Article Source: IMechE

Speaking of the Californian high-speed rail (HSR) line in 2013, entrepreneur Elon Musk said that it would be “slower, more expensive to operate and less safe than flying, so why would anyone use it?”

Yet outside the US the reality is that worldwide HSR networks totalling 30,000km carry 1.6bn passengers a year with an exemplary safety record.

In response to the Californian HSR proposal, Musk published his “Hyperloop Alpha” white paper, which proposed a transport system with minimal resistance to motion. This consisted of pods travelling at up to 1,200km/h in a vacuum tube depressurised to one thousandth of an atmosphere. The pods would have air-cushion suspension and be driven by linear induction motors. They would carry 28 passengers.

Less costly to build

Musk’s paper claimed that a Hyperloop from Los Angeles to San Francisco could be built for $6bn, compared with $77bn for the HSR line, and that its pods would cover the 560km in 35 minutes. Since Musk published his paper in 2013, various Hyperloop companies have raised hundreds of thousands of dollars. Musk’s company, SpaceX, has sponsored competitions for students to test their pods in a 1.6km tube. In 2016, 1,000 students from 20 countries took part.

Hyperloop companies are developing routes in various countries following reports from reputable consultancies showing the transformational benefits of the technology. These assume that Hyperloop will be operational within the next few years.

Yet bringing a new transport system into service is a formidable task that requires all parts to be at technology readiness level (TRL) 8, which means undergoing active commissioning.

In tests in a 500m tube, the maximum Hyperloop pod speed achieved to date is 387km/h. Such research facility testing is at TRL 4. Moreover, many aspects of the system are only at the stage of establishing basic principles (TRL 1). These include vehicle suspension, air locks, vacuum-tight tube expansion joints and switches.

Switches are required for proposed Hyperloop networks that need to route pods between tubes. Yet neither Musk’s paper nor any of the Hyperloop companies describe how their switches would work at 600km/h, the turnout speed needed to maintain the capacity of the system.

Assessing the risks

To avoid passengers being subject to excessive centrifugal acceleration, the tube radius of curvature would need to be at least 14km at this speed, resulting in a 250m splice between the tubes. Within this, the guidance system must prevent the pod hitting the joint between the tubes. Failure to do so would result in a catastrophic accident, releasing the kinetic energy of the pod equivalent to 50kg of TNT.

Other potential hazards relate to suspension, guidance, control system, braking during power failure, pod depressurisation, and passenger air supply during delays. Approval to operate would require a rigorous analysis to demonstrate that the residual risk from each identified hazard is acceptable. The absence of standards or risk data from a similar system would require extensive testing.

For these reasons the UK Department for Transport’s Science Advisory Council concluded that “an operational hyperloop is likely to be at least a couple of decades away”.

Aside from the engineering challenges, Hyperloop must deliver capacity to succeed commercially. At one-minute frequency, Hyperloop Alpha’s 28-seat pods would carry 1,680 passengers an hour. This is less than a tenth of the capacity of HS2’s 18 trains an hour.

Hyperloop has attracted much publicity claiming it will soon be in service. Yet any serious engineering analysis shows that it is decades away.

There are many reasons for this gulf between perception and reality, one of which is the need to understand that a transport system is much more than a pod in a tube.

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