145> How fast is fast?

Let’s talk about speed. How fast should PRT ideally go and why? I suppose the first question that comes to mind is how far the routes extend. I have a bit of a problem with the slower, downtown-only type systems. First, of all, many people have to travel a long way, often in heavy traffic, just to get there. Then, unless the streets are just too narrow (and this is admittedly sometimes the case) traditional public transportation can often suffice. For example buses or light rail, even with the added wait time for passengers to get on and off at each stop, actually work fairly well for short hops. It is when you have to endure that stop-and- go for mile after mile that such transit starts to get increasingly impractical. Second, it seems to me that slower systems lack optimism. If PRT is so good, won’t it end up extending far and wide? And if that is the case, isn’t designing a slower system not just putting the brakes on speed, but putting the brakes on the extent of the system as well? I know, ideally people should live downtown and this should be encouraged. After all, giving people a means to commute actually encourages environmentally destructive suburban sprawl. But we are not talking about running PRT to lots of outlying neighborhoods. The shape of the urban center has changed. It is no longer a circle or square but rather shaped more like an octopus, with many essential goods and services being located along the arteries that extend outward. PRT, it seems to me, should go where the action is.   

When you talk about these longer trips, such as commuting, speed becomes much more important. Nobody is going to leave their car behind to take a 25mph vehicle for six miles that probably won’t take them exactly to their destination - unless, of course, the traffic is absolutely horrible. PRT has a “last mile” problem on both ends, and speed along the way can make up for that. Running parallel to highways, the vehicles are moving billboards for the system, since they will either be speeding past the cars or the cars will be speeding past them. I think it is vitally important that the former, and not the latter, be the case.   

For PRT, commuting and downtown circulation would seem to have a symbiotic relationship. Each makes the other much more viable by adding passengers that would otherwise use other means.  Often travel within cities is marred by certain chokepoints that make an otherwise speedy journey by car (or bus) into an exercise in frustration. PRT must serve more than that chokepoint area itself. It must extend to areas on either side that are easily accessible. This, by definition then, means that PRT should service areas with low traffic, as convenient entry points into the system. I think this is a point that is missed by PRT designers who assume a business model with only large, high volume stations. (If there are so many pedestrians, how did they get there?) By extension the logic of distributed, easily accessible entry points leads to those easy-to-build-on corridors that divided highways often offer. At this point the cost-effectiveness of a minimal-profile rail-type system almost begs to go farther out, since the capacity vs. cost is far superior to extra freeway lanes, and “Park&Ride” lots are already in place in most cities.

The aerodynamic drag on a given vehicle roughly cubes as the speed doubles, and so there is a natural limit on the top speed that PRT vehicles ought to go, after which they are wasting energy toting around oversized, expensive motors that may seldom be used. Actually the whole concept of individual travel becomes less and less advantageous compared to large groups as you approach these speeds, since many little vehicles catch much more air than a single, larger vehicle.  So this begs the question of what, exactly, that speed limit should be. I believe that the answer should be guided as much by psychology as by physics.

People know when they are being whisked off to their destination, and they know when they are in a sluggish vehicle. Ever get driven somewhere by a senior citizen or try hitting the highway in a weak sub-compact car?  The power needed for decent acceleration, particularly up hills, will require motors that are capable of freeway speeds, and probably a bit faster. Here I would remind the reader that a hanging system enables ground level boarding, and minimal ramps are cheaper and easier to fit into the urban landscape;  This, in turn, means that it is highly advantageous to both lift and accelerate the vehicle to merging speed in a short time…something that will require substantial power. This would indicate that there is little value in making the vehicles only capable of sub-highway speeds, just on the basis of motor size.

Way back in post 56 I outlined a method of going very fast, using “engines” that would link together PRT pods into an aerodynamic “train.” Such a method seems well suited to the “diamond track” concept that was outlined in post 141. Those large, splayed outer wheels could be on the engine/connectors, and this could free the ordinary, lower-speed guide wheels from costly wear, while enabling speeds much faster than any highway. This method of connecting vehicles addresses the both motor size and aerodynamic drag issues.

