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Under pressure: hyperloop structures and vehicles

Karen Steele, Principal rolling stock engineer

Did you know…
… that the pressure inside a hyperloop tube is likely to be equivalent to the pressure at an altitude of 200,000ft? That’s almost seven Mount Everests!

Hyperloop promises journeys faster than commercial aircraft, and the engineering challenge of designing a vehicle that protects passengers from extremely low external pressures is one the air industry has faced for decades.

Air pressure at a cruising altitude of 40,000ft is around 3psi – a challenging enough design obstacle to overcome – yet some hyperloop concepts propose a tube pressure of around 0.015psi to minimise the air resistance when travelling at very high speed – that’s like an aircraft flying 160,000ft above normal cruising height.

This presents several interesting design challenges to the structural engineers, including design of a reverse pressure vessel. The hyperloop tube will have a very low pressure on the inside and atmospheric pressure on the outside, but most pressure vessels are designed with higher pressures on the inside, resulting in tensile stresses within the structure. By contrast, the hyperloop tube will be subject to compressive stresses and will need to be designed with buckling in mind. This is by no means a show-stopper – submarine hulls are designed to withstand much higher pressure differentials – but it will certainly affect the materials selection and cost of manufacturing the tube.

What about design for fatigue strength? Differences in pressure across the hyperloop vehicle structure will be significantly greater than, say, an airliner – possibly around double if the interior of the vehicle is kept at atmospheric pressure. The number of pressurisation cycles it’ll be subjected to is also likely to be higher thanks to a more intensive operational duty. The vehicle structure will have to be designed to withstand this particularly demanding environment.

There is an alternative: at least one developer intends to keep the vehicle in a de-pressurised environment full-time, using portals to interface with the door to allow passengers on and off. This will significantly reduce the number of fatigue load cycles applied to most of the structure, though the structure around the door is likely to see a significant load cycle every time it opens and must be designed accordingly. This may involve making the structure strong enough to withstand the pressure cycle fatigue loads or designing the portal such that the load applied by the door onto the structure around it is seamlessly transferred to the portal when opening the door.

And then there’s the sealing of the door. The differences in pressure between the inside and outside of the hyperloop vehicle will be greater than for an aeroplane, and the flow rate of air loss from the interior will also be higher, assuming a similar level of sealing. This flow rate may not be acceptable. If that’s the case, then more effective methods of sealing the vehicle will have to be developed.

Karen Steele

Principal rolling stock engineer

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