The future of mass transit could be trains flying 20 feet above streets and trees.
Flying trains are difficult to imagine by today's standards, but it is important to note that high speed trains must use vehicle shape to push the vehicle downward and create "aerodynamic grip" as a necessary means to increase the wheels' grip on the tracks. Race cars supplement vehicle shape with front spoilers, rear spoilers, and/or diffusers to further enhance aerodynamic grip. Poor design can lead to trains and race cars flying off the tracks at high speeds.
And so: Should trains be designed to fly?
For example, an overhead guideways could be used as a source of traction engaged by a propulsion carriage to pull an airfoil vehicle (see Figure 1), or train of airfoil vehicles. A suspension guideway is illustrated by Figure 2; this example resembles electrical power distribution lines. A modified version of the Figure 2 guideway has the upper support cables attached to lateral trusses to which multiple guideways are attached like a multilane interstate.
Infrastructure
By eliminating the rails, concrete highways, or superconductor maglev guideways; the major costs of these mass transit infrastructures are eliminated. A recent history of HSR expansion in China documents a cost of $30 million per mile, while decades of experience in the U.S. puts the larger electrical power distribution lines (towers spaced at 0.2 miles) at $2 million per mile. This is more than a 90 percent reduction in cost.
The low cost of this high-performance guideway is the differentiator that can lead to the transformation of our entire transportation infrastructure. In fact, it is possible to distribute electrical power on the same infrastructure if electrical insulators are placed between the upper support cables (Figure 2) and the lower guideway. The result is an effective guideway cost near $1 million per mile. This type of combined infrastructure can be particularly valuable for developing countries that lack basic infrastructure.
To keep the overhead guideway cheap, it is important that aerodynamic lift, vehicle control systems, and vehicle/carriage design are such that the primary forces on a guideway is a longitudinal pulling force; the guideway could be as simple as a 1.5 inch diameter cable. A flexible (i.e. tensile-straightening) guideway would only work at very high velocities if lateral or vertical forces were minimal, leading to minimal lateral or vertical movement of the guideway.
The Airfoil Vehicle
Implicit in the illustration of Figure 1 is a torque and force balance at a point of connection on top of the vehicle. An arm connects the vehicle to a propulsion carriage equipped with a linear motor that exerts a propulsion/pulling force on the guideway. Like the vehicle, the propulsion carriage has surfaces and control means so that the cumulative force on the guideway is only a longitudinal pulling force.
When the dominant force on the guideway is a pulling force, the guideway provides a straight and stable path for high speed travel. If vehicles/trains were stalled on the lines, downward displacement of the guideway would occur as part of the balancing of forces that occurs with a flexible guideway; such displacement would not be problematic because the vehicle(s) would either be stationary or moving at slow velocities. Stations would need reinforced guideways or other accommodations to support the weight of vehicles for loading/unloading and acceleration/deceleration.
Benchmark examples of liftoff velocities are 30 mph for the Write brothers first flight and 63 mph for a Cessna 150. Lift increases with the square of velocity as approximated by Equation 1.
(1) L = 0.5 W H F p v2
where L is Lift (N), W is wingspan (m), H is wing thickness (m), F is lift coefficient, ρ is air density (kg/m3), and v is velocity (m/s).
Since lift is proportional to the square of velocity; increases in velocity, ultra-light vehicles, airfoil shapes, and addition of spoilers can readily compensate for the lack of wings on the vehicles. Also, use of grid electricity for energy, rather than stored fuel, leads to major reductions in weight. Preliminary estimates indicate that smaller airfoil vehicles could achieve full aerodynamic lift at 90 mph with velocities greater than 200 mph having many methods to maintain lift for vehicles connected as train units.
Acceleration to 90 mph takes less than a quarter mile (0.2 g), and so, benefits of this approach are realized for routes as short as 1.5 miles. The same methods and infrastructure would be suitable distances of 1.5-2000 miles, commuter to trans-continental transit. One system could displace most of air, rail, bus, and automobile transit if an effective guideway switching system were feasible.
With the addition of high-speed vehicle-controlled guideway switching, this new mode of transportation could provide non-stop service from origin to destination. Such a switching method can be achieved by having a lower main travel guideway and an upper switch guideway where the propulsion carriage (containing the short stator of the linear motor) is able to engage with either. A two-part propulsion carriage (see Figure 3) could engage the upper switch guideway by raising its upper part (about 2 inches) to engage with the switch guideway (left) or not engage the upper switch guideway by not raising its upper part (right).
Details of Figure 3 include a lower guideway that is temporarily narrow to allow the lower carriage to slip up and away from the, lower, main travel guideway. An end of the, upper, switch guideway would appear at switching locations, and if the upper carriage is in the switch location the end of the guideway would enter into the linear motor channel of the upper carriage, like a thread going through the eye of a needle.
Estimated travel times for such a system include 43 minutes from the origin of 90% of the population of Washington D.C. to a destination to 90% of the population of New York City and less than an hour from Los Angeles to San Francisco. These are times from origin to destination, not central station to central station.
Routing
Aerodynamic, low-profile vehicles would be quiet and vibration-free. Routing problems are substantially overcome by these features combined with the ability to use suspension guideways above streets, buildings, rivers, and mountain hollows. Guideways can also be suspended on existing bridges, expediting implementation, further reducing costs, and further reducing environmental impact; this is possible due to the light weight of the infrastructure.
Approaches are sufficiently understood to put bench mark systems in place, but areas of technology to be advanced for more-optimal systems include:
- Airfoil vehicle designs that optimize lift-to-drag (L/D) ratios along with development of the corresponding science/technology,
- Design of short stator linear induction motors capable of propelling along a cylindrical guideway,
- Methods of attaching cylindrical guideways that do not obstruct more than 10% of the guideways' circumferences,
- Optimization of designs for transfer of grid electricity to carriages, and
- Optimization of propulsion carriages for performing guideway switches including reducing redundant components on those carriages.
For the many challenges of this new mode of transportation, there are many and adequate good solutions. The benefits include reduced travel costs, reduced travel time, reduced environmental impact, and a means to displace use of fossil fuels at a level much greater than previously thought possible.
A few 1.5 inch D cable guideways suspended 20 feet above the ground could replace the massive concrete overpasses and highways that dominate our cities. Acres of pavement and parked vehicles could be transformed to green space.
Will future generations look with wonder at pictures the huge bridges, elevated tracks, and parking lots full of vehicles? The amount of money put into beautiful building architecture is ironic when one notes that the prominent feature of most buildings at ground level are the masses of concrete, asphalt, and parked vehicles.