One issue that often comes up is the matter of travel between whole different cities. That is one that raises questions about tracking vehicles over long distances and imbalances in inventory between cities, as well as issues like rest stops, or the simple matter that longer journeys would be uncomfortable in the small vehicles that are best suited for economical city use. I suppose one thing that ought to be considered early on is the possibility of GRT (Group Rapid Transit) vehicles that are compatible with the track size. This is also consistent with both the need for less air drag per passenger and the lure of heavier freight loads than would be acceptable in town. In such a scenario the smaller “pods” would be able to travel the heavier GRT/freight track but not vice versa. One drawback would seem to be the possibility of speed-limited PRT getting in the way of faster long haul vehicles. But computers are amazing things, and with a couple of passing lanes and properly orchestrated timing, I would imagine that the two could coexist in a pinch. Going very fast, say 100 mph and above requires a track with enough inner area to allow air to get out of the way of the speeding bogie. This seems consistent, however, with a larger, stronger track, since the minimal-profile track used in the city is only for aesthetics. There is structural value in increasing the dimensions, even with the same amount of steel. I do not think changing vehicles on either end of a long journey represents much inconvenience. After all, the PRT vehicles should be waiting for your arrival, and even departures should involve minimum waiting with a relatively small number of passengers. (Or just pay extra for the whole darn thing… a sleeper car!) Anyway, that is a subject that is removed from the matter at hand, other than that a heavier, even faster track could (and probably should) be considered for track that parallels major highways.   

So, to sum things up, there are factors, some purely psychological, that push the optimal speed limit up to a range that is roughly comparable to what we experience in our cars. There are other factors, namely aerodynamic drag, that limit the speed to not much beyond that range. After that, vehicles should probably be coupled or GRT could be used, increasing efficiency dramatically. This probably would call for a modified track design. This is, however, generally consistent with what would be optimal for fast PRT routes anyway. 

Every endeavor has to start somewhere, and PRT surely has to start with very modest routing and so performance is not really an issue. That does not mean, though, that PRT should best be designed with those limitations baked in. The trick, however, is to take this speed requirement and integrate it into a design that does not sacrifice full 3D maneuverability or minimal track size. Otherwise the design would be quite simple!

144> Compromise

This is a post I have been struggling over for several weeks.  It touches on so many subjects that it is difficult to compact it into a single post.  It is basically a review of the wheel geometry issues that have taken up so much of my time and an overview of the design philosophy and limitations we face.

There are three “must have” criteria that are central to the design - inexpensive, unobtrusive and easy-to-produce track, extreme 3D maneuverability, and speed.  There are also two additional criteria that overlay the others.  These are mechanical simplicity and forward compatibility.

Each of the three initial priorities is easily attainable, but at the expense of the other two.  This leads to tough choices.  Maneuverability in 3D lends itself to a short, wheel-driven bogie.  Speed indicates large wheels, or maglev.  Minimal track cost means no maglev or LSM, and minimal track size means smaller guide wheels.  All in all, the best choice seems like some kind of wheel driven system, but there is no easy answer as to what the best guide wheel placement would be.  Making the track a bit bigger or smaller, or making the speed bit faster or slower completely changes the optimal geometry.  What is sad is that sometimes these kinds of things are never resolved.  Consider rear wheel vs. front wheel drive for automobiles. Apparently now rear wheel drive is making a comeback.  Clearly this kind of ambiguity does not engender a standards-based approach, something that was central to this whole project.  That does not mean that there isn’t a best way to design for a specific speed or track size.  It does mean, though, that a relatively exact track size, speed, and maneuverability must be decided upon before the best geometry can be determined. 

