer guesswork indicates that reaching Mars in three months or Jupiter in a year calls for a mission delta v in the range of about 30-100 km/s, and therefore some form of high specific impulse drive. Even NERVA style nuclear thermal rockets - the classic Atomic Rockets that gave the website its name - fall short of this requirement.

The time honored high specific impulse drive in science fiction is ion propulsion, used in real life to send the Dawn mission to Ceres and Vesta, but not suited to much larger human-carrying spacecraft. To a great many people, however, 'ion drive' is more or less synonymous with electric drive in general.

The most likely such drive for human missions appears to be some form of plasma jet. Unlike ion drive this is a thermal drive: The plasma has a meaningful temperature - and it is extremely hot. But the thrust chamber is a magnetic field, so it won't melt. Only the gizmos that produce the field are exposed, and they don't get up close and personal with the plasma. They and their supporting struts must have heat shielding, forming a 'lantern' structure.

(The strictly technical term for this drive is electrothermal magneto-plasma propulsion - doesn't that sound exactly like classic Trek technobabble? "I've engaged the electrothermal magneto-plasma thrusters, Keptain - she canna take much more!")

So far plasma drive has gone no further than the laboratory bench, but there don't seem (yet) to be any serious problems in scaling it up to be suitable to large spacecraft. Like many forms of electric drive it has no inherent exhaust velocity and therefore no fixed specific impulse. At least in principle these drives can be configured either to expel a relatively large flow of relatively (very relatively!) cool plasma at lower velocity, or a smaller quantity of hotter plasma at higher velocity.

The effect is very closely analogous to gearing; these drives can be set for a higher acceleration and lower specific impulse or vice versa. VASIMR is supposed to achieve this not only in principle but in engineering practice, permitting clever tweaking of engine settings to get the optimum performance in each phase of flight.

For all of its advantages, electric drive has one essential drawback. It does not produce its own energy, as chemfuels do, or even use a reactor directly to heat the propellant, as nuke thermal does. It must be plugged into an external electric power supply. This is seriously inconvenient, because it takes a lot of electric power, tens to hundreds of megawatts, to drive a big, human carrying ship even at milligee acceleration.

For travel in the inner system I am partial to solar electric power. It hums along quietly with little fuss and practically no moving parts. But the butterfly's wings must be enormous, a hectare for every few megawatts, and extremely light. Even milligee forces may be problematic when the wing structure is that big and that light. And solar electric fades rapidly with distance from the Sun, unsuitable for travel beyond Mars.

For the asteroid belt and Jupiter the practical alternative is nuclear electric drive, which was the cause of my original grump. All vivid if misleading imagery of clanking steam engines aside, nuclear power plants are heavy, filled with complex plumbing that must operate for months under fiercely hostile conditions, and produce two or three times their useful output in waste heat, which must be got rid of through large radiators with their own demanding plumbing.

That eerie green glow is produced by the disintegration of money.

There is an upside to all this downside: Ships with nuclear or solar electric drive have plenty of juice at the main switchboard, making these drives, especially nuke electric, well suited to laser stars. All you need is the laser installation; the power supply is already provided, and you can zap away as long as you want to hold down the trigger.

But the general messy inconvenience of carrying around a naval-equivalent fission power reactor accounts for much of the appeal of fusion. In principle, and popular imagination, fusion is an ideal power source for a plasma drive, because the fusion plasma and the thrust plasma can be one and the same. VASIMR in fact is a byproduct of fusion research; in a conceptual sense it is a derated fusion drive.

Fusion in practice could turn out to be another matter. What else is new? The easiest fusion reactions to sustain (and we can't yet fully sustain any of them) release most of their energy as neutrons, useless for propulsion, but - irony alert - suitable for heating a steam boiler.

On the other hand, fusion propulsion is in some respects simpler than fusion power for earthly energy needs. It does not need to be an economical means of producing electric power. In fact to serve as a drive it need not produce any electric power at all, though any fusion drive would likely produce some 'bleed' power.

There are alternatives to fusion, all about as speculative as fusion itself. Orion is arguably the least speculative of the bunch, though the organ music and black cape factor has pretty much overshadowed the actual technical challenges of building a spacecraft that must nuke itself thousands of times, at close range, in the course of normal operation. (Those have to be some badass shock absorbers!)

But on the whole the specific technical details of a high specific impulse drive matter surprisingly little. What matters is how heavy the thing is, relative to the thrust power it puts out. The benchmark here is is a specific power output of roughly 1 kW/kg, or a megawatt per ton, for the full drive installation including thrusters, power supply, and waste heat radiators. (For a complete drive bus add propellant tankage and keel structure; mate it all to a payload to get a ship.)

Example: Suppose a 100 MW, 100 ton VASIMR style drive engine. With exhaust velocity tuned to 75 km/s, specific impulse near 7500 seconds, propellant mass flow is 36 grams/second, producing about 2.7 kN of thrust, enough to push a 500 ton ship at just over a half a milligee, gradually increasing as propellant is burned off. If half the departure mass is propellant (250 tons, plus 100 tons for the drive, leaving 150 tons for tankage, structures, and payload), mission delta v is just over 50 km/s. Full power burn duration is about 80 days. This broadly corresponds to the requirement for a fast, three month orbit to Mars.

Tune the same drive to an exhaust velocity of 150 km/s, specific impulse near 15,000 seconds. Propellant mass flow falls to about 9 grams/second, producing 1.3 kN of thrust, pushing the ship at a quarter of a milligee. With the same mass proportions our ship has a mission delta v of just over 100 km/s and full power burn duration of 11 months, approximating a one year trip to Jupiter.

