Sunday, May 30, 2010
Sorry about the broken link to the video in the last post. I don’t know why it doesn’t work. If you search YouTube for it, you get the same video at the same address. Go figure. I also apologize for all of the unanswered emails. The library is supposed to be finished in a week, and rain is forecast, so stay tuned!
Sunday, May 23, 2010
I have been trying to devise the absolute best PRT track design I can come up with, with the help of you, the readers. My goal is to offer an alternative transportation infrastructure that beats the pants off of roads by every metric, save for (initially) the network effect. In my opinion, only then will politicians and transit decision-makers have the political cover to move forward with a PRT program.
The search for the best track design brings up some inevitable questions, such as “How far apart can the supports be before the stability or economy of the system is compromised?” and “How high should the track be?”
The first question leads to an examination of bridge design. Trusses with minimal girth, such as Pratt, Howe or Warren trusses) seem to top out at about 100 feet. The most economical way to span long distances, however, lies in the introduction of tension elements, such as is seen in a suspension or cable-stayed bridge. The most extreme case of a tension maximizing (compression minimizing) design would be the cable/tower arrangements that support ski-lift gondolas. Some have proposed such means as an urban transit system, and they certainly would have something there were it not for the need for multiple origins and destinations.
It would be an easy matter to design a flexible track and simply pull it tight, though sagging in the middle would be both unavoidable and unacceptable. Pulling this sag up with cables would seem to be the cheapest way to span long distances with a minimum profile track.
Suspension Bridges can traverse the longest distances, but cable-stayed bridges (above) offer other advantages. A good explanation of how the two compare is found about halfway down this Wikipedia page. ttp://en.wikipedia.org/wiki/Cable-stayed_bridge. The gist of it is that suspension bridges have vertical cables from which the load bearing structure hangs, while cable-stayed bridges’ hanging cables are angled from a support column so that the load bearing structure is compressed by its weight.
Many of you may have seen the late Hans Kylberg’s dramatic use of cable-stayed PRT track in the Bubbles and Beams video or in this illustration. Below is a picture of a curved bridge section supported by cables.
The use of cables does present a question of aesthetics. Whereas I think most people would find both types of cable supported bridges generally attractive, there is a question of too much of a good thing. Instead of a minimalist design, there would be support columns and cables everywhere. What looks good crossing a river might not look so good close-up on your street. I have given some thought to reducing the tower height and I will post my ideas for that in the near future.
Then there is the question of track height. One obvious way to mollify the NIMFYs (Not I My Front Yard) is to have the system so elevated that it is unobtrusive. An advantage to the system I have been advocating is that it can easily and steeply climb or descend to any desired level. There is the matter of emergency evacuation, but I think that is manageable. So how high is too high? Should this be a system that can whisk you along above the trees? That would be my preference, but everyone has his own sensibilities. What are yours?
Sunday, May 16, 2010
I recently read an article called “Robocars vs. PRT” posted on ITT’s PRT Debate page, which directly relates to my last (and many other) previous posts. Whereas I found more than a few dubious assertions in there, I’ll confine my response to just one. Don’t get me wrong. There is a whole lot of good stuff in the article, and I would recommend reading it. I mostly disagree with the premise. That is that there can only be one urban transportation platform that “wins,” and that will be the robocar. Is it any wonder that, in some imaginary bipolar battle for acceptance, the winner is cars/roads? They are already here. The “robo” part is just evolution.
Virtually nothing is said as to why such a competition exists. Are there also “Robocars vs. Light rail” or “Robocars vs. Buses” articles in the pipeline? This is a false choice. One of the first clues to this fact can be heard in the author’s statement, “New dedicated right-of-way is, of course, wonderful for any transportation system…” Exactly. That hits the nail on the head, because it leads to the obvious question, “What is the cheapest, most efficient, most compact, least intrusive, most flexible way to produce this “dedicated right-of-way?” Score one for PRT. His argument, however, is that roads are actually more doable for political reasons. This is certainly true, but it doesn’t make it right. This is the kind of logic that perpetuates obsolete designs and practices of all kinds. The obstacles he sites that are faced by PRT are, however, spot on. His arguments against PRT should sound a cautionary alarm to all would-be system vendors. Nobody ever said it was going to be easy.
PRT can be seen as the most efficient means of alleviating traffic congestion by bypassing it. Robocars and “smart” traffic management techniques are means to decrease that traffic in the first place. Both are sorely needed.
