Very fast high speed rail suffers from severe air resistance. The hyperloop concept can be likened to high speed rail sent by pneumatic tube transport. A promising combination of relatively low cost, high speed and limited environmental impact makes the hyperloop a possible future fifth mode of transportation.  Pilot programs are being planned – and Sweden is one of the considered locations.


A Hyperloop cross section. The capsule glides inside an elevated steel tube covered by solar panels.

Railways, roadways, airways and waterways. The transportation networks comprise various modes of transport, each with their own limitations. Shipping is slow. Air travel is fast but expensive. Cars provide flexibility at the expense of coordination. Trains can be emission efficient, but require substantial infrastructure developments. A way of moving people and goods in a fast and cost efficient manner, but without substantial emissions, would be very valuable both for the environment and for regional and economic integration.

Air resistance goes up rapidly with speed

Compared to air traffic, rail bound traffic can easily be electrified. This makes rail an attractive starting point in the quest for zero emissions. While the speed of rail bound traffic has increased, it still falls well short of airplane speeds. When speed is increased, the force needed to overcome rolling resistance and air resistance goes up rapidly. Airplanes solve this dilemma by travelling at high altitude where air pressure is significantly lower. To match their speed on the ground, other measures to overcome drag has to be found.

High speed rail was introduced in Japan in 1864, connecting Tokyo and Osaka. Since then, entire high speed rail networks have been developed in many countries. Large investments are often demanded, since separate railway lines with increased curve radius may be needed to accommodate the speeds of up to 350 km/h.

Maglev and vactrains

There are trains faster than that as well. Maglev technology can be used to eliminate rolling resistance. Maglev trains travel along a guideway, without wheels, using magnetic levitation to create both lift and forward thrust. Only one maglev line is currently in commercial operation: the Transrapid in Shanghai, China, achieving 430 km/h. At that speed, four fifths of the propulsion energy is used to overcome air resistance.

The vactrain (vacuum tube train) is a proposed further development of the maglev. This design aims to eliminate air resistance as well, by running the train inside an evacuated tube. The lack of air resistance could permit very high speeds — perhaps up to 8,000 km/h. One of the more mind-boggling suggestions building on the vactrain concept is the Transatlantic tunnel – a theoretical submerged floating tunnel between New York City and London, 50 meters below the ocean surface, allowing a vactrain to cross the Atlantic in less than one hour.

Such an enormous project would obviosly face staggering technical hurdles. The sheer cost estimates and the technical difficulty of maintaining a vacuum in a long tunnel likely prohibits even less ambitious vactrain applications. For the foreseeable future, the concept seems stuck at the drawing board.

Hyperloop – large-scale pneumatic tube transport?

In resemblance to the vactrain idea, the hyperloop concept also envisions the use of low-pressure tubes to reduce drag. In contrast, the hyperloop capsule would not be magnetically levitated, and the tunnel would only be evacuated to one millibar of pressure. It can be likened to a fusion between high speed rail and pneumatic tube transport.

Pneumatic tube transport is a proven system for transporting mail and small packages across a distance. Containers are propelled through networks of tubes by compressed air or partial vacuum. Many large cities have at times incorporated PTT in their postal service – the Paris network, for instance, exceeded 500 km of tubes.

PTT is still in use at many hospitals and similar institutions, for example to send blood samples to laboratories instead of having to deliver them manually. Modern systems have containers equipped with transponders, reporting their location to a central server so that the entire state of the system can be monitored digitally. Several major Swedish hospitals have recently invested in new systems.

This works well for small, low-weight containers. But extending the concept to handle large containers, able to carry people and goods, would require further development of the propulsion and the tube environment.

Faster than travelling by air

To reduce air resistance, the hyperloop tube would be partly evacuated to a pressure of one millibar – similar to the atmospheric pressure at 50 km altitude. The capsules are intended to reach a speed of 1100 km/h – close to the velocity of sound. (This corresponds to a travel time of about half an hour between Stockholm and Gothenburg.)

The capsules will be propelled by accelerators built into the tunnel at a spacing of about 50 km. These linear induction motors give the capsules a speed suitable for the next segment of the journey. Between accelerators, the capsules just glide – floating on a millimeter thin layer of air.

In spite of the low initial pressure, the capsule will experience a compounded pressure in front of it when it is set in motion. To handle this, an electrical fan and compressor will be put in the front of the vessel, actively moving air past the capsule. Some of the air is shunted to create the supporting air cushion. A battery pack on board drives the compressor, but the tunnel motors are entirely responsible for providing the forward thrust. The tunnel is suggested to be constructed from prefabricated steel segments, elevated on pillars and covered by solar panels to supply the accelerators with energy.

An open source project

The original white paper on the Hyperloop was released in 2013 by entrepreneur Elon Musk, the founder of Tesla Motors and Space X.

Musk’s document contained a description of the technology and a cost estimate for a hyperloop between San Francisco and Los Angeles. The route was chosen as an example of where a fifth mode of transport would be most valuable: a medium distance connection with large volumes of traffic. The cost was estimated to be less than ten percent compared to traditional high speed rail. Instead of attempting to commercialise, Musk opted to open-source the concept, encouraging others to develop it further.

A vision becoming reality

So far, at least two start-ups, Hyperloop Transportation Technologies and Hyperloop Technologies, have begun working on the technology. A five kilometer test track is under construction in Nevada, and there are plans to open a commercial track in 2020. Three additional test tracks are scheduled to be built.

Hyperloop technologies have expressed interest in locating one of the test tracks in Sweden, referring to the Swedish spirit and tradition of innovation. The track would go from Norrtälje to Åbo via the Åland Islands, and a feasibility study is set to be released in the coming autumn.

A lot of work remains before the Hyperloop can enter commercial operation. Whether it will be able to live up to the expectations still remains to be seen. But it definitely serves as a good example of the multiple innovation tracks leading towards the faster and greener transportation of the future.

The article was published in February 2016