I don't think that's right. The air bearings function based on pressure, not mass density. 50Pa air + 50Pa H2 would keep the overall tube pressure the same, so the compressors' job wouldn't change. In fact, since the H2 (in my proposed design) would be injected into the ski air stream post-compression, the compressors would have less work to do, not more. The required mass flow to the skis using an air/H2 mix would actually be substantially less than in the Alpha proposal. Again, the pressure is what's important (averaging 7kPa under the skis), not the mass per se.

Ok, let's look at an emergency scenario where a capsule (not the tube) undergoes rapid depressurization. To save the passengers, the ambient pressure in the entire tube must quickly (within a few seconds) be brought up to levels at which oxygen masks will function; about 20kPa. This can be done by flooding the tube with air evenly along its length; no tube destruction required. The question is whether a 20kPa tube atmosphere would impose problematic aerobraking forces on the capsules. At 700mph, you'd probably be ok. But at 1500mph, you'd immediately be exceeding the ambient speed of sound, which would be very bad. (Flooding the tube with a high H2 mixture to keep the speed of sound high would create a very explosive environment, so that's not an option.)

Long story short: if your capsule suddenly depressurizes at 1500mph, you're dead. But at 700mph, you might still be ok. The risks and complexities associated with hypersonic tube travel seem to outweigh the benefits, at least for now. Subsonic is good enough, really.

Keep in mind that these quotes are all excerpted from articles by bloggers, not engineers. The skis connect to the capsule through a hefty suspension, so minor bumps won't be transmitted to the passenger compartment. There will be no lateral acceleration experienced by passengers; the capsule would bank freely like a bobsled, so all banking acceleration will be perceived as vertical (as in an airplane), pressing you down into your reclined seat, thus much less barf-inducing. The noise levels shouldn't be problematic, since the ducted/bypassed air is still at extremely low pressure (2kPa, equivalent to 30km/90,000ft altitude), and can be sonically isolated from the cabin. The airflow around the exterior of the capsule is vanishingly thin, so it should produce hardly any noise at all.

The main advantage to hydrogen would be overcoming the Kantrowitz choking effect caused by supersonic flow; this is one of the most significant design constraints of the Hyperloop. Increasing the speed of sound (by mixing the air with hydrogen) increases the speed at which the ambient gas can flow around the capsule, greatly reducing the pressure that builds up in front at a given speed. Hydrogen is cheap; liquid hydrogen can be had for less than $1/kg, so cost is a non-issue.

Another design constraint of the Hyperloop is cooling the compressed ski air. The Alpha design calls for a 300kg water tank and intercooler, flash-heating the water to steam and storing it in "steam tanks", the complications of which are swept under the rug. (300kg of water would become 500 m^3 of steam at 1atm, several times the volume of the capsule itself.) By injecting liquid H2 into the air stream to cool it, the need for the intercooler and steam tanks would go away. Hydrogen has an exceptionally high specific heat; liquid hydrogen is extremely effective at removing heat from a system. Some of the compressed air + H2 could be further compressed and stored onboard the capsule to maintain the tube at 100Pa, or perhaps the excess H2 could be handled by placing adsorbent material (e.g. activated charcoal) in the tube to soak it up, and replacing/recharging this material at intervals. Since hydrogen moves so fast, placing the adsorbent material only at the endpoints might be sufficient.

The stopping distance is just fine. At 30 second intervals at full speed, the distance between capsules is about 10km. A 1g deceleration (combined force = 1.4g, equivalent to pressing hard on car brakes) will stop the capsule in less than 6km. No internal organ squishing required.

Passenger aircraft commonly bank at up to 30, which is a vertical g-force of 1.15g. The Tesla Model S P85D accelerates off the line at 1G longitudinal, for a combined g-force of about 1.4g. The Gravitron (common at carnivals) pulls 3g's continuous. Roller coasters commonly pull 4-5g's for short times. (though admittedly, that can be barf-inducing.) The Hyperloop is designed for a maximum combined g-force of 1.4g, corresponding to a 45 banking angle. With reclining seats, this should feel similar to a Model S accelerating off the line, and should be easily tolerated. The key consideration will be to minimize "jerk"; abrupt changes in acceleration. So the curves will have to be designed so the banking radius changes gradually rather than abruptly. No doubt there will be people who are intolerant of the Hyperloop, just as there are people who are phobic/intolerant of air travel. Perhaps there could be a designated "slow hour" where the capsules are run at 3/4 speed to reduce the g-forces. But with appropriate warning and visual feedback provided, I think most passengers would quickly get used to it.

The Hyperloop will bank freely like a bobsled; the passengers will experience virtually no lateral acceleration. (The same is true of an airplane in a tight bank.) This will make the ride far less barf-inducing. The lack of turbulence will also help greatly.