And for the second two design priorities … Well simplicity seems like it would go without saying, but actually it jumps right in with the initial three;  there are ways to partially remedy the conflicts between track size, speed and maneuverability that involve the addition of parts.  This is, of course, not unusual. No engineer wants to add parts unnecessarily, but functionality may demand it.  Trying to design an automatic transmission with five moving parts is obviously a fool’s errand.  Yet mechanical complexity is surely the enemy of any brand-new, yet-untested PRT system, so I have spent considerable time on such foolish pursuits!

Lastly, there is the matter of forward compatibility.  Since PRT track of this type represents new and presumably permanent infrastructure, there is a need to “get it right.”  The last time this happened, with the railroads, there was little incentive to get everything standardized around the very best engineering paradigm, but there was every incentive to get track put down ASAP so that money could be made transporting goods.  This led to many now-obsolete track specifications and, to this day, those early design choices still limit what rail can do. Too late now!

So where does all of this leave us?  There is a real maze of options to sort through.  All of the designs I have shown are basically workable, and in my mind there is a near tie between them, and all have drawbacks.  One thing that would be useful is to get a handle on exactly how small guide wheels can be and still perform well in continuous duty at specific, higher speeds. New urethane formulations, like Dupont’s Hylene PPDI have greatly extended the life expectancy of similar wheels used in rollercoasters, although I have heard of some costing well over a thousand dollars each.  Also such wheels almost universally run on round (pipe) track, and so quickly wear in the center before spreading out the load a bit.  How would they perform against a flat surface? How wide could the wheels be?  Is a very wide wheel of small diameter a decent substitute for a narrower, larger diameter one with an equal wear area?  How about asymmetrical guide wheel placement?  Could that allow for a reduced track size?

All fixed guide wheels must contact outwardly, from the center, and steering guide wheels must engage from the outside, towards the center of the track.  If the steering guide wheels double as running guide wheels, they can be large (allowing for very high speeds) the track can be small, and if both sides retract,  very tight maneuvers can be accomplished.  But this creates the problem that was pointed out regarding the “diamond track” design.  While large guide wheels outside of the track can essentially eliminate any speed limit restrictions, one side must disengage at every interchange. Is this a price worth paying for speed and smaller track? Probably, but that sure doesn’t mean I like it.

Perhaps it is possible to include designated zones on the bogey or track where future guide wheels could go.  This might enable simple, straight-forward designs for speeds that have traditionally been considered sufficient without foreclosing the option of higher speed routes in the future.  For the record, I see little problem with switching schemes such as Anderson’s nearly up to highway speeds, assuming a similar track size.   

I started this project with the belief that it was only the desire of PRT companies to quickly get to market that lowered the performance bar.  I was hoping that with persistence and an open mind, a truly simple design would emerge that could do it all.  I have learned that it will be a bit more complicated, and there will be performance limitations that will have a real impact on functionality.  These will be hard trade-offs, and the many judgment calls will mean that such a design will be ill-suited for any kind of standardization.  After all, any design that prioritizes the above criteria differently is apt to prompt significant changes throughout. Thus any standard will only emerge by the usual Darwinian means, with all of the uncertainty and waste that such experimentation entails.

For the moment I feel like starting from scratch yet again, despite the fact that it seems like I am spinning my wheels. (pardon the pun) Sometimes you get too close to a problem and can’t see the obvious. Also, it is time to “try on” at least a few, reasonable limitations to the design’s capabilities. Perhaps in the next post we can delve into the three criteria, and discuss what those limits should be and why.  ‘Till next time!

143> Waiting for SkyTran…

Recently several alert readers have called some aspects of my latest designs into question. It is extremely easy, especially with CAD software, to get on a roll with a particular set of beginning assumptions and start working out details from there, never revisiting those initial assumptions. After all, once you go through the trouble of drawing a component, it is very hard to throw it away, so you tend to modify your design to make it work. I think this may be the case with the recent conversion to the “diamond” track.  So I am taking a break to reassess the basics before going back to component design. I need fresh eyes, and to sleep on the problem for a while.

In the meantime  I thought I might share a few observations about another highly ambitious suspended PRT system under development, SkyTran. Naturally I have given the system some thought, since perhaps there is something in the design that would benefit our little project.

