Wednesday, November 30, 2011
I know I said that my next post would be about standards, but I think this topic should come first since it weighs into that discussion. There has been something in the back of my mind, in every design that I have posted on this blog. It is the recognition that progress often happens in baby steps, yet baby steps often don’t go in a straight line or a logical direction.
I am talking about forward compatibility. The vexing thing about designing PRT (or anything else) to be forward compatible is that it requires a design that allows evolution toward an end that must first be defined itself. In the case of PRT, that eventual product would ideally be very fast, silent, comfortable, able to be deployed with minimal cost, and be adaptable for every contingency a city could throw at it. These adaptions would include a variety of station types for different locations and passenger volumes, being practical within buildings, being able to be elevated to a level deemed acceptable by the effected parties, etc. The vehicles themselves can be assumed to be, in this future world, mass produced to point of enjoying economies of scale.
I am sure that many readers have dismissed my designs as being too ambitious. The vehicles, in particular, have very advanced electro-mechanical systems which are designed to remedy situations which could largely be avoided in the first place with little negative impact. Well now you know the reason. By designing the vehicles as though Toyota had been building them for decades, one can better consider the best design for the track and stations that may still be around when that day comes.
In the meantime, however, PRT will be subject to the restrictions that our current economic, political and technological realities place on it. The problem is that designs for today and designs for tomorrow are strikingly different. The main culprit is the tire wear and noise/vibration associated with speed. It is true that smooth running surfaces and track clamping emergency braking capability enable harder rubber, solid tires which don’t need to flatten out on pavement to achieve the high traction requirements associated with gripping slippery roads. Still, highly wear resistant plastics, such as are seen in rollercoaster wheels, are a recipe for a very noisy system at high speeds, especially on pipe, which is notoriously good at amplifying sound. (This is the basis for many musical instruments)
I should point out that the design I show in the Oct. 30 post enables both steering guide wheels to be raised for high speeds, eliminating contact (and therefore noise and wear) by the plastic flanges. Also, these flanges are to rotate independently of the drive wheels, so that they may make contact with the track (pipe) anywhere on their surface and create their own rotational speed based on the diameter established by that point, rather than the smaller diameter of the tire. This reduces wear on what is a tiny contact point.
The main point remains, however, that high speed systems should have larger wheels (OK, not Maglev) or risk lots of wear and/or lots of noise. A system that requires wheels or tires to be changed every few thousand miles would be a disaster. But longer wearing, larger tires means bigger track, something that is nearly as bad, in that it raises the cost and visual impact of a system which will, in the beginning, be under intense scrutiny from critics. Also initial systems will probably be slower anyway, because such trial systems will have to first prove themselves for short-distance downtown use.
Could there be a two tier system? Would it be crazy to start with a system for downtown that would preclude high speed vehicles? I know it sounds like a terrible idea, but it would probably shave 20% off of the track costs, and vehicles would be discounted considerably more. And let’s face it. The other PRT systems out there aren’t exactly fast or flexible either.
In the illustration above, the system on the left, which is obviously simplistic and incomplete, would only need to raise and lower the small pairs of wheels to steer. I do not believe that the middle “hold-down” wheels (illustrated in previous posts) would be required, so that’s really all there is. There are inexpensive “off the shelf” hub motors available that would fit in the drive wheels, and the flange and hubs could be cast as one, (in urethane) so that solid rubber tires would slip on. Such a bogie would be extremely cheap to produce. The complex (expensive) articulation capabilities of the swing-arm and gondola could also be dialed back in such a “starter” system.
The obvious problem comes from the fact that the fast vehicles wouldn’t fit in the smaller track, although the slow vehicles could run in the high speed track. So what is a city to do? Well, there are a couple of things to note here. First, the high speed track would be equally usable for GRT. (Group Rapid Transit.) A track going out to an airport, for example, might well be a good stand-alone investment used in this way. Passengers coming from the airport to downtown would need to change vehicles to use the downtown PRT, but the upside is that they didn’t have to make the long trip at 30 mph. Faster, express PRT could share the track at some point, and slow vehicles could use the track at certain times of day. Because PRT is a smart technology, if high-speed track is running through a grid of low speed track, the slow vehicles could still, in theory, get on and off without disrupting the high speed service. (Assuming sparse high speed traffic) If the track is modular, standardized and interchangeable, the slow track could be removed (during an upgrade) and be reused elsewhere. In the airport example, for instance, slow track taken from downtown could be used to build a network around that airport. In such a case changing vehicles would be a minor inconvenience for relatively few passengers. It is also noteworthy that, in a downtown environment with mixed track, fast vehicles can’t get up to speed anyway, because of sharp turns. Therefore slow vehicles sharing the (fast) track would be no problem. In such a case the system could simply send a slow vehicle if a trip would involve a stretch of slow track.
I have come to the conclusion, reluctantly, that there are theoretically reasonable migration paths from slower, inexpensive PRT to faster systems capable of tackling longer distance commuter traffic. The examples above are just a sample of the possibilities. They also show that it takes some creativity to undo what many of us would say is a very shortsighted decision. (To put down track that can’t take fast vehicles) But at least it’s better than having no forward compatibility at all! It is unfortunate that such a complicated situation should ever exist in the first place, but I have my doubts that we can ever get to PRT 2.0 without first dabbling in PRT 1.0.