The Hyperloop skis will be on a flexible suspension; the pylons can sway many centimeters (e.g. in earthquakes) before the ride quality would be affected. In other words, the skis can move quite a lot to maintain precise contact with the tube, while the passenger compartment remains stable. The raw materials and basic construction will be the expensive part; polishing the interior to the required smoothness is comparatively trivial. The vacuum part is also fairly simple; the vast majority of the tube is simple continuous inch-thick steel, which is trivial to make airtight. The complexity will be at the endpoint stations and airlocks, but given that it's only a soft vacuum being maintained (100Pa), even this is easily manageable.

You could do a mixture of air with other gases, and gain many of the advantages while still avoiding a hard air vacuum. For instance, 50Pa air + 50Pa water vapor, or even 50Pa air + 50Pa H2. A promising approach would be for the capsules to store on board some of the air they're compressing anyway, to help maintain the tube pressure. The alternate gases could even be added as part of the cooling system; if liquid nitrogen or liquid hydrogen were injected directly into the compressed air stream to cool it, it would greatly reduce the need for water intercoolers and onboard steam storage, increase the available pressure to the skis, and be balanced by onboard storage of some of the compressed air stream. And the pressures are low enough that combustion shouldn't be a problem, even with H2. Vacuum leaks are most likely to come from two sources: the end station airlocks, and the capsules themselves. Most of the tube is just Big Dumb Pipe (tm), which really shouldn't leak. And 100Pa is really not too difficult to maintain; the volume of the entire LA-SF Hyperloop tube is equivalent to a cube about 130 meters on a side.

Branching at full speed is probably not possible with the Hyperloop as designed; the skis are curved to match the diameter of the tube, with a ~1mm clearance with the tube surface, so there is no passive tube design that could accommodate a "switch". In order to continue from Section A to either Section B or Section C, you'd have to make an intermediate length of tube several hundred meters long that could be physically moved at one end from B to C, with sub-millimeter precision, with the entire thing enclosed in vacuum. By the time demand is great enough to warrant branches, it's probably more cost-effective to make a dedicated parallel tube than to re-purpose a single tube with a ridiculously complicated switch. Hydrogen (or water vapor) would be most helpful in reducing Kantrowitz effects near the sound barrier, but not necessarily in enabling higher absolute speeds. The reason is threefold: drag continues to increase at higher speeds regardless of the speed of sound, lateral acceleration increases with the square of velocity, and the vertical precision of the pipe also improves with the square of the velocity. If you consider that the steel Hyperloop pipe draped across 30m-spaced pylons will approximate a vertical sine wave, then at 700mph the allowable sag is only about 5cm between pylons before the capsule's vertical suspension is overwhelmed and it starts "bouncing". (Assuming the mass of the skis/suspension is 10% of the capsule mass, so it can accelerate vertically at 10g to keep contact with the track.) At 1500mph, the tube requires a vertical precision of 1cm between 30m-spaced pylons, and its trajectory would have to be ridiculously straight to avoid problematic lateral g-forces. Mechanical braking from 1500mph in the event of an emergency is also a non-starter; 700mph is right at the edge of what can be feasibly done without melting brakes or destroying the tube. And in the event of a rapid tube repressurization, a 700mph capsule will incur about 2g's of aerobraking deceleration; at 1500mph it would experience about 10g's, likely enough to destroy the capsule and/or kill the passengers.

Air travel between LA and SF metro areas is the busiest air route in the US (and third-busiest in the world), with about 7.7 million passengers annually, or 21,000 per day. Assuming a 12-hour day, the Hyperloop could accommodate this with one 30-passenger capsule every two minutes each way, and the system is designed to quadruple this capacity at rush hour. (one capsule every 30 seconds.) Of course, if the Hyperloop is built, it will generate plenty of its own demand. And of course, it can be modified to carry cargo too (and possibly vehicles), not just passengers. The cost as outlined in the Hyperloop Alpha document is about 1/10th that of the proposed HSR.

"Existing elevated rail" is not a valid comparison. The Hyperloop infrastructure needs to support about 1/10th the weight per meter as traditional rail, therefore it can be done with 1/10th the materials. The proposed Keystone XL pipeline is 36 inches in diameter; the Hyperloop would be about 100 inches, but hollow and empty most of the time. Oil pipelines are full of oil, therefore quite heavy relative to diameter. In practice the total weight per linear meter of oil pipeline vs Hyperloop is about the same; 1 metric ton per meter. Traditional elevated rail is about 10 metric tons per meter.

Does the Touch ID imply that it also has an NFC chip for ApplePay? (Apparently it does, and the iPad Mini 2 doesn't.) That's an odd thing to leave off the comparison chart.

16:x sucks for work.

Unless x = 12.