Tracked Hovercraft

Tracked Hovercraft was an experimental high speed train developed in the United Kingdom during the 1960s. It combined two British inventions, the hovercraft and linear induction motor, in an effort to produce a train system that would provide inter-city service with lowered capital costs compared to other high-speed solutions. Substantially similar to the French Aérotrain and other hovertrain systems of the 1960s, Tracked Hovercraft suffered a similar fate to these projects when it was cancelled as a part of wide budget cuts in 1973.

History

Genesis at Hovercraft Development

It was noticed early on in the development of the hovercraft that the energy needed to lift a vehicle was directly related to the smoothness of the surface on which it travelled. This was not entirely surprising; the air trapped under the hovercraft will remain there except where it leaks out where the lifting surface contacts the ground — if this interface is smooth, the amount of leaked air will be low.Sandie Yang
"Handling Hovercrafts: Designing a project curriculum to introduce girls to mechanical engineering"
MIT, June 2006, p. 14. This is the purpose of the ''skirt'' found on most hovercraft; it allows the fuselage to be some distance from the ground while keeping the air gap as small as possible.

The surprising discovery was that the amount of energy needed to move a given vehicle using hover technology could be lower than the same vehicle on steel wheels, at least at high speeds. Over , conventional trains suffered from a problem known as hunting oscillation that forces the flanges on the sides of the wheels to hit the rail with increasing frequency, dramatically increasing rolling resistance. Although the energy needed to keep a hovercraft in motion also increased with speed, this increase was slower than the sudden (and sometimes catastrophic) increase due to hunting. That implied that for travel above some critical speed, a hovercraft could be more efficient than a wheeled vehicle running on the same route.

Better yet, this 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, and the complexity of the suspension system could be greatly reduced. Additionally, since the load is spread out over the surface of the lifting pads, the pressure on the running surface is greatly reduced — about the pressure of a train wheel, about of the pressure of a rubber tyre 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 to support the weight on two rails. This could greatly reduce infrastructure capital costs.

In 1960 several engineers at Christopher Cockerell's Hovercraft Development Ltd. in Hythe, Hampshire, began early studies on the hovertrain concept. At the time, a major problem was selecting a suitable power source. As the hovercraft had no strong contact with a running surface, propulsion was normally provided by an aircraft-like solution, typically a large propeller. This limits the acceleration, as well as the efficiency of the system, a major limitation for a design concept that would compete with aircraft on the same routes.

Introducing the LIM

Through the same period, Eric Laithwaite had been developing the linear induction motor (LIM) at the University of Manchester. By 1961 he had built a small demonstration system consisting of a LIM reaction plate and a four-wheeled cart with a seat on top. In 1962 he started consulting with British Rail (BR) on the idea of using LIMs for high-speed trains. A November 1961 ''Popular Science'' article shows his Hovertrain concept using a LIM, the accompanying illustration shows small lift pads like those from the Ford Levapad concept, running on top of conventional rails. After moving to Imperial College London in 1964, Laithwaite was able to devote more time to this work and perfect the first working examples of large LIMs suitable for transport systems.''Laithwaite''

LIMs provide traction through the interaction of magnetic fields generated on the vehicle and a fixed external conductor. The external conductor was normally made of plates of aluminium, chosen due to its high conductivity in relation to its price. The active portion of the motor consists of a conventional electric motor winding stretched out under the vehicle. When the motor windings are energised, an opposing magnetic field is induced in the nearby reaction plate, which causes the two to repel each other. By moving the fields down the windings, the motor pushes itself along the plate with the same force that is normally used to create rotation in a conventional motor. A LIM eliminates the need for strong physical contact with the track, requiring instead a strong reaction plate. It has no moving parts, a major advantage over conventional traction.

In Laithwaite's original designs, known as ''double-sided sandwich motors'', two sets of windings were used, positioned a few centimetres apart. They were positioned so that the aluminium stator plate would fit in the gap between the windings, sandwiching it between them. The advantage to this layout is that the forces pulling one set of windings toward the plate are balanced by the opposite forces in the other set. By attaching the two sets of windings to a common frame, all of the forces are internalised.

Hovertrain

The Hovercraft Development team quickly picked up on the LIM concept as well. Their initial solution was a track shaped like an upside-down T, with the vertical portion consisting of a central concrete section with aluminium ''stator'' plates fixed on either side. Their first design concept looked like the fuselage of an airliner with two decks, riding above the stator beam, with the LIM centred in the middle of the body. Four pads provided lift, arranged two on a side fore and aft and riding on the horizontal surface of the guideway. Four more pads, above the lift pads, were rotated vertically to press against the centre beam and kept the craft centred. A test rig of this layout was built at Hythe, which was filmed in operation by British Pathé in 1963, which also showed a model of the proposed full-sized version.(https://www.britishpathe.com/video/hovertrain/query/Hythe)

As development of the testbed design continued at HDL, the problem of high-speed loads on the guideway became obvious. In spite of its light weight compared to conventional train sets, the Tracked Hovercraft operated at such high speeds that its passage induced vibration modes in the guideway that needed to be damped out. This was a relatively new field for the civil engineers that were working on the guideway design, as their field was more generally concerned with static loads.Jim Platts
"Dynamics of Hovertrain Tracks"
1971 The train layout was redesigned with a box-like main girder, with a top-mounted reaction plate being used for the LIM, and the vertical sides of the guideway being used for centring. Wing-like extensions extended down from the body of the train and covered the centring pads. A version with this layout was built as a scale model at Hythe, and featured in another Pathé film in 1966. This version was shown at Hovershow '66.