Improving on this performance will not be easy. To reduce travel time on semi-brachistochrone orbits with prolonged burns you must reach a higher peak speed in less time, and must therefore increase both thrust and specific impulse. In the flat space approximation, drive power increases as the inverse cube of travel time - that is, you need eight times the drive power output to cut travel time in half.

The good news, such as it is, is that this also works the other way. An early generation drive with a more modest 250 W/kg power output can still take a relatively fast orbit to Mars. But you pretty much need fusion drive, or an equivalent array of oscillating hands, to reach Jupiter in a few months, or for practical travel to the outer planets.

Still, the Solar System as far as Jupiter should be a decent sized playground for a while.



The image comes from a NASA publication on VASIMR.

Accelerando

Comments on our last exciting episode discussed, among many other thread drifts, the concept of an Accelerando, a speeding up of technological progress that is presumed, in many circles, to culminate in the Singularity. (See the comment thread, starting around #180.)


I will argue - and I've made this argument before - that the real Accelerando happened roughly a hundred years ago, say in the period from about 1880 to 1930.

The Industrial Revolution began a hundred years earlier, but most people in 1880, even in industrialized countries, still lived essentially postmedieval lives. (Cribbing from my own comment follows:) Railroads and steamships had transformed long distance travel, but on a day to day basis people walked, or if they were quite well off they used horses. They lived by the sun; the only artificial lighting was candles or oil lamps, the same as for centuries. A few large cities had gaslight; reputedly it made Paris the City of Lights.

By 1930, millions of people were living essentially modern lives. They drove cars to homes with electric lighting, talked on the phone, streamed entertainment content on the radio or played recorded media on the phonograph. To a person from the pre-industrial world a hand-crank telephone and an iPhone are equally magical; to a person from 1930 the iPhone is an nifty piece of 'midfuture' technology, not remotely magical. (Gee whiz, Tom, a wireless telephone with moving pictures! And it all fits in your pocket!)

Militarily a good part of the Accelerando played out in the course of World War I; people went in with cavalry and came out with tanks and aircraft. Commenter Tony handily expanded on this theme:

Murray and Millette made this point in their operational history of WWII, A War to be Won. They pointed out that a lieutenant in 1914 had little in common with the colonel that he himself had become by 1918. Yet that same colonel would have easily recognized the overall form, if not the detail, of war in the 1990s.

How do you measure an Accelerando? One handy benchmark is human travel speed. Here the Accelerando actually began a bit before the Industrial Revolution. Stagecoaches could maintain a steady speed of about 15-20 km/h by combining advanced carriage design with the infrastructure innovation of fresh horses for each stage. Ordinary travellers could thus maintain human running speed for hours.

The first steam locomotive ran in 1804. General purpose steam railroading began in 1825-30, and a locomotive appropriately called The Rocket reached 47 km/h in 1829. Rail speed data in the 19th century is amazingly sparse, but I would guess that locomotives exceeded 100 km/h by midcentury. The next doubling was reached in 1906 by a (steam!) racing car. The next doubling after that, to 400 km/h, was achieved in 1923 by an airplane.

Mach 1 was reached in 1947, and then of course things got wild. Yuri Gagarin reached orbital speed, a shade under 8 km/s, in 1961, an accelerando of 25x in 14 years, with another bump up to lunar insertion speed of 11 km/s in 1968.

Things have settled back a shade since then. Most of the 500+ human space travellers have piddled along at orbital speed, while since the retirement of Concorde the civil standard for long distance travel is high subsonic.

In this particular case the period 1880-1930 actually falls between stools - steam railroading was already pretty well developed by 1880, while aviation in 1930 was just starting to combine low drag airframes with high power engines.

Other technologies would give different results. Some, like computers, are still in the rapid transition phase of railroads around 1840 and airplanes around 1950. The overall Accelerando of the Industrial Revolution is a sort of weighted average of many individual and interrelated tech revolutions. And sometimes an older, mature-seeming tech gets a new power jolt, as has happened with railroad speed since the Japanese bullet trains of the 1960s.


Science fiction is the literary child of the Accelerando, and emerged as a distinct genre of Romance in just about the period 1880-1930. Jules Verne published From the Earth to the Moon in 1865; Hugo Gernsbach launched Amazing Stories in 1926.

In 1800 no one speculated about the world of 1900, because no one imagined it would be all that different from the world they already knew. And in 2000 there was only limited speculation about the world of 2100. Indeed the future has gone somewhat out of style, replaced in part by the enchantment of retro-futures.

The future has lost its magic not so much (if at all) because our technical progress has reached a 'decelerando,' but because we have learned to take technical progress for granted. It is a lot harder to get a Gee Whiz! reaction these days, a sort of psychological decelerando. As I've suggested in the last couple of posts, the challenge of interplanetary travel is not how to do it but why to spend the money.

(As a far more modest example of psychological discounting, where in this holiday retail season are the iPad rivals? Did Apple blow everyone else's tablet devices back to the drawing board, or has everyone else decided that tablets are a niche market they'll leave as an Apple playground? I haven't a clue.)

This is where I am supposed to wrap my arguments neatly in a bow, but I am not sure what the summation should be. So instead I will toss the question out for comments.



The image of a North American train c. 1900 comes from a public library site in Kansas.