Several of the detractors of Hyperloop have suggested that it's an attractive target for terrorists. This is silly. No more than a few dozen people ever accumulate at the station. It's the same attractiveness as a city park or the lobby of an office building. If you want to kill lots of people, Hyperloop is a difficult way to do it.
Lets look at a few cases: The tube is steel, almost an inch thick. A pipe bomb or other crude explosive, such as used in the Boston bombing, will barely scratch the surface. A shaped charge might. Somebody with a large gun--a tank, for example, could. Rednecks taking pot shots can do little harm, unless they're using a Barrett 50 or bigger. We're not really talking about "terrorists" doing this sort of thing. This is a much more sophisticated operator.
What if they do manage to poke a hole big enough to cause loss of vacuum? Well, they've just shut the thing down. If a vehicle happened to be in the path of whatever did the damage, they'd be hurt, but any other vehicles would simply see a sudden rise in air pressure and the vehicles would slow to a stop due to air friction--if they're going at top speed, this could be several Gs. By designing the vehicle so it's still pretty slippery in normal air pressure, passengers would be uncomfortable, but unhurt. Get the passengers out and begin repairs.
I'm a lot more concerned about manufacturing errors and acts of nature than I am about terrorists. A lightning hit could potentially cause some damage, although I'd think grounding it properly should be easy. Manufacturing errors could cause splits, but they should mostly be slow leaks--and most of them should be discovered before it's carrying passengers.
The tube is stiff enough that if it's missing a few support columns, such as if a wayward truck ran into one, it should be inconsequential. They should be fixed as soon as practical but there's probably no need to even shut the thing down. Of more concern is if an earthquake or flood or coordinated attack damages a bunch of them. The tube should settle gracefully to the ground, where it's most likely still operable. One special case: the supports for spans that cross rivers or big valleys or such need to be specially hardened. This is already true for more conventional bridges.
One particular vulnerability is the slowdown LIM. By breaking the circuit, vehicles would still be going extremely fast at the end of their trip and crash into the station. A certain amount of redundancy can make this more difficult to attack and reduce the chance of malfunctions. In a worst case scenario, the tube can be opened to normal air pressure and the vehicles will slow down pretty quickly.
The vehicle carries a large battery, which powers life support and a large fan, which propels the vehicle enough to maintain speed in near-vacuum, and lifts it. I suspect this can be made to still work in normal atmosphere, but it won't propel the vehicle very fast. The system should be designed with emergency exits every few miles. The vehicle would move along to the nearest and stop. There should be a mechanism which allows the passengers to get out and then move the vehicle further to allow another vehicle to use the same stop. I suspect the emergency exit would be integrated with expansion and other sorts of airlocks. more on this in another article.
What happens if the vehicle itself loses power while the rest of the system is still fully functioning? It slows down--potentially fairly rapidly, depending upon how fully power is lost. If other vehicles are still functioning normally, the next in line will smack into it pretty fast. So there needs to be active communication between vehicles. If one slows mysteriously, or contact is lost, the ones behind need to slow, until they figure it out. It may be necessary to have some alternate way of moving the disabled vehicle--e.g. the one behind pushes. I'd hope to avoid this but it may be necessary to allow the system to come to atmospheric to recover the passengers in some cases of failed vehicle. Obviously it's worth a great deal of redundancy to minimize the chance of this happening.
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