Of course SkyTran’s most notable characteristic is the Inductrack technology, which uses magnetic levitation to eliminate the need for wheels, (and the inefficient friction and periodic replacement associated with them.) But before going much further, there is one very important point to keep in mind in regards to wheels… Inflatable tires that are designed to absorb shock (and grab the road to prevent skidding) are extremely inefficient. Hard wheels, such as the steel wheels on a train, offer MUCH less rolling resistance. Perhaps some alert reader out there can quantify this with some numbers, but my gut tells me that the difference between hard wheels and maglev is not much greater than between hard wheels and soft, road-worthy tires. Of course this point is somewhat moot, since our little design requires a certain amount of “give” to the main wheels, so they probably will be of solid rubber. My best guess is that this will put us about halfway between maglev and automotive tires in terms of frictional resistance, and still give us enough cushion for a quiet, smooth ride.  That said, let’s take an “under the hood” look at Inductrack.

With Inductrack, vehicle-mounted permanent magnets pass over magnetic coils in the track to induce current. That electricity, in turn, induces magnetism that will be polarized to repel those permanent magnets, creating lift. While this has been described as “passive” levitation,” this description is a bit misleading. Actually the system is behaving as a linear generator, where it uses the power generated by moving permanent magnets over coils to create lift, instead of harvesting that power for other uses.   

Generators and motors are structurally nearly identical, and the same can be said for linear generators and linear motors. Therefore such a system, with only minor modifications, can be made to propel and brake the vehicle as well.

It sounds too good to be true. All you need to do to make the vehicle “float” is to get it going a few miles per hour, and it takes almost nothing to push a floating object! But there’s a catch… do you see it? The reason this sounds so darn good is because it has the attributes of a perpetual motion machine. It is a generator that feeds a motor that propels a generator that feeds a motor. We all know that perpetual motion is impossible – that energy must be added- and Inductrack is no exception. Inductrack is unique, though, in that levitation can be had from any outside form of propulsion, from jet engines to sliding down a hill. But for PRT, where the applied energy will undoubtedly be electrical, it raises an important question. “How efficient is this Inductrack design compared to other forms of maglev?” After all, we have established that energy needs to be applied to Inductrack to create the propulsion that creates the lift. Anyone who owns a generator knows that the motor works harder under load. In the case of Inductrack, this effect would manifest itself by creating some amount of drag on any source of propulsion, so the vehicle is not really floating freely, but rather encountering resistance because it, too, is “under load”.  Other forms of maglev simply create the lift and propulsion directly. Why should Inductrack be more efficient? Another problem I see is with linear motors in general, where the object being propelled is a big, wind catching object possibly filled with hyperactive children. That is the fact that the greater the gap between the vehicle and the track, the less efficient the linear motor. The motors I have seen need a gap of less than a quarter inch to be reasonably efficient, and half of that to be comparable to a rotary motor. 

Let’s take a brief look at maglev in general, including  SkyTran’s approach. The benefits are obvious. No noise, friction, or moving parts to wear out. The downside is that it puts a bunch of electrical hardware into the track, and that has consequences in terms of how any given transit project is routed and financed. If all of the propulsion hardware is in the vehicle, that investment is put to immediate and nearly continuous use, where it can pay for itself. If the expense is shifted to the track, then that track must be equally in constant and continuous use to achieve a timely payback. This seems problematic to me, because it would tend to inhibit expansion of routes. Or, put another way, is this the most efficient use of copper? In the vehicle (in a rotary motor) the same copper coils are used again and again, with each rotation of the wheels. In the track, between each passing vehicle the copper just sits there, paid for, but unused. The shear mass of the coils is many times what it would be if they were in the vehicles, so it is more expensive, period. More expensive, but, all else being equal, well … better.

