Hovertrain
A hovertrain is a type of high-speed train that replaces conventional steel wheels with hovercraft lift pads, and the conventional railway bed with a paved road-like surface, known as the track or guideway. The concept aims to eliminate rolling resistance and allow very high performance, while also simplifying the infrastructure needed to lay new lines. Hovertrain is a generic term, and the vehicles are more commonly referred to by their project names where they were developed. In the UK they are known as tracked hovercraft, in the US they are tracked air-cushion vehicles. The first hovertrain was developed by Jean Bertin in the early 1960s in France, where they were marketed as the Aérotrain before being abandoned by the French government.
History
Hovertrains were seen as a relatively low-risk and low-cost way to develop high-speed inter-city train service, in an era when conventional rail seemed stuck to speeds around or less. By the late 1960s, major development efforts were underway in France, the UK and the USA. While they were being developed, British Rail was running an extensive study of the problems being seen at high speeds on conventional rails. This led to a series of new high-speed train designs in the 1970s, starting with their own APT. Although the hovertrains still had reduced infrastructure costs compared to the APT and similar designs like the TGV, in practice this was offset by their need for entirely new lines. Conventional wheeled trains could run at low speed on existing lines, greatly reducing capital expenditures in urban areas. Interest in hovertrains waned, and major development had ended by the mid-1970s.Hovertrains were also developed for smaller systems, including personal rapid transit systems that were a hot topic in the late 1960s and early 1970s. In this role their ability to float over small imperfections and debris on the "rails" was a practical advantage, although it competed with the maglev concept that had the same advantages. The only hovertain to see commercial service was the Otis Hovair system. Originally developed at General Motors as an automated guideway transit system, GM was forced to divest the design as part of an antitrust ruling. The design eventually ended up at Otis Elevator who later replaced its linear motor with a cable pull and sold the resulting design for people mover installations all over the world.
Basic concept
It was noticed early on that the energy needed to lift a hovercraft was directly related to the smoothness of the surface it traveled on. This was not surprising; the air trapped under the hovercraft's skirt will remain there except where it leaks out around the bottom of the skirt where it contacts the ground – if this interface is smooth, the amount of leaked air will be low. What was surprising was that the amount of energy lost through this process could be lower than steel wheeled vehicles, at least at high speeds.At high speeds, trains suffer from a form of instability known as "hunting oscillation" that forces the flanges on the sides of the wheels to hit the sides of the rails, as if they were rounding a tight bend. At speeds of or over, the frequency of these hits increased to the point where they became a major form of drag, dramatically increasing rolling resistance and potentially causing a derailment. That meant that for travel above some critical speed, a hovercraft could be more efficient than a wheeled vehicle of the same weight.
Better yet, such a vehicle would also retain all of the positive qualities of a hovercraft. Small imperfections in the surface would have no effect on the ride quality, so the complexity of the suspension system could be reduced. Additionally, since the load is spread out over the surface of the lifting pads, often the entire underside of the vehicle, the pressure on the running surface is greatly reduced – about the pressure of a train wheel, about of the pressure of a tire on a road.
These two properties meant that the running surface could be considerably simpler than the surface needed to support the same vehicle on wheels; hovertrains could be supported on surfaces similar to existing light-duty roadways, instead of the much more complex and expensive railbeds needed for conventional trains. This could dramatically reduce infrastructure capital costs of building new lines and offer a path to widespread use of high-speed trains.
Development
Early efforts
One of the earliest hovertrain concepts predates hovercraft by decades; in the early 1930s Andrew Kucher, an engineer at Ford, came up with the idea of using compressed air to provide lift as a form of lubrication. This led to the Levapad concept, where compressed air was blown out of small metal disks, shaped much like a poppet valve. The Levapad required extremely flat surfaces to work on, either metal plates, or as originally intended, the very smooth concrete of a factory floor. Kucher eventually became VP in charge of the Ford Scientific Laboratory, continuing development of the Levapad concept throughout.It does not appear any effort was put into vehicle use until the 1950s, when several efforts used Levapad-like arrangements running on conventional rails as a way to avoid the hunting problems and provide high-speed service. A 1958 article in Modern Mechanix is one of the first popular introductions of the Levapad concept. The article focuses on cars, based on Ford's prototype "Glideair" vehicle, but quotes Kucher noting "We look upon Glideair as a new form of high-speed land transportation, probably in the field of rail surface travel, for fast trips of distances of up to about 1,000 miles ". A 1960 Popular Mechanics article notes a number of different groups proposing a hovertrain concept.