A further modification produced a guideway that looked like a rightside-up T, although the vertical section was a trapezoidal girder almost as wide as the top of the T. The reaction plate for the LIM was moved to the underside of the horizontal portion of the T on one side, extending vertically down, while the other side contained the electrical conductors that provided power. In such an arrangement, rain, snow and debris would simply fall off the plates. The new guideway design was simulated at the Atlas Computer Laboratory. This work included the generation of films showing the vehicle in-action, using a Stromberg-Carlson SC4020 microfilm recorder.

Laithwaite joins

While the hovertrain was being developed, BR was running an extensive research project on the topic of high-speed wheeled trains at their newly opened British Rail Research Division in Derby. This was the first group to characterise the hunting oscillation in detail. Their work clearly suggested that careful design of the suspension system could eliminate the problem. This would allow high-speed trains to be built using conventional steel-wheel technology.

Although high-speed travel would require new lines to be laid, expensive ones, such a train could use existing rail infrastructure at lower speeds. This would allow such a train to approach existing stations at lower speeds, greatly reducing capital costs of bringing the service into cities. The inter-city sections could be re-laid for higher speeds, where the infrastructure costs were generally lower anyway. BR also showed that the capital cost advantages of the hovertrain concept were offset by the higher vehicle costs; the tracked hovercraft concept made sense for a smaller number of vehicles or longer lines where the capital costs were concentrated in the tracks, but neither of these characterised BR's operations.

Meanwhile, having exhausted their research abilities using small models, the Hovercraft Development team had been petitioning their parent organisation, the National Research Development Corporation (NRDC), for additional funding to build a full-sized test track. NDRC was unsuccessful in raising new capital from the government and decided to put in £1 million from their own pre-assigned discretionary budget to start construction of a track, hoping that additional funding would be forthcoming from industry.''Go-Ahead'' 1967, pg. 58

On 1 April 1967, Hovercraft Development was officially transferred to National Physical Laboratory. Seeking to protect their investment, and finding little external funding, the NRDC decided to spin off the hovertrain group as Tracked Hovercraft Ltd. (THL). They also decided to spool out the funding over four years, starting with a £1 million grant for a single prototype vehicle and a short portion of the test track. Although this funding was enough only for the first stage of a track, the NRDC suggested it would be quite useful for testing low-speed intra-urban versions of the concept.

Frustrated with BR's lack of interest in his hovertrain work, and their lack of funding, in 1967 Laithwaite severed his ties with BR and joined Tracked Hovercraft as a consultant. By this time the French government had started providing major funding for Jean Bertin's Aérotrain project, which was substantially similar to the Tracked Hovercraft in concept. Laithwaite, always described as persuasive, convinced the government that they were about to lose out on this burgeoning field of high-speed transit, and eventually won £2 million in additional funding.

RTV 31

By the time construction was preparing to start in 1970, a new problem had appeared. Prior to construction most LIMs were test systems that had operated at low speeds, but as the speeds increased it was noticed that the mechanical forces of the LIM windings on the stator plate gave rise to a serious safety issue. Magnetic forces vary with the cube of distance, so any change in distance between the motor and stator plate caused it to be pulled more strongly to the closer side. At high speeds, the forces involved were so great that it was possible for the stator plate to crack along vertical joins in the plates, at which point it could strike the motor, or portions of the vehicle behind the crack point. Even without an outright failure, any mechanical motion in the plate due to the forces of the passing train could induce waves in the stator that travelled along with it. If the vehicle then decelerated these waves could catch up with it. Additionally, the passing of the train heated the plate, potentially weakening it mechanically. Laithwaite concluded that the double-sided LIM was "far too dangerous" to use.

Most systems using LIMs – there were dozens by this point – redesigned their tracks to use a single-sided LIM over a stator plate lying flat between the rails. This led to another redesign of the Hovertrain guideway as a square box girder with the LIM stator attached flat on the top of the box, and the electrical pick-ups below on either side of it. Power pick-ups extended from the rear of the vertical wing-like surfaces on either side of the vehicle, and the sparks they threw during operation are easily visible on test runs."Video of RTV 31 test run"
BBC News, February 1973

Starting in the 1970s, construction of test track started in the fens at Earith in Cambridgeshire, supported by Tracked Hovercraft Ltd offices in Ditton Walk in Cambridge city. The track was about off the ground, running along the earthworks between the Old Bedford River and the Counter Drain just to its north, between Earith and the Denver Sluice. The first long section of the planned long track was laid to Sutton-in-the-Isle. Along the full length it was expected the train would reach .

On 7 February 1973 the first test train, Research Test Vehicle 31, or RTV 31, reached on a section, in spite of the short track and a headwind. The test was heavily publicised and shown on BBC
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