Speaking of “All else being equal,” I was going to point out that maglev vehicles, like their LIM powered cousins, would tend to be less maneuverable in close quarters, especially tight vertical turns. Actually, though, I think that problem is surmountable.  This is something I wish the SkyTran people would address. As it stands, The SkyTran’s stations need to be elevated, and that means elevators. True, good maneuverability would complicate their simple design and might make the curved track segments more expensive, but I think the versatility would be worth it.

Maglev, it seems to me, requires more than a single track profile, in terms of how it is wired. Nobody, in this fast-paced world, wants to accelerate at a snail’s pace, and feeder tracks need to be kept short anyway. That means big, beefy acceleration coils in the track around stations. At speeds just under where aerodynamic drag starts to really take its toll, I would expect the track’s coils to be minimal. For very high speeds, air resistance again creates the need for high energy input. I have just mentioned that very tight turns might require special provisions to keep the vehicle centered and moving. (Actually Inductrack comes in two flavors, a high speed and a low speed type, but this still does not address the propulsion issue, at least as far as I can tell.)

Since maglev vehicles are untethered, transfer of power from the track to the vehicle is more problematic. Any onboard battery must presumably be recharged at the station, or additional magnets and coils can be used as a linear generator. (creating more resistance to propulsion) I wonder though. If there was a break in the track, cutting off power, would a SkyTran vehicle be stranded? Or is there a way to hobble to a station or emergency evacuation area on battery power alone? It is not impossible that a maglev system could be entirely battery powered, or battery powered but supplemented where higher power is required, such as acceleration ramps. But again, you can’t just harvest the energy that you are creating. Battery powered maglev would require charging stations or a way to harvest energy from an electrified track through magnetic induction, which sort of defeats the purpose.  

To summarize, it has been said the maglev is the future for transportation, and that may well be the case. SkyTran’s Inductrack technology demonstrates that the amount of power that is required to break the bonds of gravity (and loose the wheels) is not that great. (even the best rare earth magnets moving over coils at only, say, 12 mph represent VERY little generated power, since speed is everything when it comes to the amount of electricity a generator can make) The underlying principals involved in this, and other maglev designs are simple and well understood. Anyone who has ever opened up a typical electric motor has seen the components – copper coils to induce or harvest magnetism, and steel laminations to coax those fields where they are needed most, perhaps some permanent magnets.  We have been using the techniques for the last century. Using it for linear, instead of circular propulsion, and tweaking it to create repelling magnetic fields to create lift, well, that is more recent. Nonetheless, we are basically talking about geometry here. It is the clever arrangement of the magnets, coils and laminations that makes everything possible. Making it application specific – say, for a payload of 2-4 people with expectations of performance similar to their family car – that is, indeed, an undertaking! The people at SkyTran say they have it essentially nailed down, but all complex engineering tasks involve compromise, and we don’t yet know what specific trade-offs they have had to accept.

In the meantime, for what it is worth, our humble little system is suitable for a retrofit, if and when it comes to that point. And a hybrid is not out of the question! After all, their weakness, (low speeds, maneuverability) are our strengths, and our challenges (going through tires because of high speeds, too much machinery packed into a small track, getting silky-smooth high-speed switching) are their strengths. Who knows?

142> Dodging Bullets

Well, as I write this, it is (for those of us who are US citizens) Memorial Day, or as we say here in rural New Hampshire, “Better-shoot-off-our-guns” Day. And so I am “sheltering in place,” in my cabin, trying my best to ignore the gunfire and musing over the state motto, “Live free or die!”. 

In my last post I outlined a design that I am hoping ends the long quest for the perfect framework for a ultra-high performance PRT system, and I want to add a couple of additional comments. First of all, I would point out that my depictions illustrate both a track and bogie wheels, but not just for any bogie. It’s a high-speed one, and that is a very important point. You see, if we were just talking about the speeds normally associated with PRT, any one of a number of wheel designs would suffice. But I am looking for a design than can potentially well exceed highway speeds, and that explains my fixation with larger wheels. You see, I believe that the corridors created by highways are perfect for PRT, and that commercial development has tended to be along these areas anyway. In most US cities, almost all of one’s needs may be met within a block or two of a highway. I also don’t mean ONLY high speed. It must also be highly maneuverable to get everywhere you might want to go in a dense suburban environment. 

