> What happens to passengers if the seal fails on the vehicle, and then all the air is sucked into the vacuum?

I fly a lot for work. Every week, someone gives me a demonstration of what to do when this happens.

Unfortunately, that won't work. Oxygen masks only work up to a certain altitude, which is roughly the altitude where airliners cruise. At pressures lower than that, even pure oxygen isn't enough to sustain you.

You don't die instantaneously when exposed to vacuum, although it is pretty quick. I wonder if it would be possible to repressurize the tube (by blowing a big hole in it?) within 15 seconds or so.

The trouble with blowing a hole in the tube somewhere to re-pressurise is that whatever is inside the hole is hit by a wall of air travelling somewhere around the speed of sound. There are a LOT of problems with hyperloop not limited to someone walking up to the tube and shooting it with a .50 Desert Eagle, earthquakes, and the expansion and contraction of the tube as the sun rises and sets.

It's definitely the most unlikely of Elon Musk's plans.

The hyperlink won't be a vacuum. It's just a low air pressure chamber. The plan has never been to maintain a vacuum since that's too difficult at such large scales.

Hyperloop is planned to run at roughly 0.1% of sea level air pressure. For the purposes of a discussion about human survival in the event of pressurization failure, that's effectively a vacuum.

We already run vehicles in 0.14 atm environments. The issue of running these vehicles in an environment hostile to human life has been mitigated pretty well.

In regards to there being more than expected air in the tunnel - the vehicle would be slower, but not much else right?

Less air than expected inside the vehicle could be life-threatening for breathers. For humans we could solve this with an emergency oxygen hose at every seat like in planes. Pets and other oxygen dependant lifeforms wouldn't be protected by this system though. An alternative could be to have an electronic hatch at regular distances in the tunnel. So when a vehicle detects a massive oxygen leak inside the cabin, it could signal the nearby tunnel hatch to open. The whole tunnel, and thus also the vehicle [1] would fill with air pretty fast.

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[1] The vehicle would have a large hole in order for oxygen levels to suddenly go from life-sustaining to not. Thus that same hole would serve to refill the vehicle with oxygen coming from the tunnel hatch. However there could be an additional hatch for the vehicle to guarantee airflow. This could maybe even mount directly to a tunnel hatch, so that outside air would come to the vehicle and skip the tunnel.

Have you considered, that, in light of the fact that Elon wants to go to Mars, that research in having vehicles perform in a near-vacuum is a possible side-benefit for that goal?

My understanding is that labeling it a vacuum tube is a bit of a misnomer. It should be called a low-pressure environment or near-vacuum. Achieving that much reduction in air pressure will greatly reduce drag and allow for increased speeds. It doesn't need to be in a vacuum.

The maintainance of a vacuum tube this long and the consequences of a failure are a main theme in most hyperloop "debunking" videos on YouTube.

I'm guessing that any leak is likely to be small, and thus slow. And depending on how many vehicles are 'in flight' at once, and how easy it is to detect, they could be slowed down before hitting atmospheric pressure to avoid shocks or vehicle damage.