The PRT community needs to hone its message and its product offerings. The virtuous combination of being electric, automated, and point-to-point is not exclusive to PRT anymore. PRT must be a reflection what it does best – It must be (and bill itself as) the most efficient way to move stuff (like people) using the least expensive, most efficient and versatile infrastructure.
Steerable free-roaming robocars can never be as efficient as rail-based PRT. They need MUCH bigger (and heavier) batteries, time to recharge, and those batteries eventually need replacement. They need softer tires. They cannot go as fast because they have nothing to grab on to in an emergency stop situation, and could skid or roll over for the same reason. They are more at the mercy of adverse weather, again effecting speed. The road or guideway cannot be as compact or as lightweight. Moreover, privately owned robocars will evolve into as big and energy hogging a form as the law allows. If they can drive themselves, they’ll come with wide screen TVs in no time. Energy saving electric cars will become complete offices-on-the-go. Sometimes a little “top-down” infrastructure control is a good thing.
Rail based PRT need not go everywhere. Every mile of it that exists is saving the commuter and the taxpayer time, money and resources with every trip. It effectively uses an underutilized resource - the space over medians and sidewalks. It is ideally suited for being routed in 3D thus avoiding land-hogging ramps, as I mentioned in my last post. It is a minimalist solution that recognizes scarcity. Tell me THAT has no future.
Cities, states and countries are up to their eyeballs red ink and we live in a very competitive, increasingly used-up world. Doing more with less faster is the ONLY answer, so we as citizens, as communities, as countries, and even as a civilization, must get down to the business of replacing our various inefficiencies, or face a pretty dire future. We need both PRT AND Robocars. Hopefully there is somewhere in the world with the political will.
Sunday, May 9, 2010
I just wanted to expound a bit on the subject matter that was touched on in the comments section of Post 82. The conversation centered on the pros and cons of the ULTra system. (Actually mostly cons, in regard to weather, speed and aesthetics.) One statement by alert reader Bruce, in particular, got me thinking - “I think it would be rather more regrettable if the perfect were allowed to become the enemy of the good. The ULTra design is quite good enough for a wide range of transit applications.”
This has always been a worry of mine. As one who is probing the possibility of a standards-based design architecture, I am particularly averse to imposing arbitrary limits on those standards. The fact is, though, that if I had real-world budgets, deadlines and targeted customers, I, too, would have to dial back the system capabilities to get the job done. I do not want to create unrealistic expectations in regards to what I am doing or cast doubt on present systems. In other words, I do not want to be the “enemy of the good.” On the other hand, there is also the possibility that the “good” could become the enemy of any and all PRT, if it doesn’t measure up to expectations.
PRT used to have the advantage of being the only practical way to move individuals and small groups electrically. Battery technologies have changed that. Now we can expect the door-to-door convenience of a private car with the energy usage formerly attributable to PRT alone. Suddenly PRT has something else to compare itself to beside gas-guzzlers.
I believe a very strong case can still be made for PRT, but some embodiments make the case better than others. Recently this NY Times article was posted on the Transport-Innovators site. It illustrates how damaging it can be to choose the wrong PRT system for a given implementation. This has given a black eye to all PRT. The layman will read this article and assume that the PRT concept was proven unworkable. At the very least, they will take away that it is “buyer beware” when it comes to PRT. In actuality the problem wasn’t PRT per se but more with what this design was to ride on… pavement. If we lived in a paved labyrinth of levels and ramps, relatively slow robocars would be an excellent choice. But in a 2D world of limited surface area, pavement riding PRT designs must compete with electric Scooters, Segways, bicycles, pedestrians, regular electric cars, not to mention gasoline powered vehicles. Is PRT really the best use of pavement? If so, by what measure? Energy usage? Passenger throughput? Time to destination? Will it remain that way into the future?
There was a time when one main object, it seems to me, was to free up the pavement to reduce traffic and get a bit more green space. True, pavement roving PRT vehicles are smaller than the average car, so the track for such vehicles is more economically elevated. But such track could also be used productively by opening it up to ALL small, motorized vehicles. This would encourage downsizing.
At the risk of getting sidetracked, I wish to reiterate the point about being smaller and therefore more economically elevated. This is no small deal. All ground -based travel, from pedestrians to supertankers, is subject to interference based on differing directions of travel. That is a fundamental fact of 2D travel. The fact that ordinary roads must sometimes support very heavy trucks makes overpasses, (the non-stop solution to 2D interference) much more expensive. Nevertheless, making them anyway has revolutionized our way of life and greatly increased our prosperity. Imagine, for a moment, turning back the clock, and replacing all of the freeway overpasses in your town with stoplights. This would effectively draw many cities to a halt. This is a revolution that has not come down to the neighborhood level, however. We all still pay homage to the good old red light.