Giving PRT longer-haul, faster capabilities has certain design ramifications, and those larger wheels are meant to address the wear multiplied by both higher speeds and greater distances. This does not mean that the larger wheeled system is the only one that can run on the track, but rather that the track profile is equally well suited for high speeds as low. The use of off-the-shelf steel makes the running surfaces  extremely cheap and easy to fabricate, and can be housed in a variety of ways, such as trusses, a box-beam, or even open, such as in an roofed enclosure.  

The profile is advantagious because it does not force the wheels to run on round pipe, something that concentrates wear on the center of the tires or solid wheels, yet the square tubing is easy to bend using a conventional 3 roll pipe bender, as shown below. (Needs V-grooved rolls)

Of course running PRT wheels on flat surfaces is nothing new, but established designs mandate that the beam that houses the running surfaces be of sufficient size to house those various guide wheels – generally oriented sideways. This limits those wheel sizes to less than one half of the interior of the truss or box beam. This has meant a trade-off between what you had to look at overhead, and the life of those wheels. Whereas that track girth can otherwise be justified for structural reasons if the spans are to be large, if it is convenient and cost effective to space track support poles more closely, then the track itself can be thinner, cheaper, and more attractive. Turning those flat running surfaces diagonally allows for either longer-lasting guide wheels, thinner track, or a little of each. Of course placing the main guide wheels outside of the track doesn’t hurt either!

That brings up the point by alert reader Rick, who noted that there has been an alarming departure from previous designs in that there is no failsafe for the possibility of a steering guide wheel failing to respond. This is true, but I stand by my design. A feature of many early designs, such as the one depicted in this patent drawing, is that they always had one or the other the of steering guide wheels engaged. That way if the something failed, the vehicle would still go one way or the other, and would never crash by trying to go both ways or somewhere between. 

That is fine, and an excellent feature, but those wheels that stay engaged for safety sake must, in a high speed long distance system, do so for thousands of miles per week at high RPM. What if the system is to include an express shuttle that bypasses many areas that have no PRT service, so there are very long stretches with no off ramps? Would we want to be wearing out the steering guide wheels for that whole time?  

I would note that there has been somewhat of a culture change underway in recent years in regard to the trust we put in our computer controlled equipment. When Anderson patented this mechanism, meant to “snap” the steering into full left or right positions, it was in a time when making the system centrally controlled was not a choice but rather mandated by the size and cost of even limited computational power. I am on record as advocating a more autonomous control architecture than many proposed systems, and autonomy certainly plays a part in any discussion on the safety of a system that requires an action at every junction. Google’s robocar, now licensed to drive in Nevada, could, on a twisty road, certainly veer into oncoming traffic by a similar failure to actively steer. Luckily the vehicle is not driving by Google Maps alone, but by an array of sensors and onboard computers all working in concert to create a vehicle that can act and react autonomously. Now, in the days of “cloud” computing (many seperate computers drawing from, and collaborating through, a central computer) and supercomputers made of dispersed computers sharing a common program, (such as SETI) the lines have been totally blurred. With today’s technology, steering gear deployment can be assured through a number of cooperative means, including autonomously from within the vehicle, from sensors within the track, from communication with a central computer and/or any combination therein.  

Another, separate point that needs to made is that the track and bogie combination has been carefully designed to allow extremely tight turns, including changes of pitch, and the combination of this attribute with high speeds and switching is particularly challenging.

Anyway, to get back to a point I was making earlier, by dialing back the speed a bit, many other bogie designs are possible, and may be more practical than what I have shown for those speeds. My focus is to not PRECLUDE high speed (or other valuable attributes) in the track first, and then to design practical bogies for that track. I am really not sure what a downtown-use-only bogie for this track would look like at this point. I'll have to work on it between the bullets.