[jyl]

I read a couple articles on the idea.

Without being a physicist, my questions would be:

- The whole concept requires the air in the tube to be at low pressure. Not a vacuum, but low enough to greatly reduce air resistance. Otherwise the pod will require continuous propulsion, instead of being accelerated once then getting small re-accelerations at intermediate points. I am not sure how low is "low", but I'd think we are talking 30,000 foot altitude equivalent or thereabouts. How do you maintain low pressure in a thousand miles of 20 foot diameter tube? What are the structural requirements, the energy requirements? What are the safety mechanisms: if the tube is badly holed at Bakersfield and the northbound pod approaching Bakersfield is slowed from 800 mph to 300 mph by the sea-level air pressure, does the following pod crash into it? Are passengers injured by the deceleration? If the pod keeps slowing, does it get stuck between intermediate re-acceleration stations?

- Along the same lines, the pods have compressor fans on the nose, that capture the air that would otherwise slow the pod down, compress it, send some of it to the air bearings, and some to the passenger cabin, then that air is exhausted out the back of the pod. He says this is not only feasible, but can be powered by a battery carried in the pod. Assuming that is so, suppose the compressor fan in pod #1 fails, and the pod slows from 800 mph to 300 mph, what stops pods #2, 3, 4 etc from crashing into pod #1 at 800 mph? How many compressor fans, batteries, etc do we need to have for redundancy?

- How big are the pods? The artist conceptions show a narrow capsule where people sit reclined, one behind the other, like daisy-chained F1 drivers. That is an obvious non- starter, the public won't travel like that. He has also talked about pods big enough to hold cars. Okay, that's more like it - is it practical to propel a pod big enough to hold, say, 50 people and 10 occupied cars? Because it seems the pods have hold enough people that they don't have to run too close together. At 800 mph, trying to coordinate pods that are gliding on air-bearings (so, the pod itself has no independent ability to accelerate or slow) seems tricky, too tricky to have them flying along 30 seconds apart.

- Why, exactly, does this save so much money? The articles I read suggested the tube would be elevated, so it can run through farm fields and over roads etc without expensive property acquisition, railway beds, crossing infrastructure, etc. If so, why can't a bullet train's tracks be similarly lightweight and elevated, with the same savings? I suspect the reason is that the train carries 100's of people and is massive, while the pod is small and light. But that gets to the preceding question. (By the way, this does touch on something I've never understood about the CA train - why doesn't it run down the median of Hwy 5, where the state already owns the land?)

There is a long, detailed PDF that probably answers these and more questions. I've started reading it but it is long . . .