PRT carries the promise of cutting through the busy urban landscape like nothing ground-based ever could. With a system like Ultra or 2getthere, there is the flexibility to have
the system either ground-based or elevated. Ground based is cheaper, and so has that as an inherent attraction. Both companies point this out. But when once you consider that the track must be fenced, and that it will block any cross traffic from pedestrians or other vehicles, this becomes a false choice. It seems painfully obvious that this is partly what the designers at Masdar are now discovering.
Another promise of PRT is (like most automated systems) to achieve speed by eliminating human error. But we have become accustomed to dangerously small headways between very fast moving vehicles when it comes to cars, yet are extremely unlikely to ever allow such headways on automated systems that rely on simple tire traction to steer and stop. This is especially true considering the possibility of wet or icy pavement. So this promise, too, of PRT is unlikely to ever be realized in such systems. The system’s users will have to be content to go at school-zone speeds for the entire trip.
True, these problems are of little consequence for applications like airports or campuses. Creating a profitable business model around these platforms would seem to be a positive first step for PRT. But many people are holding these systems up as the urban/suburban transportation of the future, using arguments borrowed from faster, all-elevated (and sometimes purely theoretical) systems. Somewhere in the definition of PRT is the implicit supposition that the system is a viable means of urban transportation. With top speeds that are 10mph less than the current speed limit for un-posted city streets, I really have to question that, at least for the sprawling cities I know. I very much worry that such a system will be tried and then fail to live up to expectations. Imagine what the folks from light rail would say then.
Saturday, May 1, 2010
As you probably have noticed, I am a chronic designer/inventor. I really can’t help myself. It’s what I do. So, when I watch the news and see how royally someone is screwing something up, I naturally I ask myself what I would do. I often get aggravated sitting in traffic – hence my interest in PRT.
Last time my blog was diverted from its primary subject was after the earthquake in Haiti, when they couldn’t get food to the people who needed it. Now we are helpless as thousands of barrels of oil are released daily into the Gulf of Mexico. Apparently engineers are working on a “dome-like structure” to cover the leak, but it will take weeks to have it in place, according to BP. Dan the Blogger is NOT happy.
Structures and devices of all sorts can be broken down functionally into tension parts, compression parts, shear-strength parts, membrane parts, etc, Many highly useful things come out of designs that separate and maximize these functions though their geometry. Consider strength to weight ratio of a bicycle wheel. Or a simple bag, as opposed to a box. A tent, as opposed to a house. A balloon. An Umbrella. A suspension bridge. None start with the proposition of taking a pile of building materials and making a solution, although that is the human tendency. Welders think steel, masons think rock or brick, and carpenters think wood. The petroleum business is full of people who make tanks and pipes and valves and scaffolding. So they are welding together a solution.
In this case the remedy would seem to require establishment of a membrane separating the area directly around the source of the oil and the sea at large. This membrane would need to extend a mile from the ocean floor to the surface. This membrane would need to enclose the spill on all sides. The structure can therefore be regarded as a tube.
It is unreasonable to consider spanning the mile to the sea bottom with a solid, unyielding structure, like giant walls or something similar. We are by definition talking primarily tension. More specifically, tension between an anchoring means and floatation means.
Luckily, membrane material in very long lengths is easily available in the form of fabric, and that fabric can be folded upon itself to form a tube. With plenty of overlap, there are many glues of sufficient strength to make sewing unnecessary for open weave fabrics.
Tension members are readily available as cable or rope. Logically, then, the solution would seem to comprise the establishment of seafloor to surface cables and the attachment of fabric to it. It would also seem obvious that a cable running through any chute or tube would alleviate most destructive forces acting upon it, such as being stretched by ocean currents.
All this, to me, leads to an inevitable design conclusion. A tent is constructed with a chimney-like chute, with a cable running through it. The tent is tethered to the ocean floor, and the top of the chute is held up by floatation, first by buoys, and later, perhaps, by the comparatively light weight of the oil inside. No, it’s not as strong or permanent as steel, but it sure would be faster to deploy.
I’ll get back to PRT soon…
Posted by Dan at 9:25 PM