What was lacking from all of them was a suitable way to move the vehicles forward – since the whole idea of the hovertrain concept was to eliminate any physical contact with the running surface, especially wheels, some sort of contact-less thrust would have to be provided. There were various proposals using air ducted from the lift fans, propeller, or even jet engines, but none of these could approach the efficiency of an electric motor powering a wheel.
LIM
At about the same time, Eric Laithwaite was building the first practical linear induction motors, which, prior to his efforts, had been limited to "toy" systems. A LIM can be built in several different ways, but in its simplest form it consists of an active portion on the vehicle corresponding to the windings on a conventional motor, and a metal plate on the tracks acting as the stator. When the windings are energized, the magnetic field they produce causes an opposite field to be induced in the plate. There is a short delay between field and induced field due to hysteresis.By carefully timing the energizing of the windings, the fields in the windings and "reaction rail" will be slightly offset due to the hysteresis. That offset results in a net thrust along the reaction rail, allowing the LIM to pull itself along the rail without any physical contact. The LIM concept sparked considerable interest in the transportation world, as it offered a way to make an electric motor with no moving parts and no physical contact, which could greatly reduce maintenance needs.
Laithwaite suggested that the LIM would be a perfect fit for high speed transport, and built a model consisting of a chair mounted on a four-wheeled chassis on rails with a LIM rail running down the middle. After successful demonstrations, he convinced British Rail to invest in some experimental work using a LIM to power a train on rails using small lift pads similar to the Levipad system for suspension.
Momentum drag
As the various hovertrain systems were developing, a major energy use issue cropped up. Hovercraft generate lift by providing pressure, as opposed to generating lift due to the momentum of air flowing over an airfoil. The pressure of the air required is a function of the vehicle weight and the size of the lift pad, essentially a measure of overall vehicle density. A non-moving vehicle only loses this air due to leakage around the pads, which can be very low depending on the relative pressure between the pad and the outside atmosphere, and further reduced by introducing a "skirt" to close the gap between the pad and running surface as much as possible.However, as the vehicle moves another loss mechanism comes into play. This is due to the skin friction between the lift air and the ground below it. Some of the lift air "sticks" to the running surface, and is dragged out from under the pad as it moves. The amount of air that is lost though this mechanism is dependent on vehicle speed, surface roughness and the total area of the lift pads. The vehicle air pumps must supply new pressurized air to make up for these losses. As the vehicle weight and lift pad area is fixed, for a given vehicle design the volume of air that needs to be ingested by the pumps increases with speed.
The problem is that the air is at rest compared to the world, not the vehicle. In order to be used by the air pumps, it must first be brought up to vehicle speed. Similar effects occur with almost all high-speed vehicles: thus the reason for the large and complex air inlets on fighter aircraft, for instance, which slow the air down to speeds that their jet engines can ingest. In the case of a hovertrain design, the air losses at the pads increase with speed, so an increasing amount of air must be ingested and accelerated to compensate. That increasing volume of air is at an increasingly lower speed, relative to the vehicle. The result is a non-linear increase in power dissipated into the lift air.
A study by UK Tracked Hovercraft Ltd. considered the energy use of a 40-ton 100-passenger hovertrain. At and in a crosswind, they predicted that their hovertrain would require 2,800 kW to overcome aerodynamic drag, a figure that compared favourably to any other form of ground transit. However, in order to provide lift, the vehicle would need to ingest air and accelerate it to vehicle speed before pumping it into the lift pads. This produced what they called "momentum drag", accounting for a further 2,100 kW. The combined was not unheard of, existing freight locomotives of similar power were already in use. However, these locomotives weighed 80 tons, much of it constituted by the voltage control and conversion equipment, whereas the Tracked Hovercraft design was intended to be a very lightweight vehicle. THL's solution was to move this equipment to the trackside, requiring this expensive technology to be distributed all along the line. However the PTACV demonstrated that a, 60 seat vehicle needed only at for its air suspension and guidance system. At, the French I80 HV reached similar figures.