Hyperloop: Prospects and Challenges

The much-vaunted Hyperloop, sometimes described by its proponents as the ‘fifth mode of transport’, uses the premise of pods travelling through evacuated tubes to offer high-speed inter-city transport. The concept was first attributed to US-based technology entrepreneur Elon Musk, and independent backers are popping up across the globe, amid huge amounts of publicity.

On paper, the concept is a clever one, extrapolating Newton’s First Law to remove as much air resistance as possible to reduce the required motive force to achieve the desired speeds. The technology itself is not revolutionary, simply comprising a pod elevated and driven forwards by magnetic levitation in a tube pumped to a near vacuum.

Some progress has been made by the various competing Hyperloop developers. Virgin Hyperloop One, for example, has built a 500 m ‘DevLoop’ test ring in the Nevada desert, where it has demonstrated the technological union of maglev and vacuum tube. Acceleration, top speed, the pressurised cabin environment and associated emergency arrangements are all very similar to those utilised in commercial air travel. But, for all the attention lavished upon Hyperloop, there are fundamental problems that must be overcome before any commercial application is realistic.

Straight, underground

Steel-wheel high-speed rail can have a design speed of up to 400km/h (250mph), and it is common for curves to be 10km or more in radius. Whilst Hyperloop will probably permit tighter curves than a railway, it aspires to a design speed of up to 1,100km/h (690mph).

As with conventional railway alignments, Hyperloop will rotate the plane of its guideway as curvature increases to reduce the forces on passengers. Yet it seems unlikely that a guideway could be tilted enough to avoid a near-straight alignment, with inertial forces on passengers being comparable to those in a jet aircraft. This, in turn, is likely to mean the tubes would be underground in most applications.

Switches pose another huge technological hurdle. One solution includes relying solely on the guidance of the linear motors for switching – at 1,100km/h the consequences of a failure would be catastrophic and creating a successful safety case would be difficult.

Another option would involve the mechanical shifting of vacuum tube segments to create a continuous through or turnout route. However, for a switch that would be around 1,000 metres long, managing detection and the interface with the control system, all whilst maintaining a vacuum, would be immensely challenging.

The vacuum tube concept also throws up some technical gremlins. Thermal expansion effects on the tubes can be managed by using materials with a reduced thermal expansion coefficient and by constructing expansion joints between each tube segment. However, these expansion joints would have to be strong enough to withstand the pressures from the vacuum within, increasing their cost greatly.

In an emergency, or in the case of a pump failure, the tube would have to be returned to atmospheric pressure: all this requires is a valve controlling the ingress of air. The problems start when the vacuum needs to be restored. With regular airlocks, you could not run a pod at speed from a vacuum into a section at atmospheric pressure. At 1,100km/h, this would be akin to driving a car into a concrete block.

But, if pods have to sit and wait for the correct pressure conditions to be achieved mid-service, delays could be considerable: the Hyperloop One test tube needs four hours to return to a vacuum over a 500-metre alignment. Undoubtedly there will be more powerful pumps in any commercial specification, but this is still technology requiring radical development.

Energy and capacity

Japan’s Chuo Shinkansen maglev is likely to use approximately three times more energy per seat than steel-wheel high-speed rail. While Hyperloop’s vacuum tubes will remove almost all aerodynamic friction, reducing the motive power needed to reach and sustain high speed, the self-same lack of aerodynamic drag will increase the power required to slow the pods down.

On top of this, the likely power consumption of the pumps maintaining the vacuum conditions must be considered.

Yet it is passenger capacity that is arguably the most fundamental challenge. Using the UK’s planned High Speed 2 as a benchmark, high-speed rail capacity can be nearly 20,000 passengers per hour per direction, assuming 18 trains/hour, each with 1,100 seats, over a double-track alignment.

If a Hyperloop pod had 50 seats, for example, then 400 pods would need to depart every hour at a nine-second headway to match HS2’s capacity. Assuming the same number of seconds to alight from a Hyperloop pod as a train, 23 tubes would be needed to match HS2’s throughput.

None of this is to dismiss entirely Hyperloop’s prospects. Indeed, the eager and exceptional minds in organisations like VHO will doubtless continue their quest for answers. But no-one should yet claim that Hyperloop could replace steel-wheel rail, which is far from the outdated mode some would assert.

For the foreseeable future, Hyperloop is likely to remain a technological experiment meriting private backing, rather than public funding.

Gareth Dennis, a senior permanent way engineer for an international design consultancy, leads the local section of the Permanent Way Institution and is a lecturer on track systems at the National College for High Speed Rail. A version of this article first appeared in Railway Gazette International, reproduced here with permission.

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