Monday, December 12, 2011
Recently there was a posting in the Transport Innovators site that caught my eye. It was entitled “SkyTran a Sham?” I know… I really should join that group and post my thoughts on that site, and lend a little support to what I consider to be a valuable public resource. But I like to scratch my head and measure my words a bit more than most before I open up my mouth, and I am usually too busy to even consider an issue on a timely basis anyway. So I hoard my musings away, like precious little nuggets, to be used as rainy day subject matter for this blog.
I am not so interested in the charge that was made against SkyTran but rather by whom it was made. It was leveled by the author of this study, which starts out promoting and then later disparaging nearly the exact same concepts as SkyTran. In my opinion both have severe problems. The author threw in the towel. SkyTran still purports to be a practical system.
What is it about maglev that makes smart people so crazy as to think that it is appropriate for PRT? maglev’s major advantage is being frictionless, except for air. This is a very minor consideration at city speeds, and yet they want to shoehorn this high speed technology into the sharp cornered, stop and go world of urban transit. A friction free object wants to glide at a steady speed, in a straight line. Navigating a city requires something wholly different. Both systems are designed to travel in excess of 200 km/h (124 mph), far too fast for short trips. Even though he states that, because of G-force constraints, average system speed can never exceed 100km/h (62 mph) in an urban environment, he never wavers from his 200 km/h maglev design, and the inflated track cost that it entails.
In the case of Swift PRT, the author, after all kinds of analysis and simulations, concludes that the track is too expensive (Duh! It’s full of copper coils!) and that vehicles that fast must be spaced way apart to allow exiting, entering, or even simple turns. (Or, alternatively, they need 600 meter ramps to and from the stations.) He states, “If your intersection or station spacing is meant to be <1km apart, you effectively need two lanes in each direction: a fast lane, and an acceleration/deceleration lane. The net result is you have at least doubled your track costs, and the width of your system.” It seems to me that the problem lies with trying to connect 200 km/h fast lanes to every downtown station! That seems to be what SkyTran is advocating as well. Like I say, there seems to be something about maglev that makes people lose their senses.
The conclusions are what bother me most: While I have no problem with his realization that maglev PRT is not cost effective, he then applies his figures to PRT generally. His assumption is that all PRT track must cost 7m/km, even though his own figures show that one way track without the copper coils and in-track electronics would come in at 2m/km. He then, through mathematical inference, extends this inflated cost to justify only putting one station per 2.7 km, and then uses this spacing to assert that PRT (in general) cannot compete because of the long walks to get to the station. His reliance on formulas over common sense has led to the “crap-in, crap-out” phenomena.
What is curious (and unfortunate) is the illogical leap from discovering that his system is too fast and expensive to the conclusion that the future of transportation is in vehicles that run on asphalt. This argument is made without anything to back it up, save the cost and ubiquity of the road system itself. Was it not the shortcomings of the road system that lead him to explore PRT in the first place? Sure, I think we all agree that traffic problems can be reduced by using networking and AI technologies. But asphalt will always be primarily a two dimensional, stop and go system. Multilevel interchanges simply cost too much and are too big to be ubiquitous. So when he compares the cost of his 130 mph, non-stop system against asphalt, he is comparing apples and oranges. If I had to venture a guess, I would say that the author set up an experiment that he was forced to carry it out with scientific rigor, even though the basis for the experiment (his hypothetical system) was clearly flawed. Having exhausted his time and/or interest, he was in no mood to do it all over with a better system, and so hastily framed his results. These conclusions certainly do not reflect the thoughtfulness shown in the sections where he first discusses the original problem.
That being said, one other interesting result of the study highlights the parking problem, something that is often glossed over in PRT discussions. While it has been admitted that PRT vehicles will have to travel around empty sometimes, the extent to which this will occur has been a subject that has remained somewhat opaque. Obviously, during off-hours, if the system is only operating at half capacity, there are 50% empty vehicles, and they have to be somewhere. Are they traveling around in circles? Clearly these vehicles should be staged somewhere, but I have not seen this reality reflected in the various PRT designs. It occurs to me that this is still one more argument for a fully multi-axis (3D) system. Warehousing numbers of empty vehicles would be much more space-efficient if they don’t require long ramps. As with parking cars, a vehicle with a small turning radius is a plus. In PRT, such a radius might be horizontal or vertical. Compact parking is especially important if an attempt is going to be made to shelter those vehicles from the elements. A PRT design that allows a combination of tight turns and compact track switching in full 3D can clearly minimize the real estate (and roofing) required for such storage. Being able to make multiple track configurations in tight spaces would be particularly advantageous in utilizing whatever real estate might be available, including very small or oddly shaped lots. Such parking can be envisioned more in terms of a lattice or matrix, as compared to lines of cars on long parallel tracks typified by the storage of railroad cars.
I might add that tight turning radii in both vertical and horizontal axes is inherently difficult to achieve in systems that use the track as part of the propulsion. This is because of the required tight spacing between track and bogie. Magnetism loses force with distance, so using magnetism between track and bogie for propulsion will always entail a fairly tight fit. This is my major beef with linear motor propulsion, even non-maglev varieties such as simple LIMs, which don’t require coils in the track.
Anyway, to those of us not under the maglev spell, this study illustrates some of the challenges of PRT design that must be, and, indeed, can be properly addressed. The challenge of G forces, of storing and staging pods, the track cost issues, the station spacing… All of these must be carefully balanced and tweaked if PRT is to succeed without the blunt instrument of generous government subsidies. Any good PRT design must address these issues from the onset, not as afterthoughts. Otherwise it will either fail outright or be relegated to a few niche markets.